INTEGRAL Observations of the Galactic 511 keV Emission and MeV
Gamma-ray Astrophysics
K. Watanabe
NASA Goddard Space Flight Center & Univ. of Maryland College Park, USA
On behalf of the INTEGRAL/SPI Science TeamAbstract
Although there are a number of interesting phenomena, such as Nucleosynthesis in stars,
in the MeV energy region, the observations have been difficult due to a small signal tonoise (background) ratio (<1%). While NASA’s Compton Gamma-ray Observatory(CGRO) enabled us to explore the Gamma-ray universe, ESA’s INTEGRAL mission,launched in 2002, is providing us more detailed information with its superior energy andangular resolution. We will briefly discuss some of the current issues in MeV Gamma-rayAstrophysics. Then, we will focus on the Galactic 511 keV emission with the latest
INTEGRAL observations, and talk about challenges we currently have.
1. The MeV Gamma-ray Astrophysics
Some of major Gamma-ray lines in the MeV energy region we are interested in the mostinclude the 26Al line at 1.809MeV, the 60Fe lines at 1.172 & 1.333MeV, the 44Ti line at
1.156MeV due to Nucleosynthesis in massive stars, the decay line from 56Co to 56Fe at
0.847 & 1.238 MeV in Supernovae, the Neutron-Proton recombination line at 2.223 MeV
in Neutron star binaries (Bildsten, Salpeter & Wasserman 1993), the12C excitation line at
4.439 MeV and the16O excitation line at 6.129 MeV due to Cosmic Ray interactions.
Studies of Gamma-Ray Bursts, Hypernovae, Black Holes, Pulsars, Solar Flares, AGNs,and Cosmic Gamma-ray Background in this energy range have also revealed many newfacts. The biggest problem is, however, that the signal to noise (background) ratio is sosmall (< 1%). Thus, very detailed background studies must be carried out. We willdiscuss briefly some of them as follows.
1.1 26Al
The 26Al line at 1.809MeV was first detected by HEAO-C (Mahoney et al. 1984). It isnarrowly confined to the Galactic Plane with local hot spots in the Cygnus and Vela
regions [Fig. 1, also see CGRO/COMPTEL (Diehl et al. 1995; Knödlseder et al.
1999)]. Although a broadened line (5.4 ± 1.4 keV FWHM) was reported [GRIS (Naya etal. 1996)], no further evidence has been reported [RHESSI (Smith 2004), INTEGRAL(Diehl et al. 2003)]. The recent INTEGRAL/SPI observation of the Cygnus region, where
young stars and WR stars dominate, shows a little broader width than that on Inner
Galaxy (Fig.2).

Fig.1 26Al all sky map observed by COMPTEL(Plüschke et al., 2001)

Fig.2 26Al line widths (Courtesy of Diehl )Galactic rotation affects the peak energy of thespectrum. The Doppler shift of the centroid has beenobserved by SPI. The three left panels in Fig. 3 illustratethese shifts. The upper panel is a red shift (0.14±0.31keV)
at the galactic longitude between +10o and +40o and thebottom panel is a blue shift (0.44±0.24keV) at between-10o and -40o. The middle panel with no spectrum shift is
at the galactic center. The right bottom panel shows theenergy shifts as function of the galactic longitudes withindication of the three data of the left panels. The sources
are not localized. We calculate the total galactic mass of26
Al = 2.8 (±0.8) Mo. Although we observe a small linebroadening from Inner Galaxy (the middle panel in Fig.
3), the change is within the errors, hence, not significant.
Fig.3 26Al spectrum
centroid shifts due tothe Galactic Rotation. (Courtesy of Diehl )1.2 60Fe
As shown in Fig. 4 (left: RHESSI, Smith, 2004, right:SPI, Harris et al., 2005) the 60Fe
lines at 1.172 & 1.333 MeV have been marginally detected. The observed fluxes are
3.6±1.4 x 10-5 ph cm-2s-1 with 2.6σ detection (RHESSI) and 4.0±1.3 x 10-5 ph cm-2 s-1 with
3σ (SPI). Thus, they are comparable. The 60Fe/26Al ratio in the Galaxy is 0.11 ± 0.03
(Harris et al. 2005). Theoretical models of SNII, however, predict the ratio being nearunity (Limongi & Chieffi, 2003, also see Fig.5). These observations indicate that someother source of 26Al besides explosive nucleosynthesis in core-collapse SNe are required.
1.3 Cosmic Gamma-ray Background (CGB)
Fig. 6 summarizes the CGB observations made by several different space missions. In theMeV region both CGRO/COMPTEL (Kappadath, et al. 1996) and SMM (Watanabe etFig.4 60Fe observations by RHESSI (left, Smith, 2004) and SPI (right, Harris et al.,2005)

Fig.5 Theoretical predictions of the galactic
60
Fe/26Al ratio (Prantzos, 2004)

Fig.6 The CGB observations and SNIacontribution. (Watanabe, et al., 1999)

al., 1997) results show a power-law spectra. Seyfert galaxies seem to be responsible forthe energies < ~300keV (Zdziarski, et al., 1993) and, the decay chain of 56Ni => 56Co =>56
Fe in the cosmological SNIa could produce the CGB flux comparable to the observed
one (Watanabe, et al., 1999). Some recent studies (e.g. Zhang et al. 2004), however,demonstrate that SNIa could contribute only ~1% of the CGB flux due to uncertainties ofthe SNIa rates and the star formation rates. Some suggest (e.g. Ahn, et al., 2005) a Dark
Matter scenario. Since there are many unknowns, we need to await future Gamma-ray
missions, such as Advanced Compton Telescope (ACT), to conclude the origin(s).
2. INTEGRAL Mission
INTEGRAL (INTErnational Gamma-Ray Laboratory) is an ESA (European SpaceAgency) mission launched on October 17th, 2002. Its mission science is Gamma-ray
astronomy in the energy range from 15 keV to 10 MeV The payload includes two main
instruments: SPI (germanium spectrometer) and IBIS (CdTe + CsI) imager, as well as
two monitor Instruments: JEM-X (x-ray monitor) and OMC (optical monitor). Thismission has significant improvements in performance over Compton Gamma-Ray
Observatory (CGRO) with new technologies. For instance, Germanium detectors andCdTe detectors have ~ 50 times better energy resolution and ~ 15 times better angularresolution than CGRO, respectively. INTEGRAL forms complementary set with SWIFT(gamma-ray bursts) and GLAST (high-energy gamma-rays). Fig. 7 – Fig. 10 illustrate theCodedMask

BGO
ActiveShieldMechanicalCooler

Ge Detector
Array

BGO
ActiveShieldFig.8 SPI

Fig.7 INTEGRAL space craft

Fig.9 JEM-X

Fig.10 IBIS

space craft, SPI, JEM-X, and IBIS. Table 1 summaries the characteristics of thesedetectors.
SPI, IBIS, and JEM-X employ coded aperture imaging. Since Gamma-rays can not be
focused easily, INTEGRAL uses the coded mask technique. It is basically a pinholeEnergy
Range
18 keV 8 MeV

EnergyResolution
2.2keV@1.3MeVField ofView
16 degfully coded

AngularResolution
2.5 deg
FWHM

Detector
Area
500 cm2 (19 hexagonalGe detectors)

IBIS (Imager on
Board the Integral
Satellite)

15 keV 10 MeV

8%@100keV
10%@1MeV

8.3 x 8.0
deg fullycoded

12 arcmin
FWHM

JEM-X (JointEuropean X-rayMonitor)

3-35keV

1.3keV@10keV

4.8degfully coded

3 arcmin

2600 cm2(4x4x2 mm
CdTe pixels)ISGRI,
3000 cm2 (9x9x30 mm
CsI pixels)PICsIT
500 cm2 (2 imagingmicrostrip gas chambers(90% Xenon + 10%Methane)

SPI(Spectrometeron INTEGRAL

Table 1 Characteristics of each detector

camera with many holes. Incoming Gamma-rays fromsources come into detectors through the coded-masks(see Fig.11). Then, the detectors can be deconvolved
allowing us to separate and locate individual sources,and create images.

Mask

INTEGRAL has made many discoveries of various newsources. They include High Mass X-ray Binaries(HMXB):IGR J16318-4848, IGR J16320-4751, IGR
J17391-3021, Seyfert Galaxy: IGR J18027-1455, Low
Mass X-ray Binaries (LMXB):IGR J16358-4726, IGR
Detector
J16418-4532, IGR J17464-3213, Neutron Star: IGR
Array
J17597-2201, Gamma-ray sources: IGR J00291+5934,
IGR J06074+2205, IGR J07597-3842, IGR J11305-6256, Fig. 11 Coded Aperture Imaging
IGR J16167-4957, IGR J16479-4514, IGR J16558-5203, IGR J17252-3616, IGR J183250756, IGR J18450-0435,IGR J18483-0311, and IGR J21247+5058.
Some recent accomplishments are: a discovery of a faint persistent source within 1’ ofSgr A* (Bélanger et al., 2005), an identification of the Galactic TeV (H.E.S.S. ) sources:IGRJ18135-1751 (Ubertini et al.,2005) & AX J1838.0-065 (Malizia et al., 2005),
detection of fast X-ray transients of just a few hours duration: XTE J1739-302 (Sguera etal.,2005; Smith et al., 2005), discoveries of the fastest known accreting X-ray millisecond pulsar IGR J00291+5934(1.67ms, Porb= 2.46h) (e.g. Falanga et al., 2005), andAnomalous X-ray Pulsar 4U0142+614 (den Hartog et al., 2004). Gamma-Ray Bursts or
X-ray flashes have been also detected (e.g. GCN #3059, 3323, 3348, 3430, 3446, 3472,3552, 3607, 4002, 4007, 4192).
3. The Galactic 511 keV Emission
3.1 Possible Positron Sources
Although no positron sources have been identified yet, there are a several possible
sources. In massive stars (e.g. Wolf-Rayet) positrons can be created by nucleosynthesis instellar interior. They are convected to the surface and will be transported into interstellar
medium (ISM) by stellar winds. Hypernovae, which are asymmetric explosions of amassive (> 30 Msun) stars, are another possibility. Positrons can be efficiently transported
outward by strong jets and escape into low optical depth regions where they subsequently
annihilate. Pair plasmas are believed to exist in Black Holes (BH) jets. Positrons could
either be aggregate emission from many BHs in Galactic Center region or from a single
massive BH at Galactic Center. Beta-decay of radioactive nuclei created in type Ia
supernovae explosion also produces positrons, and they are transported into ISM inexpanding shell. There has been recent speculation on the possible existence of light (mc2
< 100 MeV) dark matter particles which would decay or annihilate primarily through theformation of electrons and positrons. A tangle of light superconducting strings in theMilky Way also could create positrons. In this scenario the positron production isproportional to the Galactic magnetic field.

3.2 511 keV Spectroscopy
The measured 511 keV line energy is consistent with the rest energy of mc2 = 511.00
keV. The narrow line width (2.3 keV) is also consistent with annihilation in warm
interstellar clouds (see Table 2). A step is seen in continuum from ortho-positronium.
(Fig.11).
511 keV Line Flux
(10-3 cm-2 s-1)
Line Energy (keV)
FWHM (keV)
Positronium Continuum
Flux (10-3 cm-2 s-1)
Ps fraction

1.35 ± 0.11
511.14 ± 0.08
2.34 ± 0.24
4.7 - 10.2
> 0.88

Table 2 Line characteristics.

Fig. 11 Observed 511 keV line spectrum by SPI

3.3 Annihilation Processes
Positrons lose energy mainly due to
Coulomb collisions, then most of them(> 90%) form positronium in flight. The
rest of positrons become thermalized
and go into several processes as shownin Fig. 12. Positronium is in either thepara or ortho states. Para- positronium(S=0, Ms= 0) decays into two 511 keV
photons, and ortho- positronium (S=1,Ms=-1, 0, 1), which forms, 3/4 of thetime, undergoes 3-photon decay making
continuum below 511keV. (Guessoumet al. 1991)
Fig. 12 Annihilation processes.

3.4 Line Profile Thermalized e+ and positronium annihilations create narrow lines, whilepara-positronium in-flight forms a broad continuum (see Fig. 13). Positrons can migratefrom the source and annihilate somewhere in ISM. The 511 keV line profile andpositronium fraction are diagnostic of annihilation region. Annihilation probably takes
place in warm (T ~ 104 K) partially ionized region of ISM. The recent SPI observation
using 2004 public data reveals the detection of the broad component (Jean et al., 2005).
They fit the line with combination of a narrow line, a broad line, an ortho-positronium
continuum, and a power law. Fig. 14 shows the detail. Although the detection was not sosignificant (3.2σ) with χ2 ~ 171.3 (dof 148), they find that the broadline flux is about 1/3of the total 511 keV flux of (1.07±0.03) x 10-3 g s-1 cm-2, and the positronium fraction is
96.7±2.2 % with the systematic error of ~ 0.2%.

Thermalized Ps
annihilation

Thermalized e+
directannihilation

Ps inflightannihila
tion

Ps 3γ
continuumFig. 13 Line profiles. (Churazov et al., 2004) Fig. 14 Detection of the broad line. (Jean et al., 2005)

3.5 Spatial Distribution
Fountain?

Gal.
Lat.

BulgePlane

Gal. Long.

Fig. 15 OSEE 511 map (Purcell et al.,1997)

Fig. 16 OSSE/SMM/TGRS map(Milne et al., 2001)

CGRO/OSSE measured the 511 keV distribution, which extends to the high galactic
latitude (~ 15o) as well as displaying a strong bulge concentration and emission fromthe galactic plane (Fig 15, Purcell et al.,1997) The OSSE/SMM/TGRS combinedobservations using two different algorithms (Richardson-Lucy & Singular Value
Decomposition), however, show much less “fountain” (Fig. 16, Milne et al., 2001).
Fig. 17 shows the SPI observation. The Bulge distribution data are reasonably well fit
by symmetric gaussian with FWHM = 8° ± 1° (Fig. 18). There is no evidence for theOSSE “fountain”. Although it could be diffuse, ensemble of point sources cannot be
ruled out. The concentration of flux at galactic center suggests that the origins are in
the older population.

Fig. 17 SPI 511 Map (Knödlseder et al.,2005)

Fig. 18 Symmetric Gaussian (Teegarden et al. 2005)

Fig. 19 Bulge + Disk Model (Knödlseder et al.,2005)

Fig. 20 Positronium Map (Weidenspointner, etal., 2005)

SPI marginally detects the 511 keV galactic disk component at 3-4 σ significance. The
result depends on the models we introduce, and Fig. 19 is the best fit model. The 511 keVflux is F511 = (7±5) x 10-4 cm-2 s-1 , and the Bulge to the disk flux ratio is 1 – 3.
(Knodlseder et al., 2005) A positronium map was also obtained by SPI (Fig. 20). The
measurement is more difficult than the 511 keV map due to the broader energy range, thesmaller signal-to-background ratio, and more complex background. It has the similarmorphology to the 511 keV narrow line, and consistent with the OSSE results.
(Weidenspointner, et al., 2005)
3.6 511 keV from 26Al
As we discussed earlier, the distribution of 26Al is concentrated in the galactic disk. It isthought to be produced by nucleosynthesis in massive stars. Thus, the 511 keV disk
component could be accounted for by the beta+ decay of 26Al. We estimate the 511 keV
flux
F511 = fdecay x f511 x F1809 = 3.5 x 10-4 cm-2 s-1 ,where fdecay is the β+ decay fraction (82%), f511 is number of 511 keV photons perannihilation (≈ 0.65), and F1809 is the 26Al 1.809 MeV line flux( ≈ 7 x 10-4 cm-2 s-1 ).
Therefore, the predicted 511 keV disk component flux from 26Al decay is ~ 50% ofobserved value but still within the error bar.
3.8 Searching for Tracer of 511 keV Emission
Fig. 21 shows search for correlations between the 511 keV line emission morphology andtracer maps obtained at various wavelengths. The Reduced Maximum Likelihood-Ratio(RMLR, see Knödlseder et al.,2005) is plotted as function of the tracer. There are strong
correlations at nuclear bulge, K and M giant stars, as well as gamma-ray compactobjects. RMLR versus the bulge-to-disk (B/D) ratio is shown in Fig. 22. The data favour
the large B/D ratio. HEAO-1 traces X-ray binaries, and we know that Low-mass X-raybinaries (LMXBs) have a strong concentration towards the galactic center (B/D ~ 1.8).
Candidates for the origin of positrons are LMXBs and old stellar populations such as

Reduced Maximum log Likelihood-Ratio

K + M giants
nuclearbulge

compactobjects

relativistic
electrons

ISM

atomic HRadioµ-waveszodiacallight pollution

FIRNIR

extinction

VX-raySNIa. Dark Matter is not
ruled out. The 511 keV lines
from point sources have beenmeasured by SPI. We,
however, find no significant
point sources. Table 3 is asummary for LMXBs with 3
σ upper limits.(Knödlsederet al.,2005) An independent,
comprehensive line search,using the first year’s SPI data,also does not find any 511
keV point sources.(Teegardenγ-ray& Watanabe 2005)

Fig. 21 RMLR as function of the tracer map. (Courtesy of Knödlseder)

Fig. 22 RMLR as function of the tracer map. (Courtesy of Knödlseder)
Sourcel (deg)GX 5-1

5.08°

GRS1915+105 45.37°b (deg) Distance 3 σ Flux Limit(kpc)(10-4 ph cm-2 s-1)
-1.02° ~ 8?
< 0.7-0.22° 12.5

< 1.0A 0620-00
209.96° -6.54° 1.5 - 2.5 < 3.8
Nova Muscae 295.30° -7.07° 5.5< 2.0
Cir X-1
322.12° 0.04° > 8< 1.1
Cen X-4
GX 349+2
Sco X-1

332.24° 23.89° ?
349.10° 2.75° ?
359.09° 23.78° 0.7

< 1.7
< 0.8
< 1.5

Table 3 511 keV narrow line 3 σ upper flux limits for LMXBs. (Knödlseder et al.,2005)

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