PASJ: Publ. Astron. Soc. Japan , 1–??,
c 2011. Astronomical Society of Japan.

In-Orbit Performance of MAXI Gas Slit Camera (GSC) on ISS
Mutsumi Sugizaki 1 , Tatehiro Mihara 1 , Motoko Serino 1 , Takayuki Yamamoto 1 , Masaru Matsuoka 1,2 ,Mitsuhiro Kohama 2 , Hiroshi Tomida 2 , Shiro Ueno 2 , Nobuyuki Kawai 3 , Mikio Morii 3 , Kousuke Sugimori 3 ,Satoshi Nakahira 4 , Kazutaka Yamaoka 4 , Atsumasa Yoshida 4 , Motoki Nakajima 5 , Hitoshi Negoro5 , Satoshi
Eguchi 6 , Naoki Isobe 6 , Yoshihiro Ueda 6 , and Hiroshi Tsunemi 7
1

MAXI team, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198
sugizaki@riken.jp
2
ISS Science Project Oﬃce, ISAS, JAXA, 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505
3
Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551
4
Department of Physics and Mathematics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara, Kanagawa 229-8558
5Department of Physics, Nihon University, 1-8-14, Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8308
6
Department of Astronomy, Kyoto University, Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
7
Department of Earth and Space Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan(Received 2010 November 1; accepted 2011 February 1)

Abstract
We report the in-orbit performance of the Gas Slit Camera (GSC) on the MAXI (Monitor of All-sky
X-ray Image) mission carried on the International Space Station (ISS). Its commissioning operation started
on August 8, 2009, confirmed the basic performances of the eﬀective area in the energy band of 2–30 keV,
the spatial resolution of the slit-and-slat collimator and detector with 1.5◦ FWHM, the source visibilityof 40-150 seconds for each scan cycle, and the sky coverage of 85% per 92-minute orbital period and 95%
per day. The gas gains and read-out amplifier gains have been stable within 1%. The background rate isconsistent with the past X-ray experiments operated at the similar low-earth orbit if its relation with thegeomagnetic cutoﬀ rigidity is extrapolated to the high latitude. We also present the status of the in-orbit
operation and the calibration of the eﬀective area and the energy response matrix using Crab-nebula data.
Key words: instrumentation: detectors – X-rays: general – X-rays: individual (Crab Nebula)
1.

Introduction

MAXI (Monitor of All-sky X-ray Image) is the first
astronomical mission operated on the ISS (International
Space Station) (Matsuoka et al. 2009). The payload
was delivered to the ISS by Space Shuttle Endeavour on
July 16, 2009, and installed on the Japanese Experiment
Module – Exposed Facility (JEM-EF or Kibo-EF) on July
24. The mission was designed to achieve the best sensitivity and the highest energy resolution among the all-sky
X-ray monitors performed so far by using two kinds of
X-ray slit cameras, Gas Slit Camera (GSC, Mihara et al.
2011) and Solid-state Slit Camera (SSC, Tomida et al.
2011), which work complementary.
The GSC is the main X-ray camera to cover the energy band from 2 to 30 keV with a large area. It employs
conventional slit camera that consists of Xe-gas proportional counters and slit-and-slat collimators. Twelve gas
counters achieve a large detector area of 5350 cm2 . They
are assembled into six identical units to cover wide fieldsof views (FOVs) of 1.5 × 160 degree2 . They are embodied in the MAXI payload module such that they cover
the earth-horizon and the zenith directions with an equalarea. It enables us to cover the entire sky every 92-minuteorbital period even if the GSC has to be partly downed
in a heavy particle-radiation environment such as the passage of the SAA (South Atlantic Anomaly). The Xe-gas

counters employ resistive carbon-wire anodes to acquire
one-dimensional position sensitivity. The gas volume ofeach counter is separated into six carbon-anode cells formain X-ray detectors and ten tungsten-anode cells for vetocounters. The veto cells surround the carbon-anode cells
to reduce backgrounds by the anti-coincidence-hit logic.
Total 14 anode signals in each counter are read out independently through an electronics unit and processedby on-board data processors. Figure 1 illustrates theschematic views of a single GSC camera unit and FOVs ofthe whole six units on the ISS low-earth orbit. The detailsof the instruments are described in Mihara et al. (2011).
This paper presents the GSC in-orbit performance including the operation and calibration status. The SSC inorbit performance is presented in Tsunemi et al. (2010).
2.

Operation

2.1.

Initial Operation

The main power of the MAXI payload is supplied fromthe JEM-EF via the Payload Interface Unit (PIU). It was
activated on August 3, one week later since the payload
was installed on the JEM-EF. On August 4, the GSC
electronics units (GSC-E) including the Radiation-Belt
Monitor (RBM) were switched on. The on-board RBM
parameters, thresholds of the lower discriminators andcount-rate criteria to down the gas counters, were then2

Sugizaki et al.
X-ray sources-1.5 θ

Zenith camera FOV

*

n
io

po

at

GSC_3[9]GSC_4[A]

t
ro

o

o

o

-40

Slit

+40

+1.5

Slat
Collimator

Horizon camera FOV
Left camera
GSC_0[6]GSC_5[B]
-80 o

+80

ETX)
ion (D
t
c
e
ir
d
ity
Anode
ensitiv
ition-s
s
o
p
1-D

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+40

o

u
cq

A

GSC_2[8]Right camera

Earth

:β

-40 o

MAXI

Xe-gasCounterle

ISS rotationoφ[Vol. ,

tio
isi

n
na

gle

GSC_1[7]Center camera

ISS orbitalmotio

nFig. 1. (Left): Schematic view of a single GSC camera unit and its field of view (FOV) of 1.5◦ (FWHM) × 80◦ . (Right): FOVs ofthe six camera units including 12 counters (GSC 0, ..., GSC 9, GSC A, GSC B) on the ISS low-earth orbit. Every two counters, suchas GSC 0 and GSC 6, are used as a pair for a redundancy. Left, center, and right camera units cover the area of source-acquisition
angle β of −80◦ < β < 0◦ , −40◦ < β < +40◦ , and 0◦ < β < +80◦ , respectively.

set up using the data taken for the first three days. On
August 8, the high voltage of the first gas counter was activated and increased up to the nominal operation voltageof 1650 V. The first test observation was then performed
for three hours and various analog/digital-electronic functions were verified. All the 12 counters were activated at
the nominal 1650 V since August 10 to 13 one by one.
Each gas counter has 14 anode signals for both two endsof six carbon-wire anodes and two veto signals. Each anode signal has individual pre-amplifier, a shaping amplifier, a lower discriminator (LD), and an Analog-to-Digital
Converter (ADC) which produces 14-bit pulse height. The
gain of the shaping amplifier and the threshold of the lower
discriminator of each signal can be changed by command.
These parameters are adjusted according to the gas gainof individual anode wire. The first all-sky image by all 12
counters was taken on August 15.
2.2.

RBM

GSC incorporates Radiation-Belt Monitor (RBM) tomonitor the flux of the cosmic rays in orbit. Two siliconPIN-diode detectors with 0.2-mm thickness and 5×5 mm2
area are equipped with each of GSC horizon and zenith
center units. The detector on the horizon unit (RBMH) is faced to the tangential direction of the ISS motion
along the earth horizon. The zenith unit (RBM-Z) is faced
perpendicular to the horizontal plane. The RBMs always
monitor count rates of cosmic rays with a certain energy
deposit, whose threshold can be changed by commands.
The level of the threshold is set at 50 keV, which corresponds to the half of the minimum ionization-loss energyof the relativistic particles.
Figure 2 shows the RBM count-rate maps on the ISS

orbital area by RBM-H and RBM-Z units, respectively,
averaged over the data taken for the first year. The ratioof the RBM-H to the RBM-Z count rate is shown at thebottom where contours of inclination angle of the magnetic field from the earth-horizontal plane are overlaid.
The two RBM rates agree with each other in the low
count-rate area below 0.2 Hz, where the anti-correlations
with the geomagnetic cutoﬀ rigidity (COR) are clearly
seen. It indicates that the origins are energetic cosmic
rays. The cosmic-ray flux ∼ 1 counts cm−2 s−1 and its
relation with the COR are consistent with those obtainedby ASCA (Makishima et al. 1996) and Suzaku (Kokubun
et al. 2007) in low-earth orbit although the altitude of theISS, 340–360 km, is diﬀerent from that of these satellite,
530–590 km.
While the two RBM rates are agreed at the low radiation area, they are quite diﬀerent in the high radiation
area around the SAA and the both geomagnetic poles located at the north of the north American continent andthe south of the Australian continent. The RBM-H rate ishigher by an order of magnitude than the RBM-Z rate inthese area. From the correlation between their ratio andthe inclination of the magnetic field, the diﬀerence is considered to be due to the anisotropy of the flux of trapped
particles. A large number of cosmic-ray particles trappedby the geomagnetic fields are moving circularly on thehorizontal plane around the magnetic pole. The RBM-Z
detector faced on the zenith direction is insensitive to theparticles moving on the horizontal plane. The anisotropyof the trapped particles at the high latitude has also beenrecognized by images of particle tracks taken by the SSC
CCD imager (Tsunemi et al. 2010).

No. ]

In-Orbit Performance of MAXI/GSC
RBM-H

80

2.3.

102

60

Latitude [deg]

40
20
0

1

-20
-40

Count rate [Hz]

10

10-1

-60
-80
-150

-100

-50

0

50

100

150

10-2

Longitude [deg]

RBM-Z

80

102

60

Latitude [deg]

20
0

1

-20
-40

Count rate [Hz]

10

40

10-1

-60
-80
-150

-100

-50

0

50

100

150

10-2

Longitude [deg]× North pole (incl.=+90 o)

80

RBM-H / RBM-Z

102

60
10

20
0

1

-20
-40

10-1

-60

×
South pole (incl.=-90o)

-80
-150

-100

-50

0

50

100

150

10-2

Longitude [deg]Fig. 2. RBM count-rate maps of horizon (RBM-H, top
panel) and zenith (RBM-Z, middle panel) units. Bottom
panel shows their ratio (RBM-H/RBM-Z) and contours of
the inclination angle of the magnetic fields from the horizontal plane are drawn by 10◦ step from −90◦ (south pole) to90◦ (north pole).

Ratio

Latitude [deg]403High Voltage Operation

We started full GSC operation with twelve counter
units, GSC 0, ..., GSC 9, GSC A, GSC B, on August 15,
2009. The high voltages of all the twelve gas counterswere set at the nominal 1650 V. They are reduced to 0 V
when the ISS passes through heavy particle backgroundarea defined by an on-board Radiation-Zone (RZ) map.
In the beginning of the mission, we set the RZ map onlyat the SAA.
On September 8, 2009, the analog power on the GSC 6
counter was suddenly down. It was followed by the power
down of another counter, GSC 9, on September 14. The
data indicate that a carbon-anode wire in each of thesecounters was fractured. It was also suggested that thelocation fractured on the carbon wire would be where alarge amount of discharges occurred repeatedly.
We then changed the counter operation strategy so thatany risks to cause potential damages on the carbon-anodewire such as discharge and heavy irradiation should be
avoided as much as possible. The high-voltage reduction at the high latitude above 40◦ was employed since
September 23. Two noisy counters, GSC A and GSC B,
which are considered to be fractured sooner, were alsotentatively stopped on September 26.
On March 26, 2010, the analog power on the GSC 3
counter was downed, which were considered to be due
to a carbon-wire fracture by breakdown discharge again.
Since then, the operation voltage was reduced to 1550 V
if the counter had any discharge experience.
As the results of these limited HV operation, the effective observation eﬃciency is reduced to about 40%. It
is expected to reduce the sensitivity. The three counters
suﬀering from the breakdown will be soon activated withgreat care.
3.

In-Orbit Performance

3.1.

Background

The GSC counter has a one-dimensional position sensitivity and performs scanning observation. Backgrounds
for any X-ray sources on the sky can be roughly estimated from the levels of the adjacent source-free region.
The studies of the backgrounds are important because
the residual backgrounds and its uncertainties limit thesource-detection sensitivity.
3.1.1. LD-hit Rate and Processed Event Rate
Each GSC gas counter embodies six carbon-anode cells
for main X-ray detector and the veto cells surrounding
the carbon-anode cells. The cosmic-ray backgrounds arescreened out on board by applying an anti-coincidence
matrix to hit patterns of all anode signals in each counter.
The eﬃciency of the background reduction depends on the
LD threshold of the read-out signals. The amplifier gains
and LD levels of all the read-out signals are tuned during
the initial verification operation.
The background rates of all the 12 GSC counters arefound to have a similar time variation during the entire
observation-operation period. They exhibit clear anti-

4

Sugizaki et al.

[Vol. ,

103

COR<2
20

Counts s−1 keV−1

LD-hitRate [Hz]102

Anti-coin10cidence15

10
4<COR<8
5

8<COR

cut

CXB
2

5

10
Energy (keV)

20

2-30 keV

1
0

Ti−K
2<COR<40

PHA cut
(

Cu−K

2

4

)

6
8
10
12
Cutoff Rigidity [GV]

14

Fig. 4. GSC background spectra per anode cell sorted by
COR (Cutoﬀ Rigidity) and estimated contribution of the
CXB (Cosmic X-ray Background). The data are average spectra of the entire 12 counters taken for a day on August 18,
2009. Ti-K and Cu-K fluorescent lines are seen at 4.6 keV
and 8.1 keV, respectively.

16

Fig. 3. GSC LD-hit rate, on-board processed event rate after anti-coincidence cut, and PHA-filtered event rate with theenergy band of 2–30 keV per anode cell against geomagnetic
cutoﬀ rigidity.

100 mCrabPhotons cm−2 s−1 keV−1

0.01

correlations with the geomagnetic COR in orbit. Figure 3
illustrates the LD-hit rate, the processed event rate after
the anti-coincidence cut, and the cleaned event rate with
the PHA (Pulse-Height Amplitude) range of the 2–30 keVenergy band against the COR. Each data point represents
the rate per anode averaged over six anodes in a counter,
GSC 0, every minute, taken for a day on August 18, 2009,
except while the ISS is in a heavy particle-irradiation area
detected by RBM and RZ map. Data expected to include
significant X-ray events from celestial bright X-ray sources
are also excluded. The ratio of the anti-coincidence-event
rate to the LD-hit rate is about 1:30 and almost constantover the entire period. It is quite similar with the GingaLAC case (Turner et al. 1989). The LD-hit rate is wellagreed with the RBM rate taking the detector area, 90
cm2 per anode, into account. It indicates that they are
the energetic cosmic-ray origin.
3.1.2. Energy Spectrum
Figure 4 shows energy spectra of the residual backgrounds sorted by COR and the estimated contribution of
the unresolved Cosmic X-ray Background (CXB) within
the FOV (e.g. Gendreau et al. 1995). The profile shows
the fluorescent lines of Ti-K at 4.6 keV from the gascounter body and Cu-K at 8.1 keV from the slat collimator made of phosphor bronze. These emission lines areused for the energy response calibration. The shape ofthe continuum in the energy band above 8 keV is quitesensitive to the COR. It suggests that the origins of thesecomponents would be energetic cosmic rays. The CXB
component is expected to be dominant in the energy band
below 5 keV at COR>8.
Figure 5 illustrates the background spectrum normal-

Ginga−LAC
MAXI−GSC (8<COR)
10−3

RXTE−PCA
1 mCrab2

5

10 mCrab
10
Energy (keV)

20

Fig. 5. GSC background energy spectrum normalized by
eﬀective area and comparison with those of Ginga-LAC,
RXTE-PCA, and Crab-like source spectra. Expected contributions of the CXB in each instrument are included.

ized by eﬀective area, and those of the Ginga LAC(Hayashida et al. 1989) and the RXTE PCA (Jahoda
et al. 2006), where expected contribution of the CXB
in each instrument are included. Crab-like source spectra
are shown together as the comparison. The GSC background is approximately 2 mCrab at 4 keV and 10 mCrab
at 10 keV. The level is almost comparable to that of the
Ginga LAC and slightly higher than that of the RXTE
PCA.
3.1.3. Spatial Distribution
Figure 6 shows spatial distributions of the residual backgrounds along the anode wire in 2-10 and 10-30 keV energy
bands and the estimated contributions of the CXB. Each
GSC counter has a sensitivity gap at the center of the an-

No. ]

In-Orbit Performance of MAXI/GSC

within the statistical uncertainty.
The energy scale of the PHA of all the read-out signals
are monitored at the entire location along the carbonanode wire using Ti-K and Cu-K fluorescent lines in the
background, as seen in figure 4. The time variations forthe first year was within 1% in the entire counter area.
This information is employed for a PHA-to-PI (Pulseheight Invariant) conversion in the data reduction process.

0.025

2-10 keVCounts s-1 mm-1

0.02

0.015

0.01

0

3.3.

CXB

0.005

-100

-50

0

50

100

50

100

DETX [mm]
0.045
0.04

10-30 keVCounts s-1 mm-1

0.035
0.03
0.025
0.02
0.015
0.01

CXB

0.005
0

-100

-50

0

DETX [mm]Fig. 6. Background spatial distribution in GSC gas countersalong anode wires (DETX) in each energy band of 2-10 keV(top) and 10-30 keV (bottom). Estimated contributions of
the CXBs are shown with solid lines. The dips at the center(DETX=0) are due to the shadow of the support structure of
the counter beryllium window.

ode wire due to the frame structure to support 0.1-mm
thick beryllium window. It causes the dip at the centerin the spatial distribution. The depth of the dips can bemostly explained by the extinction of the CXB component
at the support structure.
The profile of the residual background left after subtracting the CXB component is primarily flat and slightly
increases at both ends of the anode wires. It is because
the eﬃciency of the anti-coincidence reduction decreases
at the counter edge. The sensitivity is expected to getworse in these area.
3.2.

5

Energy Response and Gain Stability

Each gas counter is equipped with a calibration source
embodying radioactive isotope 55 Fe at the end of one ofsix carbon anodes (the left-side end of carbon anode #2).
It always exposes collimated X-ray beam with a diameterof 0.2 mm. The counter gas gain and energy resolution
have been continuously monitored with the 5.9-keV line
since the ground calibration test. The results show thatthe gas gain is stable within 2%, which agrees well withthe results of the vacuum-environment test on the ground.
The variation in orbit during the first year was within
0.8%. The energy resolution of 18% at 5.9 keV was stable

Detector Position Response and Stability

The incident X-ray position along a carbon-anode wireis calculated from the PHA ratio of two read-out signals
to the both anode ends. All the required calibration data
to derive the detector position from the PHA ratio werecollected in the ground tests (Mihara et al. 2011).
The stability of the detector position response has beenmonitored with the calibration source since the groundcalibration tests. The time variation of the calculated
position during the first year was less than 0.2 mm that
corresponds to an angular distance of 0.07◦ on the sky. It
is small enough for the position determination accuracy of
0.2◦ achieved. The detector position resolution of 2.0 mm(FWHM) for 5.9-keV X-ray at the nominal anode voltageof 1650 was constant within the statistical uncertainty of
0.05 mm (1 σ) since the ground tests.
3.4.

Point Spread Function in the Anode-wire Direction

The point spread function (PSF) of the GSC is determined by the angular response of the slit-and-slat collimator and the position response of the position-sensitive gas
counter along the anode wire. The collimator is designed
to have an angular resolution of 1.0–1.5◦ in FWHM, whichslightly depends on the X-ray incident angle. The position
resolution of the gas counter depends on the X-ray energy.
The resolution for X-rays with a normal incident angle isinversely proportional to the number of multiplied signal
charges and varies from 4 to 1 mm in the GSC energy
band from 2 to 30 keV (Mihara et al. 2011). For eventswith slant incident angles, the variation of the photonabsorbed depth in a gas cell can also degrade the position
resolution. The relation between the positional scale on
the detector and angular distance on the sky also depends
on the incident angle. Thus, the PSF is represented by acomplex function of X-ray energy and incident angle.
Figure 7 illustrates the cross-section profiles of PSF
models for the direction along the detector anode-wire ateach photon-incident angle, φ, of 5◦ and 33◦ in 2–10 keV(soft) and 10–30 keV (hard) energy bands (see the geometry in Figure 1). We also show the actual observed profiles
taken from the data of a bright X-ray source, Sco X-1, forcomparison. The PSF models agree with the data within
the statistical uncertainty. For events of φ = 5◦ , which
is close to the normal incident angle, the angular resolution in the hard band is better than that in the soft band
according to the better detector position resolution along
the anode wire for the higher-energy X-rays. For eventswith a slant incident angle of φ = 33◦ , the variation of theabsorbed depth yields the broad distribution along theanode wire and degrades the resolution especially in the

6

Sugizaki et al.

Normalized count rate

1

φ=5°

eﬀective areas during scan transits at typical two sourceacquisition angles of 5◦ and 72◦ , where the angular scalesof the collimator transmission function are also indicatedat the top on each panel. The observed count rates of Sco
X-1 taken at the same acquisition angle are plotted together for comparison. The calculated scan duration andthe observed period are agreed well with each other.

10-30 keV

0.8
0.6
0.4

3.6.
2-10 keV

0.2
0-4

Normalized count rate

1

-3

-2

-1

0

∆φ [degree]

1

φ=33°

2

3

4

2-10 keV

0.8
0.6

0.4

10-30 keV
0.2
0

-4

-3

-2

-1

0

∆φ [degree]

1

2

3

4

Fig. 7. PSF cross-section profiles along the detector anode-wire direction at each source-incident angle, φ, of 5◦ (top)and 33◦ (bottom) in two energy bands of 2–10 keV (solid line)
and 10–30 keV (dashed line). The power-law energy spectrumof photon index, 2.1, is assumed. Actual observed profiles of
Sco X-1 taken at each source angle are plotted with 1-σ error
bars in the 2–10 keV (circle) and 10–30 keV (triangle) bands.

hard band.
3.5.

[Vol. ,

Scan Transit Profile and PSF in the Scan Direction

The GSC typically scans a point source on the sky during a transit of 40–150 seconds with the FOV of 1.5◦ -width(FWHM) every 92-minute orbital period. The transit
time depends on the source incident angle for each GSC
counter φ as well as the acquisition angle from the ISS
rotation equator β (see the geometry in figure 1). Figure
8 illustrates the relation between the scan-transit time oneach GSC counter and the source acquisition angle fromthe rotation equator in the ISS normal attitude. The scanduration increases from the ISS rotation equator (β = 0◦ )towards the pole (β = ±90◦ ) in the side cameras such as
GSC 0 and GSC 2 according to the FOV-area angle fromthe rotation pole.
The detector area for the target changes according to
the triangular transmission function of the collimator during each transit. It modulates time variations of observed
photon count rates and represents the PSF in the scan direction if the source flux is constant. The scan direction isperpendicular to the detector anode-wire direction in thenormal ISS attitude. Figure 9 shows time variations ofExposure Map and Sensitivity

An exposure map on the sky for a given period is calculated from the counter HV-operation history, the detector
eﬀective area as a function of a photon incident angle, andinformation of the ISS orbit, attitude, and configuration
parameters interfering the FOV such as the solar paddles, the space-shuttle vehicle docked on the ISS. Figure
10 shows the actual exposure maps by all operated GSC
counters for 92 minutes of an ISS-orbital cycle, one day,and 27 days since January 2, 2010, 00:00 (UT), calculated
from all the required information during these periods.
GSC covers approximately 85% of the whole sky for one
orbit except for the orbits including the SAA passage, and
95% for a day. The daily map has uncovered area for the
Sun direction, the solar-paddle shadow, and the rotationpole that drift on the sky according to the precession of
the ISS orbit. We can achieve the 100% full coverage inevery three weeks.
An actual exposure time for an arbitrary direction on
the sky was typically 4000 cm2 s per day. It is about one
third of the initial expectation because the HV operation
at the high latitude above 40◦ was stopped and four counters out of 12 were disabled in these period. The daily 5-σ
source sensitivity is expected to become 15 mCrab, whichis three times worse than that of the pre-flight simulation(Matsuoka et al. 2009).
4.

Calibration Status

We here present the status of the GSC response calibration for data taken at the nominal HV of 1650 V. We
also exclude data detected by anodes #1 and #2 of all
GSC counters because some of them show complicatedenergy-PHA relations at low PHA, which is probably due
to feedback crosstalks on the circuit board. The responses
functions for the data taken at the reduced HV of 1550
V and by the anode #1 and #2 are being developed andwill be updated soon.
4.1.

Alignment and Position Localization Accuracy

The attitude of the MAXI payload module iscontinuously measured with the on-board Attitude
Determination System (ADS), which consists of a VisualStar Camera (VSC) and a Ring Laser Gyroscope (RLG).
Its accuracy is estimated to be better than an arc minutefrom the ground calibration tests (Horike et al. 2009).
The source location of an incident X-ray on the sky is
calculated from the position coordinates on the detector
where the X-ray is absorbed and the attitude parameters at the event time. The alignments of the collimators and detectors on the payload module are calibrated

No. ]

In-Orbit Performance of MAXI/GSC

7

Scan−transit time [s]

150

100
GSC_0

50

GSC_2

GSC_1

0
−50
0
50
β: Source−acquisition angle [degree]
Fig. 8. Scan-transit time as a function of source-acquisition
angle for each GSC unit of GSC 0, GSC 1, and GSC 2, in the
ISS normal attitude.

∆θ [degree]-1.5 -1 -0.5 0 0.5 1 1.5
5

β=5°

GSC_1

Area [cm2]

4

GSC_2

3
2
1
0
-60

-40

-20

0
TIME [s]

20

40

60

Fig. 10. GSC exposure map by scans of 92-minute orbital
period (top), 1 day (middle), and 27 days (bottom) since 2010
January 2 00:00 (UT). The color scales of all three panels
are identical and represent the eﬀective exposure for the sky
direction in the units [cm2 s]. The uncovered areas for therotation pole, the Sun direction, the SAA, and the interferencewith the solar paddles are indicated with arrows.

∆θ [degree]
-1
5

-0.5

0

0.5

β=72°
GSC_2

4
Area [cm2]1

using standard X-ray sources whose positions and intensities are well-known. The accuracy of 0.2◦ in the 90%
containment radius of the best determined position has
been confirmed so far using several bright-source samples
selected randomly (Morii et al. 2011).

3
2
1
0
-60

-40

-20

0
TIME [s]

20

40

60

Fig. 9. Time variation of eﬀective area during each scantransit at source-acquisition angle of 5◦ for GSC 1 and GSC 2
units (top) and 72◦ for GSC 2 units (bottom). The angular scale of each transit is shown at the top of each panel.
Observed raw event rates of Sco X-1 by each GSC unit areplotted together with 1-sigma error bars.

4.2.

Eﬀective Area and Light curve

The visibility and the area of each GSC unit for a given
X-ray source on the sky are always changing according
to the ISS orbital motion and interferences with otherrelevant ISS activities, as shown in section 3.5. To derive the intrinsic time variation of a certain X-ray source,
the time-dependent eﬀective area for the target has to be
known. The light-curve response builder calculates thearea curve of each GSC unit from the source coordinates,
the ISS location, attitude, and the area of the GSC unit
as a function of the source incident angle. The area of all
the GSC units were calibrated on the ground using pencil

Sugizaki et al.

4.3.

Energy Response Matrix

The energy response matrix is represented by a productof the eﬀective area and the dispersion relation between
the incident X-ray energy and the PHA of the output signal. The dispersion relation as well as the gas gain dependon the 3-dimensional location where the X-ray photon isabsorbed in a detector gas cell. The spatial gain variation and the time variation in orbit are corrected event by[Vol. ,
GSC 4-10 keV

Photons cm-2 s-1

X-ray beams (Morii et al. 2006).
The top panel of figure 11 shows the obtained light
curve of Crab nebula in the 4–10 keV energy band since
the mission start on August 15, 2009. The data are collected from the area-corrected photon count rates for eachday. Backgrounds are estimated from the count rates forthe adjacent sky region, which corresponds to the dataobtained from the successive time periods. A power-law
energy spectrum with a photon index, Γ = 2.1, is assumed
in the eﬀective-area correction. The light curve should
be approximately constant and consistent with the fluxof the standard Crab model. The measured average flux
over the entire period, 1.25 photons cm−2 s−1 , agrees withthe standard Crab value (Kirsch et al. 2005; Weisskopf etal. 2010). However, the fit to a linear model results in a
reduced chi-squared χ2ν = 1.74 for 364 degrees of freedom(DOF), which is not accepted within the 90% confidence
limit. It is due to the limit on the eﬀective-area calibration
accuracy on the daily basis. The error on the daily fluxin the 4–10 keV band is estimated to be 3% with the 1-σrange. The calibration over the entire 2–30 keV energy
range is in progress.
Recently, Wilson-Hodge et al. (2011) presented evidences of flux variations of Crab nebula by ∼7% on a
3-year timescale, consistently observed by Fermi/GBM,
RXTE/PCA, Swift/BAT, and INTEGRAL/ISGRI. The
flux in 2–15 keV band observed by RXTE/PCA exhibits
a 5.1% decline from MJD 54690 to 55435. We examinedthe variation with the GSC data. The bottom panel of figure 11 shows the variation of the obtained GSC 4–10 keV
flux by each interval of about 100 days. The start and theend of each interval are slightly adjusted according to theobservation-time coverage. A continuous decline is seenclearly. The fluxes averaged by the long term are derived
from large event data taken by various camera angles anddetector positions. Therefore, the calibration uncertaintyof the eﬀective area on daily basis, 3% in 1 σ, is supposed
to be smeared out in it and the observed decline is considered to be real.
We fitted the data to a linear function and obtained
the decline ratio of 2.2 ± 0.5% (1-σ statistical uncertainty)
from MJD 55058 to 55558. The ratio is slightly smaller
than that extrapolated from the 5.1% decline in MJD
54690–55435 observed by the RXTE/PCA in 2–15 keVband. We tried to fit the GSC data to a linear modelwith the fixed slope determined by the RXTE/PCA, as
shown in figure 11. The fit is marginally unacceptablewith χ2ν = 1.92 for 5 DOF. It might imply the dependenceof the variation amplitude on the energy band, as reported
in Wilson-Hodge et al. (2011).

Binsize = 1 day

1.6
1.4
1.2
1

χ2 ν=1.74 (dof=364)

0.8

Binsize = 40 - 100 days
Photons cm-2 s-1

8

1.26
1.25
1.24

χ2 ν=0.32 (dof=4)

1.23
1.22

MJD

55100

08/13
UT 2009

55200

55300

55400

55500
12/30
2010

Fig. 11. (Top) Crab nebula light curve by GSC in 4–10
keV band in one-day time bin. The vertical errors represent
the 1-sigma statistical uncertainties. Solid line represents thebest-fit linear model. The reduced chi-square (χ2ν ) and thedegrees of freedom (DOF) of the fit is shown on the panel.
(Bottom) The same Crab nebula light curve but in 40–100-day
time bin and the best-fit linear model. The model with a slope
extrapolated from the 5.1% decline from MJD 54690 to 55435
observed by RXTE/PCA in 2–15 keV band (Wilson-Hodge etal. 2011) is shown together with dashed line.

event in the first data-reduction process using calibration
data collected from the ground and in-orbit test results.
All the dispersion relations required to derive the response
matrix were taken in the ground test (Mihara et al. 2002).
The energy response builder calculates the eﬀective area
and the energy redistribution matrix for a target sourceat given sky coordinates during a given observation period. The program accumulates instantaneous response
functions for the source location according to the variation of the incident angle during the active observation
period.
Figure 12 shows a Crab spectrum by GSC taken fora day on August 15, 2009 (UT) and the best-fit powerlaw model with an interstellar absorption, folded by
the response matrix obtained by the response builder.
Backgrounds are extracted from events in the adjacentsky region. The best-fit parameters are summarized intable 1. The model-to-data ratio shows that the obtained
spectrum is well reproduced within 10% over the 2-30
keV energy range. The best-fit absorption column density, 0.72 ± 0.22 × 1022 cm−2 is slightly higher than thestandard Crab value, 0.35×1022 cm−2 (Kirsch et al. 2005)
although the fit with the absorption fixed at the standard
value is also accepted within the 90% confidence limit.
The discrepancy is considered to be in inaccuracy of theeﬀective-area calibration at the low energy band. The finecalibration is in progress.

No. ]

In-Orbit Performance of MAXI/GSC

9Table 1. Best-fit parameters for GSC Crab-nebula spectrum

Model
NH a
Γb
Norm.cχ2ν (DOF)
Power-law, NH : free
0.72 ± 0.22 2.15 ± 0.05 11.4 ± 1.0 1.08 (43)Power-law, NH : fixed
0.35
2.08 ± 0.03 9.9 ± 0.4
1.25 (44)All errors represent 90% confidence limits of statistical uncertainty.
aAbsorption hydrogen column density in units of 1022 cm−2 .
bPower-law photon indexcPower-law normalization in units of photons cm−2 s−1 keV−1 at 1 keV.

0.1
0.8Counts s−1

Counts s−1 keV−1

GSC 2−30 keV

0.01

0.6
−3

10
1.2
Ratio

1.1
1

0

0.9
0.8
2

5

10
Energy (keV)

4.4.

TimingIn-orbit timing calibrations are performed using signals
from X-ray pulsars or binaries, such as Crab pulsar and
Cen X-3 with 33 millisecond and 4.8 second period, respectively. Events within the PSF of these pulsars are
extracted and applied to the barycentric correction. The
pulsation of Crab pulsar can be detected for every one-day
interval. Figure 13 show the folded pulse profiles since
January 1 to January 5, 2010. The absolute timing andthe relative stability are estimated by the relative pulse
phase of the Crab pulsar to those with the radio wavelength (Lyne et al. 1993) and RXTE/PCA (Rots et al.
2004), and comparison of the Cen X-3 pulse period andphase with those of the Fermi/GBM (Meegan et al. 2009).
We confirmed the timing stability of an order of 10−9 .
5.

All-sky image

Figure 14 shows the GSC all-sky X-ray image taken
for the first year. The red, green, and blue color scales
represent the intensity in 2-4, 4-8, and 8-20 keV energy
band, respectively. The diameter of point source images
roughly represents the relative brightness according to the
PSF spread. We can easily see more than a hundred of

1
Phase

1.5

2

Fig. 13. Folded light curve of the Crab pulsar in 2–30 keVband obtained with GSC during a period from January 1 toJanuary 5, 2010. The vertical count rate represents the event
rate from the target direction over the entire elapsed period.

20

Fig. 12. GSC Crab-nebula spectrum taken for a day on
August 15, 2009 (UT) and the best-fit power-law model witha fixed absorption of 0.35 × 1022 cm2 (top). Data-to-model
ratio (bottom).

0.5

discrete X-ray sources over the whole sky as well as unresolved Galactic ridge emission along the Galactic planewith a scale height of 1–2◦ .
6.

Summary

The GSC in-orbit performance on the MAXI mission
payload is verified from the initial commissioning operation. Table 2 briefly reviews the relevant parameters.
They are summarized by the following points.
1. The optimal configurations of the read-out electronics and on-board data processing at the nominal
high voltage of 1650 V are established. The gas gain
and the amplifier gain are confirmed to agree withthose expected from the ground calibration tests.
The gain change during the first year is confirmed
to be less than 1 %, which is small enough for therequirements of the calibration accuracy.
2. Two gas counters out of the twelve were turned oﬀ
for the high-voltage breakdown during the first twomonths commissioning operation and another breakdown occurred in the ninth month. These breakdown would be presumably caused by repeated discharges. The HV operation under the high particlebackground, which includes area at the high latitude above 40◦ as well as the SAA, were stopped
since September 27, 2009 in order to avoid any dam-

10

Sugizaki et al.

[Vol. ,

Fig. 14. GSC all-sky image taken for the first year. Red, green, and blue color maps are scaled logarithmically and represent theintensities in 2-4, 4-8, and 8-20 keV bands, respectively.

age that can develop into the discharges. The three
counters suﬀering from the breakdown are now operated with a limited sensitivity using alive-anodepartitions.
3. The background cosmic-ray rate is consistent withthat extrapolated from the past experiments in the
low-earth orbit using the relation with the geomagnetic COR. The eﬃciency of the on-board background rejection by the anti-coincidence-hit logic isalso found to be comparable to those of the similar
instruments, Ginga-LAC.
4. The PSF with an FWHM of 1.5◦ , the scan transit
time from 40 to 150 seconds per orbital cycle, the allsky coverages of 85% per 92-minute orbital period
and 95 % per day are verified, which agrees with thepre-flight design. The eﬀective exposure time for acelestial target is typically 4000 cm2 s. The daily
5-σ source sensitivity is expected to be 15 mCrab in
the condition.
5. The instrument response in orbit has been calibrated using Crab nebula data. The fitting results
verified that the uncertainty of the eﬀective area andthe energy response would be less than 10% if dataof a few anodes suﬀering from cross talk on the circuit board are ignored. The relative timing accuracy
was also confirmed to be normal.
We are grateful to Mark Finger and Fermi/GBM pulsar
project team for providing the pulsar ephemeris data fortiming calibration. We also thank Keith Jahoda for stimulating and constructive comments. This research was partially supported by the Ministry of Education, Culture,
Sports, Science and Technology (MEXT), Grant-in-Aid
for Science Research 19047001, 20244015, 21340043,
21740140, 22740120 and Global- COE from MEXT
“Nanoscience and Quantum Physics” and “The Next

Generation of Physics, Spun from Universality andEmergence”.
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No. ]

In-Orbit Performance of MAXI/GSC

Table 2. Summary of GSC in-orbit performance

Spatial resolution
Source localization accuracyAll-sky coverage
Exposure
Energy resolution
Eﬀective area accuracy
Sensitivity

1.5◦ (FWHM)
0.2◦ (90% containment radius)
85% per orbit (nominal 92 minutes), 95% per day
4000 cm2 s per day (typical)
18% (1σ) at 5.9 keV
3% (1σ) at 4–10 keV
15 mCrab per day in 2–30 keV (typical)

11