A Study of Thermal and Nonthermal X-ray
Structures in the Galactic Center Region
Atsushi Senda
Department of Physics, Graduate School of Science, Kyoto University
Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japansenda@cr.scphys.kyoto-u.ac.jp

Reprinted from the doctoral thesis submitted to
the Department of Physics, Graduate School of Science,
Kyoto University
on January 5, 2005
in partial fulfillment of the requirements
for the degree of Doctor of Philosophy in physics.

Abstract
We present the result of an X-ray observational study of small scale structures in thecentral region of our galaxy. From the region, we resolved two discrete thermal X-ray
structures (G0.570−0.018 and G359.8−0.3) and tenth of nonthermal X-ray structures
for the first time mainly using Chandra X-ray Observatory.
G0.570−0.018 is a thermal source characterized by extraordinary high temperature(kT ∼ 6 keV) and shell-like morphology with a small diameter (less than 1 pc at the

8.0 kpc). The observed ionization parameter suggests the X-ray emitting plasma is
far from collisional ionization equilibrium. These unusual physical conditions are wellunderstood if G0.570−0.018 is an extremely young supernova remnant (SNR) before
reaching adiabatic expansion phase (Sedov phase). G359.8−0.3 is a nonthermal radio
shell located at the ∼ 30 pc south of the Galactic center. We successfully detected thethermal X-rays from the central region of the shell for the first time. X-ray spectrum of
G359.8−0.3 obtained with Chandra and XMM-Newton exhibits strong line emissions
from Si, S, Ar, and Ca. The physical properties deduced from observable parameters
are consistent with the interpretation that G359.8−0.3 is middle-aged (t ∼ 104 year)

SNR. Furthermore, we revealed that the nature of G359.8−0.3 can be well explainedin the framework of the “mixed-morphology” SNR.

In addition to these thermal structures, we discovered a number of nonthermal Xray features within the central 40 pc × 40 pc region of the galaxy using archival dataof Chandra. The series of Chandra data consists of 12 pointings and total eﬀective

exposure is ∼ 600 ksec, which provides us with the finest quality of X-ray spatial

information of the Galactic center(GC) so far. Eight of the discovered nonthermal Xray structures exhibit knot-like features with characteristic scales similar to each other.
In particular, radio counterparts are found to be associated with at least two of them.
Moreover, we also found two rim-like nonthermal structures in the southeast region of
the Galactic nucleus and a nonthermal clump embedded with the 6.4-keV clump in thenorth region. Photon indices of these nonthermal structures are determined to Γ ∼ 2or harder values. In the case of synchrotron emission, due to the extreme short lifetime(1–10 yr) of synchrotron X-rays under the strong magnetic field (B ∼ 1 mG) of theGC, continuous energy injection by high energy particles is required to maintain theobserved nonthermal structures. In other words, These X-ray knots should be the traceof the cosmic-ray accelerators at the GC. While other possible emission mechanism,

3i.e. inverse Compton scattering or nonthermal bremsstrahlung, cannot be excluded as
their origin so far.
Among them, the most outstanding features are the three X-ray knots which seem tobe aligned on a straight line from Sgr A∗ to northwest direction. The X-ray propertiesof the knots indicate their origin is associated with high energy activities of Sgr A∗ in
the recent past. These results are not only the discovery of the nonthermal structure
closest to the Galactic nucleus (d < 1 pc), but also have a possibility of the firstdetection of jets ejected from the nearest super massive black hole.

Contents
1 Introduction

13

2 Review : The Center of our Galaxy

19

2.1

∗

Sgr A : The Nucleus of our Galaxy . . . . . . . . . . . . . . . . . . . .

19

2.1.1

Discovery of Sgr A* . . . . . . . . . . . . . . . . . . . . . . . . .

19

2.1.2

Evidence for a super massive black hole . . . . . . . . . . . . . .

20

∗

2.1.3

X-ray observations of Sgr A . . . . . . . . . . . . . . . . . . . .

20

2.1.4

Emission mechanism of Sgr A∗ . . . . . . . . . . . . . . . . . . .

23

X-ray Diﬀuse Emission from the GC . . . . . . . . . . . . . . . . . . .

26

2.2.1

Large scale hot plasma . . . . . . . . . . . . . . . . . . . . . . .

26

2.2.2

6.4-keV emission from giant molecular clouds. . . . . . . . . .

27

2.3

Sgr A East . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

28

2.4

Nonthermal Radio Structures . . . . . . . . . . . . . . . . . . . . . . .

30

2.4.1

The Galactic Center Radio Arc (GCRA) . . . . . . . . . . . . .

30

2.4.2

Nonthermal Radio Filaments (NTRFs) . . . . . . . . . . . . . .

33

2.4.3

Strength of the magnetic field . . . . . . . . . . . . . . . . . . .

33

Gamma-ray Emissions from the GC region . . . . . . . . . . . . . . . .

34

2.2

2.5

26

2.5.1

Extended emission of 1.809 MeV

Al line . . . . . . . . . . . .

34

2.5.2

511 keV electron-positron pair annihilation line . . . . . . . . .

35

2.5.3

High Energy Gamma-rays from the GC region . . . . . . . . . .

36

3 Instrumentation
3.1

39

Chandra X-ray Observatory . . . . . . . . . . . . . . . . . . . . . . . .

39

3.1.1

Mission Overview . . . . . . . . . . . . . . . . . . . . . . . . . .

39

3.1.2

HRMA – High Resolution Mirror Assembly. . . . . . . . . . .

40

3.1.3

ACIS – Advanced CCD Imaging Spectrometer . . . . . . . . . .

43

1

2

CONTENTS

4 Thermal Structures in the GC region
4.1 X-ray Study of G0.570−0.018 . . . . . . . . . . . . . . . . . . . . . . .
4.1.1 Observations and data reductions . . . . . . . . . . . . . . . . .
4.1.2 X-ray morphology . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.3 X-ray spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.4 Physical parameters . . . . . . . . . . . . . . . . . . . . . . . .
4.1.5 The nature of G0.570−0.018 . . . . . . . . . . . . . . . . . . . .
4.2 X-ray study of G359.8−0.3 . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.2 Observations and data reductions . . . . . . . . . . . . . . . . .
4.2.3 X-ray and radio morphology . . . . . . . . . . . . . . . . . . . .
4.2.4 X-ray spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.5 The nature of G359.8−0.3 . . . . . . . . . . . . . . . . . . . . .
4.2.6 Distance of G359.8−0.3 . . . . . . . . . . . . . . . . . . . . . .

51
51
51
52
56
58
59
62
62
62
63
64
69
70

5 Nonthermal Structures in the GC region
5.1 Observations and Data Reductions . . . . . . . . . . . . . . . . . . . .
5.2 Jet-like Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1 X-ray spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.2 Radio counterparts . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.3 Proper motion measurements . . . . . . . . . . . . . . . . . . .
5.3 Outflow Structures in the Southeast Region . . . . . . . . . . . . . . .
5.3.1 Radial profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.2 X-ray spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4 GC “Fishhook”: A Nonthermal/6.4-keV Clump . . . . . . . . . . . . .
5.5 Other Nonthermal Structures . . . . . . . . . . . . . . . . . . . . . . .

71
71
74
78
78
79
83
83
83
89
92

6 Discussion
6.1 Brief Summary of the Results . . . . . . . . . . . . . . . . . . . . . . .
6.2 Contribution to the GC Diﬀuse Emission . . . . . . . . . . . . . . . . .
6.3 Emission Mechanism of Nonthermal Structures . . . . . . . . . . . . . .
6.4 Possible Relation to the Galactic Nucleus . . . . . . . . . . . . . . . . .
6.4.1 Spatial distribution of nonthermal structures . . . . . . . . . . .
6.4.2 Implication on past activities of the Galactic nucleus . . . . . .

99
99
100
101
101
101
106

7 Conclusion

107

List of Figures
1.1

2.1

The extinction (absorption plus scattering) cross section by interstellar
medium from the infrared (10 µm) to hard X-ray (0.1 nm) band (Ryter,1996). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

14

Orbit of nearby star (S2) around Sgr A∗ for about 10 years. Data points
between 1992-2001 and 2001.50 are obtained with SHARP camera on the
ESO NTT, while 2002.25, 2002.33, 2002.40, 2002.58 and 2002.66 are withCONICA/NAOS on the ESO VLT (Schödel et al., 2002). . . . . . . . . . .

2.2

21

Enclosed mass as a function of projected distance from Sgr A∗ Schödel et al.
(2002). An upward-pointing rectangle obtained from proper motions observed
Keck Observatory Ghez et al. (1998). Gray filled rectangles and open circles
are mass estimates by Genzel et al. (2000). Open rectangles is taken from a
result of Chandran (2001).

2.3. . . . . . . . . . . . . . . . . . . . . . . . .

22

Chandra ACIS view of the central 1′ × 1′ region of our Galaxy (Baganoﬀ etal., 2003). Energy band is 0.5−7 keV. The black cross represents the radio

position of Sgr A∗ determined by Reid et al. (1999). . . . . . . . . . . . . .

2.4

24

Upper two panels: Hard(a) and soft(b) band light curves of the X-ray flareobtained with Chandra ACIS on 26−27 October 2000. Both plots are constructed from counts accumulated within a radius of 1.′′ 5 of Sgr A∗ . Lower
two panels: Hard/soft band ratio within annulus of inner and outer radii
0.5′′ (c) and 2.5′′ (d). (Baganoﬀ et al., 2001) . . . . . . . . . . . . . . . . .

2.5

Spectrum of the diﬀuse emission from 1◦ × 1◦ region of the GC obtained with

ASCA. (Koyama et al., 1996). . . . . . . . . . . . . . . . . . . . . . . . .

2.6
2.7

Surface brightness map of the 6.4-keV line emission. (Koyama et al., 1996). .

25
26
26

Adaptively smoothed X-ray image of Sgr B2 cloud with Chandra ACIS-I.
Overlaid contours indicate 13 CO line flux. The positions of the resolved point
sources are shown by small circles (Murakami et al., 2001b).

3. . . . . . . .

28

4

LIST OF FIGURES
2.8 X-ray spectrum of Sgr B2 cloud. The source and background regions areshown in the rectangle and the dotted circle in Figure 2.7, respectively.
Dashed line indicates the contribution of all the resolved point sources (Murakami et al., 2001b). . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.9

VLA 20 cm image of Sgr A complex with 1.3′′ × 2.5′′ resolution (Pedlar etal., 1989).

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

28
29

2.10 Same region of Figure 2.9viewed with VLA 6 cm. Spatial resolution is1.3′′ × 2.5′′ (Pedlar et al., 1989). . . . . . . . . . . . . . . . . . . . . . . .

29

2.11 VLA 330 MHz (90 cm) image of the central ∼ 1.0◦ × 1.2◦ region of the Galaxy.

The beam size of the observation is 12′′ × 7′′ and the rms noise is 1.6 mJybeam−1 . Color scale is linear between -2 and 50 mJy beam−1 (Nord et al.,2004). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

31

2.12 A schematic of the distribution of nonthermal radio filaments (NTRFs) near
the Galactic center based on th VLA 90 cm wide-field image. A dashed line
represents the Galactic plane and an asterisk represents the position of Sgr A∗ .
Confirmed NTRFs are shown in Heavy dark lines, while NTRF candidates
are shown in thin lines (LaRosa et al., 2004).

. . . . . . . . . . . . . . . .

32

2.13 A 26 Al line (1.809 MeV) intensity map of the Milky Way. Continuum emission
is subtracted (Diehl et al., 1995). . . . . . . . . . . . . . . . . . . . . . . .

35

2.14 Background-subtracted 511 keV count rate map as a function of Galactic
Longitude for the region |b| < 20◦ obtained with INTEGRAL/SPI (Teegarden

et al., 2004). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.15

Gamma-ray significance map within 3◦ × 3◦ of the GC centered on Sgr A∗

obtained with the H.E.S.S. observation (Aharonian et al., 2004). . . . . . .

35
37

2.16 Energy spectrum of gamma-rays from the Galactic center (Aharonian et al.,2004). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

37

3.1 The overall view of Chandra X-ray Observatory. This image is credited toNASA/CXC/SAO. http://chandra.harvard.edu/resources/ . . . . . . . . . .

39

3.2 A cross-sectional view of HRMA mirror assembly. (Chandra Proposers’ Observatory Guide 2003) . . . . . . . . . . . . . . . . . . . . . . . . . . . .

41

3.3 The HRMA, HRMA/ACIS, and HRMA/HRI eﬀective areas as a function of
X-ray energy (Chandra Proposers’ Observatory Guide 2003) . . . . . . . . .

42

LIST OF FIGURES
3.4

5

The HRMA eﬀective area as a function of oﬀ-axis angle for selected energies.
Data represents azimuthal averaged value and normalized to that of on-axis
area for each energy (Chandra Proposers’ Observatory Guide 2003). . . . . .

3.5

42

The Chandra HRMA on-axis PSFs of several representative incident X-ray
energies. The fraction of the encircled energy as a function of angular radius
is presented. (Chandra Proposers’ Observatory Guide 2003).

3.6. . . . . . . .

44

The dependences of the Chandra HRMA/ACIS-I PSF on oﬀ-axis angle for
given incident X-ray energies. The 50% and 90% encircled energy radii as afunction of oﬀ-axis angle at two incident X-ray energies (1.49 and 6.40 keV)are presented. (Chandra Proposers’ Observatory Guide 2003). . . . . . . . .

3.7

45

(Top) A photo of the whole view of the ACIS instrument. (Bottom) the
configuration of the ACIS array on the focal plane. Insight to the terminology
is given in the lower left (Chandra Proposers’ Observatory Guide 2003).

3.8

. .

Vignetting eﬀect(defined the ratio of oﬀ-axis to on-axis eﬀective area) as afunction of energy for several oﬀ axis angles in arcminutes. . . . . . . . . .

3.9

46
48

The ACIS pre-launch energy resolution as a function of incident X-ray energy. 48

3.10 A Non X-ray Background (NXB) spectra taken by ACIS CCDs. Chip S2 issimilar to the ACIS-I chips (denoted I023 in the figure) and not shown forclarity. Line features are due to fluorescence of material in the telescope andfocal plane instrument. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.1

50

ASCA GIS image of G0.570−0.018 with 6.0−7.0 keV band. The spectrum is
extracted within the 3′ circle (solid line), while the background is from theannulus between the solid and dashed circles (3′ < r < 4′ ). The dotted lines
are the Galactic coordinates with grid spacing of 5′ .

4.2

. . . . . . . . . . . .

Chandra ACIS image of G0.570−0.018. The energy band is 4.0−7.0 keV.
Gaussian smoothing of σ = 2 pixels is applied for the image. . . . . . . . .

4.3

. . .

55

X-ray spectra of G0.570−0.018. Black crosses indicate the data point obtainedwith Chandra, while orange crosses indicate ASCA data.

4.5

54

A radial profile of the ring-like structure of G0.570−0.018 resolved with Chandra ACIS. The center position of the profile is mentioned in the text.

4.4

53

. . . . . . . . . .

57

Morphological evolution of the X-ray remnant of SN 1987A from Dec. 2000
to Dec. 2002 with Chandra(Park et al,, 2004). . . . . . . . . . . . . . . . .

61

6

LIST OF FIGURES
4.6 Chandra ACIS image of G359.8−0.3 (color map) with logarithmic scale. 1.5−4.0 keV
photons are accumulated to construct the image. A solid gray zigzag line indicate the boundary of ACIS CCDs. White Contours indicate intensity of 10 cmradio continuum emission obtained with Nobeyama Radio Observatory (NRO). 65

4.7 XMM-Newton EPIC image of G359.8−0.3 (color map) with logarithmic scale.
X-ray photons detected from MOS1a MOS2 and pn detector are all combined
to construct the image. Exposure and vignetting eﬀect are collected. A solid
gray circle indicates FOV of X-ray mirror of XMM-Newton. White Contours
indicate intensity of radio 10 cm continuum emission obtained with NRO.

.

66

4.8 A radial profile of G359.8−0.3 obtained with XMM-Newton EPIC. 1.5−4.0
keV X-ray Photons accumulated with MOS and pn detectors are used to draw
the profile. Exposure and vignetting eﬀect are collected. Center position of
the profile is at the peak point of the X-ray excess. . . . . . . . . . . . . .

67

4.9 Upper panel: The X-ray spectrum extracted from the central region of G359.8−0.3.
Black, red, blue, and green crosses represent data points obtained with Chandra ACIS, XMM-Newton EPIC MOS1, MOS2, and pn detector, respectively.
Parameters of the Fitting model (NEI ) are fixed to the same value for all
data. The best-fit model is shown in solid lines. Lower panel: Residuals fromthe best-fit model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

68

5.1 Three-color image of the central 17′ × 17′ region of our Galaxy with Chan-

dra ACIS. Red (6.50−7.07 keV) represents hot plasma components, Green(6.30−6.50 keV) represents fluorescent emission from neutral Fe-Kα, and Blue
(3.42−6.30 keV) represents continuum-dominated emission. Point sources are
removed then the holes are filled with an extrapolation of nearby emission. .

73

5.2 An adaptively smoothed 3.0−8.0 keV image of the northwest region of the
Galactic center with Chandra ACIS. An adaptive smoothing and a correction
for exposure variations were performed. . . . . . . . . . . . . . . . . . . .

75

5.3 X-ray images (left panels) and spectra (right panels) of the jet-like structures(knot 1–3) with Chandra ACIS. Left panels: The images shows 3.0–8.0 keV
X-ray photons from 3 to 300 count par pixel with a logarithmic scale. Allof detected point source is excluded from the image. Right panels: Source
photons of each spectrum (solid crosses) within the solid elliptical region
shown in the corresponding left panel. Solid line represents the best-fit powerlaw model with ISM absorption. Parameters are shown in Table 5.2. . . . .

76

LIST OF FIGURES
5.4

7

Merged X-ray spectrum of knot1–3. The best-fit power-law model convolved
with interstellar absorption is given in solid line. The best-fit parameters areshown in Table 5.2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5.5

77

The adaptively smoothed 3.0−8.0 keV image of the northwest region of the
Galactic center with Chandra ACIS. Overlayed contours indicate radio continuum intensities of 6 cm (left) and 20 cm (right) band obtained with VLA.
The black point of each image represents the position of Sgr A∗ .

5.6

. . . . . .

80

Linear profile of knot1–3 made from 3–8 keV photons. Red data shows represents the profile of Period 1, while black dat represents the profiles of Period
2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5.7

82

Chandra ACIS-I image of the Southeast outflow region in logarithmic. Energy
band is 3.42–6.30 keV and all detected point sources are excluded from theimage. A center of the small gray circle indicates the position of Sgr A∗ . . .

5.8

84

The radial profile of the southeast outflow region, which are extracted from a
sector described with solid lines in Figure 5.7. The inner and outer annuli inthe sector represent the source region of “Inner rim” and “Outer rim”, while
an annulus between dashed-lined arcs is that of “Whole outflow”.

5.9. . . . .

84

X-ray spectra of the Outer rim (left) and Inner rim (right) in the southeast
outflow. Solid line in the upper panel of each image indicates the power-law
model with the best fit parameters shown in Table 5.3. . . . . . . . . . . .

86

5.10 X-ray spectrum of the whole region of the Southeast Outflow. X-ray Emissions from inner and outer rims are excluded. Dashed lines in the upper panelof the image indicate the 2-kT NEI (red and green) + 6.4-keV Gaussian (blue)
model with the best-fit parameters shown in Table 5.4. While the lower panel
shows residuals from the model. . . . . . . . . . . . . . . . . . . . . . . .

87

5.11 Chandra ACIS-I image of the GC “Fishhook” in logarithmic scale. Energy
band is 3.42–6.30 keV. Source and surrounding background regions for the
spectra shown in Figure 5.12is shown with solid lines.

. . . . . . . . . . .

90

5.12 X-ray spectra of the “Fishhook” region. Top (upper left panel), Middle (upper
right), Bottom (lower left), and East (lower right) is presented. Solid, dashed,
and dotted-dashed line represent the fitting model with the best parameters
shown in Table 5.5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

91

8

LIST OF FIGURES
5.13 The spatial distribution of the X-ray knots discovered near the Galactic centerwith Chandra ACIS-I. The gray scale represents intensity of 3.42–6.30 keV
photons in logarithmic scale. The detected point sources are all excluded
from the image (see the section 5.1). The center of the gray circle indicatesthe position of Sgr A∗ . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

93

5.14 Close up X-ray image (top) and X-ray spectrum (bottom) of knot4 with
Chandra ACIS-I. Top: The X-ray image are constructed from 3.0–8.0 keV
X-ray photons from 3 to 300 counts par pixel with a logarithmic scale. The
detected point sources are excluded from the image. Source photons of thespectrum shown in bottom are accumulated within the solid line ellipse. Bottom: Crosses and a solid line in the upper panel show data points and best-fit
power-law model, respectively. while crosses in the lower panel give residuals
from the best-fit model. . . . . . . . . . . . . . . . . . . . . . . . . . . .

94

5.15 Close up X-ray image (top) and X-ray spectra (bottom) of knot5 and 6 with
Chandra ACIS-I. Top: The X-ray image are constructed from 3.0–8.0 keV
X-ray photons from 3 to 300 counts par pixel with a logarithmic scale. The
detected point sources are excluded from the image. Source photons of thespectrum shown in bottom are accumulated within the solid line ellipse. Bottom: X-ray spectra of knot5 (left) and knot6 (right). Crosses and a solidline in the upper panel show data points and best-fit power-law model, respectively. while crosses in the lower panel give residuals from the best-fit
model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

95

5.16 Close up X-ray image (top) and X-ray spectrum (bottom) of knot7 with
Chandra ACIS-I. Top: The X-ray image are constructed from 3.0–8.0 keV
X-ray photons from 3 to 300 counts par pixel with a logarithmic scale.The
detected point sources are excluded from the image. Source photons of thespectrum shown in bottom are accumulated within the solid line ellipse. Bottom: Crosses and a solid line in the upper panel show data points and best-fit
power-law model, respectively. while crosses in the lower panel give residuals
from the best-fit model. . . . . . . . . . . . . . . . . . . . . . . . . . . .

96

LIST OF FIGURES

9

5.17 Close up X-ray image (top) and X-ray spectrum (bottom) of knot8 with
Chandra ACIS-I. Top: The X-ray image are constructed from 3.0–8.0 keV
X-ray photons from 3 to 300 counts par pixel with a logarithmic scale. The
detected point sources are excluded from the image. Source photons of thespectrum shown in bottom are accumulated within the solid line ellipse. Bottom: Crosses and a solid line in the upper panel show data points and best-fit
power-law model, respectively. while crosses in the lower panel give residuals
from the best-fit model. . . . . . . . . . . . . . . . . . . . . . . . . . . .

6.1

6.2

97

The Angle of major axis with respect to the Sgr A*–knot line (|θ − φ|)is
described by a function of the distance from Sgr A∗ . The definition ofθ and φ is shown in the Table 6.1. The number above black crosses
represent knot ID. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
Photon index of each knot is described by a function of the distancefrom Sgr A∗ . The number above black crosses represent knot ID. . . . . 104

List of Tables
4.1
4.2

ASCA and Chandra observation log for G0.570−0.018 . . . . . . . . . . . .
Best-fit parameters of G0.570−0.018 for NEI model obtained from the combined fitting of ASCA and Chandra spectra . . . . . . . . . . . . . . . . .

4.3

52
57

Comparison of the parameters between G0.570−0.018 and the X-ray remnantof SN 1987A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.4The observation log of G359.8−0.3 . . . . . . . . . . . . . . . . . . . . . .

59
63

4.5
4.6

Best fit parameters of G359.8−0.3 for NEI and MEKAL model . . . . . . .

64

5.1
5.2

Comparison between G359.8−0.3 and other MM SNRs near the Galactic center region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

70

The observation log of the GC deep exposure with Chandra . . . . . . . . .

72

Best fit parameters for the straight-line X-ray knots fitted with a power-law
model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5.3Best fit parameters for the outer and inner rims of the southeast outflow with
diﬀerent models

5.4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Best fit parameters for the southeast outflow with thermal models . . . . .

5.5Best fit parameters for the GC “Fishhook” with a power-law + two Gaussiansmodel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5.6

79
86
88
90

Best fit parameters for the Nonthermal X-ray Filaments fitted with a powerlaw model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

92

6.1

The summary of X-ray properties of Nonthermal X-ray knots . . . . . . 102

6.2

Summary of radio properties of NTRFs / NTRF candidates detected at
the GC region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

11

Chapter 1
Introduction
As we all know, there are countless galaxies in the universe. Also as we know, our solar
system belongs to one of them; the Milky Way Galaxy, hence the center of our Galaxy(abbreviated to the Galactic center, or the GC, afterward) is the closest center of all thegalaxies. For example, the Galactic center is about one hundred times closer to us than
the nearest spiral galaxy (M31; Andromeda Galaxy) and more than a thousand times
closer than the nearest Active Galactic Nucleus (NGC 1068). In spite of its vicinity, we
had not been able to know much about the GC for the long years, because of the heavy
optical extinction of the interstellar medium which lie between the Solar system andthe GC. Assuming the distance from us to the GC is 8.0 kpc (McNamara et al., 2000)
and averaged number density of the galactic plane is 1 cm−3 , hydrogen column densityfrom the GC to us is amount to 5 × 1022 cm−2 . Therefore, as is shown in Figure 1.1,
we can see through the GC region with limited energy band; below λ ≃ 2 µm (radio
and near-infrared) and above E ≃ 2 keV (hard X-ray and Gamma-ray).
Radio observations opened up the first window to see the GC region, In 1933, C.
G. Jansky detected the first cosmic (extra-solar) radio emission and discovered that its
intensity is increased toward the Galactic center (Jansky, 1933). In 1950’s, the GC is
identified as the radio source and named Sgr A (Piddington & Minnett, 1951; McGee &Bolton, 1954), which is the strongest radio emitting source in the whole sky. Until now,
numerous higher spatial/spectral resolution radio observations have been performed.
They have revealed that complex features are highly concentrated on a few hundred
parsecs of the GC region.
Then near-infrared and X-ray observations have found a number of sources which
may have possible relation to high energy activities of the GC region. In particular,
13

14

CHAPTER 1. INTRODUCTION

Figure 1.1: The extinction (absorption plus scattering) cross section by interstellar
medium from the infrared (10 µm) to hard X-ray (0.1 nm) band (Ryter, 1996).

15
the Galactic nucleus (Sgr A∗ ) must play a crucial role for the high energy activitiesof the GC. As will be mentioned in Chapter 2, many observational results have been
accumulated to confirm that Sgr A∗ is a super massive black hole over this twenty years.
With Near-infrared high-resolution observations, proper motions of near-infrared stars
in the immediate vicinity (≃ 30 light days) of Sgr A∗ were successfully detected(Schödelet al., 2002). According to the recent measurement of Ghez et al. (2005), the shape of
their Keplerian orbits indicates that ∼ 3.7 (± 0.2) × 106 M⊙ is concentrated within the

45 AU at the position of Sgr A∗ . Recent high-resolution X-ray observations succeeded

in detecting point-like X-ray emission at the position of Sgr A∗ . However, its Xray luminosity is far dimmer than that expected from its enclosed mass (Eddington
luminosity) by 10−12 orders of magnitude (Baganoﬀ et al., 2001; Goldwurm et al.,
2003; Porquet et al., 2003).
On the other hand, strong 6.4-keV line emissions have been detected from giant
molecular clouds near the GC. The most plausible explanation for the 6.4-keV line
nature is provided by the X-ray reflection nebulae (XRNe) model: X-rays from past
activities of Sgr A∗ have been irradiating the nearby GMCs and they are now emitting
fluorescence of neutral iron and reflected X-rays (Koyama et al., 1996; Murakami et al.,2000, 2001a,b). The XRN model also suggest that Sgr A∗ is as luminous as 1039 ergs s−1
about 300 years hundred years ago. Thus, the nucleus of our Galaxy is far from static,
but exhibits dynamic characteristic with closely interacting with nearby environment.
These 6.4-keV features are not observed in the Galactic plane or bulge, but unique tothe GC, hence the origin of the neutral iron line must have a close relation with the
Galactic nucleus and/or its vicinities.
Another enigmatic feature within the central hundred parsecs region is a large scale
diﬀuse X-ray emission, discovered with Ginga (Koyama et al., 1989; Yamauchi et al.,1990) and ASCA (Koyama et al., 1996). Assuming that the extended X-ray emissionis thermal origin, the electron temperature is as high as kT ∼ 10 keV and their total
thermal energy should be up to Eth ∼ 1054 ergs. The huge energy corresponds with

explosion energy of 103 Supernovae. From the time scale of the diﬀuse hot plasma((5–10) × 104 year), the rate of SN must be 1 event / 50–100 year. However, such alarge number of supernova remnants have not been found in the GC region so far. An
alternative explanation is that some energetic explosion(s) of Sgr A∗ generate diﬀuse
hot gas in the GC. In this case, some gradient of electron temperature and ionization

16

CHAPTER 1. INTRODUCTION

parameter should be observed as a function of distance from the Sgr A∗ , however, wehave not obtained such indications so far. Thus, its origin remains an open issue overthe decade.
In contrast with the large thermal structures, a number of nonthermal radio emissions from linear filaments have been detected around the Galactic center (Yusef-Zadeh
et al., 1984; Yusef-Zadeh & Barry, J., 1989; Morris, 1994; LaRosa et al., 2000, 2004).
Their spatial distribution seems localized within only central hundred parsecs. Theiremission must be originated from synchrotron radiation from high energy particles.
Thus, these feature lead us to suspect that the Galactic center holds particle accelerators. In addition, it also suggests that the magnetic field strength around the GC is
extremely strong (B ∼ 1 mG) inferred from the extreme linearity of these filaments.

Recently, the discovery of non-thermal X-ray filaments (NTXFs) has been reportedwith Chandra and XMM-Newton observations (Wang et al., 2002; Sakano et al., 2003;
Lu et al., 2003), while their origins and connection of radio filaments are still unclear.
The origin of cosmic-ray is a long standing problem which has possible relation withthe high energy activity of the Galactic center. This serious issue has been unsolved
over a century since the discovery of cosmic-rays (Hess, 1911, 1912). Recently, X-ray
and TeV Gamma-ray observations have found robust evidence that shock fronts ofseveral young supernova remnants accelerate particles. So far, supernova shock fronts
are the first and the only case that a site of cosmic-ray acceleration is spatially identified(e.g., SN1006 (Koyama et al., 1995; Tanimori et al., 1998), RX J1713.7−3946 (Koyama
et al. , 1997; Enomoto et al., 2002) and RCW 86 (Bamba et al., 2000a; Borkowski etal., 2001a)). On the other hand, The cosmic-ray spectrum has a significant break atthe energy of ∼ 1015.5 eV, so called “knee energy”. The origin of cosmic rays under

the knee energy, at least, is thought to be Galactic, because an average magnetic fieldstrength of a galaxy is suﬃcient to confine them. It is no doubt that the GC region
has the deepest gravitational potential field in the Galaxy, hence we suspect that the
GC harbors the highest cosmic-ray accelerator in our Galaxy.
The objective of this thesis is to unveil the origin of high energy activities in the
GC region. More specific, to answer questions as shown below;- What injects the huge energy to the large scale X-ray diﬀuse emission; multiple

17supernovae occurred within the recent past or the energetic explosion(s) of the central
massive black hole ?
- What is the origin of the radio/X-ray nonthermal structures ?
- Is the Galactic center region the site of cosmic-ray acceleration ?
To give answers to these long standing problems, we think it is the best strategy to focus
attention on the X-ray diﬀuse structures resolved high spatial resolution observations.
Hard X-ray observation is essential for tracing high energy activities of the Galactic
center in the following reasons;- An X-ray photon energy is higher than that of radio and infrared by an order of 6−9.
- Recent technical advance can enable us to carry out high resolution X-ray imaging(∆θ ∼ 0.5′′ ) and spectral (E/∆E ∼ 50) study.
- At the GC, only hard X-ray band can cover both thermal emission from the hot gas
and nonthermal emission from the high energy particles.
Thermal structures may be relics of some heating process occurred in the past hundredsthousands years. On the other hand, the nonthermal X-ray structures can be the direct
evidence for the cosmic-ray accelerations. Hence we used archival data of Chandra inmain and performed systematical search for thermal and nonthermal X-ray structuresnear the GC region.
The road map of this thesis is as follows. In Chapter 2, We at first review the GC
sources which have close relation with high energy activities at the Galactic center.
Chapter 3 is devoted to summarize the basic characteristics of science instruments ofChandra X-ray Observatory. In Chapter 4, we present the results of X-ray analyses ofthermal features which we discovered in the Galactic center region; G0.570−0.018 andG359.8−0.3. The origins of these sources are also discussed in the chapter. In Chapter
5, We present the data analyses of the deepest exposure observations of Chandra.
Then we report the discovery and the X-ray properties of nonthermal X-ray features.
In Chapter 6, possible relations between thermal/nonthermal features and high energy
activities of the GC as well as the nature of the nonthermal structures are discussed.
Finally, we briefly summarize our work in Chapter 7.

Chapter 2
Review : The Center of our Galaxy
Since too many components are concentrated within the deep gravitational potential of
the Milky Way; central massive black hole, HII regions, massive star clusters, supernova
remnants, cold molecular clouds, diﬀuse hot gas, nonthermal sources, and etc, we
cannot cover all of these features in the limited space. Fortunately, we can find several
good reviews about the past study of GC region (Mezger et al., 1996; Morris & Serabyn,1996). Therefore, in this chapter, we briefly review the selected sources/structures
which are thought to be closely related to our work.

2.1

Sgr A∗: The Nucleus of our Galaxy

Many galaxies are thought to harbor super massive black holes (MBH ≥ 106 M⊙ )at their dynamical centers. Sgr A∗ is not only the nearest galactic nucleus, but thehighest probable super massive black hole candidate. Note that detailed reviews of Sgr
A∗ are provided by Genzel et al. (1994) and Melia & Falke (2001), hence here we give a
brief summary for the past results and mention in details only for recent observational
results.

2.1.1

Discovery of Sgr A*

Sgr A∗ was discovered by Balick & Brown (1974) as a bright (brightness temperature≥ 107 K), compact (θ ≤ 0.′′ 1) radio source. A position of the source is in the directionof the inner 1 pc core of a dynamical center of our Galaxy. Ever since Sgr A∗ hasbeen the primary candidate for the nucleus of our Galaxy. In fact, radio luminosity of
Sgr A∗ (∼ 1034 ergs s−1 ) is within range with the nuclear radio sources of other nearby
normal galaxies such as M31.
19

20

CHAPTER 2. REVIEW : THE CENTER OF OUR GALAXY

2.1.2

Evidence for a super massive black holeA strong evidence for mass concentration at the position of Sgr A∗ has been provided byproper motion measurements of nearby stars. According to Eckart & Genzel (1997) andGhez et al. (1998), stellar speeds in excess of 1000 km s−1 at a distance of 0.015 pc fromSgr A* indicate a central dark mass of ∼ 2.6 × 106 M⊙ confined to a volume of at most
10−6 pc−3 . The huge mass concentration within such a compact region is consistent

with a model that Sgr A∗ is associated with a super massive black hole. Its nonthermal radio spectrum(Serabyn et al., 1997) and extremely slow proper motion(Backer& Sramek, 1999; Reid et al., 1999) also support the model.
Recently, Schödel et al. (2002) reported a result of high-resolution near-infraredobservations which have been tracing a proper motion of the closest star (named S2)around Sgr A∗ for about 10 years. The positional data of S2 trace two-thirds of a closed
elliptical orbit and robustly explained by the Keplerian orbit around a central point
mass located at the position of Sgr A∗ (Figure 2.1). An orbital period and a pericenter
distance of S2 are determined to be 15.2 years and 17 light hours (∼ 124 AU), respectively. This proper motion measurement strictly constrains a mass distribution of Sgr
A∗ . As shown in Figure 2.2, pericenter data of S2 probes the mass concentration within
2100 times schwarzshild radius of 3 × 106 M⊙ black hole. The best fit mass distribu-

tion model for all observational data shown in Figure 2.1 is that (2.6 ± 0.2) × 106 M⊙

point-like mass plus a stellar cluster of central density 3.9 × 106 M⊙ pc−3 (solid curvein Figure 2.2).

At the stage, alternative models which have been previously discussed (dense clusterof dark stellar objects or massive degenerate fermion ball) are excluded for the mass
origin of Sgr A∗ .

2.1.3

X-ray observations of Sgr A∗

Poor spatial resolution had prevented us from estimating an intrinsic X-ray luminosityof Sgr A∗ for a long time. In 1990’s, ASCA achieved a hard X-ray imaging spectroscopy
with moderate spatial resolution (θ ∼ 3′ ) for the first time. However, no clear X-ray

counterpart can be found with ASCA, because of a serious contamination from nearby

variable hard X-ray source AX J1745.6−2901, which was turned out to be a low mass

2.1. SGR A∗ : THE NUCLEUS OF OUR GALAXY

Figure 2.1: Orbit of nearby star (S2) around Sgr A∗ for about 10 years. Data points

between 1992-2001 and 2001.50 are obtained with SHARP camera on the ESO NTT,
while 2002.25, 2002.33, 2002.40, 2002.58 and 2002.66 are with CONICA/NAOS on
the ESO VLT (Schödel et al., 2002).

21

22

CHAPTER 2. REVIEW : THE CENTER OF OUR GALAXY

Figure 2.2: Enclosed mass as a function of projected distance from Sgr A∗ Schödelet al. (2002). An upward-pointing rectangle obtained from proper motions observed
Keck Observatory Ghez et al. (1998). Gray filled rectangles and open circles are
mass estimates by Genzel et al. (2000). Open rectangles is taken from a resultof Chandran (2001).

2.1. SGR A∗ : THE NUCLEUS OF OUR GALAXY

23

X-ray binary located at ∼ 1.′ 5 southwest of the position of Sgr A∗ (Maeda et al., 1996).
An upper limit of an X-ray luminosity of Sgr A∗ inferred from ASCA observations is
3 × 1035 ergs s−1 in 2−10 keV band (Maeda, 1998; Sakano et al., 2002). This value isconsistent with the result of a Beppo-SAX measurement Sidoli & Mereghetti (1999).
After 2000, thanks to an excellent angular resolution of Chandra, an X-ray source
was resolved at the position of Sgr A∗ for the first time(Baganoﬀ et al., 2003). A
coincidence between the X-ray source and the radio position of Sgr A∗ is within 0.′′ 27(Figure 2.3).
In addition, X-ray flaring events from Sgr A∗ have been recently reported with
Chandra (Baganoﬀ et al., 2001) and XMM-Newton (Goldwurm et al., 2003; Porquet
et al., 2003). Figure 2.4 shows a light curve of the first detected X-ray flare of SgrA∗ (Baganoﬀ et al., 2001). Although some diﬀerences exist between these flares, they
seem to share basic properties; flare duration time-scales (103 -104 sec) and luminosity
ratios of a flaring state to a quiescent (∼ 50−150). These X-ray features support theidea that Sgr A∗ is associate with a super massive black hole.

2.1.4

Emission mechanism of Sgr A∗

A bolometric luminosity of Sgr A∗ is below 3 × 10−8 of its corresponding Eddington
Luminosity of 3.4 × 1044 ergs s−1 (e.g. Melia & Falke (2001)), which is far fainterthan those of typical extra-galactic AGNs. Moreover, An X-ray luminosity of Sgr A∗
is estimated to ∼ 2.4 × 1033 ergs s−1 (2−10 keV) in a quiescent state (Baganoﬀ et al.,2003) or ∼ 2.4 × 1035 ergs s−1 even in the brightest flaring state so far detected (Porquet
et al., 2003). These X-ray luminosities are fainter than its Eddington luminosity by
about 9−11 orders of magnitude.
The weakness of emission from Sgr A∗ requires a lack of accretion matter, radiative
ineﬃcient accretion flow, or some other mechanisms. Various models of radiation
mechanisms have been proposed to explain the spectral energy distribution (SED) of
Sgr A∗ on the basis of radiative ineﬃcient accretion flows (RIAFs); Bondi-Hoyle flows(Melia, 1992; Melia & Falke, 2001), advection dominated accretion flows (Narayan etal., 1996), RIAFs with non-thermal electrons(Yuan et al., 2003), etc.
On the other hand, alternative models have been proposed. They are constructedfrom the standpoint that not only accretion flows, but also outflows (such as a jet) playa significant part in the observed emission (Falke & Markoﬀ, 2000), such as Jet-RIAF
model (Yuan et al., 2002).

24

CHAPTER 2. REVIEW : THE CENTER OF OUR GALAXY

Figure 2.3: Chandra ACIS view of the central 1′ × 1′ region of our

Galaxy (Baganoﬀ et al., 2003). Energy band is 0.5−7 keV. The black
cross represents the radio position of Sgr A∗ determined by Reid et al.
(1999).

2.1. SGR A∗ : THE NUCLEUS OF OUR GALAXY

Figure 2.4: Upper two panels: Hard(a) and soft(b) band light curves of

the X-ray flare obtained with Chandra ACIS on 26−27 October 2000. Bothplots are constructed from counts accumulated within a radius of 1.′′ 5 of
Sgr A∗ . Lower two panels: Hard/soft band ratio within annulus of innerand outer radii 0.5′′ (c) and 2.5′′ (d). (Baganoﬀ et al., 2001)

25

26

CHAPTER 2. REVIEW : THE CENTER OF OUR GALAXY

2.2

X-ray Diﬀuse Emission from the GC

2.2.1

Large scale hot plasma

The first X-rays from inner few degrees of the GC were discovered with UHURU satellite (Kellogg et al., 1971). However, the specific X-ray feature of the Galactic centerregion have not been debatable before 1980’s because of technical diﬃculties of the hard
X-ray imaging and spectroscopy. With Ginga satellite, Koyama et al. (1989) discovered
diﬀuse X-rays with a strong iron line from an extended region near the Galactic Center.
The total thermal energy is ∼ 1054 ergs with an electron temperature of kT ∼ 10 keV.

Figure 2.5: Spectrum of the diﬀuse emission

from 1◦ × 1◦ region of the GC obtained with
ASCA. (Koyama et al., 1996).

Figure 2.6: Surface brightness map of the
6.4-keV line emission. (Koyama et al., 1996).

The ASCA observations confirmed the Ginga result and revealed more detailed
structures in the GC region (Koyama et al., 1996). As shown in Figure 2.5, The strong
iron line in a ∼ 1◦ × 1◦ region was resolved into [ three lines at 6.9, 6.7, and 6.4 keV,which are attributable to H, He-like, and neutral irons, respectively. The former two
are due to high temperature plasma, while the latter indicates the presence of cold gas
in a strong X-ray field. The intensities of 6.9 and 6.7 keV lines suggest that the theplasma temperature is a few–10 keV, roughly consistent with Ginga result. Since sucha high-temperature plasma cannot be bounded by the Galactic gravity, the plasma
will expand. It takes 5 × 104 year for the X-ray emitting plasma to expand observed

2.2. X-RAY DIFFUSE EMISSION FROM THE GC

27

size (∼ 80 pc) with the sound speed of 10 keV (∼ 1600 km s−1 ). This time-scale may
be considered to be the age of the plasma. The total thermal energy equivalent with
explosion energy of 103 supernovae. These huge energy and short escape time lead us
to suspect that the Galactic center has experienced energetic explosion(s) of Sgr A∗
or multiple supernovae within the past 5 × 104 years. Chandra observation resolved
a number of point sources were resolved in the GC region with its excellent spatial
resolution, however, the contribution of the resolved point sources is less than 10 % of
total Galactic center X-ray emission (Wang et al., 2002; Muno et al., 2003). Hence,
the origin of the X-ray diﬀuse emission is still an open issue.

2.2.2

6.4-keV emission from giant molecular clouds

The ”X-ray quiet” Sgr A∗ is in sharp contrast to the surrounding high temperatureplasma represented by strong 6.9 and 6.7 keV line emissions. The 6.4-keV line emission
can be a clue to connect the quiet Sgr A∗ and its violent surroundings. ASCA foundthat the 6.7-keV and 6.9-keV emission is uniformly distributed, while the 6.4-keV
emission is not. As shown in Figure 2.6, the brightest regions are spatially coincident
with the position of “Sgr B2 region” and “Radio Arc region”, both are known to be
the site of giant molecular clouds (GMCs). They both located in the positive galactic
longitude. On the other hand, strong 6.4-keV emission has also found from Sgr C
with ASCA (Murakami et al., 2001a), which is GMC located in the negative Galactic
longitude, where the allmost mirror position of Sgr B2 with reference to the Galactic
nucleus (Sgr A∗ ).
Chandra has made it possible to study quantitatively the X-ray spectra and morphologies of these cold molecular clouds. Murakami et al. (2001b) revealed that diﬀuse
X-rays from the Sgr B2 have a concave shape pointing to the GC direction as shownin Figure 2.7, then confirmed that the X-ray characteristics of Sgr B2 can well be explained by the “X-ray Reflection Nebula (XRN)” model; strong external X-rays fromthe GC direction hit a cold molecular cloud, then it reflects Thomson scattering X-rays
and emit 6.4-keV line emissions via photo-electric absorption and fluorescence by cold
(neutral) iron atoms (Figure 2.8).
One problem is that we can find no external X-ray source which is strong enoughto explain the 6.4-keV from the XRNe. To explain the luminosity of Sgr B2 cloud,required luminosity of an irradiating source is ∼ 1039 ergs s−1 , which is larger than
Eddinton luminosity of a neutron star. The most plausible scenario is that Sgr A∗ is

28

CHAPTER 2. REVIEW : THE CENTER OF OUR GALAXY

X-ray spectrum of Sgr B2
cloud. The source and background regions
are shown in the rectangle and the dotted
circle in Figure 2.7, respectively. Dashed line
indicates the contribution of all the resolved
point sources (Murakami et al., 2001b).

Figure 2.8:
Figure 2.7: Adaptively smoothed X-ray im-age of Sgr B2 cloud with Chandra ACIS-I.
Overlaid contours indicate 13 CO line flux.
The positions of the resolved point sources
are shown by small circles (Murakami et al.,2001b).

faint at present but was bright in the past equivalent with light traveling time fromSgr A∗ to each XRN. In this case, we are observing delayed X-rays, traveling extra
distance from Sgr A∗ to the molecular clouds. Taking projected distance to the each
XRNe into account, Sgr A∗ must be as bright as 1039 ergs s−1 until recent past of
100–300 years (Murakami et al., 2001b).

2.3

Sgr A East

Sgr A East is a bright, nonthermal, shell-like structure with a scale of 3.5′ × 2.5′ just at
the Galactic center (Ekers et al., 1975). As shown in Figure 2.9 and 2.10, in projection,
Sgr A East include the radio structure Sgr A West, which contains the nonthermal
point source Sgr A∗ and a three-arm spiral structure called the “mini spiral” which isthought to be orbit around Sgr A∗ . Although the center of the Sgr A East radio shellis oﬀsetted from the position of Sgr A∗ and Sgr A West by about ∼ 50′′ , a numberof observations suggest that Sgr A∗ and Sgr A West is physically located very near or
possibly embedded with Sgr A East (Yusef-Zadeh, 2000).
The radio morphological and spectral properties of Sgr A East can be explained in
the context of SNR (Jones, 1974; Ekers et al., 1983). However, the proximity of Sgr2.3. SGR A EAST29

A East and Sgr A∗ evoke alternative interpretation of the origin; e.g. the remnant ofan energetic explosion triggered by some gravitational interactions occurred in Sgr A∗(Yusef-Zadeh & Morris, 1987a; Khokhlov & Melia, 1996).

Figure 2.9: VLA 20 cm image of Sgr A com-plex with 1.3′′ × 2.5′′ resolution (Pedlar etal., 1989).

Figure 2.10: Same region of Figure 2.9
viewed with VLA 6 cm. Spatial resolution
is 1.3′′ × 2.5′′ (Pedlar et al., 1989).

With the hard X-ray (2−10 keV) observation of ASCA, Koyama et al. (1996) remarked on an extended emission of 2′ × 3′ oval-shaped region around Sgr A∗ . The

surface brightness of the X-ray enhancement is higher than surrounding diﬀuse X-ray
emission by a factor of about five.
With Chandra observation, Maeda et al. (2002) resolved diﬀuse X-rays which isclearly associated with Sgr A East from the other complex structures in the region for
the first time. The X-ray emission from Sgr A East fills central ∼ 2 pc of the nonthermal

radio shell. Its X-ray spectrum exhibits strong Kα lines from highly ionized S, Ar, Ca,

and Fe, which can be well reproduced by a thin thermal plasma model (MEKA) with
an electron temperature and metal abundance of kTe ∼ 2 keV and Z ∼ 4 solar. Froman emission measure, total mass of X-ray emitting gas is estimated to be ∼ 2 η 1/2 M⊙ .
The overabundance of heavy elements and the total gas mass support the hypothesis

30

CHAPTER 2. REVIEW : THE CENTER OF OUR GALAXY

that Sgr A East is a remnant of single supernova, perhaps Type II SN of a 13−20 M⊙
progenitor star.
The recent result of XMM-Newton observation implies multi-temperature thin thermal plasmas are coexist in Sgr A East, which can be approximated with two temperature components of 1 keV + 4 keV (Sakano et al., 2004). They also discovered aspatial distribution of metal abundances. The iron abundance Sgr A East is as highas ZFe ∼ 4 solar at the core region, while sub-solar at the outer region. On the other

hand, the abundance of lighter elements (S, Ar, and Ca) stay at 1−3 solar at the entire

region. Their interpretation for the origin of Sgr A East is a Type I SN or a Type II
SN of relatively small progenitor mass, which is consistent in principle with the above
Chandra result (Maeda et al., 2002).

2.4

Nonthermal Radio Structures

2.4.1

The Galactic Center Radio Arc (GCRA)The Galactic Center Radio Arc (GCRA), discovered by Yusef-Zadeh et al. (1984) toward the ∼ 50 pc (= 20′ ) northeast direction from the Galactic center, is one of themost distinguished radio structure in the central hundred parsecs of the Galactic cen-ter. The absence of radio recombination line emission and the presence of strong linear
polarization indicate synchrotron emission from GCRA (e.g. Tsuboi et al. (1985, 1986);
Sofue et al. (1987); Seiradakis et al. (1989)). The radio spectrum in the frequency range
from 330 MHz to 43 GHz can be reproduced by a power law with a spectral index ofα ∼ 0.3 (Lesch et al., 1988; Reich et al., 1988; Anantharamaiah et al., 1991; Tsuboi

et al., 1995). Synchrotron emission indicates the presence of magnetic fields and highenergy particles at the GCRA.
The GCRA may be also the part of even larger structure known as the GC Ωlobe (Sofue & Handa, 1984; Sofue, 1985), which is a loop-like structure extended of∼ 200 pc diameter in the direction of the positive Galactic latitude. The GCRA is

located at a foot point of the Ω-lobe, while another foot point coincides with a giant
molecular cloud Sgr C, where a bright nonthermal filament (see next subsection) isembedded. However, a physical connection between these nonthermal radio filamentsand Ω-lobe is still uncertain.

2.4. NONTHERMAL RADIO STRUCTURES

31

Figure 2.11: VLA 330 MHz (90 cm) image of the central ∼ 1.0◦ × 1.2◦ region of the Galaxy.

The beam size of the observation is 12′′ × 7′′ and the rms noise is 1.6 mJy beam−1 . Color
scale is linear between -2 and 50 mJy beam−1 (Nord et al., 2004).

32

CHAPTER 2. REVIEW : THE CENTER OF OUR GALAXY

Figure 2.12: A schematic of the distribution of nonthermal radio filaments (NTRFs) near

the Galactic center based on th VLA 90 cm wide-field image. A dashed line represents the
Galactic plane and an asterisk represents the position of Sgr A∗ . Confirmed NTRFs areshown in Heavy dark lines, while NTRF candidates are shown in thin lines (LaRosa et al.,2004).

2.4. NONTHERMAL RADIO STRUCTURES

2.4.2

33

Nonthermal Radio Filaments (NTRFs)

After the discovery of GCRA (Yusef-Zadeh et al., 1984), a number of nonthermal radio
filaments (NTRFs) have been resolved with high resolution observations performed by
VLA; G0.08−0.15 and G259.96+0.09 named Northern and Southern threads (Morris & Yusef-Zadeh, 1985; Anantharamaiah et al., 1991), G359.1−0.2 so called the“Snake” (Gray et al., 1995), NTRF259.40−0.07 near the Sgr C (Liszt & Spiker, 1995),
G359.54+0.18 (Yusef-Zadeh et al., 1997), and G358.85+0.47 called the “Pelican” (Langet al., 1999).
The NTRFs are unique to the GC region within ∼ 150 pc and share the following
characteristics; (1) extreme high length / width ratio with typically a few tens of parsecs
for the length, yet less than 1 pc for the width, (2) orientation perpendicular to the
Galactic plane within the angle of ∼ 20◦ , (3) strong (up to ∼ 50%) linear polarization,(4) rather negative or flat spectral indices of α ∼ −1.0 to 0.5, (5) large rotation measures(grater than 1000 rad m−1 ), and (6) alignment of the intrinsic magnetic field along thelong axis of the filament.
Recently, LaRosa et al. (2000) successfully resolved a number of filamentary structures and small diameter sources from the region of the central ∼ 300 pc of the Galaxy
with applying a new image restoration technique. In these sources, twenty sources
are listed as candidates of new NTRFs based on the evaluation of their morphologicalfeature (Nord et al., 2004). LaRosa et al. (2004) detected a linear polarization fromsome of these candidates with VLA 6 cm pointing observations. Now 9 NTRFs and 20
NTRF candidates are identified within the 300 pc of the GC region. Table 7.2 shows
radio properties of NTRFs/NTRF candidates, while 90 cm radio image and schematicsof their spatial distribution are shown in Figure 2.11 and Figure 2.12.
Although a few tens of NTRFs have been discovered, a number of issues remainunresolved and still debatable. Most fundamental of these is, what is the source ofrelativistic particle and the mechanism of their acceleration ? Then, what do the
NTRFs suggest about the condition of magnetic field at the GC, and what can confine
NTRFs within such a narrow structures.

2.4.3

Strength of the magnetic field

Several estimates for the strength of NTRF magnetic fields have been made. Basedon radio continuum measurements, several NTRFs require a few tenths of a milligauss

34

CHAPTER 2. REVIEW : THE CENTER OF OUR GALAXY

assuming energy equipartition of magnetic field and synchrotron emitting electrons (e.g.
Gray et al. (1995), Lang et al. (1999)). On the other hand, The absence of any bending
or distortion of the NTRFs against the strong ram pressure of the GC molecular clouds
indicates field strength of NTRFs of ∼ 1 mG (e.g. Yusef-Zadeh & Morris (1987),
Chandran (2001)).
Such field strengths of NTRFs are considerably strong by over two orders of magnitude comparing with a general interstellar magnetic field, which is no more than a
few tens of µG. Thus, strong magnetic pressure of NTRFs must expand NTRFs beyond their observed width within the synchrotron lifetime of electrons unless they are
confined by some comparable pressures. The confinement problem is not an issue if
the 1 mG magnetic field are distributed among the GC region. In this case, NTRFs
are explained as the flux tubes which are illuminated by high energy electrons generated from some local interactions of molecular clouds and ISM. If this is the case,energy density of magnetic field of the GC region should be consequently extremely
high (uB ∼ 4 × 10−8 ergs cm−3 ) and total magnetic energy of the GC region is amountto EB ≃ 6 × 1054 ergs (R/75 pc)2 (L/300 pc). Alternatives to the pervasive magnetic
field explanations have been proposed based on the idea that some local enhancementof field strength occur around the site of NTRFs (Shore & LaRosa, 1999; Yusef-Zadeh,2003).

2.5

Gamma-ray Emissions from the GC region

2.5.1

Extended emission of 1.809 MeV 26 Al line

The COMPTEL instrument on-board Compton Gamma Ray Observatory; CGRO discovered 1.809 MeV 26 Al line emission from the GC region (Diehl et al., 1995). The emission seems extended along the Galactic plane at the scale of a few tens of degrees (Figure 2.13). The the lifetime of 26 Al−26 Mg decay process is short (τ1/2 ∼ 7.2 × 106 year)and 26 Al is thought to be produced by a nucleosynthesis of massive stars. Therefore observed Gamma-ray line emission is originated from Type II supernovae ( explosions of
12−30 M⊙ stars) and/or stellar wind of Wolf-Rayet stars (>30 M⊙ stars was evolved)(Diehl et al., 1995; Prantzos & Diehl, 1996; Naya et al., 1996). It also should be theevidence for a massive star formation occurred at the GC region no longer than a few
million years ago. Recently, Diehl et al. (2003) reported a clear detection of 26 Al line

2.5. GAMMA-RAY EMISSIONS FROM THE GC REGION

35

at a significance of 5−7σ from a inner l ∼ ± 30◦ and b ∼ ± 20◦ region of the Galaxy
with SPI spectrometer on INTEGRAL satellite, which confirms the COMPTEL result.

Figure 2.13: A 26 Al line (1.809 MeV) intensity map of the Milky Way. Continuum emission

is subtracted (Diehl et al., 1995).

Figure 2.14: Background-subtracted 511 keV count rate map as a function of Galactic
Longitude for the region |b| < 20◦ obtained with INTEGRAL/SPI (Teegarden et al., 2004).

2.5.2

511 keV electron-positron pair annihilation line

An enhancement of electron-positron annihilation radiation in the direction of the
Galactic center was first reported with balloon-borne experiments over 30 years ago(Johnson et al., 1972). In 1990s, OSSE experiment on the CGRO observed Galactic
center region many times. These data are well described by bulge distribution plus disk

36

CHAPTER 2. REVIEW : THE CENTER OF OUR GALAXY

component; a Gaussian of FWHM ∼ 5◦ plus an elongated Gaussian with a FWHM of∼ 35◦ in longitude (Kinzer et al., 2001). Although early results of OSSE claimed a possible enhancement in the positive latitude, so-cold “annihilation fountain” (e.g. Purcell
et al. (1997)), later analyses can found little or no evidence for the feature (Kinzer etal., 2001).

2.5.3

High Energy Gamma-rays from the GC region

The EGRET detector on-board CGRO, which have a sensitivity at the energy rangeof 30 MeV−100 GeV, detected high energy gamma-ray emission from the GC region (Mayer-Hasselwander et al., 1998). This source is extended in the scale of ∼ 85 pc (0.6◦ )at the peak position is within ∼ 30 pc (0.◦ 2) from the position of Sgr A∗ . The sourceis listed as a unidentified EGRET source 3EG J1746−2851 in the third EGRET catalog (Hartman et al., 1999). The possible origin of the strong gamma-ray source arethought to be (1) a single or a few young pulsars (t < 103 yr), (2) an inverse Compton
process between high energy electrons and FIR photons, and (3) neutralino annihilation of WIMP particles at the core of the Galaxy.
TeV gamma rays have been recently discovered in the direction of the GC withground-based experiments using Imaging Air Cerenkov Telescopes (IACTs). CANGAROOII group succeeded at a significant detection of sub-TeV gamma-rays with an energy threshold of 250 GeV (Tsuchiya et al., 2004). An Energy spectrum between
250 GeV−2 TeV is determined to be quite steep (∝ E −4.6 ± 0.5 ) and flux is lower
than 0.1 Crab at 1 TeV. Although a peak position of the gamma ray excess is coincide with both Sgr A∗ , Sgr A East, and 3EG J1746−2851, the angular resolution of
CANGAROO-II telescope (∼0.◦ 32) is not enough to decide whether its origin is. Using
26 hours data accumulated over a period of 1995−2003 with Whipple 10 m telescope,
Kosack et al. (2004) reported an gamma ray excess of 3.7 σ significance above 2.8 TeV
at the GC. The 95% confidence region is spatially extended about 15′ which include
the position of Sgr A∗ . Recently, H.E.S.S. group reported the detection of very highenergy gamma-rays from the Galactic center (Aharonian et al., 2004). They resolveda point-like source within 1′ of Sgr A∗ . The center position of the gamma-ray excess
is RA = 17h 45m 41.3s ± 2.0s and Dec. = 29◦ 00′22′′ ± 32′′ (Figure 2.15). An energy
spectrum of the point-like sources can be reproduced by a power-law with a spectral
index of α = −2.2 ± 0.09 ± 0.15, which is significantly flatter than the result of

2.5. GAMMA-RAY EMISSIONS FROM THE GC REGION37CANGAROO-II (Tsuchiya et al., 2004), as shown in Figure 2.16. A measured flux
(1.82 ± 1.2) × 10−7 m−2 s−1 above 165 GeV threshold is also diﬀer from previous
results.
In addition to the point-like source, Figure 2.15 exhibits some diﬀuse-like enhancement of significance along the Galactic plane, which is under investigation.

Gamma-ray significancemap within 3◦ × 3◦ of the GC centeredon Sgr A∗ obtained with the H.E.S.S. observation (Aharonian et al., 2004).

Figure 2.15:

Figure 2.16: Energy spectrum of gamma-rays
from the Galactic center (Aharonian et al., 2004).

Chapter 3
Instrumentation
3.1

Chandra X-ray Observatory

3.1.1

Mission Overview

The Chandra project, which is managed by NASA / MSFC (Marshall Space FrightCenter), is casted as the X-ray component of NASA’s four great observatories The otherthree are Habble Spece Telescope (Optical,NIR), Compton Gamma Ray Observatory(Gamma-ray), and Spitzer (IR). The Chandra Spacecraft is successfully launched by
NASA’s Space Shuttle Colombia on July 23, 1999.

Figure 3.1: The overall view of Chandra X-ray Observatory. This image is credited toNASA/CXC/SAO. http://chandra.harvard.edu/resources/

39

40

CHAPTER 3. INSTRUMENTATIONThe orbit of Chandra is highly elliptical and varies as time goes by. At the point of

October 2003, The distance of the apogee and peregee from the earth was ∼ 132,000 km

and ∼ 32,000 km. The orbit makes it possible to perform high eﬃciency observation

because Chandra can spend ∼ 75% of its period above the radiation belt around theearth. Moreover, the long period (∼ 63.5 hour) allow for continuous observations aslong as 160 ksec.
Chandra is comprised of a spacecraft and a telescope / science-instrument payload.
The spacecraft includes six subsystems; (1) structures and mechanical subsystem, (2)thermal control subsystem, (3) electrical and power subsystem, (4) communication,command, and aspect determination subsystem, (5) pointing control and aspect determination subsystem, and (6) integral propulsion subsystem. On the other hand,
The principal element of the observatory is an X-ray telescope (HRMA), focal planescience instruments (SIs) and the objective transmission gratings (HETG and LETG).
SIs consist of two types of X-ray detectors; CCD imaging spectroscopy arrays (ACIS)
and micro channel plate imaging arrays (HRC). The position of each component isshown in Figure 3.1.
In the following subsections, we concentrate on describing HRMA and ACIS instrument. Detailed description of Chandra can be found in the Chandra Proposers’
Observatory Guide (The latest version is 6.0, Dec. 15, 2003).

3.1.2

HRMA – High Resolution Mirror Assembly

The most distinguished feature of Chandra is its excellent angular resolution (∆θ ∼ 0.5′′ ),which is the highest value among that of the other past and present X-ray satellites

so far. High Resolution Mirror Assembly (HRMA) make it possible for Chandra toachieve the unprecedented spatial resolition.
HRMA represents a logical progression from the X-ray mirror of Einstein (Giacconi
et al., 1979) and ROSAT (Trümper, 1983; Aschenbach, 1991). HRMA comprised of anested set of four paraboloid-hyperboloid (Walter Type-I; Aschenbach (1985) reviews
detailed of the X-tay optics) grazing-incidence X-ray mirror pairs. The eight mirrors
are fabricated from Zerodur1 glass, polished and coated with iridium on a binding layerof chromium. A schematic configuration of HRMA is shown in Figure 3.2. The Focal
1

AG.

Zerodur is a glass ceramic with an extremely low thermal expansion coeﬃcient produced by Schott3.1. CHANDRA X-RAY OBSERVATORY

41

Figure 3.2: A cross-sectional view of HRMA mirror assembly. (Chandra Proposers’ Observatory Guide 2003)

42

CHAPTER 3. INSTRUMENTATION

length of each mirror is ∼ 10 m and the pair diameters are range from ∼ 0.65 to 1.23 m.

HRMA Eﬀective Area
The unobscured geometric clear aperture of HRMA is 1145 cm2 . Although supporting struts of the mirror nests obstruct the HRMA clear aperture, the eﬀect is less than
10%. The HRMA throughput is a function of input X-ray energy and oﬀ-axis angle,
because reflectivity on the mirrors depend on X-ray energy as well as grazing angle.
Figure 3.3 represents an energy dependence of the on-axis HRMA eﬀective area. The
convolved eﬀective areas; the product of HRMA eﬀective area and quantum eﬃciencies
of a focal plane instrument (ACIS or HRS), are also presented. Figure 3.4 describes theeﬀects of oﬀ-axis vignetting on the eﬀective area for the various energies. The plotted
data are averages over the azimuthal angle.

Figure 3.3: The HRMA, HRMA/ACIS,

and HRMA/HRI eﬀective areas as a function of X-ray energy (Chandra Proposers’
Observatory Guide 2003).

Figure 3.4: The HRMA eﬀective area as afunction of oﬀ-axis angle for selected energies. Data represents azimuthal averaged
value and normalized to that of on-axis
area for each energy (Chandra Proposers’
Observatory Guide 2003).

3.1. CHANDRA X-RAY OBSERVATORY

43

Point Spread Function (PSF)
The point spread function (PSF) represents the distribution of accumulated X-ray
photons on the focal plane instruments at a given energy. A shape of a PSF can beapproximated with a two-dimensional Gaussian. One of the most useful parameters toevaluate the sharpness of the PSF is the encircled energy fraction as a function of radiusfrom the PSF center. As shown in Figure 3.5 and 3.6, the PSF and encircled energy
fraction depend upon incident X-ray energy and oﬀ-axis angle. The PSF broadens andencircled energy fraction decreases as the oﬀ-axis angle increases and as the incident
X-ray energy increases. The former eﬀect is due to mirror aberrations, while the latteris due to large X-ray scattering.
Note that the ACIS-I surfaces are not axially symmetric with respect to the HRMA
optical axis, because the aim-point of HRMA is located at the inner corner of the oneof the four ACIS-I array, ACIS-I3.

3.1.3

ACIS – Advanced CCD Imaging Spectrometer

The Advanced CCD Imaging Spectrometer (ACIS) is the focal-plane detector which
has a capability of acquire high resolution images and moderate resolution spectra at
the same time. ACIS can also be used in conjunction with the High Energy Grating
Transmission (HETG) or Low Energy Grating Transmission (LETG) to obtain higher
resolution X-ray spectra of point sources.
Figure 3.7 shows the configuration of the ACIS instrument. ACIS consist of 10
planer 1024 × 1024 pixels CCDs; four arranged in a 2 × 2 array (ACIS-I0, I1, -I2 and-I3 ) used for imaging, and six arranged in a 1 × 6 array (ACIS-S0, -S1, -S2, -S3, -S4,
and -S5) used either for imaging or as a grating readout. The pixel size of the CCDs
is 24.0 microns, equivalent to the angular scale of 0.492′′ . Since the size of the chipboundaries is equivalent to 22 pixels for ACIS-I and 18 pixels for ACIS-S, the arraysize is 16.9 × 16.9 arcmin for ACIS-I and 8.3 × 50.6 arcmin for ACIS-S. The spatial
resolution for on-axis imaging with ACIS is limited by the size of the CCD pixels andnot the HRMA. All CCDs are utilized for Front Illuminated (FI) except for two Back
Illuminated (BI) chips (ACIS-S1 and -S3).

Quantum Eﬃciency and Eﬀective Area

44

CHAPTER 3. INSTRUMENTATION

Figure 3.5: The Chandra HRMA on-axis PSFs of several representative incident X-ray

energies. The fraction of the encircled energy as a function of angular radius is presented.
(Chandra Proposers’ Observatory Guide 2003).

3.1. CHANDRA X-RAY OBSERVATORY

45

Figure 3.6: The dependences of the Chandra HRMA/ACIS-I PSF on oﬀ-axis angle for given

incident X-ray energies. The 50% and 90% encircled energy radii as a function of oﬀ-axis
angle at two incident X-ray energies (1.49 and 6.40 keV) are presented. (Chandra Proposers’
Observatory Guide 2003).

46

CHAPTER 3. INSTRUMENTATIONACIS FLIGHT FOCAL PLANE
not constant with Z~22 pixels ~11"22 pixels
= 11"

I0

I1

w203c4r
0

w193c2
1x

I2
w158c4r

w215c2r

2

3

S0

S1

S2

w168c4r
4

w140c4r
5

w182c4r
6}

I3

ACIS-I(aimpoint on I3 = (962, 964))

330 pixels = 163"

S3+
w134c4r
7

S4S5

w457c48

w201c3r
9}

ACIS-S(aimpoint on S3 = (252, 510))+∆Y
18 pixels = 8".8

Image Region

Bottom.

column

three

two

one

Top

BI chip indicator

node zero

+Z

.

row

TargetOffsetCoordinates+Z
Pointing+∆Z

Coordinates-Z

Pixel (0,0)+Y

Sim Motion

Frame StoreCCD Key

NodeDefinitions

Row/Column
Definition

Coordinate
Orientations

Figure 3.7: (Top) A photo of the whole view of the ACIS instrument. (Bottom) the configuration of the ACIS array on the focal plane. Insight to the terminology is given in the lower
left (Chandra Proposers’ Observatory Guide 2003).

3.1. CHANDRA X-RAY OBSERVATORY

47

The quantum eﬃciency (QE) for the FI chips varies somewhat with row number(not shown), and decreases by 5-15% farthest from the readout at energies above about
4 keV. This is due to the migration of good grades to bad grades produced by chargetransfer ineﬃciency, which varies with row number. The QE variation with position
for the BI chips is much smaller.
Figure 3.8 shows the vignetting (defined as the ratio of oﬀ-axis to on-axis eﬀectivearea) as a function of energy at several oﬀ-axis angles. The detector was appropriately
oﬀset for each oﬀ-axis angle so that the data were obtained at the same focal position,
minimizing the eﬀects of any spatially-dependent variations in the CCD response.

Energy Resolution
The ACIS FI CCDs originally approached the theoretical limit for the energy resolution at almost all energies, while the BI CCDs exhibited poorer resolution. The
pre-launch energy resolution as a function of energy is shown in Figure 3.9. However,subsequent to launch and orbital activation, the energy resolution of the FI CCDs has
become a function of the row number, being near pre-launch values close to the frame
store region and substantially degraded in the farthest row. The loss of energy resolution was due to increased charge transfer ineﬃciency (CTI) caused by low energy
protons, encountered during radiation belt passages and reflecting oﬀ the X-ray telescope (HRMA) onto the focal plane. Subsequent to the discovery of the degradation,
operational procedures were changed, the ACIS is not left at the focal position during
radiation belt passages. Since this procedure was initiated, no further degradation in
performance has been encountered beyond that predicted from pre-launch models. The
BI CCDs were not impacted and this result is consistent with the proton-damage scenario - it is far more diﬃcult for low-energy protons from the direction of the HRMA
to deposit their energy in the buried channels of the BI devices, since the channels
are near the gates and the gates face in the direction opposite to the HRMA . Thus
the energy resolution for the two BI devices remains at their pre-launch values (thediﬀerence in energy resolution of the BI flight devices compared to pre-launch is < 1
ADU at the time of writing).
The position-dependent energy resolution of the FI chips depends significantly on
the ACIS operating temperature. Since activation, the ACIS operating temperature
has been lowered in steps and is now set at the lowest temperature thought safely andconsistently achievable ( ∼ −120◦ C).

48

CHAPTER 3. INSTRUMENTATION

Figure 3.8: Vignetting eﬀect(defined the ratio of oﬀ-axis to on-axis eﬀective area) as afunction of energy for several oﬀ axis angles in arcminutes.

Figure 3.9: The ACIS pre-launch energy resolution as a function of incident X-ray energy.

3.1. CHANDRA X-RAY OBSERVATORY

49

On-Orbit Background and Sensitivity Limit
There are three components to the on-orbit background. The first is that due to thecosmic X-ray background (a significant fraction of which resolves into discrete sources
during an observation with Chandra). The second component is commonly referred
to as the charged particle background. This latter arises both from charged particles,
photon and other neutral particle interactions that ultimately deposit energy in the
instrument. The third component is the “readout artifact” which is a consequence of
the “trailing” of the target image during the CCD readout.
Several “observations” were carried out with the ACIS in the stowed position,
shielded from the sky by the SIM structure, collecting data in normal imaging TE
VFAINT mode at -120C. Chips I0, I2, I3, S1, S2, S3 were exposed. The SIM position was chosen so that the on-board calibration source did not illuminate the ACIS
chips. This allowed us to characterize the non-celestial contribution to X-ray background (i.e., from charged particles). The resulting spectra from diﬀerent chips areshown in Figure 3.10. We have not observed any background flares in the stowed position. Therefore, the ACIS stowed background is a good representation of the quiescent
non-X-ray background in the normal focal position and can be used for science observations.
In real observations, two more components to the background come into play. The
first is the cosmic X-ray background which, for moderately long (∼ 100 ks) observations
will be mostly resolved into discrete sources (except for the diﬀuse component below
1 keV) but, nevertheless, contributes to the overall counting rate. The second is atime-variable “flare” component caused by any charged particles that may reflect fromthe telescope and have suﬃcient momentum so as not to be diverted from the focal
plane by the magnets included in the observatory for that purpose, or from secondary
particles.

50

CHAPTER 3. INSTRUMENTATION

Figure 3.10: A Non X-ray Background (NXB) spectra taken by ACIS CCDs. Chip S2 issimilar to the ACIS-I chips (denoted I023 in the figure) and not shown for clarity. Linefeatures are due to fluorescence of material in the telescope and focal plane instrument.

Chapter 4
Thermal Structures in the GC
region
4.1

X-ray Study of G0.570−0.018

In this chapter, we reports the X-ray analyses and discussion for the peculiar X-ray
source, G0.570−0.018, located near a giant molecular cloud Sgr B2. Although existence of the X-ray source is recognized as a strong iron line emitting object with
ASCA observation (Sakano et al., 1999), while the nature of G0.570−0.018 cannot be
debated because of limited photon statistics and spatial resolution of ASCA. Using
ASCA and Chandra data, we investigate the morphological and spectral properties of
G0.570−0.018 for the first time.

4.1.1

Observations and data reductions

We both use ASCA GIS and Chandra ACIS data for the analyses of G0.570−0.018
presented in the following sections. The observation log is shown in Table 4.1.
As for ASCA, three pointings of GIS (GIS2 and 3) data are available. Unfortunately,G0.570−0.018 is placed on the center of the SIS detector, where there is the central
gap of SIS CCDs, hence we cannot use SIS data for the analysis. For the data of GISs,
nominal bit assignment of PH mode was used in the high and medium bit rate. After
applying the standard data reduction, we finally obtained the eﬀective exposure timeof 17.5 ksec for the 1993 October observation and 80.0 ksec for the 1994 September
observation, respectively.
As for Chandra, the data of ACIS-I field of Obs.ID=944 is used for the following
analysis. The same method of Sgr B2 analysis (Murakami et al., 2001b) was applied
51

52

CHAPTER 4. THERMAL STRUCTURES IN THE GC REGION

for standard data processing in creating level=2 event file. In addition, G0.570−0.018
is placed at the furthest point of the detector readout of ACIS-I3 CCD, hence is heavily
aﬀected by a Charge Transfer Ineﬃciency (CTI). In order to minimize the degradationby the CTI eﬀect, we applied the software to correct the CTI developed by (Townsley
et al., 2000). We also corrected the energy gain using the instrumental emission lines
from Ni-Kα (7.5 keV) and Au-Kα as calibration sources, then achieved an accuracy of< 0.5 % (90 % confidence).
Table 4.1: ASCA and Chandra observation log for G0.570−0.018
Sequence IDa

Obs. start timeb(UT)

ASCA GIS
60011000
1993-10-01-21:16
52006000
1994-09-22-03:45
52006001
1994-09-24-02:05
Chandra ACIS
944
2000-06-03-29-09:44

Nominal point(Ra, Dec)J2000

Exposure(ksec)

17 47 09.58 28 32 24.0
17 46 59.26 28 27 54.7
17 46 59.26 28 27 54.7

19.9
63.1
25.0

17 47 06.70 -28 26 47.29

98.9

a Corresponding with Observation ID in case of Chandra.
b Year-Month-Date-Hour-Minutes in UT.

4.1.2

X-ray morphology

In the ASCA GIS image pointed at the Sgr B2 cloud, we find a local excess in the westof Sgr B2. This excess is clearly seen if we limit the X-ray energy band of 6.0–7.0 keV,which include the iron K-shell transition line. Figure 4.1 gives an intensity map of 6.0–
7.0 keV band X-ray photons obtained with ASCA GIS. The brightest source located at
the northeast of the image is the giant molecular cloud Sgr B2 (Murakami et al., 2000).
The second brightest excess located near the center of the image is a newly discovered
X-ray source. Due to the limited photon statistics and spatial resolution of ASCA GIS,
error of the peak position of the diﬀuse structure is rather large of 0◦ .55 ≤ l ≤ 0◦ .57,
−0◦ .02 ≤ b ≤ 0◦ .00.
In order to investigate the accurate position and fine spatial structure of this diffuse source, we inspect the hard X-ray image observed with Chandra ACIS. Thanks
to the fine spatial resolution of Chandra, As is shown in Figure 4.2, we see a complex

4.1. X-RAY STUDY OF G0.570−0.018

53

160.5

+0°40′+0°05′+0°35′-0°10′+0°30′ -0°05′+0°00′

104.5
94.5
150.5
Figure 4.1: ASCA GIS image of G0.570−0.018 with 6.0−7.0 keV band. The spectrum is
BLOW BLOW IMSMO(Gauss) g3_6-7.img0

extracted within the 3′ circle (solid line), while the background is from the annulus between
the solid and dashed circles (3′ < r < 4′ ). The dotted lines are the Galactic coordinates withgrid spacing of 5′ .

305.554

CHAPTER 4. THERMAL STRUCTURES IN THE GC REGION

X-ray structure with a ring of about 10′′ radius and an east-to-west tail from the ring.
The center coordinate of the ring structure is determined to be R.A.= 17h 47m 02s.6,
Dec= −28◦ 27′ 33′′ (epoch 2000), which corresponds to (l, b) = (0◦ .570, −0◦ .018), thesame position of the ASCA diﬀuse source within the statistical error. We hence desig-

nate this source as G0.570−0.018/CXO J174702.6−282733. To investigate the statistical significance of the shell-like structure, we make a radial profile with the center at(17h 47m 02s .6, −28◦ 27′ 33′′ )J2000 .

-0°00′20″+0°34′20″+0°34′-0°01′20″-0°01′+0°33′40″-0°00′40″

220.5
179.5
Figure 4.2: Chandra ACIS image of G0.570−0.018. The energy band is 4.0−7.0 keV. 320.5
Gaussian smoothing of σ = 2 pixels is applied for the image.
BLOW BLOW BLOW IMSMO(Gauss) BLOW SNR_6-7keV_1bin.img0

Figure 4.3 shows the radial profile of the ring-like structure of G0.570−0.018 in the
4.0−7.0 keV band. The excess around r ∼ 10′′ is statistically significant, hence thering-structure is real. On the other hand, a faint point-like structure at the shell center

seen in Figure 4.2 is not significant because it contains only 3 photons. Unlike Sgr B2,
no radio nor any other wave band counterpart is found for G0.570−0.018 (Oka et al.,1998a; Oka et ak., 1998b; Tsuboi et al., 1999; Lis & Carlstrom, 1994; Mehlinger et al.,
1992, 1993; Seiradakis et al., 1989).

normalized photon number (counts/bin)

4.1. X-RAY STUDY OF G0.570−0.018

55

0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0

5

10

15

20

25

30

/ REBIN PROR:CTS:SNR_acisf944_filter2_4-7keV_2bin.img0

distance (arcsec)
Figure 4.3: A radial profile of the ring-like structure of G0.570−0.018 resolved with Chandra
ACIS. The center position of the profile is mentioned in the text.

56

4.1.3

CHAPTER 4. THERMAL STRUCTURES IN THE GC REGION

X-ray spectra

The ASCA X-ray spectrum is made using the data in a circle of 3.0′ radius and subtracting the background data in an annulus region of 3′ < r < 4′ as is shown in
Figure 4.1. The spectrum exhibits prominent line at 6.0−7.0 keV and large absorption at low energy band. We then fit with a phenomenological model of a thermal+0.12
bremsstrahlung plus a Gaussian line. The line energy is determined to be 6.60−0.09
keV+3.0
with an equivalent width of 3.7−1.2
keV (here and after, errors are 90% confidence unless otherwise noted), indicating a Kα line from iron atoms with lower ionization states
than helium-like (6.7 keV). An electron temperature is constrained to be higher than
2 keV. Therefore the equilibrium ionization state of iron should be helium or hydrogen
like with the line energy of 6.7−6.9 keV. We thus conclude that the plasma is still inan ionizing phase or in a non-equilibrium ionization (NEI).
The Chandra X-ray spectrum is made from the same area as with ASCA, while thebackground spectrum is taken from a source-free region with the same CTI eﬀect as
the source region. The background-subtracted flux is 20400 counts and the spectrum
shows a clear iron line as is already found with ASCA. In the diﬀuse source region,
we also find many faint point-like sources. We hence execute the CIAO ’wavdetect’software of a wavelet method (Freeman et al., 2002) and resolve 18 point sources. The
integrated flux (2.0−10.0 keV) of all the point sources is 410 counts, which is only ≃ 2%of the diﬀuse X-rays. Therefore a contamination of the point-sources can be ignored
practically. The Chandra spectrum is well fitted with the same phenomenological model
as used for the ASCA spectrum, a bremsstrahlung plus a Gaussian line. The plasma+0.03
temperature is constrained to be > 3.4 keV, while the line energy is 6.50−0.03
keV with+1.4
the equivalent width of 4.1−1.0 keV. Thus the best-fit parameters are consistent with,and are more accurate than the ASCA results.
Since both the ASCA and Chandra spectra are fitted with the same model andthe line center energy is consistent with an NEI plasma, we simultaneously fit thetwo spectra with a more realistic NEI plasma model (Borkowski et al., 2001). In
addition to a conventional thin thermal plasma model, an NEI model includes anotherparameter, which is called the ionization parameter τ = nt, where n and t are plasma
density and elapsed time after the plasma is heated-up. As the parameter τ becomeslarger than 1012 ∼ 1013 cm−3 s, an NEI plasma approaches to a collisional ionization
equilibrium (CIE) plasma. In addition to the strong iron line, the Chandra spectrum
includes emission lines at the energies of K-shell transition of argon, calcium atoms. We

4.1. X-RAY STUDY OF G0.570−0.018

57

Figure 4.4: X-ray spectra of G0.570−0.018. Black crosses indicate the data point obtainedwith Chandra, while orange crosses indicate ASCA data.

Table 4.2: Best-fit parameters of G0.570−0.018 for NEI model obtained from the combined
fitting of ASCA and Chandra spectra
Parameter
Unit
kT
[keV]
Metal Abundancesa [solar]
ne t[cm−3 s]NH
[cm−2 ]b
Flux
[ergs cm−2 s−1 ]cLuminosity[ergs s−1 ]χ2 /d.o.f.
—

Value
6.1 (3.1–25.8)
4.5 (1.6–9.9)
1.7 (1.3–2.7) × 1010
13.9 (10.7–17.2) × 1022
8.2 × 10−13
1.3 × 1034
41.3/40

All the errors given in parentheses indicate the 90% confidence limits.
a Abundance ratio relative to the solar value (Anders & Grevesse, 1989).
b X-ray flux in 2.0–10 keV. Absorption is not corrected.
c The absorption corrected luminosity in 2.0−10 keV.

58

CHAPTER 4. THERMAL STRUCTURES IN THE GC REGION

therefore vary the abundances of all the elements collectively fixing the relative ratio tobe solar (Anders & Grevesse, 1989). Free parameters are the electron temperature (kT ),the abundance (Z), the ionization parameter (nt), and interstellar absorption (NH ). For
the other free parameter, we allow one normalization factor to be fitted both the ASCA
and Chandra fluxes simultaneously. The NEI fit is acceptable with the best-fit spectra
and parameters shown in Figure 4.4 and Table 4.5. The data excess of Chandra anddeficit of ASCA above and below the iron line are attributable to the gain uncertaintyof these instruments. However this small systematic diﬀerence is not a serious problem
in the present analysis. The ionization parameter of τ =1.7×1010 cm−3 s is very small,while the abundance is significantly larger than the solar value.

4.1.4

Physical parameters

ASCA discovered a diﬀuse X-ray source (G0.570−0.018) with a strong iron line near
the Sgr B2 cloud, then Chandra found a ring-like structure of 10′′ -radius. The hydrogen+3.3) × 1022 H cm−2 .
column density is determined to be NH = (13.9−3.2

Since the NH value is nearly equal to those of the Galactic center sources of thesame Galactic latitude (Sakano, 2000), G0.570−0.018 would be located near at the
Galactic center region.
We hence assume the source distance to be 8.5 kpc. The X-ray luminosity is thenestimated to be ≃ 1034 ergs and the size of the ring radius is 0.4 pc. The spectra are

well fitted with an NEI model with plasma temperature (kT ) of 6.1 keV, ionization
parameter (τ ) of 1.7×1010 cm−3 s, and metal abundances (Z) of 4.5 solar. Therefore,
together with the ring-like morphology, G0.570−0.018 is likely a young SNR, either
in an adiabatic or a free expansion phase. We first apply a Sedov self-similar model

assuming in an adiabatic phase. This model gives the emission measure (E.M. = n2 V ),radius (R) and electron temperature (kT ) as follows (Ostriker & McKee, 1988);

R) [cm−3 ]
12
na
ts
E
)1/5 ()2/5 [pc])−1/5 ( 3
R = 5.0 × ( 51−3
10 ergs
1 cm
10 years
n2 V = 4πR2 (4na )2 (kT = 4.5 × (E1051 ergs)2/5 (

nats)−6/5 [keV])−2/5 ( 3−3
1 cm
10 years

(4.1)
(4.2)
(4.3)

4.1. X-RAY STUDY OF G0.570−0.018

59

Where ts , na and E are the age of the plasma (in unit of year), ambient density (inunits of cm−3 ) and explosion energy of the supernova (in units of ergs), respectively.
Using the observed values of R = 0.4 pc, kT = 6.1 keV and E.M. = 8.1 × 1056 cm−3 ,we obtain ts , na and E to be 70 years, 5.1 cm−3 and 3.5 × 1048 ergs, respectively. The
swept-up mass is then calculated to be na × 4/3πR3 ≃ 0.03M⊙ , which is extremely

smaller than that of the SN ejecta of a few M⊙ . The estimated explosion energy ofG0.570−0.018 is also extremely small compared with a usual SNR. These indicate that
only a tiny fraction of the explosion energy (∼ 1051 ergs) and ejected mass (a few
×M⊙ ) have been converted to the thermal plasma. These facts strongly indicate that
G0.570−0.018 is not in an adiabatic phase but still in a free expansion phase.

4.1.5

The nature of G0.570−0.018

The X-ray ring structure of G0.570−0.018 is similar to that of SN 1987A (Burrows
et al., 2000; Park et al,, 2002, 2004), where a strong stellar wind from a massive
progenitor might produce a gas ring of sub-pc radius, and was heated by the collisionof supernova ejecta. From the radial profile in Figure 4.3, the thickness of the ring
(FWHM) is about 6′′ , or 0.2 pc, hence the plasma volume is 1.2×1055 cm3 . Since
the X-ray flux from the ring is 67% of that of the whole area of G0.570−0.018, theemission measure (E.M.) from the ring is 5.5×1056 cm−3 . Then using the best-fit
ionization parameter (nt) of 1.7×1010 cm−3 s, the plasma density (n) and ionizationage (t) of the X-ray ring are estimated to be 6.7 cm−3 and 80 year, respectively. In
Table 4.3, we compare the physical parameters of the ring of G0.570−0.018 with thoseof SN 1987A. The ring size and age are about 3−6 times larger than those of SN 1987A.
Therefore G0.570−0.018 would be a ”future SN 1987A” after a free expansion with the
expected speed of v = R/t ≃ 4900 km s−1 .
Table 4.3: Comparison of the parameters between G0.570−0.018 and the X-ray remnant of
SN 1987A

Source nameG0.570−0.018
SN 1987Aa

Distance
[kpc]
8.5
50

Radius of X-ray shell[parsec]
0.4
0.12

a The parameters are taken from Burrows et al. (2000).

Age[year]
80
13

60

CHAPTER 4. THERMAL STRUCTURES IN THE GC REGION
One problem of this scenario for G0.570−0.018 is an additional X-ray structure,

the east-to-west X-ray tail. We suspect that the structure would be made by someinstability of dense stellar wind from progenitor, and was heated-up by the collisionof the SN ejecta. In fact, the surface brightness of X-ray remnant of SN 1987A is not
uniform and varying as time progresses (Figure 4.5). In this case, the front of free
expanding ejecta would be about two times larger than the ring radius, hence the ageof SNR is doubled. This age is still in a very young phase, thus the above discussion
based on a free expansion phase is essentially not changed.
In the young SNR scenario, we expect nonthermal radio emissions associated withthe thermal X-rays, However we see no significant radio continuum flux from G0.570−0.018.
In general, if the X-ray emission is thermal origin, then the observed flux intensity isdetermined by the emissivity of the thermal bremsstrahlung. Hence X-ray flux is proportional to the square of electron density (n2e ), assuming that ne ∼ nH . On the other
hand, an intensity of radio continuum emission is determined by synchrotron emissivity, thus the flux density is proportional to the square of magnetic field, B 2 . Assumingthat magnetic field is proportional to the ambient gas density (the magnetic field isfrozen to the ambient gas), the flux ratio between X-ray to radio is roughly constantamong shell-like SNRs. Referring the radio and X-ray data of typical young shell-like
SNRs, Cas A, Tycho and Kepler, we estimated the radio flux of G0.570−0.018 to be
0.1−1 Jy (except Cas A) at 1 GHz, using the X-ray flux of ∼10−12 ergs s−1 .
SN 1987A has almost the same X-ray flux as that of G0.570−0.018, and the radio
flux density is ∼40 mJy at 4.7 GHz (Burrows et al., 2000). These radio flux density
would be below the current detection limit of the radio SNRs located near the GC,
where the radio background is extremely high (∼0.1−0.5 Jy). Thus we encourage moredeep and fine spatial resolution search for radio emissions near at G0.570−0.018.

4.1. X-RAY STUDY OF G0.570−0.018

61

Figure 4.5: Morphological evolution of the X-ray remnant of SN 1987A from Dec. 2000 to
Dec. 2002 with Chandra(Park et al,, 2004).

62

CHAPTER 4. THERMAL STRUCTURES IN THE GC REGION

4.2

X-ray study of G359.8−0.3

4.2.1

Introduction

G359.8−0.3 is a nonthermal radio shell which is first detected at the 30 pc south of
the Galactic center with the 10 GHz radio continuum survey performed by Nobeyama
Radio Observatory (NRO) (Sofue , 1988). The diameter of the radio shell is about 30′
and it has a almost flat spectral index (α ∼ 0.0) at least between 5 to 10 GHz. Althougha nature of the radio shell has been unclear, it is thought to be (1) extended HII region/ stellar wind shell, (2) shell-type supernova remnant, or (3) a bubble structure causedby the Energetic activity of the GC.
Yusef-Zadeh et al. (1986) reported that jet-like structure emanating from the galactic nucleus with 160 MHz radio observation of Culgoora Circular Array. The lowfrequency radio structure seems to lie along the rotation axis of the Galaxy and extend
at least ∼ 30 pc toward negative galactic latitude. Sofue (1988) also claimed thatthe terminal point of the structure is coincide with G359.8−0.3 radio shell, hence theorigin may have possible relation to the past activity of the nucleus of our Galaxy.
From these radio continuum data only, however, it is impossible to further study ofthe source. X-ray observations should give crucial information as for the nature of
G359.8−0.3.

4.2.2

Observations and data reductions

The western part of the G359.8−0.3 is covered with FOVs of Chandra observations,while the eastern part is covered with FOVs of XMM-Newton. Both observations havealready performed and archived. We used two observations of the Chandra ACIS data
which are the part of GC Survey (PI: Wang), and an observation of XMM-Newton
EPIC data (PI: Turner). The detailed accounts of the each observation are shown in
Table 4.4. Data acquisition from ACIS was made in timed-exposure faint mode withchip readout every 3.24 s. We carried out Chandra data reduction and analysis using
Chandra Interactive analysis of Observations software package (CIAO; version 2.3).
We used level 2 processed event file pipelined by Chandra X-ray Center.
From the XMM-Newton observation, we used and analyzed the EPIC data which
consists of the two MOS cameras (Turner et al., 2001) and the pn camera (Struder etal., 2001). During the observation, Each camera was operated with full-frame mode

4.2. X-RAY STUDY OF G359.8−0.3

63

and acquired with the medium filter and imaging mode. Tha Standard Analysis System (SAS version 5.4.1) was used for processing and analyzing the EPIC data. As aevent list, we used pipe-line processing data distributed from ESA.
After performing high background periods rejection, total live time is amount to
8.3 ks, 7.7 ks, and 7.4 ks for MOS1, MOS2, and pn instrument, respectively. Moreover,
we selected events with pixel pattern of 0−12 (single to quadruple) for MOS and 0−4(single and double) for pn to the subsequent analysis.
Table 4.4: The observation log of G359.8−0.3
Obs. ID

Obs. Start Time
(UT)

Chandra/ACIS
2282 (GCS13) 2001-07-18 11:13:58
2291 (GCS16) 2001-07-18 20:49:28
2293 (GCS17) 2001-07-19 00:01:18
2296 (GCS19) 2001-07-19 06:41:38
XMM-Newton/EPIC
0112971001
2000-09-24 04:34:32

4.2.3

Nominal Point(Ra, Dec)J2000

Exposure(sec)

17 46 22.90 -29 02 15.4
17 45 55.03 -29 12 15.1
17 45 09.10 -29 06 28.4
17 45 26.88 -29 22 33.6

10625
10625
11118
11118

17 46 40.18 -29 13 12.7

17192

X-ray and radio morphology

With Chandra observations, an X-ray excess is clearly seen in the soft (1.5−4 keV)band. This can be resolved into two components. At the center of the Figure 4.6,
diﬀuse emission is extended about 10′ . On the other hand, there is another excess
which is located in the south edge of the ACIS chip. Since it is on the edge of the FOV
and its short exposure time, the Chandra observation could not determine whether
this source is point-like or extended. Moreover, as shown in Figure 4.6, FOVs of
Chandra GC survey covered the only western part of the radio shell of G359.8−0.3,
hence an X-ray property of the eastern part cannot reveal with Chandra GC survey
data. Fortunately, XMM-Newton EPIC Observation covered a complementary region
as is shown in Figure 4.7.
Our analysis revealed that there is no significant X-ray excess seen in the easternpart of the radio shell. From the EPIC image (Figure 4.7, we also confirmed that thecentral excess has an extended feature while the southern excess is resolved into two

64

CHAPTER 4. THERMAL STRUCTURES IN THE GC REGION

point-like sources. From spectral analysis, we conclude that both of point sources isforeground object because the column densities of them are smaller than the expectedvalue of GC source by factor 2−3. On the other hand, the observations of the both
instruments (Chandra ACIS and XMM-Newton EPIC), the position of the central X-ray
excess coincides with the center of radio 10 GHz continuum shell G359.8−0.3 reported
by Sofue (1988). Figure 4.8 represents a X-ray photon intensity profile with reference
to the peak position of X-ray surface brightness. Hereafter, we focused on analyzing
the central excess and discussing its nature.

4.2.4

X-ray spectra

We extracted the spectra from the 6.8′ × 4.0′ elliptical region of the X-ray northexcess of the central radio shell. The background was taken from the same galactic
latitude region. X-ray spectra obtained with Chandra/ACIS and XMM-Newton/EPIC
are extracted from the same sky region to confirm the consistency of the result. The
following spectral analyses are performed with XSPEC version 11.2.
Table 4.5: Best fit parameters of G359.8−0.3 for NEI and MEKAL modelParameterkT
Abundancea
log ne t
NH
fluxb
Lcx
χ2 /d.o.f.

Unit
(keV)
(solar)(cm−3 s)1022 (cm−2 )(ergs cm−2 s−1 )(ergs s−1 )—NEI
1.6 (1.3–1.9)
1.0 (1.3–1.9)
11.3 (11.1–11.5)
4.4 (4.1–4.7)
1.8 × 10−12
1.6 × 1035
204.0/181

MEKAL
1.04 (0.98–1.11)
1.3 (0.9–2.2)—
4.6 (4.3–4.8)
1.6 × 10−12
2.6 × 1035
220.4/182

All the errors given in parentheses indicate the 90% confidence limits.
a Abundance ratio relative to the solar value (Anders & Grevesse, 1989).
b X-ray flux in 0.5–10 keV. Absorption is not corrected.
c The absorption corrected luminosity in 0.5−10 keV, assuming a source distance of
8.0 kpc. (McNamara et al., 2000).

To check the consistency between Chandra ACIS and and XMM-Newton EPIC, we
fitted them with a same physical model. We at first tried to reproduce the spectral datawith a model of thermal plasma in collisional equilibrium (MEKAL; Mewe et al. (1985,
1986); Kaastra (1992); Liedehl et al. (1995)), but it could not yield an acceptable
fit (χ2 /dof = 220.3/182). Therefore we fitted the combined spectra with a single

4.2. X-RAY STUDY OF G359.8−0.3

65

Figure 4.6: Chandra ACIS image of G359.8−0.3 (color map) with logarithmic scale.

1.5−4.0 keV photons are accumulated to construct the image. A solid gray zigzag line
indicate the boundary of ACIS CCDs. White Contours indicate intensity of 10 cm radio
continuum emission obtained with Nobeyama Radio Observatory (NRO).

66

CHAPTER 4. THERMAL STRUCTURES IN THE GC REGION

Figure 4.7: XMM-Newton EPIC image of G359.8−0.3 (color map) with logarithmic scale.

X-ray photons detected from MOS1a MOS2 and pn detector are all combined to construct
the image. Exposure and vignetting eﬀect are collected. A solid gray circle indicates FOV of
X-ray mirror of XMM-Newton. White Contours indicate intensity of radio 10 cm continuum
emission obtained with NRO.

4.2. X-RAY STUDY OF G359.8−0.3

67

Figure 4.8: A radial profile of G359.8−0.3 obtained with XMM-Newton EPIC. 1.5−4.0

keV X-ray Photons accumulated with MOS and pn detectors are used to draw the profile.
Exposure and vignetting eﬀect are collected. Center position of the profile is at the peak
point of the X-ray excess.

68

CHAPTER 4. THERMAL STRUCTURES IN THE GC REGION

Figure 4.9: Upper panel: The X-ray spectrum extracted from the central region of
G359.8−0.3. Black, red, blue, and green crosses represent data points obtained with Chandra
ACIS, XMM-Newton EPIC MOS1, MOS2, and pn detector, respectively. Parameters of the
Fitting model (NEI ) are fixed to the same value for all data. The best-fit model is shown insolid lines. Lower panel: Residuals from the best-fit model.

4.2. X-RAY STUDY OF G359.8−0.3

69temperature NEI model convolved with interstellar absorption. For the NEI model, we
adopted the model version of 2.0 in XSPEC, which takes the emission lines from argon
into account.
As a result, single-temperature thin-thermal NEI model with kT ∼ 1.6 keV could

well reproduced the combined X-ray spectra. The combined X-ray spectra obtainedwith Chandra and XMM-Newton and best-fit NEI model is shown in Figure 4.9. Bestfit parameters of the NEI model is shown in Table 4.5. The suﬃciently large absorption
column (NH ∼ 4 × 1022 H cm−2 ) indicated that G359.8−0.3 is located at the distanceof the GC(D ∼ 8.0 kpc; McNamara et al. (2000)). The value of NEI parameter issmaller than 1012 s cm−3 , by about an order of magnitude, which suggested that theplasma is not still reached thermal equilibrium.

4.2.5

The nature of G359.8−0.3

From the best fit value of an ionization parameter (log nt ∼ 11.3 [cm s−1 ]), and anemission measure (nH ne V ∼ 6.7 × 1057 cm−3 ), we estimated an electron density (ne )and an age (t) of the X-ray emitting plasma to be 0.5 cm−3 and 1.3 × 104 year,
respectively. We assumed a density of Hydrogen ion is equal to that of an electron (nH= ne ) and a shape of X-ray emitting plasma is spherical with a diameter of the source
extracting region. The total thermal energy which is radiated by X-rays is estimatedto be Ethermal = 3ne kTe V ∼ 1 × 1050 ergs. Total mass of the X-ray gas was also

determined to be M = 1.4mH nV ∼ 16 M⊙ .

The morphology of G359.8−0.3; a shell-like structure in radio and the center-filled
in X-rays, reminds us of the “Mixed-Morphology” SNRs (MMSNRs) proposed by Rho& Petre (1998). They pointed out common characteristics of MMSNRs as follows; (1)shell-like structure in radio band, (2) filled-center structure in X-rays, and (3) absence ofa compact source in its center. The Radio and X-ray properties of G359.8−0.3 satisfy allof their criteria. In addition, the X-ray spectra of MMSNRs show thermal feature withseveral emission lines, which means the X-ray emission process is essentially diﬀerent
from the case of Crab-like . Thermal X-ray spectrum of G359.8−0.3 is also consistent
with the MMSNR interpretation.

70

CHAPTER 4. THERMAL STRUCTURES IN THE GC REGION

Table 4.6: Comparison between G359.8−0.3 and other MM SNRs near the Galactic centerregionParameter
Size (radio shell)Size (X-ray)
NH
kTe
X-ray fluxb
ne
t
Eth

Unit
(arcmin)arcmin1022 (cm−2 )(keV)

G359.8-0.3
21 × 21
6.8 × 4.0
4.4 (4.1–4.7)
1.6 (1.3–1.9)

Sgr A Easta
3.5 × 2.5
1.3 × 1.3
11.4 (10.5–12.3)
2.1 (1.9–2.4)(ergs cm−2 s−1 )(cm−3 )
(year)(ergs)

1.8 × 10−12
0.5
1.3 ×104
1.0 × 1050

1.6 × 10−126
1.0 × 104
2 × 1049

G359.1-0.5a
24 × 24
12 × 125.9 (4.1–8.4)
0.6 (0.4–0.9)
4.4(2.2–13.2)d
1.4 × 10−12 c
0.5
1.8 × 104

All the errors given in parentheses indicate the 90% confidence limits.
a Values are quoted from Maeda et al. (2002) for Sgr A East, and Bamba et al. (2000) forG359.1−0.5, respectively.
b X-ray flux in 0.5–10 keV. Absorption is not corrected.
c 0.7–10 keV flux. d Two thermal components (Bamba et al., 2000).

4.2.6

Distance of G359.8−0.3

So far, 231 Galactic SNRs are discovered in the all sky (Green, 2004). At least 8% of
all Galactic SNRs and as much as 25% of X-ray-detected Galactic SNRs are possible
MMSNRs (Rho & Petre, 1998). On the other hand,four SNRs were discovered within acentral degree of the Galaxy (G359.1-0.5, Sgr A East, G0.3+0.0, and G0.9+0.0), threeof them are detected with X-rays, and two of them (G359.1-0.5 and Sgr A East) are
identified as MMSNRs. Table 4.2.5 shows a physical quantities of G359.8−0.3 derived
from X-ray observations with applying MMSNR interpretation and compare them tothat of the other MMSNRs located near the GC region.
On the other hand, Sofue (1988) reported that a radio spectral index of G359.8−0.3
is almost flat (α ∼ 0.0) at least between 5 to 10 GHz. Assuming the same spectral
index can be extrapolated to the lower frequency band, an 1 GHz surface brightnessof G359.8−0.3 is estimated to be Σ1GHz ∼ 6 × 10−21 W m−2 Hz−1 str−1 . By applyingΣ − D relation (Case & Bhattacharya, 1998), a distance of G359.8−0.3 is determined
to D ∼ 5 kpc. The value is rather smaller than the distance of the GC (8.0 kpc).

However, the uncertainty of the source distance derived from the Σ − D relation israther large, typically ∼ 3 kpc, hence it is not inconsistent with our estimation of the
distance We conclude that G359.8−0.3 is located at the distance of the GC.

Chapter 5
Nonthermal Structures in the GC
regionThanks to the high spatial resolution and good photon statistics of Chandra deep
exposure observations, we have successfully resolved detailed diﬀuse structures at thecentral 17′ × 17′ (40 pc × 40 pc) of the Galaxy for the first time. In this chapter,
we focus on the nonthermal features located in the inner 20 parsecs of the Galactic
center; Jet-like structure ejected from the Galactic nucleus (Section 5.2), Outflows in
the southeast direction of Sgr A∗ (Section 5.3), A complex system of nonthermal–6.4keV fluorescent clumps which we named “Fishhook” (Section 5.4), and a number ofother nonthermal features (Section 5.5).

5.1

Observations and Data Reductions

To extract the results presented in the following sections, we use eleven Chandra ACIS-I
observations as shown in Table 5.1 except for the observation of Obs.ID 1561b because
the data is suﬀered from the contamination of the bright X-ray transient (probably
GRS 1741.9−2853 (Pavlinsky et al., 1994)). All of these observations were performedin Timed-Exposure Faint mode except for Obs ID=242, which was worked on TimedExposure Very Faint mode.
The data reduction and analyses were performed on the platform of the Chandra
Interactive Analysis of Observations (CIAO)1 .
We start to retrieve level=1 event files of the each observation provided by Chandra
X-ray Center (CXC) to create level=2 event list of each data. Our data reduction process is as follows. First, we run the standard processing pipeline of acis process events
1

http://cxc.harvard.edu/ciao/

71

72

CHAPTER 5. NONTHERMAL STRUCTURES IN THE GC REGION

then correct CTI and time dependency of the gain. Second, we select the events of
ASCA 02346 grade as X-ray events. Third, we examined background particle flaring
time, and excluded the flaring intervals. Finally, we obtain level=2 event list of eachdata with its eﬀective exposure time is shown in Table 5.1.
Table 5.1: The observation log of the GC deep exposure with Chandra
Obs. ID
242
1561a
1561b
2943
2951
2952
2953
2954
3392
3393
3663
3665

Obs. start time(UT)
1999-09-21 02:43:00
2000-10-26 18:15:11
2001-07-14 01:51:10
2002-05-22 12:25:04
2002-02-19 14:27:32
2002-03-23 12:25:04
2002-04-19 10:39:01
2002-05-07 09:25:07
2002-05-25 15:16:03
2002-05-28 05:34:44
2002-05-24 11:50:13
2002-06-03 01:24:37

Nominal point(Ra, Dec)J2000
17 45 39.32 -29 00 46.8
17 45 39.22 -29 00 46.1
17 45 39.22 -29 00 46.1
17 45 40.78 -29 00 14.6
17 45 40.48 -29 00 12.1
17 45 40.55 -29 00 12.4
17 45 40.61 -29 00 12.6
17 45 40.65 -29 00 13.5
17 45 40.78 -29 00 14.7
17 45 40.78 -29 00 14.6
17 45 40.78 -29 00 14.6
17 45 40.78 -29 00 14.7

Exposurea
(sec)
45923
35705
13504
37978
12370
11859
11785
12455
166691
15802637959
89928

a Eﬀective exposure time after the GTI filtering is applied.

Figure 5.1 represents the spatial distribution of thermal and nonthermal diﬀuse
components within central 17′ × 17′ (40 pc × 40 pc) of our Galaxy. The proceduresfor constructing the “three-color” X-ray image is described below.

At first, we combined eleven level=2 event files made above analyses with reference
to the position of Sgr A∗ (R.A.J2000 = 17h 45m 40s .0409, Dec.J2000 = −29◦ 00′ 28′′ .118,

according to Reid et al. (1999)). Since we improved astrometry by correcting the
known oﬀset reported by CXC2 , a positional error should be smaller than 1′′ . The
total eﬀective exposure of the combined event file is amount to 598 ksec.

Second, we run the point source searching script wevdetect (Freeman et al., 2002),
which is designed on the basis of wavelet method. The searching wavelet radii are set√√√
to 1, 2, 2 2, 4, 4 2 pixels and the significance threshold was fixed to 10−7. As theresult, 1557 X-ray point sources are detected in the 0.5−8.0 keV band.
Third, we sliced the “hole-filled” event file into three diﬀerent energy bands, thensmoothed each image. We selected three energy band for constructing “three-color”
2

http://cxc.harvard.edu/ciao/threads/arcsec correction/

5.1. OBSERVATIONS AND DATA REDUCTIONS

73

Figure 5.1: Three-color image of the central 17′ × 17′ region of our Galaxy with Chan-

dra ACIS. Red (6.50−7.07 keV) represents hot plasma components, Green (6.30−6.50 keV)
represents fluorescent emission from neutral Fe-Kα, and Blue (3.42−6.30 keV) represents
continuum-dominated emission. Point sources are removed then the holes are filled with an
extrapolation of nearby emission.

74

CHAPTER 5. NONTHERMAL STRUCTURES IN THE GC REGION

image; 3.42−6.30 keV, 6.30−6.50 keV, and 6.50−7.07 keV. The highest and middle
energy band contains, He/H-like and neutral Fe-Kα line, respectively. Therefore, thehighest band image should represent emission from hot plasma, while the middle band
image should trace the cold matter distribution, such as X-ray reflection nebulae reported by Murakami et al. (2000, 2001a,b). On the other hand, no significant emission
lines exists within the lowest energy band, hence, only in the band, it is possible for
faint nonthermal emissions to dominates without being distracted by thermal emission lines. An image smoothing is performed in order to make diﬀuse features visible
clearly. Before the smoothing process, we removed the all point sources detected fromthe merged image file. The excluding region for each point source is an ellipse. As acenter coordinate and major/minor radii of the excluding region, we applied the peak
position and three-sigma width which was determined by a two-dimensional gaussian
fitting for the profile of each detected point source. Then, we filled the hole of each
excluded region with an extrapolation of the X-ray emission around the region. Afterthese procedure, the “hole-filled” image was adaptively smoothed by csmooth routine.
Minimum and maximum S/N ratio of the signal under the kernel are set to 3 and 5
sigma, respectively.
Finally, we merged three diﬀerent energy band images which were adaptively smoothed
into the composite three-color image shown in Figure 5.1.

5.2

Jet-like Structures

In Figure 5.1, we can see diﬀuse hot gas represented by red color, which seems pervade
around the Galactic center region. In addition, a number of small scale X-ray features
shown in blue color are resolved elsewhere. Among them, in this section, we focused on
three X-ray linear structures running out from the GC to the north-northwest direction.
Figure 5.2 gives the close up X-ray view of the northwest region of the Galactic
center. The image size is 3.4′ ×4.1′ and the selected energy band is 3.0–8.0 keV. To
construct the image, we merged eleven Chandra ACIS-I data sets same as Figure 5.1
Then an adaptive smoothing and a correction for exposure variations were performed.
We identify three prominent features in Figure 5.2 (marked them as knot 1, 2, and
3). Knot 1, 2, and 3 are not only on the straight line (described as the dashed-line of
Figure 5.2), but also elongated to the same direction of the line. In addition, Theirangular scale is almost the same with major and minor axes of ∼ 6′′ and ∼ 1.5′′ ,respectively.

5.2. JET-LIKE STRUCTURES

75

n
tio

nta

ie
Or
of
the
tic
lac

Ga

1' = 2.3 pc

nepla
Figure 5.2: An adaptively smoothed 3.0−8.0 keV image of the northwest region of the
Galactic center with Chandra ACIS. An adaptive smoothing and a correction for exposure
variations were performed.

76

CHAPTER 5. NONTHERMAL STRUCTURES IN THE GC REGION-28:59:00
02

knot 104
06
08-28:59:10
12
14
16
39
18
46

38

17:45:38

38

37

48
knot 2-28:57:50
52
54
56
58
-28:58:00
02
04
36

35

17:45:35

35

34

16
18
knot 3-28:57:20
22
24
26
28
-28:57:30
32
34
5:34
36

17:45:34

17:45:34

33

Figure 5.3: X-ray images (left panels) and spectra (right panels) of the jet-like structures(knot 1–3) with Chandra ACIS. Left panels: The images shows 3.0–8.0 keV X-ray photons
from 3 to 300 count par pixel with a logarithmic scale. All of detected point source is excluded
from the image. Right panels: Source photons of each spectrum (solid crosses) within thesolid elliptical region shown in the corresponding left panel. Solid line represents the best-fit
power-law model with ISM absorption. Parameters are shown in Table 5.2.

5.2. JET-LIKE STRUCTURES

77

Figure 5.4: Merged X-ray spectrum of knot1–3. The best-fit power-law model convolved withinterstellar absorption is given in solid line. The best-fit parameters are shown in Table 5.2.

78

CHAPTER 5. NONTHERMAL STRUCTURES IN THE GC REGION

5.2.1

X-ray spectra

As a source region for spectral analyses of knot1–3, we applied elliptical region along
the enhancement of each knot as shown in left panels of Figure 5.3. While backgroundphotons of each knot are extracted from surrounding con-central elliptical annularregion whose minimum and maximum radii correspond with 1 and 3 times of that ofsource ellipse. To avoid contaminations by point sources, all of the point source region
were eliminated beforehand.
The background-subtracted spectrum presented in the right panels of Figure 5.3
exhibits no emission lines. Hence, we fitted the X-ray spectrum of each knot with apower-law model convolved with the eﬀect of interstellar absorption. Best-fit parameters are shown in Table 5.2. Then we made the merged X-ray spectrum of three knots(as shown in Figure 5.4) then performed fitting with the same model. The result showsa beautiful power-law profile with a photon index of ∼ 1.5 (Table 5.2). Although we
tried fitting each spectra with a thin-thermal plasma model (MEKAL model; Meweet al. (1985, 1986); Kaastra (1992); Liedehl et al. (1995)), best-fit value of a plasma
temperature was determined to be > 70 keV, which is too high for the temperatureof a thermal plasma. Therefore we thought that the application of a thermal model
is not realistic. The best-fit column density of each knot exhibit almost same value ofNH ∼ 1.0 × 1023 cm−2 , which is consistent with the value when they are located at the
distance of GC. The possibility that these knots are foreground object is robustly rejected. These results lead us to conclude that knot 1–3 are a nonthermal X-ray sources
located near the Galactic center. Moreover, not only column density, but the photon
index of each knot exhibits almost same value (Table 5.2). These spectral properties
support the idea that they should have the same origin, which is already inferred fromthe linear alignment of them.

5.2.2

Radio counterparts

As shown in a dashed line of Figure 5.2, we can see three X-ray knots are alignedwith a straight line from the direction of the Galactic center to north-northeast direction. Assuming that X-ray emission of the knots is nonthermal origin, the existence ofsynchrotron radio emission is also expected. Therefore we try to comparing X-ray andradio continuum intensity map. The result are given in Figure 5.5. The 6 cm and 20 cm
radio contour maps are created from VLA archival data taken from NCSA/ADIL 3 .
3

http://adil.ncsa.uiuc.edu/

5.2. JET-LIKE STRUCTURES

79

Table 5.2: Best fit parameters for the straight-line X-ray knots fitted with a power-law model
knot 1knot 2knot 3
mergedSource Counts
photons
501
485
295
1281

Column Density (NH )1022 H cm−2
8.5 (4.3–14)
16 (9.1–21)
10 (5.3–23)
12 (9.5–15)

Photon Index (Γ)
N (E) ∝ E −Γ
1.3 (0.3–2.2)
2.3 (1.7–3.1)
1.8 (0.3–2.7)
1.5 (1.1–2.0)

X-ray flux aergs cm−2 s−1
4.1 × 10−14
3.4 × 10−14
3.8 × 10−14
1.0 × 10−13

All the errors given in parentheses indicate the 90% confidence limits.
a An absorbed X-ray flux of 2−10 keV.

Both X-ray color and radio contours are shown in logarithmic scale. Note that the
20 cm contour map around Sgr A∗ is saturated.
6 cm radio continuum counterparts were found at the position of the knot 1−3.
Although knot 2 and 3 have only marginal counterparts in the radio band, knot 1
has a clear correlation between X-ray and radio continuum (both 6 cm and 20 cm;
Figure 5.5).
Interestingly, when the alignment of the knots extrapolates toward the direction of
the GC, it is smoothly connected to the “northern arm” of the radio structure called
Sgr A West, which is comprised of three spiral-like arm features (Seen in the left panelof Figure 5.5). In projection, Sgr A∗ is located near the intersection of the three “arms”
extended to east, west, and north directions. These arms consist of ionized gas, which
exhibits emission lines in the radio and infrared bands (Ekers et al., 1983). Recent
measurements of both proper motion in the continuum band and radial velocity in the
H92α radio recombination line suggest that ionized gas of “northern arm” has a large
variation in the velocity distribution which cannot be expected from Keplerian motion.
This means that the “northern arm” is not bound to the gravitational potential of the
Galactic nucleus (Yusef-Zadeh et al., 1998; Yusef-Zadeh, 2000).
Although the origin of Sgr A West is still a matter of debate, there remain apossibility that both “northern arm” of Sgr A West and knot1-3 are the relics of somemass-ejection activities of the the Galactic nucleus Sgr A∗ .

5.2.3

Proper motion measurements

A measurement of a proper motion is the best method for proving whether these X-ray
knots originated from mass ejection activities of the Galactic nucleus (Sgr A∗ ). To

80

CHAPTER 5. NONTHERMAL STRUCTURES IN THE GC REGION

Figure 5.5: The adaptively smoothed 3.0−8.0 keV image of the northwest region of the

Galactic center with Chandra ACIS. Overlayed contours indicate radio continuum intensitiesof 6 cm (left) and 20 cm (right) band obtained with VLA. The black point of each image
represents the position of Sgr A∗ .

5.2. JET-LIKE STRUCTURES

81

search for the proper motions, we examined a Chandra data. As listed in Table 5.1,
Chandra had continually observed GC region for about 2.7 years; from Sep. 1999 toJun. 2002. At first, we divided the all data sets into the early and the recent periods
and merged as follows;• Period 1: Sep. 1999 − Oct. 2000
• Period 2: May. 2002 − Jun. 2002
Second, We took surface brightness profiles projected to the line (white dashed line
in Figure 5.2). The projection width from which we accumulate the photon is 6′′ .
The result is shown in Figure 5.6. Comparing peak positions of each knot profile
of Period 1 and Period 2, we estimated the proper motion of these knots for about
2 years intervals by fitting each peaks of gaussians, however, no significant propermotion is detected. A typical statistical error of the position oﬀset is ± 0.5 arcsec. It

corresponds to ± 0.06 light years, assuming that the distance from the GC to us is
8.0 kpc (McNamara et al., 2000). From the above estimation, the upper limit of the
projected velocity of the knots’ proper motion is determined to be ∼ 0.03c.

On the other hand, it is interesting that the knot1 has a slight but significant oﬀset(∼ 3′′ ) from its radio counterpart(Figure 5.5). The VLA observation was performed in1985 by Yusef-Zadeh (Yusef-Zadeh & Morris, 1987), it means that there is an intervalof about 15 years between the radio and X-ray observations. If the oﬀset is due to theproper motion of the knot1, its projection velocity is estimated to be 0.025c, which isthe consistent with the upper limit shown in the above estimation.

82

CHAPTER 5. NONTHERMAL STRUCTURES IN THE GC REGION

Figure 5.6: Linear profile of knot1–3 made from 3–8 keV photons. Red data shows represents
the profile of Period 1, while black dat represents the profiles of Period 2.

5.3. OUTFLOW STRUCTURES IN THE SOUTHEAST REGION

5.3

83

Outflow Structures in the Southeast Region

In contrast to the several jet-like features in the northwest direction of the Galactic
center, the southeast region exhibits an X-ray enhancement which seems outflow-likestructure as if ejected from Sgr A∗ (Figure 5.1). We presented the X-ray analysis of
the southwest region in the following subsections.

5.3.1

Radial profiles

As shown in Figure 5.1, there are two components in the southwest outflow, pervasive
hot gas (shown in red) and two rim-like features (shown in blue). The X-ray morphology
of both the inner and the outer rim indicate that they are the part of the shell centeredon the Galactic center. In addition, hot diﬀuse gas (shown in red) seems fill the region
within outer rim. These morphological features leads us to suspect that the southeast
outflow has possible relation with some activities of the central compact object.
A radial profile shown in the right panel of Figure 5.8 supports the above relation.
The profile is taken from the sector whose center is on the position of Sgr A∗ . The
angular width of the profile is 56◦ with minimum and maximum angle of 205.1◦ <θ < 261.1◦ , which is adjusted with the angular size of rims.

5.3.2

X-ray spectra

We extracted X-ray spectra from the three diﬀerent regions; “Outer rim”, “Inner rim”,
and “Whole outflow” region . Source regions of two rims are taken from partial annuli
along with shape of each enhancement. For the purpose of determining the radialwidth of source annulus region of each rim, we performed a gaussian fitting of two
peaks, which are correspond with the enhancement of Inner and Outer rim as is shownin Figure 5.8). As the width of source region for each rim, 1 σ value of the best-fit
gaussian applied, while the width of the background region is selected from 1 σ < r < 3
σ of the surroundings from corresponded source region. These source and backgroundregions are shown in Figure 5.7.
“Outer rim” shows featureless spectrum, hence we tried to fit it with a power-law
model convolved with a interstellar absorption eﬀect then successfully reproduceed thespectrum. The best-fit result is shown in Figure 5.9 and Table 5.3. The best-fit valueof the column density NH ∼ 1023 H cm−2 is consistent with an assumption that “Outerrim” is the source situated at the distance of Galactic center.

84

CHAPTER 5. NONTHERMAL STRUCTURES IN THE GC REGION-29:00:00

01:00

02:00

Inner Rim
Outer Rim

03:00

04:00

05:00

Whole Outflow

06:00

07:00
10

17:46:00

45:50

40

30

Figure 5.7: Chandra ACIS-I image of the Southeast outflow region in logarithmic. Energy
band is 3.42–6.30 keV and all detected point sources are excluded from the image. A centerof the small gray circle indicates the position of Sgr A∗ .

Figure 5.8: The radial profile of the southeast outflow region, which are extracted froma sector described with solid lines in Figure 5.7. The inner and outer annuli in the sector
represent the source region of “Inner rim” and “Outer rim”, while an annulus between dashedlined arcs is that of “Whole outflow”.

5.3. OUTFLOW STRUCTURES IN THE SOUTHEAST REGION

85

Although we also try to apply a thin thermal model (MEKAL) and it can also
yield an acceptable fit. The lower limit of an electron temperature is determined tobe 11.2 keV, which is even too high for the temperature of the hot component of the
GC plasma (e.g. Koyama et al. (1996); Muno et al. (2004)). Then we tried to fit
the spectrum of “Inner rim” with the same model of “Outer rim”, however, we canrecognize no significant diﬀerence between power-law model and MEKAL model interms of a chi-square estimation due to the large systematic error of high backgroundemission. Thus we cannot determine whether the X-ray emission of “Inner rim” isthermal or nonthermal from our analysis.

86

CHAPTER 5. NONTHERMAL STRUCTURES IN THE GC REGION

Figure 5.9: X-ray spectra of the Outer rim (left) and Inner rim (right) in the southeast

outflow. Solid line in the upper panel of each image indicates the power-law model with thebest fit parameters shown in Table 5.3.

Table 5.3: Best fit parameters for the outer and inner rims of the southeast outflow with
diﬀerent models
Parameter
Power-law
Photon IndexNH
Fluxa
χ2 /d.o.f.
MEKALbkTNH
χ2 /d.o.f.

Unit

Outer rim

Inner rimΓ1022 cm−2
ergs s−1 cm−2—

2.2 (1.2–3.9)
11.8 (7.1–21.1)
1.2 × 10−13
10.1/25

2.8 (2.3–3.4)
6.2 (5.0–7.7)
1.4 × 10−13
33.3/23

keV
1022 cm−2—

35.5 (> 11.2)
8.4 (6.1–11.4)
12.0/25

2.8 (2.1–4.0)
5.5 (4.6–6.7)
33.7/23

All the errors given in parentheses indicate the 90% confidence limits.
a An observed flux of 2−10 keV.
b The metal abundance is fixed to a solar value.

On the other hand, An X-ray spectrum extracted from the “Whole outflow” (except
for the source regions of the two rims) exhibits apparent emission lines from highly
ionized Si, S, Ar, Ca, and Fe. The spectral profile qualitatively shares the characteristicof the GC pervasive hot plasma (Koyama et al., 1996; Muno et al., 2004). Hence weapplied the two temperature thermal model plus a 6.4-keV gaussian to reproduce theobserved spectrum. As a model of the X-ray spectrum, we employ two diﬀerent models
to reproduce the result; Case of collisional ionization equilibrium (MEKAL model)

5.3. OUTFLOW STRUCTURES IN THE SOUTHEAST REGION

87

and non equilibrium of ionization (NEI model). The best-fit result for each fitting isshown in Figure 5.10 and Table 5.4. Both model cannot be reproduce the spectrum of“Whole outflow”, perhaps because of the complexity of spatial distribution of plasmas.
To obtain an acceptable fit, we must put into more parameters such as a filling factor
for each component or another temperature components. However, it is not importantfor us to determine a physical condition of “Whole outflow” itself here. The essential
result is that the X-ray spectral property of “Outer rim” (and maybe also “Inner rim”)
is clearly diﬀerent from that of “Whole outflow”, in spite of their spatial proximity.

Figure 5.10: X-ray spectrum of the whole region of the Southeast Outflow. X-ray Emissions
from inner and outer rims are excluded. Dashed lines in the upper panel of the image indicate
the 2-kT NEI (red and green) + 6.4-keV Gaussian (blue) model with the best-fit parameters
shown in Table 5.4. While the lower panel shows residuals from the model.

88

CHAPTER 5. NONTHERMAL STRUCTURES IN THE GC REGION

Table 5.4: Best fit parameters for the southeast outflow with thermal models
Parameter Unit
2-kT MEKAL + 6.4-keV Gaussianb model
kT1
keV
Z1
solar
keV
kT2
Z2
solar
NH
1022 cm−2aFluxergs s−1 cm−2
χ2 /d.o.f.
—
2-kT NEI + 6.4-keV Gaussianb modelc
kT1
keV
Z1
solar
(ne t)1s cm−3
kT2
keV
Z2
solar
(ne t)2s cm−32
χ / d.o.f. —Value
0.22 (0.21–0.24)
0.43 (0.1–1.0)
1.7 (1.6–2.0)
0.5 (0.4–0.6)
12.9 (11.7–13.7)
1.6 × 10−12
321.3/217
1.2 (0.9–1.4)
0.43 (fixed)
1.9 (1.2–6.4) × 1011
4.3(3.7–5.8)
0.5 (fixed)
3.9 (>0.09) × 1013
344.4/217

All the errors given in parentheses indicate the 90% confidence limits.
a An observed flux of 2−10 keV.
b The center energy and line width are fixed to 6.40 keV and zero, respectively.
c N and metal abundance are fixed to the best fit value obtained with 2-kT MEKAL +H
6.4-keV Gaussian fitting.

5.4. GC “FISHHOOK”: A NONTHERMAL/6.4-KEV CLUMP

5.4

89

GC “Fishhook”: A Nonthermal/6.4-keV Clump

With Chandra observation we have resolved clumpy X-ray structures in about 7′ ofnortheast direction from the Sgr A∗ . it shows curious morphology such as fishhook,hence we named the X-ray structure GC “Fishhook”. We divided the GC “Fishhook”
into four diﬀerent region named “Top”, “Middle”, “Bottom”, and “East” as given in
Figure 5.11. Interestingly, in the continuum band (3.42–6.30 keV), we can recognize
all four components. In the neutral Fe-K band image (6.30–6.50 keV), on the other
hand, “Top” component is quite dim while the rest three components exhibit clear
enhancement.
This sharp contrast is also apparent in their X-ray spectra (Figure 5.12). The“Fishhook-Top” exhibits featureless spectrum. On the other hand, spectrum extracted
from the rest of three region exhibit strong emission line at the energy near 6.4-keV,
which indicates an existence of cold matter is embedded with the X-ray emitting region.
If 6.4-keV emission is originated from fluorescence Kα of neutral iron, then Fe Kβ line
emission is also expected. Hence, in order to reproduce their spectra, we employed amodel of a power-law plus two Gaussians with taking absorption of ISM into account.
The Center energy of the two Gaussians are fixed to 6.40 keV (Fe-Kα) and 7.06 keV(Fe-Kβ), respectively. The width of the both Gaussians are fixed to zero.
The fitting results are shown in Table 5.5. All but “Fishhook-Top” exhibit verystrong 6.4-keV emission with equivalent widths of larger than 1 keV. In addition,“Fishhook-Bottom” exhibit 7.06-keV line whose intensity is weaker than 6.4-keV by
an order of magnitude. The observed line ratio of 6.40 keV/7.06 keV is fully consistent with that of Kα/Kβ of neutral iron. They also have large absorption column
in common except for “Fishhook-Top”. Even the lower limit of their column density(≥ (1–2) × 1023 cm−2 ) is larger than that of interstellar column by twice or more.

These result indicates that X-rays from the three part of GC “Fishhook” are comefrom deep interior of cold molecular cloud, as the case of Sgr B2 (Murakami et al.,2001b).
The best-fit result for “Fishook-Top”,in contrast, rejected the existence of bothemission line of 6.40 keV and 7.06 keV. Absorption column is also rather smaller than
the rest part of GC “Fishhook”.

90

CHAPTER 5. NONTHERMAL STRUCTURES IN THE GC REGION

54:30

54:30

GC Fishhook

GC Fishhook-28:55:00-28:55:00

30

30-28:56:00-28:56:00

Top
MiddleEast
3030-28:57:00-28:57:00

30

30-28:58:00-28:58:00

30

30-28:59:00
05

Bottom-28:59:00
17:46:00

45:55

17:45:50

45

05

17:46:00

45:55

17:45:50

45

Figure 5.11: Chandra ACIS-I image of the GC “Fishhook” in logarithmic scale. Energy
band is 3.42–6.30 keV. Source and surrounding background regions for the spectra shown in
Figure 5.12 is shown with solid lines.

Table 5.5: Best fit parameters for the GC “Fishhook” with a power-law + two Gaussians
model

Parameter
Photon Index
EWa (6.40 keV)
EWa (7.06 keV)NH
Fluxb
χ2 /d.o.f.

Unit
Γ
eV
eV
1022 cm−2
ergs s−1 cm−2—Top
2.1 (1.6–3.0)——
8.8 (6.0–13.2)
5.2 × 10−14
21.6/20

Middle2.0 (0.3–3.5)
1180—
17.7 (10.0–37.2)
7.7 × 10−14
25.7/54

All the errors given in parentheses indicate the 90% confidence limits.
a Equivalent width of the line. b An observed flux of 2−10 keV.

Bottom1 (0.5–1.8)
1170
103
24.9 (21.9–28.1)
4.8 × 10−13
94.5/79East
1.3 (0.7–1.6)
1040—
16.0 (11.2–19.0)
7.3 × 10−13
92.6/95

5.4. GC “FISHHOOK”: A NONTHERMAL/6.4-KEV CLUMP

91

Figure 5.12: X-ray spectra of the “Fishhook” region. Top (upper left panel), Middle (upper
right), Bottom (lower left), and East (lower right) is presented. Solid, dashed, and dotteddashed line represent the fitting model with the best parameters shown in Table 5.5.

92

CHAPTER 5. NONTHERMAL STRUCTURES IN THE GC REGION

5.5

Other Nonthermal Structures

Our detailed analyses have discovered faint X-ray features near the Galactic center.
In this section, we present brief results of X-ray analyses of another five linear Xray features (named knot4, 5, 6, 7, and 8) which should be new nonthermal X-ray
knots. For X-ray imaging and spectral analyses of these knots, we follow the sameprocedure which we applied for knot1–3. Figure 5.13 gives the spatial distributionof these knots. Figure 5.14, 5.15, 5.16, and 5.17 present close-up view of the eachknot and the accumulated X-ray spectrum. The best-fit results of spectral fitting aresummerized in Table 5.6.
Table 5.6: Best fit parameters for the Nonthermal X-ray Filaments fitted with a power-law
model

knot4
knot5
knot6
knot7
knot8

Source Countsa
photons
800
562
782
183
419

Photon Index (Γ)
N (E) ∝ E −Γ
1.8 (1.3–2.1)
3.2 (2.2–4.6)
3.1 (2.8–4.3)−2.4 (< 1.5)
0.9 (−0.4–1.2)

Column Density (NH )1022 H cm−2
12.9 (8.8–16.6)
21.4 (15.6–30.2)
8.7 (6.0–12.5)
1.1 (< 37.0)
10.2 (7.0–18.6)

Fluxb
ergs cm−2 s−1
5.3 × 10−14
3.7 × 10−14
3.4 × 10−14
5.2 × 10−14
3.7 × 10−14

All the errors given in parentheses indicate the 90% confidence limits.
a Total counts from the source region after subtracting background counts. The subtracted
background counts are normalized with an angular size of the region. b An absorbed X-ray
flux of 2.0–10.0 keV.

5.5. OTHER NONTHERMAL STRUCTURES

93-28:57:00
Or

knot3

no

tio

ta
ien

30

c
cti
ala

-28:58:00

eG

f th

knot2

ne
pla

30

knot4
knot1

-28:59:00

knot5
knot6

30

-29:00:00

30

knot7

knot8

-29:01:00

17:45:50
30

48

46

44

17:45:40

38

36

34

17:45:3

Figure 5.13: The spatial distribution of the X-ray knots discovered near the Galactic centerwith Chandra ACIS-I. The gray scale represents intensity of 3.42–6.30 keV photons in logarithmic scale. The detected point sources are all excluded from the image (see the section 5.1).
The center of the gray circle indicates the position of Sgr A∗ .

94

CHAPTER 5. NONTHERMAL STRUCTURES IN THE GC REGION

58:30.0
35.0
knot 4
40.0
45.0
50.0
55.0
-28:59:00.0
05.0
10.0

43.0

17:45:42.0

41.0

Figure 5.14: Close up X-ray image (top) and X-ray spectrum (bottom) of knot4 with Chandra

ACIS-I. Top: The X-ray image are constructed from 3.0–8.0 keV X-ray photons from 3 to
300 counts par pixel with a logarithmic scale. The detected point sources are excluded fromthe image. Source photons of the spectrum shown in bottom are accumulated within thesolid line ellipse. Bottom: Crosses and a solid line in the upper panel show data points andbest-fit power-law model, respectively. while crosses in the lower panel give residuals fromthe best-fit model.

5.5. OTHER NONTHERMAL STRUCTURES

95

25.0
knot 5-28:59:30.0
35.0
knot 6
40.0
45.0
50.0
55.0
-29:00:00.0
46.0

17:45:45.0

44.0

Figure 5.15: Close up X-ray image (top) and X-ray spectra (bottom) of knot5 and 6 with

Chandra ACIS-I. Top: The X-ray image are constructed from 3.0–8.0 keV X-ray photons from
3 to 300 counts par pixel with a logarithmic scale. The detected point sources are excluded
from the image. Source photons of the spectrum shown in bottom are accumulated within
the solid line ellipse. Bottom: X-ray spectra of knot5 (left) and knot6 (right). Crosses anda solid line in the upper panel show data points and best-fit power-law model, respectively.
while crosses in the lower panel give residuals from the best-fit model.

96

CHAPTER 5. NONTHERMAL STRUCTURES IN THE GC REGION

25.0
00:30.0
35.0
40.0
45.0
knot 750.0
55.0-29:01:00.0
05.0

42.0

41.0

17:45:40.0

Figure 5.16: Close up X-ray image (top) and X-ray spectrum (bottom) of knot7 with Chandra

ACIS-I. Top: The X-ray image are constructed from 3.0–8.0 keV X-ray photons from 3 to
300 counts par pixel with a logarithmic scale.The detected point sources are excluded fromthe image. Source photons of the spectrum shown in bottom are accumulated within thesolid line ellipse. Bottom: Crosses and a solid line in the upper panel show data points andbest-fit power-law model, respectively. while crosses in the lower panel give residuals fromthe best-fit model.

5.5. OTHER NONTHERMAL STRUCTURES

97

35.0

-29:00:40.0

knot 8

45.0

50.0

36.8

36.4

17:45:36.0

35.6

Figure 5.17: Close up X-ray image (top) and X-ray spectrum (bottom) of knot8 with Chandra

ACIS-I. Top: The X-ray image are constructed from 3.0–8.0 keV X-ray photons from 3 to
300 counts par pixel with a logarithmic scale. The detected point sources are excluded fromthe image. Source photons of the spectrum shown in bottom are accumulated within thesolid line ellipse. Bottom: Crosses and a solid line in the upper panel show data points andbest-fit power-law model, respectively. while crosses in the lower panel give residuals fromthe best-fit model.

Chapter 6
Discussion
6.1

Brief Summary of the Results

In previous sections, we have presented the X-ray analyses of the small scale diﬀuse
structures discovered near the GC.
G0.570−0.018 has a small diameter shell in X-rays, and the emission spectrum iswell reproduced by a thin thermal hot plasma of NEI. The significantly small ionization parameter of the NEI model and X-ray morphology lead us to suspect that
G0.570−0.018 is a very young supernova remnant which has not reached the adiabatic
phase (so-called Sedov phase), but is still in the free expansion phase.
The X-rays from G359.8−0.3 are detected from the position coincident with thecentral region of a radio shell. Unlike the nonthermal property of the surrounding
radio shell, the X-ray spectrum shows thermal feature; strong emission lines fromthe Si, S, Ar, Ca, and Fe. The X-ray and radio properties of G359.8−0.3 can beunderstood within the framework of Mixed-Morphology SNR scenario proposed by(Rho & Petre, 1998). Although the emission mechanism of a MMSNR is still debatable,
their observational characteristics (rather old (t ≥ 104 year), tendency to be distributed
along the Galactic plane) support the idea that G359.8−0.3 is a MMSNR. In fact,other X-ray SNRs previously observed near the GC (G359.1−0.5 and Sgr A East) areclassified with MMSNRs.
In addition to these thermal structures, we have discovered a number of Nonthermal
X-ray structures within 40′ × 40′ parsecs of the GC. They exhibit various shape suchas a knot-like feature (knot1–8), rim-like feature (inner and outer rim of the southeast
region), and a part of the nonthermal/6.4-keV clump (“Fishhook-Top”). Among them,
99

100

CHAPTER 6. DISCUSSION

the spatial distribution and X-ray spectra of knot1–3 indicate that they have a possible
association to past activities of Sgr A∗ in the recent past.

6.2

Contribution to the GC Diﬀuse Emission

Now we resolved two thermal discrete sources and tens of nonthermal structures in the
GC region. We at first evaluate how many portions of Galactic diﬀuse emission can be
attributable to these discrete sources.
We estimated the X-ray luminosities of the newly found thermal structures, G0.570−0.018
and G359.8−0.3 to be 1.3 × 1034 ergs s−1 and 1.6 × 1035 ergs s−1 , respectively. (2–10 keV, unless otherwise mentioned, from here on). In addition, the contribution of
Sgr A East, SNR near the Galactic center (8 × 1034 ergs s−1 ) (Maeda et al., 2002), theintegrate X-ray luminosity from discrete SNR candidates is 2 × 1035 ergs s−1 .

On the other hand, integrated luminosities of nonthermal X-ray knots (knot1–8)

are amount to 5.1 × 1033 ergs s−1 . Other nonthermal structure of “Outer rim” ofthe southeast outflow and “Fishhook-Top” have luminosities of 2.0 × 1033 ergs s−1

and 1.3 × 1032 ergs s−1 , respectively. In addition, The detection of three nonthermal

X-ray features are reported until now; G359.89−0.08 (1.0 × 1034 ergs s−1 ; Sakano etal. (2003)), G359.54+0.18 ( 2.4 × 1033 ergs s−1 ; Lu et al. (2003) ), and G0.13−0.11(3.2 × 1033 ergs s−1 ; Wang et al. (2002)). Thus, the total X-ray luminosity of nonther-

mal components from the Galactic center can be estimated to ∼ (2–3) × 1034 ergs s−1 .
According to the ASCA observations (Koyama et al., 1996), the X-ray luminosityof the diﬀuse emission within central 1◦ × 1◦ region of our Galaxy is estimated to
LX ∼ 1037 ergs s−1 . Early phase of Chandra observations resolved thousands of point

sources from the 17′ × 17′ field of the GC region. The integrated luminosity of thesepoint sources is up to 1 × 1035 ergs s−1 . which contributes only 5 % of the X-ray

luminosity of the GC (Muno et al., 2003). Although a point-like flaring source which

spatially coincident with Sgr A∗ is detected, the X-ray luminosity of its quiescent stateis as faint as ∼ 2 × 1033 ergs s−1 . Assuming that the luminosity ratio of diﬀuse andintegration of point sources almost constant whole the GC region, the ASCA estimationof the diﬀuse intensity is essentially not changed.

6.3. EMISSION MECHANISM OF NONTHERMAL STRUCTURES

101

As a result, the detected SNR contribute 2% of the diﬀuse component of central
1 × 1◦ (150 pc × 150 pc) region, and the contribution of detected nonthermal components turns out to be only 0.2%. This means that unresolved thermal diﬀuse X-rays
dominate the luminosity of the GC.
As for nonthermal structures, however, their spatial distribution tends to be concentrated on the central region. For example, we identified 9 nonthermal X-ray knots frominner r < 20 pc of the Galactic center, but all of the knots are actually located within
10 pc from Sgr A∗ in a projected distance. Within the central r < 20 pc region of our
Galaxy, the X-ray luminosity from diﬀuse component is amount to 2 × 1036 ergs s−1 ,therefore nonthermal X-rays contribute ∼ 1%.
◦

6.3

Emission Mechanism of Nonthermal Structures

The discrete nonthermal X-ray structures which we found in the Assuming that nonthermal X-rays are originated from synchrotron emission from high energy electrons,
the characteristic photon energy and energy-loss time scale can be described as follows;Eph ∼ 2.0 × (

Ee 2
B
)(
) [keV]1mG 10TeV(6.1)

B −2 Ee −1) () [year]
(6.2)
1mG
10TeV
Since observed strength of the magnetic field in the GC region is extremely high,typically B ∼ 1 mG (e.g., (Morris, 1994)), electron energy which emits synchrotron
X-rays is roughly estimated to ∼ 10-100 TeV. This results in extremely short lifetimeof nonthermal X-ray structures, which is about ∼ 1–10 year. The observed X-ray knots
exhibits no significant variation on flux level during our observations with Chandra,
spaced by a time interval of 2.7 year, which indicate that particle acceleration has beenongoing in the central several dozen pc region of our galaxy.
τloss ∼ 1.3 × (6.4

Possible Relation to the Galactic Nucleus

6.4.1

Spatial distribution of nonthermal structures

The nonthermal X-ray structures discovered through this work are located at the closest
position to Sgr A∗ in all known nonthermal diﬀuse structures, including nonthermal
radio filaments (NTRFs). Table 6.1 gives the summary of the X-ray properties of the

102

CHAPTER 6. DISCUSSION

Table 6.1: The summary of X-ray properties of Nonthermal X-ray knots

knot1
knot2
knot3
knot4
knot5
knot6
knot7
knot8
G359.89−0.08†
G0.13−0.11‡
G359.54+0.18⋆

distancea
parsecs
3.3
6.4
7.8
3.9
2.9
3.2
0.8
2.1
9.3
27
70

Sizeb
arcsec11.8 × 2.6
12.4 × 3.4
11.0 × 3.2
24.8 × 4.0
17.2 × 2.0
8.8 × 2.4
14.8 × 2.4
9.2 × 2.2
18 × 858 × 1427 × 10

Inclinationb
degree (θ)
19
31
19
93
111
109
2
24
21
0
15

Directionc
degree (φ)
39
35
34
74
71
64
3
46
59
70
60

Spectral Index
I(E) ∝ E −α0.3(−0.7–1.2)
1.3(0.7–2.1)
0.8(−0.7–1.7)
0.8(0.3–2.1)
2.2(1.2–4.6)
2.1(1.8–3.3)−3.4(<0.5)−0.1(−1.4–0.2)
1.0(0.1–2.1)
0.8(0.4–1.5)∼1Luminosityd
ergs s−1
4.6 × 1032
5.4 × 1032
3.8 × 1032
7.7 × 1032
1.3 × 1033
6.3 × 1032
5.4 × 1032
5.5 × 1032
1.0 × 1034
3.2 × 1033
2.4 × 1033

Projected distance from Sgr A∗ to the center of a source region assuming it is located at the distanceof the GC (D = 8.0 kpc).
bLength and width are length of major and minor axes of elliptical source region, while inclination
indicates the angle of major axis with respect to the normal of the Galactic plane.
c
The angle of the line from Sgr A∗ to the center of a source region with respect to the normal of the
Galactic plane.
dAbsorption-corrected 2–10 keV Luminosity.
†
Values are quoted from Sakano et al. (2003).
‡
Values are quoted from Wang et al. (2002).
⋆
Values are quoted from Lu et al. (2003).
a6.4. POSSIBLE RELATION TO THE GALACTIC NUCLEUS

103

Table 6.2: Summary of radio properties of NTRFs / NTRF candidates detected at the
GC region
Nonthermal Radio Filaments
Size
Plane anglea Flux Density Spectral Indexc(arcmin)(degree)(mJy)α (Sν ∝ ν −α )Radio Arc
32.0 × 4.0
5
24000
−0.3
G0.08+0.15⋆
21.1 × 0.3
0, 10b4990
0.6 ± 0.1
Sgr C filament
11.5 × 0.2
154680
0.55 ± 0.4d
G359.79+0.17
8.3 × 0.5352540
0.6 ± 0.1
G359.1−0.2†
22.7 × 0.4
0, 10b1540−0.3 – 0.8e
G359.96+0.09⋆⋆ 12.5 × 0.4
151450
0.04 ± 0.03
G359.54+0.18
6.5 × 0.8251420
0.8 ± 0.1
G0.087−0.087
2.5 ×0.3
5390—
G358.85+0.47‡
3.2 × 0.8
90
106
0.6f
Candidates of Nonthermal Radio Filaments
G359.88−0.07
1.6 × 0.2
5930
G0.39−0.12
10.1 × 0.3
5731
G359.12+0.66
15.6 × 0.2
35647
G359.22−0.16
1.8 × 0.5
55
269
G359.43+0.13
2.4 × 0.3
0, 90
265
G0.39+0.05
4.1 × 0.3
5232
G359.40−0.07
1.7 × 0.3
40229
G0.02+0.04
2.0 × 0.3
0228
G359.66−0.11
3.5 × 0.520226G359.86−0.24
8.1 × 0.2
35205
G359.59−0.34
2.3 × 0.2
25
188
G359.85−0.02
1.8 × 0.2
90173
G0.06−0.07
2.1 × 0.2
15163
G359.90+0.19
2.4 × 0.235
129G0.37−0.07
1.1 × 0.3
5128
G359.40−0.03
1.8 × 0.2
593.8
G359.99−0.54
8.6 × 0.2
30
88.8
G359.33−0.42
2.4 × 0.2
55
81.0
G359.36+0.09
2.5 × 0.260
65.2
G0.43+0.01
1.6 × 0.3
5
43.9
Name

Size, angle, and flux density of all sources are quoted from VLA results Nord et al. (2004) except forG0.087−0.087 (Eﬀersberg 100 m observation; Reich (2003)). a Angle of the NTRF with respect tothe normal to the Galactic plane.
b
Shows significant kinks, therefore they have two orientations.
cMean spectral index of 90−20 cm quoted from (Anantharamaiah et al., 1991) unless otherwise
mentioned.
d
90−18 cm spectral index quoted from (Anantharamaiah et al., 1991).
e20−6 cm spectral index quoted from (Gray et al., 1995).
f
90−20 cm spectral index quoted from (Lang et al., 1999).
⋆
Also named “Northern thread”. † So called the “Snake”.
⋆⋆
Also named “Southern thread”. ‡ So called the “Pelican”.

104

CHAPTER 6. DISCUSSION

Figure 6.1: The Angle of major axis with respect to the Sgr A*–knot line (|θ − φ|)is
described by a function of the distance from Sgr A∗ . The definition of θ and φ is shownin the Table 6.1. The number above black crosses represent knot ID.

Figure 6.2: Photon index of each knot is described by a function of the distance fromSgr A∗ . The number above black crosses represent knot ID.

6.4. POSSIBLE RELATION TO THE GALACTIC NUCLEUS

105

nonthermal X-ray knots. The nearest structure is located only 0.8 pc from Sgr A∗
in projection (knot 7), which is the closest nonthermal structure near the Galactic
nucleus so far. On the other hand, a number of detection of NTRFs has been reported.
Table 6.2 summarizes the radio properties of NTRFs /NTRF candidates. As was shownin Figure 2.11, NTRFs are distributed at least 1◦ × 1.2◦ (140 pc × 170 pc) region. In
addition, Their spatial distribution shows no clear tendency to gather in the central
region, which contrast starkly with that of the nonthermal X-ray structures.
Moreover, there is another contrast between the spatial distribution of X-ray andradio nonthermal structures. As shown in Figure 2.11 and Table 6.2, almost all of theradio filaments aligned perpendicular to the Galactic plane. This can be interpreted
as the evidence for the existence of poloidal magnetic field of the GC region (Morris &Serabyn, 1996). On the other hand, X-ray knots seem radiate from the position of Sgr
A∗ . Figure 6.1 show a plot of angles of major axes with respect to the Sgr A*–knotline (|θ − φ| in Table 6.1) by a function of the distance from the Galactic nucleus. For
the nearest knot, the value |θ − φ| is almost zero. On the other hand, |θ − φ| has atendency to become larger as a distance increases.
Interestingly, the nonthermal X-ray knots which are moderately distant from Sgr
A have clear radio counterparts, such as G359.89−0.08 (d = 9.3 pc) and G0.13−0.11(d = 27 pc), on the other hand, X-ray knots which are closer (d < 7 pc) to Sgr A∗
have marginal or no radio counterparts and vice versa. These indicate that the energyof high energy particles is declined as a distance from the Galactic nucleus increases.
This support the scenario that particles are accelerated in the vicinity (r < 10 pc) ofor/and at the Galactic nucleus. Near Sgr A∗ , electrons may have enough energy (∼ 10100 TeV) to emit synchrotron X-rays because they have been “fleshly” accelerated
not yet reach their extremely short (< 1-10 yr) lifetime. On the other hand, as the
distant from the site of the particle acceleration up to a hundred pc, the electrons havelost their initial energy down to ∼ GeV order, and emit via synchrotron process only
in radio band. The existence of the clear radio counterparts for G359.89−0.08 andG0.13−0.11 can be understood as the middle-point of the above two cases.
It should be noted that the detection limit of the faint radio structures as anotherpossibility for the lack of radio counterparts near the Galactic nucleus. The backgroundradio emission is extremely high near the Sgr A∗ especially within r < 10 pc region(Figure 5.5), where the synchrotron emission of the shell of young supernova remnant
Sgr A East, which make it diﬃcult to resolve faint structures.
∗

106

6.4.2

CHAPTER 6. DISCUSSION

Implication on past activities of the Galactic nucleus

The concentration of X-ray knots on the GC region lead us to suspect that their origin
is closely related to the high energy activities of Sgr A∗ . Figure 6.2 gives the photon
indices of the X-ray knots as a function of the distance from Sgr A∗ . Although statisticalerrors are rather large, photon index of each knot is similar to that of Sgr A∗ in theflaring state. Without some exceptions (knot 5 and 6), we also see the tendency thatindex become steep as the distance increases.
One explanation for the origin of X-ray knots are originated from jets ejected fromSgr A∗ . As already presented in Section 5.2., knot1–3 is most plausible source as thejets. Our Chandra 2.7 year measurements of proper motion of knot1–3 give an upper
limit of these knots of ∼ 0.03c/sin Θ, where Θ is the angle between our line-of-sightand moving direction of the knots. On the other hand, projected distance from Sgr A∗
to each knots are 11, 21, and 25 light year, respectively. Therefore we can infer thatknot1–3 had been ejected from Sgr A∗ about (400–900)/sin Θ years ago. It is interestingthat the XRN model predicts that Sgr A∗ is as luminous in X-rays as 1039 ergs s−1
about 300 years ago from observation of nearby molecular clouds (Murakami et al.,2001b). In addition, Maeda et al. (2002) claimed that high density blast wave of the
SNR A East passed over Sgr A∗ about 103 years ago, which triggered mass accretionon the central massive black hole. These time-scale are coincident with our estimationof timing when knot1–3 had been ejected.
If the above scenario is true, this may be the first detection of the jet structure fromSgr A∗ . For further confirmation, however, monitoring X-ray observation by Chandra
and detection of positive proper motion are required.

Chapter 7
Conclusion
The main conclusion of our X-ray spatial and spectral study of thermal and nonthermal
structures in the GC region are summarized as follows.
• Using the data acquired with Chandra, with the help of ASCA and XMMNewton, we have newly discovered two thermal structures named G0.570−0.018
and G359.8−0.3, and eleven nonthermal structures/candidate nonthermal structures from the GC region.
• G0.570−0.018 has been found to be extraordinary high temperature (kT ∼6 keV), small diameter (less than 1 pc) X-ray shell-like source. The physical conditions can be well understood with the scenario of an extremely young supernova
remnant (SNR) before reaching adiabatic expansion phase (Sedov phase).
• The X-rays of G359.8−0.3 have been found from the central region of nonthermal radio shell. The observable parameters consistently explained in theframe work of the “mixed-morphology” SNR with the total energy and age of
Ethermal ∼ 1 × 1050 ergs and 1.3 × 104 year.
• Nonthermal structures exhibit various shape such as a knot-like feature (knot1–
8), rim-like feature (inner and outer rim of the southeast region), and a partof the nonthermal/6.4-keV clump (“Fishhook-Top”). They all show hard X-ray
photon indices of Γ ∼< 2.
• The spatial distribution and photon index distribution of X-ray/radio nonthermal
structures, indicate that the particle acceleration has been ongoing in the vicinityof the Galacitic nucleus. Jet-like X-ray nonthermal structures also imply that
107

108

CHAPTER 7. CONCLUSION
they have a possible association with the past activities of the Galactic nucleus(Sgr A∗ ) which have been predicted by past X-ray observations of giant molecular
clouds near the GC.

Bibliography
Aharonian et al., 2004, Astron. Astrophys. , 425, L13
Anantharamaiah, K. R., Pedlar, A., Ekers, R. D., & Goss, W. M., 1991, Monthly
Notices Roy. Astron. Soc. , 249, 262
Anders, E., & Grevesse, N., 1989, Geochim. Cosmochim. Acta., 53, 197
Aschenbach, B., 1985, Rep. Prog. Phys., 48, 579
Aschenbach, B., 1991, Rev. Mod. Astron., 4, 173
Backer, D. C., & Sramek, R. A., 1999, Astrophys. J., 524, 805
Baganoﬀ, F. K., Bautz, M. W., Brandt, W. N., Chartas, G., Feigelson, E. D., Garmire,
G. P., Maeda, Y., Morris, M., Ricker, G. R., Townsley, L. K., & Walter, F. 2001,
Nature, 413, 45
Baganoﬀ, F. K., Maeda, Y., Morris, M., Bautz, M. W., Brandt, W. N., Cui, W., Doty,
J. P., Feigelson, E. D., Garmire, G. P., Pravdo, S. H., Ricker, G. R., & Townsley, L.
K., 2003, Astrophys. J., 591, 915
Balick, B. & Brown, R. L., 1974, Astrophys. J., 194, 265
Bamba, A., Yokogawa, J., Sakano, M., & Koyama, K., 2000, Publ. Astron. Soc. Japan, 52, 259
Bamba, A., Tomida, H., & Koyama, K., 2000, Publ. Astron. Soc. Japan , 52, 1157
Borkowski, K. J., Lyerly, W. J., & Reynords, S. P., 2001, Astrophys. J., 548, 820
Borkowski, K. J., Rho, J., Reynords, S. P., & Dyer, K. K., 2001a, Astrophys. J., 550,
334
109

110

BIBLIOGRAPHY

Burrows, D. N., et al., 2000, Astrophys. J., 543, L149
Case, G. L., & Bhattacharya, D., 1998, Astrophys. J., 504, 761
Chakrabarty, D., & Saha, P., 2001, Astron. J. , 122, 232
Chandran, B. D. G., 2001, Astrophys. J., 562, 737
Diehl, R., Dupraz, C., Bennett, K., Bloemen, H., Knödlseder, Lichti, G., Morris, D.,
Ryan, J., Scönfelder V., Steinle, H., Strong, A., Swanenburg, B., Varendorﬀ, M., &Winkler, C., 1995, Astron. Astrophys. , 298, 445
Diehl, R., Knödlseder, J., Lichti, G. G., Kretschmer, K., Schanne, S., Schönfelder, V.,
Strong, A. W., von Kienlin, A., Weidenspointner, G., Winkler, C., & Wunderer, C.,
2003, Astron. Astrophys. , 411, L451
Eckart, A., & Genzel, R., 1997, Monthly Notices Roy. Astron. Soc. , 284, 576
Ekers, R. D., Goss, W. M., & Schwarz, U. J., 1975, Astron. Astrophys. , 43, 159
Ekers, R. D., van Gorkom, J. H., Schwarz, U. J., & Goss, W. M., 1983, Astron.
Astrophys. , 122, 143
Enomoto, R., et al. 2002, Nature, 416, 823
Falke, H., & Markoﬀ, S., 2000, Astron. Astrophys. , 362, 113
Freeman, P. E., Kashyap, V., Rosner, R., & Ramb, D. Q., 2002, Astrophys. J. Suppl.
, 138, 185
Genzel, R., Hollenbach, D., & Townes, C. H., 1994, Rep. Prog. Phys. 57, 417
Genzel, R., Pichon, C., Eckart, A., Gerhard, O., & Ott, T., 2000, Monthly Notices
Roy. Astron. Soc. , 317, 348
Ghez, A. M., Klein, B. L., Morris, M., & Becklin, E. E., 1997, Astrophys. J., 509, 678
Ghez, A. M., Salim, S., Hornstein, S. D., Tanner, A., Lu, J. R., Morris, M., Becklin,
E. E., & Duchêne, G. 2005, Astrophys. J., 620, 744
Giacconi, R., et al., 1979, Astrophys. J., 230, 540

BIBLIOGRAPHY

111

Goldwurm, A., Brion, E., Goldoni, P., Ferrando, P., Daigne, F., & Decourchelle, A.,
2003, Astrophys. J., 584, 751
Gray, A., Nicholls, J., Ekers, R. D., & Cram, L., 1995, Astrophys. J., 448, 164
Green, D. A., 2004, A Catalogue of Galactic Supernova Remnants, 2004 January
version. Available at http://www.mrao.cam.ac.uk/surveys/snrs/
Hartman, R. C., et al., 1999, Astrophys. J. Suppl. , 123, 79
Hess, V. F., 1911, Phys. Zeits., 12, 998
Hess, V. F., 1912, Phys. Zeits., 13, 1084
Jansky, K. G. 1933, New York Times, Vol. LXXXII, No. 27495 (May 5), 1
Johnson, W. N., Harnden, F. R., & Haymes, R. C., 1972, Astrophys. J., 172, L1
Jones, T. W., 1974, Astron. Astrophys. , 30, 37
Kaastra, J.S., 1992, An X-Ray Spectral Code for Optically Thin Plasmas (Internal
SRON-Leiden Report, updated version 2.0)
Khokhlov, A., & Melia, F., 1996, Astrophys. J., 457, L61
Kellogg, E., Gursky, H., Murray, S., Tananbaum. H, & Giacconi, S., 1971, Astrophys.
J., 169, L99
Kinzer, R. L., Milne, P. A., Kurfess, J. D., Strickman, M. S., Johnson, W. N., &Purcell, W. R., 2001, Astrophys. J., 559, 282
Kosack et al., 2004, Astrophys. J., 608, L97
Koyama, K., Awaki, H., Kunieda, H., Takano, S., Tawara, Y., Yamauchi, S., Hatsukade, I., & Nagase, F., 1989, Nature, 339, 603
Koyama, K., Petre, R., Gotthelf, E. V., Hwang, U., Matsuda, M., Ozaki, M., & Holt,
S. S., 1995, Nature, 378, 255
Koyama, K., Maeda, Y., Sonobe, T., Takeshima, T., Tanaka, Y., & Yamauchi, S. 1996,
Publ. Astron. Soc. Japan , 48, 249

112

BIBLIOGRAPHY

Koyama, K., Kinugasa, K., Matsuzaki, K., Nishiuchi, M., Sugisaki, M., Torii, K.,
Yamauchi, S., & Aschenbach, B., 1997, Publ. Astron. Soc. Japan , 49, L7
Lang, C. C., Anantharamaiah, K. R., Kassim, N. E., & Lazio, T. J. W., 1999, Astrophys. J., 521, L41
LaRosa, T. N., Kassim, N. E., Lazio, T. J. W., & Hyman, S. D. 2000, Astron. J. ,119, 207
LaRosa, T. N., Nord, M. E., Lazio, T. J. W., & Kassim, N. E., 2004, Astrophys. J.,
607, 302
Lesch, H., Schlickeiser, R., & Crusius, A., 1988, Astron. Astrophys. , 200, L9
Liedahl, D.A., Osterheld, A.L., & Goldstein, W.H., 1995, Astrophys. J. Lett. , 438,
115
Lis, D. C., & Carlstrom, J. E., 1994, Astrophys. J., 424, 189
Liszt, H., & Spiker, R., 1995, Astrophys. J. Suppl. , 98, 259
Lu, F. J., Wang, Q. D., & Lang, C., 2003, Astron. J. , 126, 319
Maeda, Y., Koyama, K., Sakano, K., Takeshima, K., & Yamauchi, S., 1996, Publ.
Astron. Soc. Japan , 48, 417
Maeda, Y., 1998, PhD Thesis, Kyoto University
Maeda, Y., Baganoﬀ, F. K., Feigelson, E. D., Morris, M., Bautz, M. W., Brandt, W.
N., Burrows, D. N., Doty, J. P., Garmire, G. P., Pravdo, S. H., Ricker, G. R., &Townesley, L. K., 2003, Astrophys. J., 570, 671
Mayer-Hasselwander, H. A., et al., 1998, Astron. Astrophys. , 335, 161
McGee, R. X., Bolton, J. G., 1954, Nature, 173, 985
McNamara, D. H., Madsen, J. B., Barnes, J., Ericksen, B. F. 2000, Publ. Astron. Soc.
Pacific , 112, 202
Mehlinger, M., Yusef-Zadeh, F., Palmer, P., & Goss, W. M., 1992, Astrophys. J., 401,
168

BIBLIOGRAPHY

113

Mehlinger, M., Palmer, P., Goss, W. M., & Yusef-Zadeh, F., 1993, Astrophys. J., 412,
684
Melia, F., 1992, Astrophys. J., 387, L25
Melia, F., Fatuzzo, M., Yusef-Zadeh, F., & Markoﬀ, S. 1998, Astrophys. J. Lett. , 508,
L65
Melia, F., & Falke, H., 2001, Annu. Rev. Astron. Astrophys., 39, 309
Mewe, R., Gronenschild, E. H. B. M., & van den Oord, G. H. J., 1985, Astron.
Astrophys. Suppl. , 62, 197
Mewe, R., Lemen, J.R., & van den Oord, G.H.J. 1986, Astron. Astrophys. Suppl. , 65,
511
Mezger, P. G., Duschl, W. J., & Zylka, R., 1996, Astron. Astrophys. Rev., 7, 289
Morris, M., & Yusef-Zadeh, F., 1985, Astron. J. , 90, 2511
Morris, M., 1994, in the Nuclei of Normal Galaxies, ed. R. Genzel & A. I. Harris(NATO ASI Ser. C, 445; Dordrecht: Kluwer), 185
Morris, M., & Serabyn, E., 1996, Annu. Rev. Astron. Astrophys. 34, 645
Muno, M. P., et al., 2003, Astrophys. J., 589, 225
Muno, M. P., et al., 2004, Astrophys. J., 613, 326
Murakami, H., Koyama, K., Sakano, M., Tsujimoto, M., & Maeda, Y., 2000, Astrophys. J., 534, 283
Murakami, H., Koyama, K., Tsujimoto, M., Maeda, Y., & Sakano, M., 2001, Astrophys. J., 550, 297
Murakami, H., Koyama, K., & Maeda, Y., 2001, Astrophys. J., 558, 687
Narayan, R., Mahadevan, R., Grindlay, J. E., Popham, R. G., & Gammie, C., 1996,
Astrophys. J., 492, 554
Naya, J. E., Barthelmy, S. D., Bartlett, L. M., Gherels, N., Leventhal, M., Parsons,A., Teegarden, B. J., & Tueller, J., 1996, Nature, 384, 44

114

BIBLIOGRAPHY

Nord, M. E., Lazio, T. J. W., Kassim, N. E., Hyman, S. D., LaRosa, T. N., Brogan,
C. K., & Duric, N., 2004, Astrophys. J., 128, 1646
Oka, T., Hasegawa, T., Hayashi, M., Handa, T., & Sakamoto, S., 1998a, Astrophys.
J., 493, 730
Oka, T., Hasegawa, T., Sato, F., Tsuboi, M., & Miyazaki, A., 1998b, Astrophys. J.
Suppl. , 118, 455
Ostriker, J. P., & McKee, C. F., 1988, Rev. Mod. Phys., 60, 1
Park, S., Burrows, D. N., Garmire, G. P., Nousek, J. A., McCray, R., Michael, E., &Zhekov, S, 2002, Astrophys. J., 567, 31
Park, S., Zhekov, S. A., Burrows, D. N., Garmire, G. P., & McCray, R., 2002, Astrophys. J., 567, 31
Pavlinsky, M. N., Grebenev, S. A., & Sunyaev, R. A., 1994, Astrophys. J., 425, 110
Pedlar, A., Anantharamaiah, K. R., Ekers, R. D., Goss, W. M., van Gorkom, J. H.,
Schwarz, U. J., & Zhao, J., 1989, Astrophys. J., 342, 769
Piddington J. H., & Minnett, H. C., 1951, Aust.J. Sci. Res. A, 4, 459
Porquet, D., Predehl, P., Aschenbach, B., Grosso, N., Goldwurm, A., Gokdoni, P.,
Warwick, R. S., & Decourchelle, A., 2003, Astron. Astrophys. , 407, L17
Prantzos, N., & Diehl, D., 1996, Phys. Rep., 267, 1
Purcell, W. R., et al. 1997, Astrophys. J., 491, 725
Reich, W., Sofue, Y., Wielebinski, R., & Seiradakis, J. H., 1988, Astron. Astrophys. ,191, 303
Reich, W., 2003, Astron. Astrophys. , 401, 1023
Reid, M. J., Readhead, A. C. S., Vermeuren, R. C., & Treuhaft, R. N., 1999, Astrophys.
J., 524, 816
Rho, J., & Petre, R., 1998, Astrophys. J., 503, L167

BIBLIOGRAPHY

115

Reid, M. J., Readhead, A. C. S., Vermeulen, R. C., & Treuhaut, R. N., 1999, Astrophys.
J., 524, 816
Ryter, Ch. E., 1996, Astrophysics and Space Science, 236, 285
Sakano, M., et al., 1999, Proc. of Japanese-German Workshop on High Energy Astrophysics, ed. W. Becker & M. Itoh, (MPE Rep. 270; Garching bei Muenchen: MPE),
113
Sakano, M., 2000, PhD thesis, Kyoto University
Sakano, M., Koyama, K., Murakami, H., Maeda, Y., & Yamauchi, S., 2001, Astrophys.
J. Suppl. , 138, 19
Sakano, M., Warwick, R. S., Decourchelle, C., & Predehl, P., 2003, Monthly Notices
Roy. Astron. Soc. , 340, 747
Sakano, M., Warwick, R. S., Decourchelle, C., & Predehl, P., 2004, Monthly Notices
Roy. Astron. Soc. , 350, 129
Schödel, R., et al. 2002, Nature, 419, 694
Seiradakis, J. H., Reich, W., Wielebinski, R., Lasenby, A. N., & Yusef-Zadeh, F., 1989,
Astron. Astrophys. Suppl. , 81, 291
Serabyn, E., Carlstrom, J., Lay, O., Lis, D. C., Hunter, T. R., & Lacy, J. H., 1997,
Astrophys. J., 490, L77
Shore, N. S., & LaRosa, T. N., 1999, Astrophys. J., 521, 587
Sidoli, L., & Mereghetti, S., 1999, Astron. Astrophys. , 349, L49
Sofue, Y., & Handa, T., 1984, Nature, 310, 568
Sofue, Y., 1985, Publ. Astron. Soc. Japan , 37, 697
Sofue, Y., Reich, W., Inoue, M., & Seiradakis, J. H., 1987, Publ. Astron. Soc. Japan ,39, 95
Sofue, Y., 1988, Comments Astrophys., 13, 29

116

BIBLIOGRAPHY

Struder, L., et al. 2001, Astron. Astrophys. , 365, L18
Tanimori, T., et al. 1998, Astrophys. J., 497, L25
Teegarden, B. J., et al., 2004, to be appeared in Astrophys. J., astro-ph/0410354
Townsley, L. K., Broos, P. S., Garmire, G. P., & Nousek, J., 2000, Astrophys. J., 534,
L139
Trümper, J., 1983, Adv. Space Res., 2(4), 241
Tsuboi, M., Inoue, M., Handa, T., Tabara, H., & Kato, T., 1985, Publ. Astron. Soc.
Japan , 37, 359
Tsuboi, M., Inoue, M., Handa, T., Tabara, H., Kato, T., Sofue, Y., & Kaifu, N., 1986,
Astron. J. , 92, 818
Tsuboi, M., Kawabata, T., Kasuga, T., Handa, T., & Kato, T., 1995, Publ. Astron.
Soc. Japan , 47, 829
Tsuboi, M., Handa, T., & Ukita, N., 1999, Astrophys. J. Suppl. , 120, 1
Tsuchiya et al., 2004, Astrophys. J., 606, L115
Turner, M., et al. 2001, Astron. Astrophys. , 365, L27
Wang, Q. D., Lu, F., & Lang, C., 2002, Astrophys. J., 581, 1148
Wang, Q. D., Gotthelf, E. V., & Lang, C. C., 2002, Nature, 415, 148
Yuan, F., Markoﬀ, S., & Falcke, H., 2002, Astron. Astrophys. , 383, 854
Yuan, F., Quataert, E., & Narayan, R., 2003, Astrophys. J., 598, 301
Yamauchi, S., Kawada, M., Koyama, K., Kunieda, H., & Tawara, Y., 1990, Astrophys.
J., 365, 532
Yusef-Zadeh, F., Morris, M., & Chance, D. 1984, Nature, 310, 557
Yusef-Zadeh, F., Morris, M., Slee, O. B., & Nelson, G. J., 1986, Astrophys. J., 300,
L47

BIBLIOGRAPHY

117

Yusef-Zadeh, F., & Morris, M., 1987, Astrophys. J., 320, 545
Yusef-Zadeh, F., & Morris, M., 1987, Astrophys. J., 322, 721
Yusef-Zadeh, F., & Barry, J., 1989, in IAU Symp. 136, The Center of the Galaxy, ed.
M. Morris (Dordrecht: Kluwer), 197
Yusef-Zadeh, F., Wardle, M., & Parastaran, P., 1997, Astrophys. J., 475, L119
Yusef-Zadeh, F., Roberts, D. A., & Biretta, J., 1998, Astrophys. J., 499, L159
Yusef-Zadeh, F., Melia, F., & Wardle, M., 2000, Science, 287, 85
Yusef-Zadeh, F., 2003, Astrophys. J., 598, 325

Acknowledgments
First, I am deeply grateful to Katsuji Koyama for his continuous guidance and encouragement throughout the five years of my graduate school period.
I also give my special thanks to Hiroshi Murakami and Aya Bamba for their continuous instruction and valuable advices. My gratitude is also for Shigeo Yamauchi,
Yoshitomo Maeda, and Masaaki Sakano. Without their pioneer works, this thesis
would not have been even started. I would like to express my thank to Yoshiaki Sofueand Masato Tsuboi. They provide me with radio data of the Galactic center region. I
express gratitude to Ken Ebisawa for the discussion about the Galactic diﬀuse X-ray
emission. Thanks are also due to Rhota Takahashi for the comment on the theoreticalmodel about Sgr A∗ . I wish to express my appreciation to Toru Tanimori, Shin Mineshige, Kentaro Miuchi, and Ken-ichi Imai for their careful and suggestive refereeingof this thesis. I am also grateful to Yuji Tazawa, Takeshi Tsuru, Hironori Matsumoto,
Hidetoshi Kubo, Jun Kataoka, Jun Yokogawa, Kensuke Imanishi, Masahiro Tsujimoto,
Akihiro Asahara, Makoto Kohno, Hideaki Katagiri, Tsutomu Nagayoshi, Shin-ichiro
Takagi, Daisuke Nishida, Shio Watanabe, Hiroshi Nakajima, Atsushi Takada, Ayami
Nakao, Ayako Furumori, Yuka Nakakoji, and all the members of the Cosmic-ray laboratory in Kyoto University, for their heartfelt instruction and support. I am also deeply
grateful to my excellent colleagues, Masaru Ueno and Reiko Orito for their insightfulcomments and continuous encourgement.
This work is financially supported by Japan Society for the Promotion of Science for
Young Scientists, the Hayakawa Satio Foundation of the Astronomical Society Japan,
and a Grant-in-Aid for the 21st Century COE “The Center for Diversity and Universality in Physics”.
Finally, I express my sincere gratitude to my family and friends, for their invisible
but invaluable support.
119