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ASCA Guest Observer Facility

An Overview of the Gas Imaging Spectrometer (GIS)

--by K. Makishima, University of Tokyo

The experiment

The GIS experiment is comprised of two sensors (GIS-S2 and -S3) and an analog/digital electronics unit (GIS-E). The S2/GIS-E combination also provides the RBM (radiation belt monitor) function. Compared with the SIS, the GIS has a better hard X-ray efficiency (by a factor of 2 at 7 keV), higher time resolution, higher signal saturation flux, and a four-times-wider field of view. However, but the GIS has a lower soft X-ray efficiency, a somewhat worse position resolution, and a factor 2-4 poorer energy resolution. It has been developed mainly by University of Tokyo, ISAS, and Meisei Electric Co., Ltd.

The sensors utilize the technique of the gas scintillation proportional counter, originally flown onboard the Tenma mission. The 25mm-deep gas cell ]is filled with 96% xenon plus 4% helium to 1.20 atm pressure (at 0 degree C). High-energy quantum efficiency is 94% at 7 keV and 67% at 10 keV. Low-energy efficiency is improved (14% at 0.8 keV, 32% at 1 keV and 80% at 2 keV) by the use of 10 micron thick beryllium foil (Yamaha Co., Ltd.) for the window. The window diameter of 52 mm ensures a wide (50 arcmin diameter) field of view. A high voltage of -6kV to -8kV is applied to the window.

Scintillation ultraviolet photons from the xenon gas are detected by a position -sensitive phototube (Hamamatsu R4268). Pulse-height analysis in the GIS-E gives a FWHM energy resolution of about 8.0% at 5.9 keV, which scales as inverse square root of the energy. X and Y positions of a detected photon are calculated by a CPU in the GIS-E using outputs from 32 multi-wire anodes (16 X and 16 Y) of the phototube. The FWHM position resolution is about 0.5 mm at 5.9 keV, and scales as inverse square root of the energy. Background rejection is performed through hard-wired rise-time discrimination (RTD), augmented by CPU-based spread discrimination (SPD).

Each detected event is tagged with 32-bit information and sent out to telemetry. The 32 bits consist of commandable combinations of sensor ID (1bit), Pulse Height (8-10 bits), X and Y Positions (each 4-8 bits), Rise Time (0-8 bits), SPread (0-16 bits), and photon arrival TiMe (0-10 bits). TM=10 affords an absolute timing accuracy of 61 microsec. The telemetry limit is 128, 32, and 8 c/s/GIS for high-, medium- and low-bit rates, respectively. Apart from this, the instrumental deadtime is typically 0.6, 2.7, 22 and 60% for total incident counting rates of 10, 30, 100 and 300 c/s/GIS, respectively (1 mCrab is roughly 0.8 c/s/GIS). In addition to the PH (pulse-height) mode described above, a CPU-free spectrum mode (MPC mode) is available for very bright sources under the sacrifice of position information.

In-orbit performance and calibration

The RBM function started operation on February 26 and the remaining low-voltage part was activated on March 2 and 3. The high-voltage (HV) supplies were turned on successfully in the middle of March. After various calibration and verification steps, the standard operation mode was roughly established by the end of April. However, the SPD (spread discriminator) was tightened on May 28, 1993 to accomplish the final background reduction. Therefore, analysis of the data acquired before May 28 needs particular attention.

The standard PH mode employs: a phototube HV of 1110V (level 4) ; a gas-cell HV of -6 kV (level 3); an RTD (rise-time discriminator) window of 159-218 with factor 2 bit compression; and an electronics gain of 48 for S2 and 40 for S3 (out of 0-63). These define a PH bandpass of 0.3-12 keV, which is somewhat wider than the actual XRT+GIS sensitivity band. The normal telemetry bit assignment is {ID=1, PH=10, XP=YP=8, RT=5, SP=TM=0}, while for timing observations a telemetry bit assignment of {ID=1, PH=8, XP=YP=8, RT=0, SP=0, TM=7} with a time resolution of 0.49 ms often has been used.

In-orbit sensor gains have been monitored using built-in Fe-55 isotopes (about 0.3 c/s/GIS), attached to the edge of the field of view. The gain exhibits a temperature dependence of about 1% per degree C, and a position dependence up to 20%. These have been calibrated in orbit down to 2%. The temperature- corrected gains have been stable within some 4% over 9 months. The absolute gain has been cross-confirmed within ~1% for both sensors in reference to the Au-M edge in the XRT and the Xe-L edge in the GIS, as well as iron lines in various cosmic sources, including the supernova remnant W49B. The pulse height can be corrected for the dependences on temperature and position through ASCALIN software, which also corrects the position data for the image distortion intrinsic to the experiment.

The GIS background has been studied in orbit by accumulating data for blank sky and night earth. After application of the onboard RTD and SPD, and further tightening the RT window in data analysis, the 0.3-12 keV non X-ray background (NXB) becomes about 0.07 c/s/GIS within 17 mm from the field center, corresponding to (5-7) 10-4 c/s/cm2/keV. The NXB spectrum is fairly flat above 1 keV but rises below 1 keV. The NXB is time stable within 30% or so inside the 17 mm, but toward the field edge it increases by 30-50% and becomes somewhat time variable depending on the cosmic-ray cutoff rigidity. The cosmic X-ray background is 0.082 c/s/GIS for the whole filed of view and the entire energy range.

The flat-field characteristic of the GIS is affected up to 15-20% by the shadows cast by support ribs of the gas cell window. This has been calibrated using extended sources with high surface brightness, including X-ray bright sunlit earth, the Coma cluster of galaxies, the supernova remnant IC443, and so on. These effects are taken into account by the exposure map.

Standard calibration using the Crab Nebula was conducted in early April. The XRT optical axes were determined for telescopes 2 and 3, and their relative alignment was calibrated, all to an accuracy of 0.2 arcmin. The measured on-axis Crab counting rate of 880 /261 50 c/s/GIS over the entire energy range agrees within 10% with the prediction. Using the high-time-resolution mode, the Crab pulsar was detected precisely at the predicted period, and the absolute pulse arrival times agreed with the radio prediction within 1 ms. For a number of Crab data sets acquired at various off-axis positions, the currently available GIS response function gives the photon index in the range 2.0-2.1 and the interstellar absorption of (3-4) 1021 cm-2. These values are consistent with previous measurements.

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