S. Ueno, et al., “Development status and performance estimation of MAXI”
in UV and Gamma-Ray Space Telescope Systems, Proc. SPIE 5488, pp. 197-208, 2004.

Development status and performance estimation of MAXI
Shiro Uenoa , Hiroshi Tomidaa , Naoki Isobea , Haruyoshi Katayamaa , Kazuyoshi Kawasakia ,Takao Yokotaa , Naoyuki Kuramataa , Masaru Matsuokaa , Tatehiro Miharab , Ikuya Sakuraib ,Motoki Nakajimab , Mitsuhiro Kohamab , Hiroshi Tsunemic , Emi Miyatac , Nobuyuki Kawaid ,Jun Kataokad , Yuuri Serinod , Yoshihisa Yamamotod , Atsumasa Yoshidae , and Hitoshi Negorof
a Japan Aerospace Exploration Agency, 2-1-1 Sengen, Tsukuba, 305-8505, Japan;
b The Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako, 351-0198, Japan;
c Osaka University, 1-1 Machikaneyama, Toyonaka, 560-0043, Japan;d Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, 152-8551, Japan;e Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara, 229-8558, Japan;
f Nihon University, 1-8-14 Kanda-Surugadai, Chiyoda-ku, 101-8308, Japan

ABSTRACTMonitor of All-sky X-ray Image (MAXI) is an X-ray all-sky monitor, which will be delivered to the International
Space Station (ISS) in 2008, to scan almost the whole sky once every 96 minutes for a mission life of two years.
The detection sensitivity will be 7 mCrab (5σ level) in one scan, and 1 mCrab for one-week accumulation. At
previous SPIE meetings, we presented the development status of the MAXI payload, in particular its X-ray
detectors. In this paper, we present the whole picture of the MAXI system, including the downlink path andthe MAXI ground system. We also examine the MAXI system components other than X-ray detectors fromthe point of view of the overall performance of the mission. The engineering model test of the MAXI X-ray slit
collimator shows that we can achieve the position determination accuracy of <0.1 degrees, required for the easeof follow-up observations. Assessing the downlink paths, we currently estimates that the MAXI ground system
receive more than 50% of the observational data in “real time” (with time delay of a few to ten seconds), andthe rest of data with delay of 20 minutes to a few hours from detection, depending on the timing of downlink.
The data will be processed in easily-utilised formats, and made open to public users through the Internet.
Keywords: X-ray Astronomy, All-sky X-ray Monitor, ISS, JEM, Kibo, MAXI

1. INTRODUCTION
The Japanese Experiment Module (JEM) “KIbo” is a multi-purpose laboratory of the International Space Station(ISS). The Exposed Facility of JEM can be used as a site of space observatories. Considering advanced spacesciences which can be performed under several limited conditions of the ISS, all-sky monitoring experiments are
the best solution. Since the ISS always faces the bottom side to the Earth, the sky view from the ISS is rotating
all the time. Moreover, there will be a small vibration in attitude. Thus the ISS is not a good base for precise
pointing observations. Therefore the mission has become an all-sky monitor. The rotating sky enables us to scanalmost the whole sky in every orbit without having any moving mechanism. The ﬁelds of view are free from the
Earth occultation, which makes the observation eﬃcient.
The X-ray sky is very variable and is only observable above the atmosphere. MAXI is to perform thesystematic survey of the X-ray variabilities to study the nature of the active celestial objects. MAXI can detect
the X-ray transient phenomena and rapidly inform the world about them.

2. MAXI MISSION
Monitor of All-sky X-ray Image (MAXI) is an all-sky X-ray monitoring mission, which was proposed by the X-ray
astronomy group of the Institute of Physical and Chemical Research (RIKEN) in 1996, and was selected in 1997
Further author information: (Send correspondence to Shiro Ueno)
Shiro Ueno: E-mail: ueno.shiro@jaxa.jp, Telephone: +81 29 868 3721

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Figure 1. The MAXI system. We have two downlink paths for the MAXI data. The MAXI team is responsible for
the MAXI payload and its ground system. The MAXI mission, including the rapid distribution of observational results,
requires us to study the nature of downlink paths between the payload and the ground system in detail. The MAXI data
will be transmitted to the Internet through a secure one-way path. The cultivation of public users is also an importanttask of the MAXI team.

as a ﬁrst-generation payload for the Exposed Facility of the Japanese Experiment Module “Kibo”(JEM “Kibo”),
part of the International Space Station (ISS). In the development of the MAXI system (Fig. 1), we have alreadyﬁnished the preliminary and critical design reviews of the payload, and partially started the fabrication and thecalibration of its ﬂight hardware. On the other hand, most components of the MAXI ground system are stillin the design phase. In our current schedule, MAXI will be delivered to the ISS by a Japanese H-IIA Transfer
Vehicle (HTV) launched in 2008. The mission life is at least two years, and might be extended depending on its
performance, cost, and resource availability on the JEM “Kibo”.

3. MISSION OBJECTIVESMAXI will detect thousands of active X-ray emitting sources in the 0.5 to 30 keV band and monitor themwith twenty times higher sensitivity than previous all-sky X-ray monitors. The 5σ detection sensitivity will be
7 mCrab for one scan and 1 mCrab for one week. This will be the highest sensitivity ever achieved for an all-sky
monitor observation. The mission objectives of MAXI are:1) to make a time-resolved catalog of thousands of X-ray sources;
2) to search for time variability of active galactic nuclei (AGN);
3) to make complete light curves of X-ray novae;
4) to ﬁnd and monitor transient objects;5) to make a spectral mapping of galactic hot gas with the X-ray CCD camera;6) to detect Gamma-ray bursts and their afterglows;7) to monitor long-term variation of ﬂaring stars;8) to study the distribution of distant AGN.
Objectives 1) through 5) are solid goals with the MAXI sensitivity, Regarding Objective 6), the probabilityof Gamma-ray bursts (GRB) occur in the MAXI ﬁelds of view is rather low (about ﬁve GRB a year), because
MAXI views less than 2 % of the whole sky at any given moment. Yoshida et al (1999)1 still estimates that MAXI
will detect about 25 GRB aftergrows a year. To reach Objectives 7) and 8) requires the precise calibrations ofeﬀective areas over the ﬁeld of view, and the accurate determination or reproduction of instrumental backgroundlevels.
The MAXI ground system will transmit an alert through the Internet on detecting any signiﬁcant transientphenomenon, so that other observatories or satellites can turn their telescopes on the source to make follow-up

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Figure 2. The Monitor of All-sky X-ray Image (MAXI) payload, which will be attached to the International Space
Station in 2008.

observations. A frequently asked question is how rapidly MAXI will distribute the observation results, including
X-ray nova alerts. It depends on characteristics of the MAXI downlink paths, which are assessed in Subsection 6.3.

4. MAXI PAYLOAD AND COMPONENTS
4.1. The MAXI Payload
Fig. 2 shows the MAXI payload, which weighs 490 kg with dimensions of 185 cm (length) × 80 cm (width) × 77 cm(height). It carries simple X-ray eyes, eight combinations of a slit and orthogonally arranged collimator plates,
which produce one-dimensional X-ray images along sky great circles on twelve position-sensitive proportionalcounters (Gas Slit Camera; GSC) in the 2–30 keV band and two X-ray CCD units (Solid-state Slit Camera; SSC)
in the 0.5–10 keV band. (See Appendix A to ﬁnd what we mean by “sets”, “units”, and “counters” of GSC and
SSC.)
MAXI will be attached to one of the ten ports of the JEM “Kibo” Exposed facility (Fig. 3). The attachment
port, called the Equipment Exchange Unit (EEU), has a mechanism to hold a payload, and provides various
resources such as electricity, communication channels, and ﬂuid of a single-phase heat pipe. The resource assigned
to MAXI is electricity power of maximum 400 W (120 VDC), survival power of maximum 100 W (120 VDC),
data transmission rates of 25 kbps (through the MAXI MIL1553B port) and 200–600 kbps (through the MAXI
Ethernet port), and coolant Perﬂuorocarbon (FC72) with the ﬂow rate of 155 kg/hr. The CCD camera (SSC)
is cooled with the Loop Heat Pipe and Radiator System (LHPRS) carried by MAXI itself, and the rest parts of
MAXI are cooled by the coolant FC72 circulated by the JEM.

4.2. Gas Slit Camera (GSC)
We have started the fabrication and the calibration of the ﬂight model of GSC counters. We plan the counter-level
calibration for 2004, and the camera-unit-level calibration, in combination with slit collimators, for 2005.
Table 1 presents the characteristics of GSC. The GSC has detection thresholds of 2 mCrab for one day and0.6 mCrab for 10 days in the 2–30 keV band. The GSC has two ﬁelds of view toward two directions, 84 degrees
apart from each other: a horizontal and a zenithal ﬁelds of view (see Fig. 3). The directions of the fan-shapedﬁelds of view are ﬁxed on the International Space Station (ISS). Since the ISS orbits with its bottom side always
facing the Earth, MAXI automatically scans almost the whole sky every orbit with a period of about 96 minutes.

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Table 1. Characteristics of the MAXI X-ray cameras.

Detector

Gas Slit Camera

Solid-state Slit Camera

Proportional Counter

X-ray CCD

Xe+CO2 (1%) gas
Energy Range

2–30 keV

Detector Area

5350 cm2

0.5–10 keV

Field of View

1.5 × 160 degrees

1.5 × 90 degrees

Energy Resolution

18 %<150 eV at 5.9 keV

Position Resolution

1 mm

0.025 mm (pixel size)5-σ detection limit for one scan

7 mCrab

20 mCrab

200 cm2

While MAXI is inside high background regions such as the South Atlantic Anomaly, it cannot make no usefulobservations. The sky region missed by either of the horizontal or the zenithal ﬁeld of view due to such highbackground environments will be covered by the other ﬁeld of view within one orbit.
Fig. 4 shows one of six GSC units, and illustrates how to produce one-dimensional position sensitive ﬁeld ofview with a slit and collimator plates. Each GSC unit consists of two counters and a slit collimator, and has aﬁeld of view of 1.5 degrees × 80 degrees. Three GSC units cover 1.5 degrees × 160 degrees together (Fig. 3). The
X-ray entrance window of each GSC counter is made of a 100 micron Be sheet with dimensions of 272×190 mm(446 cm2 ). The counter is ﬁlled with Xe and CO2 (1%) gas mixture at 1.4 atm (0◦ C). In each counter, six carbon
wires are used as anodes surrounded by ten veto cells. The incident X-ray position is obtained by the charge
division method. Each GSC counter carries a radioactive source (Fe55) as an X-ray energy calibrator.

4.3. Solid-state Slit Camera (SSC)
The performance of the SSC electronics, CCD and its readout and drive circuits, have been veriﬁed using theengineering model. We make some modiﬁcation in the thermal design to keep pre-amp temperature within thepermitted temperature range in the cold case of the MAXI thermal environment. We will start the fabricationof the ﬂight hardware after the modiﬁcation of thermal design in 2004.
Table 1 presents the characteristics of SSC. The performance of SSC is presented in detail by Katayama et al(2005).2 The SSC consists of two units: a horizontal and a zenithal ones. Each unit consists of a slit collimator
providing a 1.5×90-degree ﬁeld of view, and 16 CCD chips as a one-dimensional position sensitive detector. Each
CCD chip is 1-inch square of a format of 1024 × 1024 pixels with pixel dimensions 24×24 µm. Its architecture is
full frame transfer. To achieve the energy resolution of 143 eV FWHM at 5.9 keV, the CCDs must be operated
below −60◦ C. To cool CCD chips, a one-stage Peltier cooler is attached to the backside of each CCD chip to
produce a temperature gradient of ∆T = −40◦ C between the chip and a SSC unit body. Each Peltier cooler
consumes typically 1 W (in total 32 W for two SSC units). The heat from the Peltier coolers are transported
by a Loop Heat Pipe (LHP) from the SSC units to two radiators on MAXI (this system is called Loop Heat
Pipe and Radiator System, LHPRS). The JEM “Kibo” provides an active thermal control system (ATCS), but
its coolant temperature is 16 to 24◦ C, which is not suitable to cool the SSC unit bodies. LHPRS has a thermal
diode function, which is suitable in the ISS thermal environment where sky sink temperature for the MAXI
radiators changes with a large amplitude and a short period (96 minutes). Each SSC unit carries a radioactive
source (Fe55) as an X-ray energy calibrator.

4.4. Onboard Data Processor (DP)The test of the Data Processor (DP) is ongoing using a prototype and its software. The prototype does not haveinterfaces with the support sensors, such as a star sensor and a ring laser gyro unit.

200Figure 3. The computer graphic picture of the Japanese Experiment Module(JEM) overlaid with visualised ﬁelds ofview of the MAXI Gas Slit Camera (GSC). Position angles are shown along the ﬁelds of view. The rotation and the orbit
motion of the International Space Station are synchronised at the same period of about 96 minutes. Not shown are theﬁelds of view of the other type of camera, the MAXI Solid-state Slit Camera (SSC). The ﬁelds of view of SSC covers asmaller position-angle range from −45 deg to +45 deg.

The DP will have communications with the MAXI ground system and the MAXI components, such as GSC,
SSC, a star sensor, a ring laser gyro unit, and a GPS receiver. The DP formats the X-ray observation data from
GSC and SSC into photon event data, in which a minimum unit is a set of properties of a single photon detection(detection position, arrival time, and photon energy). The DP does not perform any scientiﬁc analysis, such as
X-ray nova search. It downlinks the event data to the MAXI ground system.
There are two communication interfaces on the MAXI DP: a medium-bit-rate interface (10Base-T Ethernet)
and a low-bit-rate interface (MIL1553B). In the MAXI mission design, we suppose that the low-bit-rate connection is more reliable than the medium-bit-rate one in many aspects. Thus we use the low-bit rate channel
to downlink essential part of the MAXI data, and use the medium-bit-rate channel to downlink the full set ofdata. The low-bit data enable the MAXI ground system to perform the X-ray nova search to issue alerts. The
medium-bit-data enable us to conduct further analyses, including the diagnostics of the MAXI instruments.
The DP is constructed on the VME bus and consists of four CPU modules. When one CPU module fails,
the other CPU modules can continue the task of the failed CPU with a reduced observation capability. Every
CPU is a RISC chip, R3081 (25MIPS). The four CPU modules share a memory module to store the observation
parameters and the information about the CPU module conﬁguration.

5. MAXI GROUND SYSTEM AND ITS DEVELOPMENT STATUS
The MAXI ground system will be made of the following four components (see Fig.1):
1. Database inside the JAXA Operations Control System (OCS) zone, storing time-sorted event data forquick look and nova search;
2. Nova search system inside the JAXA OCS zone, searching for signiﬁcant X-ray transient events, such as
X-ray novae;
3. Database on the Internet, providing images, spectra, and light curves in response to the public user accessusing web browsers;

201

Figure 4. One of the six Gas Slit Camera units (GSC units). Each GSC unit has a ﬁeld of view of 1.5 deg × 80 deg. A
set of collimator plates deﬁnes the 1.5-degree width (FWHM) with no imaging capability across it, and a slit produces an
80-degree long one-dimensional image of the sky on the counter along its position sensitive direction.

4. Nova alert system, issuing alerts on detecting signiﬁcant transient events to registered users.
Components 1 and 2 are connected to the Operational Control System network, isolated from the Internet for
security reasons. To transfer the MAXI data to Components 3 and 4 on the Internet eﬃciently, we are planing
to use a secure one-way data path. The candidates are a two-port hard disk and a photocoupler. We have tested
a two-port hard disk, which introduces acceptable time delay of a few seconds in data transfer.
Of the four components listed above, Components 2–4 are still in the conceptional design phase. We havealready coded Component 1 as software in advance to the other components, so that we can start an interface
test with the OCS, an external system that the MAXI team is not responsible for. Component 1 has ﬁve tasks:• Data reception from the Operating Control System (OCS);• First reduction of the raw data, and the storage in a database;• Distribution of the ﬁrst reduction data to other computers;• Data backup;• System diagnosis.
Component 1 takes 0.2 – 0.3 seconds to process one-second data in the 20-kbps data stream, with PC-Linux
Vine-2.5 (kernel 2.4.20) on a DOS/V machine with a Pentium-4 2.2-GHz CPU and 512 MB memory.

6. MAXI PERFORMANCE
6.1. Performance Evaluation Based on Simple Photon Statistics
By Monte Calro simulation, Mihara et al (2002)3 have already evaluated the performance of the MAXI GSC
observations based on photon number statistics. Fig. 5 shows the simulated all-sky images of GSC. In thesimulations, all the MAXI structure were assumed to be in the positions as designed in drawings. In other words,
the simulation has not predicted any systematic errors possibly caused by misalignments of parts, especiallycollimator plates and slit, and by limitations of our ground and in-orbit calibrations.
In the rest of this section, we examine the collimator and its calibration from the point view of the MAXI
performance. We also mention the MAXI system ability of quick data distribution.

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Figure 5. Simulated images of the MAXI Gas Slit Camera (GSC) plotted in the Galactic coordinates. Panel (a) is for
90-minute (one-orbit) data accumulation; panel (b) for two-month data accumulation.

6.2. Estimation of Position and Flux Determination Accuracy
6.2.1. Collimator
The MAXI slit collimator (Fig. 4) was thought to be a source of systematic errors against position and ﬂux
determination accuracy. The 100-µm thin collimator sheets are supported in place using spring coils, which
provides tension to keep the sheets ﬂat. Even with the tension, the sheets bend by their own weight if thecollimator is placed with the sheets parallel to the ground. Thus we have tested the engineering model, with the
sheets vertical to the ground, in the laboratory using an X-ray beam line.4
Of the 64 collimator sheets over a GSC counter, every pair of adjacent sheets makes a triangular responseof X-ray transmission with width of 1.5 degrees (FWHM at position angle of 0 deg). At every position angle
examined in the ﬁeld of view, the X-ray transmission directions of plate pairs are misaligned with a 3σ deviationof 0.12 degrees. The directions of the X-ray transmission, averaged for all collimator plates at each position
angle, should be on a single great circle in the sky. The 3σ deviation of the ﬁeld of view from a great circle ismeasured to be 0.03 degrees for the engineering model. With an estimated attitude determination accuracy of
0.05 deg (3σ) for the GSC and SSC units, the collimator quality is good enough to achieve position determination
accuracy of 0.1 degrees in the scanning direction. On the other hand, the accuracy in the position angle direction
depends on the quality of the slit and the X-ray counter. We also achieve the accuracy of 0.1 deg in this direction.
Preliminary results of the slit collimator test imply that the ground calibration provides the ratios of eﬀective
areas, with accuracy of a few percent, for diﬀerent position angles of a slit collimator. This means that, evenwhen MAXI can determine absolute ﬂux with worse accuracy (for example, 10%), it still has potential to monitor
the ﬂux variation of a given object with accuracy of a few percent.
6.2.2. In-orbit calibration
For MAXI, the in-orbit calibration is crucial to achieve a position determination accuracy better than 0.1 degrees
and a ﬂux determination accuracy better than 10%.
Some astronomical satellites can point to desired calibration sources in the sky. As noted above, however,
we cannot control the MAXI attitude, hence the direction of the ﬁelds of view. The ISS orbits with the periodof 96 minutes, the orbit inclination of 51.6 degrees, and the precession period of about two months. Thus every
celestial source change its detected position in the MAXI ﬁeld of view cyclically with a period of two months.
The source is detected in a limited region of a ﬁeld of view, depending on its declination angle in the equatorial
coordinate system.
Fig. 6 shows the coverage of ﬁeld of view with a given celestial source (Panel a) and the frequency of Crab
Nebula’s crossing the MAXI ﬁeld of view at a given position angle (Panel b). To calibrate the whole ﬁeld of viewof MAXI in orbit, we need more than one celestial source as ﬂux and position calibrators.

203

Figure 6. The coverage of the ﬁeld of view with a given celestial source.
Panel (a): The declination angle of a given celestial X-ray source, and its coverage of the MAXI ﬁeld of view (FOV).
For example, Crab nebulae, a useful calibration source in X-ray astronomy, moves across the FOV at the position angles
between −29.6 degrees and +73.6 degrees.
Panel (b): Frequency of crossing the ﬁeld of view for Crab Nebula

6.3. Quick Distribution of Observation Results
One of the MAXI objectives is the quick distribution of observation results, including the transmission of X-ray
nova alerts to the Internet. If we have a real-time connection 24 hours a day from the MAXI payload to theground system, the place of data analysis does not matter to realise the quick distribution. It is not the case for
MAXI.
On average, the 50–70% of the observational data will be downlinked in real time (with delay of a few to ten
seconds). The rest of data, stored on the ISS for next downlink, takes 20 minutes to a few hours to arrive the
MAXI ground system, depending the timing of data transmission. The fastest way to distribute nova alerts is
to perform nova search onboard the MAXI payload, and downlink the results with priority whenever a real-rime
connection is established. Instead, the MAXI team has chosen to perform an X-ray nova search on the groundusing more ﬂexible hardware and software environments.
Fig. 7 shows the low-bit-rate downlink path, which is essential for the MAXI mission. The medium-bit-rate
path is not shown. The MAXI data are duplicated, and the identical data set is output to both the NASA link
and the JAXA link. The NASA link provides real-time connection with the ground system for more than 50%
of a day using a ﬂeet of data relay satellites, and the JAXA link provides it for about 20% of a day with asingle data relay satellite. The duration of the real-time connection depends on the operation plans of the data
relay satellites, which are not dedicated to the ISS, but shared with other satellites. When the ISS is out ofreal-time contact, data are stored in the onboard recorders, and then replayed during next contact. Channels 2
and 3 in Fig. 7 are for real-time data, and Channels 1 and 4 are for the data stored in the onboard recorders
during communications outage. The MAXI ground system receives the real-time data through Channel 5. The
stored data in the onboard recorders can be already 0–200 minutes (two orbits) old at the beginning of downlink.
The NASA-stored data are collected by the MAXI ground system through Channel 6 at a very slow transferrate, one-second observation data per second. The JAXA-stored data is received through Channel 7 from theData Archives and Retrieval System (DARS) which spends 20 minutes to a few hours preparing data ﬁles for
the MAXI ground system. To make the data distribution as fast as possible, we are considering the following
modiﬁcations:
1. We modify the DP software to perform onboard attitude determination using Visual Star Camera (VSC)and Ring Laser Gyro (RLG). We planed to downlink raw data of VSC and RLG, and determine the attitude

204

Figure 7. The downlink channels of the MAXI low-bit-rate data, which is primary data used in X-ray nova search. The
MAXI data are duplicated on board the ISS, and the identical data set is output to both the NASA link and the JAXA
link (JAXA link is shown as JEM/ICS link). Both links utilise data relay satellites in the geostationary orbit. Channels
2 and 3 are for real-time downlink data. Channels 1 and 4 are for the data sets stored in on-board recorders during thecommunications outages of the ICS and the NASA link, respectively.

of MAXI on the ground. The RLG data are used to extrapolate the VSC absolute attitudes during theperiods of no VSC visibility. However, we noticed that this method introduces a delay in nova search,
because there is a case that the MAXI ground system has received the latest X-ray event data and RLG
data, but has to wait for the VSC data to determine the sky positions of X-ray events, when the phase of
no VSC visibility overlaps the beginning of the real-time downlink;
2. We consider asking the modiﬁcation of DARS (the data source of Channel 7 in Fig.7) to speed up its ﬁlepreparation of the JAXA stored data (at present, it takes 20 minutes to a few hours);
3. We consider a way to obtain the NASA stored data quickly (at present, very slow at one-second observation
data per second through Channel 6 in Fig.7).

6.4. Photon Arrival Time Accuracy
We have evaluated that the accuracy of the GPS time tag on each X-ray photon is 120 µs. The breakdown is
55 µs from the photon absorption to the collection of the ADC signal, 50 µs for the least signiﬁcant bit of the
GSC clock, 0.1 µs for latching the GSC clock on the GPS PPS signal arrival, and 15 µs for the GPS PPS signal
distribution to the GSC data processor.
The time resolution of SSC is limited by the CCD readout speed, ranging from 2 to 16 seconds depending
the readout patterns.

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Figure 8. Point spread functions (PSF) of the MAXI Gas Slit Camera. Panel (a) is for the point source at the position
angle of 0 degrees along the ﬁeld of view; Panel (b) is for 20 degrees. Along the position angle axis of Panel (b), the PSF
shows asymmetry caused by slant incidence of photons into the gas cells.

7. FUTURE PLAN
7.1. Studying Data Analysis Methods
We need to study the data analysis methods using simulated MAXI data. For example, we have to study how toanalyse a photon-accumulated image, in which photons are collected from diﬀerent scans, hence with diﬀerent
point spread functions (PSF). Fig.8 shows the PSF of GSC at two position angles along the ﬁeld of view. Panelb of 8 shows asymmetry caused by slant incident of photons into the gas cells. The shape and the position along
a GSC anode wire depends on the photon energy. This means that there is no unique correspondence between
the positions on the anode wire and the sky. We have to determine the correspondence and the ﬁtting method
at the same time to obtain the best position and ﬂux accuracy.
The SSC does not show the PSF asymmetry caused by slant incident of photons, but we will have to consider
various PSF for SSC as well. In the nominal ISS attitude, the sky scanning direction and the CCD chargetransfer direction are the same. In the permitted ISS attitude range, however, they can be diﬀer by ±15 degrees,
requiring us to constructing appropriate PSF according to the ISS attitude.

7.2. Analysing the MAXI mission scenarios
From the launch of MAXI to the end of the mission, there are many ISS speciﬁc issues to be assessed from thepoint of view of the MAXI mission success. Some of them are beyond the control of the MAXI team, and what
we can do are just assessment and preparation for them. One example is the installation of the MAXI payload
to the ISS with a robot arm, during which the MAXI instruments have to survive with no electricity supplied.
We will make speciﬁc operation scenarios, and will verify that the MAXI mission withstands the worst cases.

7.3. Preparing for Quick Start-up of the MAXI System after the Launch
Taking account of the two-year mission life and the role as an all-sky monitor, it is essential to establish the
distribution of reliable and easily-utilised data in the early stage of the MAXI mission. As we noted, the MAXI
ground system for the scientiﬁc analysis and distribution is still in the conceptional design phase. We needto complete the ground system, publish the detailed speciﬁcation of the MAXI system, and cultivate potential
MAXI users well in advance to the launch of MAXI scheduled for 2008.

206

Figure 9. Components trees of the MAXI X-ray instruments.

8. CONCLUSION
We presented the overall picture of the MAXI mission system. The design and performance of the X-ray detectorsof GSC and SSC are veriﬁed using the engineering models. We have ﬁnished the preliminary and critical reviewof the MAXI payload. The MAXI system components other than the X-ray detectors were examined fromthe point view of the mission performance. We conﬁrmed that the quality of the collimator meets our mission
requirement. The distribution of the MAXI data might speed up by modiﬁcations in the data transfer pathto the MAXI ground system. In the 2004–2005 time frame, our workforce will be gradually shifted from thedevelopment of X-ray detectors to the calibration of them and the construction of the ground system including
data analysing software.

APPENDIX A. COMPONENT TREES OF THE MAXI X-RAY INSTRUMENTS
To make clear what components we refer to by “sets”, “units”, and “counters” of GSC and SSC, we shows thecomponent trees in Fig. 9.

ACKNOWLEDGMENTS
We thank B.C. Rubin and W. Yuan for developing the MAXI simulation software. We also thank H. Nagai forassistance in developing the MAXI loop heat pipe.

REFERENCES
1. A. Yoshida, T. Mihara, M. Matsuoka, N. Kawai, T. Kotani, H. Negoro, H. Kubo, Y. Shirasaki, B. C. Rubin,
K. Torii, H. Tsunemi, S. Kitamoto, K. Hayashida, E. Miyata, and M. Yamauchi, “Observations of GRB
afterglows with MAXI on JEM/ISS,” Astron. Astrophys. Suppl. Ser. 138, pp. 439–440, 1999.

207

2. H. Katayama, H. Tomida, M. Matsuoka, H. Tsunemi, E. Miyata, D. Kamiyama, and N. Nemes, “Performanceof the engineering model of the MAXI/SSC,” in High-Enegy Detectors in Astronomy, Proc. SPIE 5501, p. inpress, 2005.
3. T. Mihara, N. Kawai, A. Yoshida, I. Sakurai, T. Kamae, M. Matsuoka, Y. Shirasaki, M. Sugizaki, W. Yuan,
and I. Tanaka, “Performance of the GSC engineering-counter for MAXI/ISS,” in X-Ray and Gamma-Ray
Instrumentation for Astronomy XII, K. A. Flanagan and O. H. Siegmund, eds., Proc. SPIE 4497, pp. 173–
186, 2002.
4. N. Isobe, M. Matsuoka, S. Ueno, H. Tomida, K. Kawasaki, H. Katayama, T. Mihara, M. Kohama, I. Sakurai,
M. Nakajima, N. Kawai, J. Kataoka, A. Yoshida, D. Takahashi, M. Uzawa, H. Tsunemi, E. Miyata, and
I. Tanaka, “Observational response of MAXI onboard ISS,” in X-ray and Gamma-ray Instrumentation for
Astronomy XIII, K. A. Flanagan and O. H. Siegmund, eds., Proc. SPIE 5165, pp. 354–365, 2004.

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