The HXD-II Wide-band All-sky Monitor (WAM)
1. Detectors 
The WAM is a large and thick anticoincidence shield of the Hard X-ray
Detector onboard Suzaku (Fig. I). The main role of the WAM is background rejection
for the HXD main detectors, i.e. PIN diodes and GSO. It also has a wide
field of view of 2pi str. and a large geometrical area of 800 cm2 per
one face, which can be utilized as an all-sky monitor
aiming at GRBs, solar flares, and bright X-ray transients.
The sensitive energy range for the WAM is from 50 keV to 5000 keV
in gamma-rays. The excellent feature of the WAM is its large effective
area of 400 cm2 even at 1 MeV (Fig. II), which enables us to study the high
energy radiation of the GRBs at MeV range (Gonzalez et al. 2003) and
determine a peak energy above 300 keV of the synchrotron emissions.
The WAM detector performace as a GRB monitor is summarized as Table I in
comparison with other similar types of GRB detectors.
The WAM data is transferred to the ground station at only 5 times when Suzaku
passes over Uchinoura Space Center (USC) in Japan. Hence, no rapid response to the GRBs such as Swift and HETE2 can
be expected. However, we will provide GRB position information by combining counting rates detected with four faces and IPN as soon as possible.
Fig.I The schematic view of the HXD scintillation counters.
The Wideband All sky Monitor (WAM) consists of the surrounding 20 BGO
Fig.II Energy dependence of the effective areas of the Suzaku/HXD-II(WAM)
detectors. In comparison with other hard X-ray and gamma-ray instruments onboard
astronomical satellites, the WAM instrument exhibits relative large effective
area in 300 to 5000 keV band.
Table I: Detector performance of WAM compared with other GRB detectors
|Crystal ||BGO ||NaI(T1) ||CsI(Na) || BGO||BGO
|Energy range (keV)||50-5000||20-2000||40-700||>75||150-30000|
|Energy resolution||30%@662 keV||20%@662 keV||14%@662 keV||-||14%@662 keV |
|Effective area (cm2)||800@100 keV|
|Time resolution||31.25 ms||2 ms ||7.8125 ms||50 ms||5 micro sec.|
2. Data types 
There are two types of the WAM data (Table II): transient data (TRN data) and
gamma-ray burst data (GRB data). The GRB data is transferred to the
data recorder only when the onboard GRB trigger occurs, while the TRN data
is done every 1 s. The GRB light curves with a fine time resolution of 1/32 s
but coarse 4 energy channels can be obtained from the time history (TH) data in the GRB data.
The energy range corresponding to TH channels (TH0, 1 2 and 3) is roughly
defined in the Table III. The 55-ch energy spectra can be derived from pulse height (PH)
data in the GRB and TRN data. These light curves and energy spectra will be put
into our web page. For GRBs with a long duration of more than 112 s and un-triggered GRBs,
the TRN data will be processed and analyzed by the Suzaku-WAM team.
Table II: Characteristics of the WAM data (TRN data and GRB data)
|Energy channel||Time resolution||Time coverage||Purpose
|GRB ||4 ch ||1/32 s (TH: Time History)||128 s (16 s before and 112 s after the trigger)||GRB|
| 55 ch||1 s (PH: Pulse Height) |
|TRN ||55 ch||1 s (PH: Pulse Height)
||Always transferred to the telemetry every 1 s
||monitor for background and transient phenomena|
Table III: Rough energy range of the TH channel
TH0: 50-110 keV
TH1: 110-240 keV
TH2: 240-520 keV
TH3: 520-5000 keV
Note that this energy range can be changed in flight by the
gain variation of the photomultiplier tubes.
3. Current calibration status 
The HXD-II team have constructed the mass model based on the GEANT4
by comparing between the pre-flight calibration results and their simulation.
The WAM energy response with a various incident angle is calculated using
this mass model.
The WAM in-flight calibration is now under way. The energy-scale calibration
is performed using the 511 keV line from surrounding activated materials on the
We always monitor the gain of all the units at once a day. The flux
calibration, i.e. fine-tuning to the mass model, can be performed using solar
flares and GRBs which is simultaneously detected with other satellites.
HXD team is now performing the flux calibration, utilizing the
simultaneous data of such as GRB 051008 between Swift/BAT and WAM.
The flux accuracy is currently aimed at 20%.
 K. Yamaoka et al., IEEE Trans. Nucl. Sci., vol. 52, no. 6, p.2765-2772, 2005
 M. Ohno et al., IEEE Trans. Nucl. Sci., vol. 52, no. 6, p.2758-2764, 2005
 Y. Terada et al., IEEE Trans. Nucl. Sci., vol. 52, no. 4, p.902-909, 2005
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