PASJ: Publ. Astron. Soc. Japan 61, 999–1010, 2009 October 25
c 2009. Astronomical Society of Japan.


The MAXI Mission on the ISS: Science and Instruments
for Monitoring All-Sky X-Ray Images
Masaru M ATSUOKA,1 Kazuyoshi K AWASAKI,1 Shiro U ENO,1 Hiroshi T OMIDA,1 Mitsuhiro KOHAMA,1,2 Motoko S UZUKI,1
Yasuki A DACHI,1 Masaki I SHIKAWA,1 Tatehiro M IHARA,2 Mutsumi S UGIZAKI,2 Naoki I SOBE,2 Yujin NAKAGAWA,2
Hiroshi T SUNEMI,3 Emi M IYATA,3 Nobuyuki K AWAI,4 Jun K ATAOKA,4 Mikio M ORII,4 Atsumasa YOSHIDA,5Hitoshi N EGORO,6 Motoki NAKAJIMA,6 Yoshihiro U EDA,7 Hirotaka C HUJO,2 Kazutaka YAMAOKA,5
Osamu YAMAZAKI,5 Satoshi NAKAHIRA,5 Tetsuya YOU,5 Ryoji I SHIWATA,6 Sho M IYOSHI,6
Satoshi E GUCHI,7 Kazuo H IROI,7 Haruyoshi K ATAYAMA,8 and Ken E BISAWA9
1 ISS Science Project Office, ISAS, JAXA, 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505

matsuoka.masaru@jaxa.jp
2 Cosmic Radiation Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351–198
3 Department of Earth and Space Science, Osaka University , 1-1 Machikaneyama, Toyonaka, Osaka 560-0043
4 Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551
5 Department of Physics and Mathematics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara, Kanagawa 229-8558
6 Department of Physics, Nihon University, 1-8-14, Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8308
7 Department of Astronomy, Kyoto University, Oiwake-cho, Sakyo-ku, Kyoto 606-8502
8 Earth Observation Research Center, JAXA, 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505
9 Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510(Received 2009 March 9; accepted 2009 May 28)

Abstract
The Monitor of All-sky X-ray Image (MAXI) mission is the first astronomical payload to be installed on theJapanese Experiment Module — Exposed Facility (JEM-EF or Kibo-EF) on the International Space Station. It has
two types of X-ray slit cameras with wide FOVs and two kinds of X-ray detectors consisting of gas proportionalcounters covering the energy range of 2 to 30 keV and X-ray CCDs covering the energy range of 0.5 to 12 keV.
MAXI will be more powerful than any previous X-ray All Sky Monitor payloads, being able to monitor hundredsof Active Galactic Nuclei. A realistic simulation under optimal observation conditions suggests that MAXI will
provide all-sky images of X-ray sources of  20 mCrab (7  1010 erg cm2 s1 in the energy band of 2–30 keV)from observations during one ISS orbit (90 min),  4.5 mCrab for one day, and  2 mCrab for one week. The final
detectability of MAXI could be 0.2 mCrab for two years, which is comparable to the source confusion limit of the
MAXI field of view (FOV). The MAXI objectives are: (1) to alert the community to X-ray novae and transient X-ray
sources, (2) to monitor long-term variabilities of X-ray sources, (3) to stimulate multi-wavelength observations ofvariable objects, (4) to create unbiased X-ray source cataloges, and (5) to observe diffuse cosmic X-ray emissions,especially with better energy resolution for soft X-rays down to 0.5 keV.
Key words: catalogs: X-ray source catalogue — instrumentation: All Sky Monitor (ASM) —stars: X-ray novae — stars: X-ray transients — X-ray: AGN — X-ray: GRB
1. Introduction
All Sky Monitors (ASMs) for X-ray observations havea long history (Holt & Priedhorsky 1987). The ASM aboard
Ariel 5 (a British satellite) was the first dedicated pioneer
payload to observe X-ray novae and transients (Holt 1976).
The Ariel 5 ASM, which has two sets of one-dimensional scanning pinhole cameras, discovered several novae and transient
X-ray objects. Thereafter, the terms “X-ray nova” and “X-ray
transient” have become well-known in X-ray astronomy. X-ray
instruments with a wide field of view (FOV) as well as ASM
can not only detect X-ray novae and transients, but also monitor
long-term X-ray variabilities of X-ray sources (Priedhorsky &Holt 1987).


Present Address: Research Institute for Science and Engineering, Waseda
University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555.

Although Vela A & B satellites with a wide FOV discovered the first gamma-ray burst (GRB) before the Ariel-ASM(Klebesadel et al. 1973), those detectors were not very suitable for monitoring GRBs as well as X-ray novae and transients. Subsequently, dedicated GRB monitors confirmed thata considerable number of mysterious GRBs are produced in
the universe. GRB monitors with a wide FOV have advanced
greatly since a wide-field camera aboard Beppo-SAX discovered a GRB afterglow (Costa et al. 1997). Since then, special
GRB satellites, such as HETE-2 (Ricker et al. 2002; Shirasaki
et al. 2003) and Swift (Gehrels et al. 2004), have been realized.
The ASM, however, gradually advanced as a supplemental
payload. Ariel-ASM, with pin-hole cameras, operated successfully for seven years with the main payload, which made spectrum observations of bright X-ray sources (Holt 1976). In
1987 the ASM aboard the Japanese satellite Ginga (Tsunemi
et al. 1989) succeeded the Ariel-ASM. Ginga-ASM was able

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to observe the spectra of X-ray novae as it scanned thesky from 60ı through 360ı once every day with multi-slat
collimators. Ginga-ASM greatly advanced the science of
black-hole binaries with the discovery of nova-like black-holebinaries (Tsunemi et al. 1989; Kitamoto et al. 1992; Kitamoto
et al. 2000). Ginga-ASM was successfully operated for
4.5 years, and some of transients discovered by ASM wereobserved in detail by the main large-area counters aboard
Ginga (Turner et al. 1989).
Since 1996, RXTE-ASM (a NASA satellite) has monitored
known X-ray sources in addition to X-ray novae and transients(Levine et al. 1996). Systematic long-term data of Galactic
variable sources provide quasi-periodic properties of the accretion disc (Zdziarski et al. 2007a, 2007b). RXTE-ASM has
provided much useful data for X-ray variable sources for 13
years, but the detection limit is around 10 mCrab. Therefore,
the main targets of RXTE-ASM were Galactic X-ray sources,of which monitoring was suitable for performing detailed
observations of these targets with an RXTE prime instrument, a large-area proportional counter array (Remillard &McClintock 2006).
We now have long-term light curves for bright X-ray sourcesfrom the above-mentioned three ASMs. Some sources haverevealed periodic or quasi-periodic components of long timescale with combined analyses over the last 30 years (Paul et al.
2000). Future ASMs taking over the three ASMs can continue
to investigate further long-term behaviour of X-ray sources.
Although the above-mentioned three ASMs have been in operation, they cannot adequately monitor AGNs because of their
low sensitivity. Thus far, Galactic X-ray novae and transients
data have been accumulated, although the information is notcompletely clear. No long-term variability of AGN has been
observed with sufficient data. Therefore, the most importantrole for future ASMs is to achieve better the detection limits
for AGN monitoring. Considering this historical situation,MAXI was proposed as a payload of ISS (Matsuoka et al.
1997). It will be one of the ASMs responsible for AGN monitoring. A slit-hole camera with a large detection area meets
this requirement. Since MAXI has a composite structure ofslit holes and slat collimators without a mirror system, theangular resolution is not good (i.e., the FWHM of slat collimators is 1:ı 5), but the localization accuracy can be 0:ı 1 forsources of sufficient statistics (e.g., bright sources and sources
with enough counts accumulated for optimum time). A mirrortype ASM, such as the Lobster-eye ASM, is promising forthe future, although its energy band is limited to soft X-rays(Priedhorsky et al. 1996).
The MAXI to be attached to the Japanese Experiment
Module (JEM; Kibo) on the ISS is ready to be launched. In
this paper we present an overview of MAXI and the new scientific contributions expected of it. The first part of this paper
considers the astronomical science expected of MAXI. The
second part describes the MAXI instrumentation, and observational simulations.
2. Key Science
The MAXI mission enables the investigation of ASMs andsurveying. MAXI will alert astronomers of GRBs, X-ray[Vol. 61,

novae, and flare-up increases of X-ray sources if they occur.
Long-term data of X-ray sources will enable us to determine a special time scale of variability, e.g., long-term periodic or quasi-periodic motions of X-ray sources. MAXI can
promote multi-wavelength observations in collaboration withother space and ground observatories, such as X-ray, infrared,
and optical satellites, radio, and optical ground observatories.
MAXI systematic observations of the variable activity of blackhole binaries and AGNs are used to investigate how and where
they produce their variable activities.
MAXI provides unbiased X-ray source catalogues over all of
the sky. Monthly or biannual X-ray catalogues could contributeto the long-term study of the variable behavior of AGN for
the first time. A catalogue accumulated for two years could
provide all-sky AGNs corresponding to a moderately deep
survey of the entire sky. This unbiased AGN cataloge will
far surpass an investigation of the distribution and evolution of
AGNs for the HEAO-1 A2 catalogue, which has been utilized
for a long time (Piccinotti et al. 1982). MAXI is also able
to make an all-sky X-ray map with soft X-rays and mediumenergy X-rays. The soft X-ray map provides line features, suchas Oxygen X-ray lines, which are useful in researching geocoronal recombination lines (Fujimoto et al. 2007) as well as
the evolution of hot gas in the Galaxy (McCammon & Sanders
1990; Tanaka & Bleeker 1977).
The current Swift (Gehrels et al. 2009; Burrow 2009) andINTEGRAL (Ubertini et al. 2009) satellites with wide FOVs
of hard X-ray detectors have provided X-ray source catalogues including AGNs in addition to a considerable numberof transients. Since these results are complementary to thoseof MAXI, it could be promising to science that these three
ASM missions will operate simultaneously. Furthermore,
the recently developed gamma-ray large-area space telescope,
the Fermi Gamma-Ray Space Telescope (Abdo et al. 2008;
Thompson et al. 2009), is a gamma-ray ASM with an energy
band complementary to that of MAXI. All-sky images withboth ASMs may reveal new information.
2.1. X-Ray Novae and GRBs Alert
More than 90% of black-hole candidates are X-ray transients or novae (McClintock & Remillard 2006). In the last 20
years, 29 have been discovered, mostly with RXTE-ASM andpartly with Ginga-ASM, while about 30% of them have been
serendipitously discovered by pointing observatories (Negoro2009). Thus far, observed X-ray novae are located near thegalactic center and near the solar system; i.e., their distances
are within about 6 kpc. Distant X-ray novae appear to be weak,
but MAXI could detect these weak sources, since it can observe
X-ray novae from distant regions about three times as far. Thus,
it is expected that the nova discovery rate may increase by one
order of magnitude.
X-ray light curves with energy spectra of X-ray novae haveprovided instability and phenomena deviating from the standard accretion-disc model (Tanaka & Shibazaki 1996). Dataof both low and high luminosities have resulted in the creationof a new theory concerning accretion discs (Mineshige et al.
1994), but these samples are not enough. We also expectthat MAXI could observe X-rays from classical novae. Multiwavelength observations of classical novae could provide

No. 5]The MAXI Mission on the ISS

useful information about the bursting mechanism of classicalnovae. Although classical novae are faint and soft in the X-ray
band, the durations are of months to years (Mukai et al. 2008).
Thus, MAXI could detect some of them.
Swift has discovered many GRBs, and has consequently
enabled us to make rapid follow-up observations of them.
Nevertheless, the energy band of the Swift detector is 15 to
200 keV (Gehrels et al. 2004). MAXI can cover the soft X-ray
band of GRBs, and this information is important for understanding X-ray rich GRBs and X-ray flashes. It is expected thatthe population of the X-ray rich GRB is comparable to that of
the GRB with an ordinary energy band (Sakamoto et al. 2005).
The instant FOV of MAXI is not large, and thus MAXI will beable to detect 3.5 prompt emissions and 2.5 X-ray afterglows
of GRB per year (Suzuki et al. 2009).
MAXI is also able to observe transient X-ray binary pulsars
with high orbital eccentricity and transient low-mass X-ray
binaries with neutron stars. Furthermore, Anomalous X-ray
Pulsars (AXPs) are still enigmatic objects that appear as Soft
Gamma Repeaters (SGRs) with enormous flux during a shorttime (Nakagawa et al. 2009; Morii 2009). Considering therecent Swift discovery of a new SGR (Barthelmy et al. 2008;
Enoto et al. 2009), we could even expect MAXI to detect
additional SGRs with weaker flux. Thus, MAXI may reveal
a new frontier of AXPs (i.e., magnetars) by detecting numerous
new magnetar candidates (Nakagawa et al. 2009). Recently,
special interest concerning long type-I X-ray bursts has arisenwith rare events of carbon-fueled super bursts and heliumfueled intermediately long bursts. These bursts are promisingfor investigating the deeper neutron star envelope (Keek &in’t Zand 2008; Keek et al. 2008). MAXI may discover suchrare occurrences of X-ray bursts.
2.2. Long-Term Variability of X-Ray Sources
Most X-ray sources with compact objects are variable, due tothe instability of the accretion disc, and other reasons. Periodic
or quasi-periodic long-term variability is sometimes createdfrom interactions between the accretion disc and the binary
system. If there were a third object in the X-ray sources, we
could expect some other periodicity in addition to the binary
period (Zdziarski 2007a). Such a third body might be usefulfor inspecting the accretion disc. However, no one has discovered such a triple-body system in X-ray sources. A composite
time scale of variability would help to generate this knowledge, and/or may create new knowledge. An unusual X-ray
transient from the galactic center region (Smith et al. 1998) is
progressing in a new class of recurrent and fast X-ray transient
sources (i.e., SFXTs). These transients sometimes occur inhigh-mass X-ray binaries associated with super-giant companions. It is promising for further investigations that MAXI, in
addition to INTEGRAL and Swift, monitors these objects witha wide FOV (Ebisawa 2009; Ubertini et al. 2009). Generally,a time scale of variability of AGNs or supermassive black holes
is longer than that of X-ray binaries. Non-recurrent X-ray
flares on time scale of days have been observed from several
non-active galaxies since ROSAT survey (e.g., Komossa 2002).
Tidal disruption of a star by a supermassive black hole (Ress
1998) is the favored explanation of these unusual events. Thesemight be also detected by the soft X-ray camera of MAXI

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although very soft and weak.
2.3. Multi-Wavelength Observations of Variable Objects
The knowledge of bursts, transients, and variable objects has
progressed considerably with multi-wavelength observations.
Research of GRB afterglows has advanced greatly as a result of
rapid follow-up observations in X-ray, optical, and radio bands(Gehrels et al. 2004), but there are still just a few samples ofshort GRBs and X-ray rich GRBs. Coordinate observationsof Blazars in radio, infra-red, optical, X-ray, and ultra-highenergy (TeV) gamma-rays have confirmed a Synchrotron SelfCompton (SSC) model for highly variable periods (Kubo et al.
1998; Kataoka et al. 1999; Takahashi et al. 2000). However, theemission mechanism of other AGNs as well as Blazars has not
yet been completely understood because of complicated correlations among multi-wavelength observations to help investigate this mechanism (Maoz et al. 2002). TeV gamma-ray
observations by Ćerenkov telescopes have progressed remarkably since TeV gamma-rays from some Blazars were detected(Petry et al. 1996; Aharonian et al. 1997). GeV gamma-ray
observations by Fermi-GLAST are now available as an ASM(Thompson et al. 2009). MAXI can promote further simultaneous multi-wavelength observations of Galactic active objects
as well as AGNs with gamma-rays (GeV & TeV) to radio bands(Fender 2009; Madjeski et al. 2009).
2.4. Unbiased X-Ray Source Catalogues
The nominal beam size (i.e., angular resolution by FWHM)
of MAXI is 1:ı 5  1:ı 5. It is estimated that the confusion
limit of X-ray sources is 5  1012 erg cm2 s1 from a recent
log N –log S plot of X-ray sources (Ueda et al. 2003); i.e.,
0.2 mCrab in the energy band of 2 to 20 keV. Thus, we canset a detection limit of 0.2 mCrab as an ideal goal, although
the period of its achievement depends on the intrinsic
background and the systematic error. To estimate theobservation time we conducted a realistic MAXI observational
simulation. MAXI achieved a detection limit of 0.2 mCrab
with a two-year observation (Ueda et al. 2009). Other simulations suggest that it is possible to detect 20 mCrab for one
orbit (90 min), 4.5 mCrab for one day, and 2 mCrab for one
week (Hiroi et al. 2009; Sugizaki et al. 2009). Some regionsin the sky are covered by a bright region from the Sun, andare sometimes affected by the South Atlantic Anomaly (SAA).
Therefore, detection sensitivities are estimated under the best
condition. The detectability of MAXI is not uniform in theentire sky, but slightly depends on the direction. The MAXI
simulation indicates that we can obtain 30–40 AGNs every
week (Ueda et al. 2009). This sample is comparable to thenumber from HEAO 1-A2 (Piccinotti et al. 1982). We arealso able to estimate about 1000 AGNs with two-year observations. MAXI observes those AGNs in a harder energy band
than observed by ROSAT. Thus, MAXI could give an unbiased
population ratio of Type I and Type II AGNs. Furthermore,
if weekly or monthly catalogues are created, we can discover
the intensity variability for a considerable number of AGNs.
It would be possible to detect a flare-up of Blazars, and tothen follow their light curves. The time sequence of X-ray
unbiased catalogues is very useful for researching the evolution of AGNs as well as their long-term variability. Here,1002

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[Vol. 61,

the next comment is noted. Although much deeper surveysof AGNs in the 2–10 keV band with ASCA (e.g., Ueda et al.
1999) and Chandra / XMM-Newton (Brandt & Hasinger 2005
and references therein) have presented log N –log S plots forresearching the evolution of AGNs, they are limited to a small
portion of the sky, and hence cannot constrain that of brightAGNs with small surface densities.
2.5. Diffuse Cosmic X-Ray Emissions
The problem of the diffuse cosmic X-ray background (CXB)
has a long history. A recent deep survey of X-ray sourcesin a medium energy band of 2 to 10 keV (Ueda et al. 2003)
suggests that the CXB may be due to the superposition ofAGNs, as proposed theoretically (Morisawa et al. 1990). It
is still unknown how some kinds of AGNs are distributed in
the universe. The global CXB distribution could be comparedwith that of optical AGNs and/or an infrared map. Thus, we
can investigate the evolution or emission mechanism of the allsky AGN distribution. If we can obtain some difference in the
distribution in different energy bands, we can investigate the
distribution of Type I and Type II AGNs.
The diffuse emission of soft X-rays less than 1 keV isattributed to geo-coronal gas as well as hot bubbles fromsupernova remnants (Tanaka & Bleeker 1977; McCammon& Sanders 1990). ROSAT obtained a precise all sky map
with a broad energy band of soft X-rays. MAXI can also
obtain all-sky maps of soft X-rays but with better energy
resolution (e.g., resolving Oxygen K-line, and Neon K-line).
Recent Suzaku observations have revealed a strong contribution of recombination K-lines of oxygen and carbon in the geocoronal region (Fujimoto et al. 2007). MAXI is able to observe
the oxygen line with a seasonal variation and solar activity.
Therefore, MAXI observations make it possible to discriminate
between the contribution of the geo-corona and that of supernova remnants. Thus, we could investigate geo-coronal science
as well as element-evolution of supernova remnants from soft
X-ray line observations.
3. MAXI Project
The large-scale Space Station (SS) project started in 1985
with the collaboration of the USA, Japan, Canada, and theEuropean Space Agency (ESA). Planning of the JEM was also
initiated at that time. In 1995, Russia began to take part in
the SS project. The project was subsequently reduced to its
present scale, and then renamed the International Space Station(ISS). JEM planning and designing have continued, but actual
construction and basic tests of the engineering model began atthat time. JEM consists of a pressurized module for microgravity experiments and an exposed facility (EF).
ISS rotates synchronously in its orbit so that one side always
points towards the center of the Earth, and the opposite sideviews the sky. Therefore, the sky side of JEM-EF surveys
a great circle every ISS orbit. Themes for JEM-EF include
the space science payload for astrophysics and Earth observations, and space technology experiments, such as robotics.
However, JEM-EF does not provide a perfectly stable platform because of unknown factors of some attitude fluctuation.
The payload size and weight are limited to the capacity of theFig. 1. Overview of MAXI; major subsystems are indicated.

JEM-EF. A payload can be suitable for survey observations,but not for pointing. Although a considerably wide FOV isavailable, the ISS structure and solar paddle partially block theview of JEM-EF. Since the ISS carries various experimental
instruments and payloads, each instrument and payload has toaccommodate many interfaces. Resources such as communication and power for each experiment or payload are also limited.
Considering these problems, MAXI was proposed andfinally accepted in 1997 by the National Development Space
Agency of Japan (NASDA), now known as the Japan
Aerospace Exploration Agency (JAXA). Thus, MAXI is thefirst astronomical payload for JEM-EF aboard ISS. Although
the launch was scheduled for 2003, the space shuttle, Columbia
accident, and the ISS construction delay resulted in a postponement of the MAXI launch by several years. The MAXI
science instruments consist of two types of X-ray cameras, theGas Slit Camera (GSC) and the Solid-state Slit Camera (SSC).
These instruments and the support instruments on the MAXI
payload are shown in figure 1, and their characteristics arelisted in table 1. The support instruments consist of a VisualStar Camera (VSC), a Ring Laser Gyroscope (RLG), a Global
Positioning System (GPS) and a Loop Heat Pipe and Radiation
System (LHPRS). The VSC and the RLG determine the directions of the GSC and the SSC as precisely as a few arc-seconds
every second. The GPS attaches the absolute time as precisely
as 0.1 ms to GSC photon data. The LHPRS is used for heattransportation and heat radiation from thermo-electric coolers(Peltier elements) to cool the CCD.
The main role of the GSC is to perform as an X-ray ASM,
which has the best detectability than previous ASMs have fora time scale longer than hours. All-sky X-ray images obtainedby GSC are also useful for new X-ray variable catalogues. On
the other hand, the main mission of SSC is to make all-sky
X-ray maps for extended sources with better energy resolution
than ROSAT maps, although the detection area and live time
for discrete sources are less by 1=20 than those of GSC. We
also expect to detect transients in the soft X-ray band, although
the detectability of transients with SSC is also poorer by 1=20
than that of GSC.
MAXI will be carried by the Space Shuttle, Endeavour,No. 5]The MAXI Mission on the ISS

1003

Table 1. Specification of MAXI slit cameras.

GSC : Gas Slit Camera
X-ray detector
X-ray energy range
Total detection area
Energy resolution
Field of view
Slit area for camera unit
Detector position resolution
Localization accuracy
Absolute time resolution
Weight12 pieces of one-dimensional PSPC;
Xe + CO2 1 %2–30 keV
5350 cm2
18% (5.9 keV)
1:ı 5  160ı
20.1 cm21 mm
0:ı 1
0.1 ms (minimum)
160 kg

SSC : Solid-state Slit Camera
32 chips of X-ray CCD;
1 square inch, 1024  1024 pixels
0.5–12 keV
200 cm2
 150 eV (5.9 keV)
1:ı 5  90ı
1.35 cm2
0.025 mm (pixel size)
0:ı 1
5.8 s (nominal)
11 kg

 FWHM  Full-FOV.

 MAXI total weight: 520 kg.

 GSC consists of 6 camera units, where each unit consists of two PCs. SSC consists of two camera units, SSC-Z and SSC-H.

along with the JEM (or Kibo)-EF from Kennedy Space Centerin the middle of 2009. MAXI will finally be mounted on
JEM-EF within two weeks of Endeavour launch. At that time
all of Kibo’s modules will have been installed on ISS. After thebasic arrangement of the structure and infrastructure of JEM,MAXI will conduct performance tests for about three months.
MAXI has a nominal lifetime of two years, but the expectedgoal is five or more years to achieve long-term monitoring.
MAXI data will be down linked through the Low Rate Data
Link (LRDL; MIL1553B), and the Medium Rate Data Link
(MRDL; Ethernet). MAXI is operated through the Operation
Control System (OCS) at Tsukuba Space Center (TKSC),
JAXA. MAXI data are not only processed and analyzed at
TKSC, but they are also transferred to the Institute of Physicaland Chemical Research (RIKEN) MAXI data facility. Generalusers of MAXI can request scientific data from RIKEN (amain port) as well as from JAXA (a sub-port) whenever they
desire. To search where X-ray novae, transients, or flaring
phenomena appear suddenly in the sky we will conduct automatic data analysis at TKSC by using down-linked data from
LRDL. If such a source is discovered, we will report this event
to astronomers and dedicated users worldwide via the Internet
from TKSC. This nova alert system has been developed toachieve automatic alerts (Negoro et al. 2008). The MAXI teamwill also maintain archival data at RIKEN using the data from
LRDL and MRDL, such as all-sky X-ray images, X-ray light
curves, and spectra of dedicated X-ray sources. In principle, allastronomical data of MAXI will be available for public distribution (Kohama et al. 2009).
4. X-Ray Mission Instruments
A specific object is observed by MAXI with a slit camera
for a limited time during every ISS orbit. The X-ray detectorof the camera is sensitive to a one-dimensional image throughthe slit, where the wide FOV through the slit spans the sky
perpendicular to the ISS moving direction, as shown in figure 2.
A scanning image of an object is obtained with the triangularresponse of a slat collimator according to the ISS movement.
An intersection of the slit image and the triangular response

Fig. 2. Principle of the MAXI Slit Camera; it consists of slit & slat
collimators and a one-dimensional position sensitive X-ray detector.

image corresponds to a source location in the sky as a Point
Spread Function (PSF). This is shown in figure 3.
Objects located along a great circle stay for 45 s in the
FOV of the MAXI cameras, where the time of stay is theshortest in the MAXI normal direction (for a great circle). For
objects in the slanted field from the great circle, they achievea slightly longer observation. Any target will come repeatedly
in each field of the horizontal and zenithal cameras with every
ISS orbital period. In this situation MAXI can intermittently
monitor a short-time scale variability for bright X-ray sources,
such as X-ray pulsars and low-mass X-ray binaries. It canmonitor their variability of weak sources on a time scale of
90 min or longer if we integrate the data. MAXI has two typesof X-ray cameras: the GSC and the SSC, which are describedin the following subsection.

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Fig. 5. X-ray detection efficiency of the MAXI proportional counterfor energy; a solid line indicates the efficiency for normal-incident
X-rays, while a dashed line indicates that of incident X-rays from 40ı .

Fig. 3. Typical sample of PSF (Point Spread Function); a slat collimator has a triangular response (ISS scanning direction), while a slit
collimator has a trapezoidal response (the slit direction corresponding
to ˇ in figure 6).

Fig. 4. GSC unit and a gas-proportional counter; the unit consists of
two gas-proportional counters, slit & slat collimators.

4.1. Gas Slit Camera (GSC)
GSC is the main X-ray camera, and consists of six unitsof a conventional slit camera, as shown in figure 1 andtable 1 (Mihara et al. 2009). The GSC unit consists of two
one-dimensional proportional counters (produced by Oxford
Instruments Co. in Finland), and slit & slat collimators, as
shown in figure 4. Thus, twelve proportional counters havea 5350 cm2 detection area in total. Each counter has six cellsof resistive carbon wires that are guarded by a veto-detector
region in the bottom and on both sides (Mihara et al. 2002),
while a carbon wire divides the charge from the signal into
both terminals for one-dimensional determinations. The X-ray
detection efficiency of the proportional counter (Xe 1.4 atmospheres with 1% CO2 ) is plotted against X-ray energy infigure 5. The observational energy band of GSC is set to be
2–30 keV in the standard operation mode where the detection

efficiency for X-rays in this band is above 10%, as shown infigure 5. The observation above the Xe K-edge of 34.6 keV
is possible as a special mode by adjusting the amplifier gain
and the high voltage. Although a response function above
the Xe K-edge is complicated, due to the escape peak, a test
for this function has been performed using synchrotron X-ray
beams. The slat collimator response is 3:ı 5 in bottom-tobottom, and a slit image corresponds to 1:ı 5, covering a wide
field of between 40ı and +40ı. The proportional counters
detect incident X-ray photons from the vertical to the ˙40ıslant direction.
Two FOVs of GSC-H and GSC-Z are placed in the horizontal (forward) and zenithal directions to compensate when
the sky is unobservable to one FOV or the other (due to highradiation background, such as SAA). The FOV of one camera
is 80ı . Two camera units can cover 160ı , but the FOV of eachcentral camera of GSC-H and GSC-Z overlaps with each halfof the FOV of both side cameras to even the exposures, as
shown in figure 6, which shows the product of the effective
area and the dwell time according to one orbit scan. The exposure time per orbit depends on the direction of a star by a factor
of 1=cosˇ, where ˇ is the angle of the star slanted from scanning a great circle. Both edges of 10ı in this figure are omitted,
due to a shadow of the ISS structure, where the scanning loss is
1.5%. The forward FOV is tilted up by 6ı so as not to observe
Earth even when the ISS attitude changes. Thus, the FOV of
GSC-H can scan the sky with ISS rotation without Earth occultation. By making the observation time longer than 90 min, we
made the instrument simpler, and then optimized it to searchin the AGN discovery space for a longer-term AGN variability(> 1.5 hr).
Considering this directional performance, we conductedvarious laboratory tests of all proportional counters. Here,
we explain some of the results. The position resolution wastested at every 2 mm segment of two-dimension on the incident window for all proportional counters. The data were takenusing 0.1 mm pencil X-ray beams with X-ray energies of 4.6,
8.0, and 17.4 keV. The total length of the carbon anode wire isNo. 5]The MAXI Mission on the ISS

1005

Fig. 7. SSC consisting of a horizontal SSC (SSC-H) and a zenithal
SSC (SSC-Z), and a CCD chip is depicted separately.
Fig. 6. The solid line shows the total product of effective area anddwelt time according to one orbit scan for a point source for theangle (ˇ) slanted from scanning a great circle. This value is common
to GSC-Z and GSC-H. The chain line indicates the contribution ofa central GSC unit only, while the dashed line and the dotted line arethose of the right side GSC unit and the left side GSC unit only, respectively.

32 cm; thus, the total resistance is 31 to 37 kΩ. The difference in the resistance depends on a slight difference of thewire diameters. The energy response also differs for different
anode wires. We took comprehensive data useful for theenergy response and the position response (Mihara et al. 2002;
Isobe et al. 2004). We also took slat collimator response datausing X-ray pencil beams (Morii et al. 2006). The data-base,including all of these data, is regarded as being the response
function at the time of data analysis. In actual data analysis,
we will use each response function for the two-dimensionalsurface of all carbon wires. The pulse-height response depends
on the X-ray energy, where hard X-rays suffer the effect ofanomalous gas amplification (Mihara et al. 2002).
The simultaneous background for a localized source ismeasured in two regions separated from the source on thesame detector in the FOV direction. Another background ismeasured in two regions just before and after observing thesource on the scanning path, where the two regions are separated from the source. In this case, both measurement locationsof the background on the detector are the same as the measurement location on the detector of the source. The backgroundis employed if there are no appreciable sources in these directions. In addition, intrinsic background is necessary to obtainthe cosmic diffuse background. The intrinsic background isgradually changing in orbit, and for a time even in the same
direction (Hayashida et al. 1989). There is the small portion
covered by a window frame of each detector in which cosmic
diffuse X-rays as well as source X-rays are never irradiated.
A small portion of the North Pole direction is also shadowed by
the ISS structure. Although these intrinsic background countrates are not enough to measure for an instant of time, the data
accumulated for days or more could be used to make a reasonable model of the intrinsic background, as obtained for Ginga
proportional counters (Hayashida et al. 1989).

Fig. 8. X-ray the detection efficiency of MAXI CCD for energy; thesolid line is the efficiency for a normal-incident X-ray, while the dashedline indicates that for X-rays incident from 40ı .

4.2. Solid-State Slit Camera (SSC)
Each unit of SSC-H / Z consists of 16 CCDs, where each
CCD acts as a one-dimensional position-sensitive detector forthe slat collimator and the slit hole, similar to that of the GSC
system presented in figure 1 and table 1 (Tomida et al. 2009).
The SSC-H camera is tilted up 16ı so as not to view the upper
atmosphere. The response of the slat collimator scanning direction is 3ı for bottom-to-bottom, while a slit image corresponds
to 1:ı 5, covering a wide FOV of 90ı , as shown in figure 7. One
orbit scan of the SSC corresponds to 90ı  360ı . Thus, it takes
SSC 70 days to scan the entire sky, depending on the precessionof the ISS orbital plane, except for the bright region aroundthe Sun. Thus, SSC will acquire actual all-sky images every
half year.
The X-ray CCD chip of SSC is produced by Hamamatsu
Photonics K.K. The CCD depletion layer is about 70 m, of
which the X-ray detection efficiency is indicated in figure 8.
The observation energy band of SSC is set to be 0.5–12 keV in
the standard mode, but observations above 12 keV are possible
in a special mode. However, since SSC is characterized by

1006

M. Matsuoka et al.

a low-energy band, the energy band may be slightly shiftedto a lower energy than 0.5 keV, depending on the temperature
for thermal noise. The CCD with 1024  1024 pixels, anda pixel size of 24 m  24 m is a two-dimensional array,but SSC requires only one-dimensional position information.
Hence, multiple rows are summed in a serial register at thebottom of the imaging region, and the summed charges in theserial register are transferred to a read-out node. 8, 16, 32,
or 64 summed rows can be selected in normal observations
by commands. The larger is this number, the better is thetime resolution, and the better is the angular resolution in the
X-ray sky map.
The CCD is sensitive to particle radiation, but an irradiation test of the simulated radiation belt fluence suggests that theCCD can survive for the expected three-year mission (Miyata
et al. 2003). Furthermore, it is possible to inject charges in the
CCD before it becomes damaged due to irradiation (Miyata
et al. 2003). Charge-injection effectively restores the degraded
performance of X-ray CCDs (Tomida et al. 1997).
To achieve better energy resolution with a CCD, all CCDs
are cooled to 60ı C using thermo-electric coolers (Peltier
devices) and the LHPRS. The maximum power of the thermoelectric cooler is 1 W/CCD. The LHPRS can automaticallytransfer heat from the Peltier device, and emit heat from theradiation panel. Performance tests in the laboratory have beensatisfactorily conducted (Miyata et al. 2002; Katayama et al.
2005). The nominal energy resolution width of X-ray spectra
is 150 eV for Mn K X-rays (5.9 keV).
4.3. Support Sensor
Since the ISS has a huge structure of  108 m and 420 tons, the attitude determined in a certain part of theISS may be slightly different from that in a distant part, due
to shaking of the structure. MAXI is designed to determine
a target location as precisely as less than 0:ı 1 Thus, it is necessary to determine the attitude of the MAXI coordinate by lessthan 0:ı 1 every time. For this purpose, MAXI, itself, hasa VSC and an RLG. The VSC can observe three or more stars,
and it can determine a MAXI coordinate by 0:0 1. When the
VSC is not available for a measurement due to solar radiation, the RLG will extrapolate the attitude from a certain resultof the VSC to the following result. This attitude determination is performed automatically by onboard software including
a Kalman filter on the MAXI data processor (Ueno et al. 2009;
Horike et al. 2009).
The high voltage of a proportional counter of GSC will
be reduced to 0 V when solar radiation or SAA radiation isextremely strong. The command signal for this reduction will
be issued by working of the Radiation Belt Monitor (RBM).
This high-voltage reduction is also useful for setting programcommands in advance, because of possible predictions of thesolar position and the SAA location.
The time precision of photon acquisition from GSC is 0.1 ms
by referring to GPS signals. This precise absolute time is used
for milli-second pulsars and burst acquisition analysis. Thus,
only one GPS is installed in MAXI for precise time reference
without any unknown lag. However, the time resolution of SSC
is 5.8 s in the normal observation mode.

[Vol. 61,

5. MAXI Simulation and Expected Performance
A realistic simulation has been conducted with the recently
developed MAXI simulator, which generates fully simulated
data of MAXI instruments on the ISS (Eguchi et al. 2009).
The simulator takes into account various conditions on the ISS,i.e., the occultation of the sky with solar panels, particle andintrinsic background, and the response function of the X-ray
cameras. The attitude data and absolute time are attachedto each event data on the ground. Thus, we can plot each
event with energy information in a certain direction in the sky.
Integrated events from a certain direction for a certain timecorrespond to intensity, including background from the direction. Here, we demonstrate some of the simulated results (Hiroi
et al. 2009; Sugizaki et al. 2009).
First, we consider the detectability of the GSC. Figure 9
illustrates one-orbit and one-week observations. The results
indicate that we can detect a 20 mCrab source in one-orbit ata confidence level of 5-sigma, and a 2 mCrab source in oneweek at a level of 5-sigma. These simulations are estimatedfor the energy band of 2–30 keV of the Crab nebula spectrum. This simulation is ideal, because the particle and intrinsic
background of 10 counts s1 per counter steady, except for
statistical fluctuation during the observation time, and thisbackground is subtracted with the statistical fluctuation from
integrated events of the source. The spectrum of an intrinsic
background is referred to the laboratory test result. In reality,
we must consider the changeable background on the ISS orbit
and systematic errors for attitude determination and response
functions. The final detectability could be 0.2 mCrab for a twoyear observation, which means a source confusion limit for the
angular resolution of 1:ı 5.
Here, it is noted again that the above 5-sigma detectability isunder the best condition for a known source. The detectability
depends on the intrinsic background or trapped particle background. Either of the zenithal and horizontal cameras canobserve most directional sources, even if the ISS passes
through the SAA. This makes the detectability uniform formost directions, except the direction around the Sun if observations are repeated by orbits. In fact, a deviation from
uniformity of the detectability of one orbit becomes worse
by about 65% for sources observed on the SAA path, whilethat of one day becomes uniform within 10%–20% for any
direction, except for the solar region. We evaluated anotherdetectability, excluding observation in the region less thanthe cut-off rigidity of 8 (e.g., aurora region). The situation of this detectability is not so much different from thatof the aforementioned detectability. Lastly, it is noted thatthe detectability for an unknown source is not significantly
different from that for known sources. The one-sigma location error for unknown 5-sigma sources is about 0:ı 2 for both
one-day and one-week observations.
Second, we performed a simulation to derive the energy
spectra of a source of 1 Crab for a one-orbit and a one-day
observation, as shown in figure 10. This simulation is also
ideal, because the background is steady only with a statisticalfluctuation, and an ideal determination of the attitude without
an unknown systematic error is assumed. Nevertheless, it isconcluded that reasonable spectra from a considerable number

No. 5]The MAXI Mission on the ISS

1007

Fig. 9. (Left) Simulation used to detect a source of 20 mCrab for one ISS orbit. Contours are smoothed in the upper panel, while a cut figure througha peak is indicated with data points in the lower panel. (Right) Simulation used to detect a source of 2 mCrab in one week. Contours are smoothed in theupper panel, while a cut figure through a peak is indicated with data point in the lower panel.

of sources can be obtained by MAXI.
Another important goal of MAXI is to make X-ray source
catalogues for every period of concern. We can make light
curves with the time bin for various X-ray sources. The light
curves of most galactic X-ray sources are obtained with timebins of one orbit or one day. If the time bin is one month, it
is possible to make light curves of a considerable number of
AGNs brighter than 1 mCrab, as shown in the lower panel of
figure 11. It is not easy at a glance to discriminate between
weak sources and the CXB, but 1–2 mCrab sources are significantly detected, as simulated in figure 9b. If we integrate all
event data from the sky for two years, we can create an X-ray
source catalogue brighter than 0.2 mCrab at a confidence levelof 5-sigma. In general, since the time scale of the variabilityof AGNs is longer than that of galactic sources, catalogues
with time bins of one month and one year will provide new

information about long-term variable sources, such as a supermassive Binary Black Hole (BBH). Actually, an all-sky image
for every six-month observation is compatible with the resultof HEAO-1 A2.
6. Concluding Remarks
MAXI is the first payload to be attached to JEM-EF (KiboEF) of the ISS. It consists of two kinds of X-ray cameras: theGas Slit Camera (GSC), which has one-dimensional position
sensitive gas-proportional counters with a 2 to 30 keV X-rays
energy band, and the Solid-state Slit Camera (SSC), which
has X-ray CCDs with a 0.5 to 12 keV X-rays energy band.
MAXI will provide an all-sky X-ray image acquisition capability under the best conditions for every ISS orbit. If MAXI
scans the sky for one week, it could make a 2 mCrab X-ray

1008

M. Matsuoka et al.

Fig. 10. Simulated spectra of Crab nebula for one-orbit (lower spectrum) and one-day (upper one) observations.

[Vol. 61,

all-sky image, excluding the bright region around the Sun.
Thus, MAXI will not only rapidly inform astronomers worldwide about X-ray novae and transients, if they occur, but also
observe the long-term variability of galactic and extra-galactic
X-ray sources. At the same time, MAXI will provide X-ray
source catalogs accumulated for various periods such as six
months, one and more years.
All MAXI instruments are ready for launch. Data processing
and analysis software, including an alert system on the ground,are being developed by the mission team. MAXI will be
attached to JEM(Kibo)-EF in mid-2009. Anyone can participate in follow-up or multi-wavelength observations of MAXI
objects with ground optical and radio telescopes, and at spaceobservatories. Please contact the present authors for more
information and further collaboration.
Finally, please refer to the Proceedings of Astrophysics
with All-Sky X-Ray Observations — 3rd International MAXI
Workshop, RIKEN, Wako, Japan, June 10–12, 2008, ed.,Fig. 11. An all-sky X-ray source image (raw data) simulated for one-orbit observation (upper panel) and an image (exposure corrected) for one-month(lower panel) on the galactic coordinate. The undetected (dark) region on the left bottom corresponds to the solar direction. The two circular dark regions
on the upper panel correspond to unobservable regions around both end directions of the ISS pitch axis.

No. 5]The MAXI Mission on the ISS

N. Kawai, T. Mihara, M. Kohama, and M. Suzuki1 for further
information on MAXI the instrumentation and a preliminary
guideline for MAXI data.
Note added in proof (2009 August 20):MAXI was launched with Endeavour on 2009 July 16.
Preliminary performance test until 2009 August 18 indicatesthat all systems of MAXI are working normally and that thecontents of this paper need not to be revised.
MAXI fabrication and testing are also supported by thestaff members at the JEM project office and the Space utilization office. We thank them for their valuable guidanceand cooperation in maximizing the scientific output from

1009

MAXI. We also thank former colleagues who developed andsupported this long-term project of MAXI, although thesemany colleagues (staff and graduate students) have not been
included as co-authors.
We also thank the following companies and institutions fordeveloping the MAXI sub-systems: NEC Co. for the MAXI
bus system, Meisei Electric Co. for the mission instruments,
Oxford Instrum. Co. and Japan I.T.S. Co. for the gas proportional counters, Hamamatsu Photonics K.K. for the CCDs,
ATK Space (former Co.: Swales Aerospace) for the LHPRS,DTU (Technical University of Denmark) for the Optical Star
Sensor, the Institute of Aerospace Technology in JAXA for theGPS, and other cooperating companies and institutions.

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