GIS

Each Gas Imaging Spectrometer (GIS) is a xenon-filled imaging gas scintillation proportional counter. Two of these instruments, being provided by the University of Tokyo and ISAS, will make up the other half of the instrumental payload. The GIS will complement the SIS: above the xenon L edge (approximately 5 keV) the GIS has a detection efficiency which is both greater than the SIS and which falls off less rapidly; unlike the SIS, the GIS will observe the entire field of view of the telescopes (30 x 30 arcminutes), covering an area 1.7 times larger than the SIS. At 5.9 keV the resolution of the GIS is 8 per cent.

The GIS has four different modes: pulse height (PH) mode, multi-channel pulse count (MPC) mode, position calibration (PCAL) mode, and memory-check mode. Observations will normally be carried out in PH mode.

• PH mode: In this mode, the on-board CPU calculates the event position and discards background events using rise-time and position information. Only accepted X-ray events are sent to the ground.
• MPC mode: This mode is meant for extremely bright sources (> 1 Crab), optimizing temporal resolution at the expense of dispensing with positional information: only pulse-height information is recorded. This is also a back-up mode against CPU failure, since the data are processed without the intervention of the CPU. 256-channel pulse-height spectra for each detector are produced.
• PCAL mode: This mode is for the on-board calibration of multi-wire outputs. All the ADC outputs from the multi-anode data are sent to the ground, which can be used to determine the event position. Only one event is sent in a frame. If the source is weak (< 1 mCrab), this mode still enables position determination on the ground.
• Memory-check mode: This mode is for checking the onboard memory after loading the program. It dumps all the GIS CPU memory.