The HXD-II Wide-band All-sky Monitor (WAM)

1. Detectors [1][2]

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 scintillation counters.


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) BGOBGO
Energy range (keV)50-500020-200040-700>75150-30000
Energy resolution30%@662 keV20%@662 keV14%@662 keV-14%@662 keV
Effective area (cm2)800@100 keV
400@1 MeV
2000@100 keV
150@1 MeV
100@1 MeV
120@200 keV
Time resolution31.25 ms2 ms 7.8125 ms50 ms5 micro sec.
Reference[1][2][3] link link link link

2. Data types [1]

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 channelTime resolutionTime coveragePurpose
GRB 4 ch 1/32 s (TH: Time History)128 s (16 s before and 112 s after the trigger)GRB
55 ch1 s (PH: Pulse Height)
TRN 55 ch1 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 [2][3]

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 satellite. 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%.

5. References
[1] K. Yamaoka et al., IEEE Trans. Nucl. Sci., vol. 52, no. 6, p.2765-2772, 2005
[2] M. Ohno et al., IEEE Trans. Nucl. Sci., vol. 52, no. 6, p.2758-2764, 2005
[3] Y. Terada et al., IEEE Trans. Nucl. Sci., vol. 52, no. 4, p.902-909, 2005

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