In-Orbit Performance of the Gas Imaging Spectrometer onboard ASCA

Abstract

The in-orbit performance and calibration of the Gas Imaging Spectrometer (GIS), located on the focal plane of the X-ray astronomy satellite ASCA, are described. An extensive in-orbit calibration has confirmed its basic performance, including a position resolution of 0.6 mm (FWHM) and an energy resolution of 7.8% (FWHM), both at 6 keV. When combined with the X-ray telescope, the GIS sensitivity range becomes 0.7-10 keV. The in-orbit non X-ray background of the GIS has been confirmed to be as low as (5-9) × 10−4 cs−1 cm−2 keV−1 over the 1-10 keV range. The long-term detector gain has been stable within a few % for nearly 3 years. Extensive observations of the Crab Nebula and other sources have provided accurate calibrations of the position response, photometric capability, dead time, and timing accuracy of the GIS. Furthermore, the overall energy response, including the temporal and positional gain variations and the absolute gain scale, has been calibrated to ∼ 1%. Thus, the GIS is working as an all-round cosmic X-ray detector, capable of X-ray imagery, fine X-ray spectroscopy, X-ray photometry with a flux dynamic range covering more than 5 orders of magnitude, and fast X-ray photometry with a time resolution up to 60 μs.

Search for hot gas in the local group with ASCA

“…However, they are still subject to sporadic NXB variations, e.g., on day-by-day time scales, that are not clearly reflected in the H02 counts. This systematic effect limits the accuracy of the NXB subtraction Makishima et al 1996). The expected NXB reproducibility is limited to ∼ 7% in terms of the 90% confidence systematic errors.…”

“…We performed a further event screening utilizing the GIS monitor counts H02 (i.e., H0+H2; see Makishima et al 1996), Radiation-Belt-Monitor counts, and COR. This additional selection, called the H02 method, improved the NXB reproductivity, mainly by rejecting data affected by unexpected increases in the NXB counts, which are probably due to a concentration of charged particles on the satellite orbit.…”

The Gas Imaging Spectrometer on Board ASCA

“…The grid itself can stand against several atmospheric pressures. The shadow of the grid onto the X-ray image is not a major problem since the wall thickness is much thinner than the position resolution of the GIS (~ 0.5 mm FWHM at 6 keV), as described in Makishima et al (1996) in detail. The finesupporting mesh has 1.2 mm pitch and 87% transmission with a wire thickness of about 80 /xm.…”

“…The gain uniformity of the phototube has been measured by scanning the photocathode with a blue LED (light emitting diode) and it varies by about ±15% over the whole area. This effect is incorporated in the GIS response function (Makishima et al 1996).…”

“…This paper describes the design of the GIS system and its ground performance. The accompanying paper (Makishima et al 1996) shows the in-orbit performance and background properties of the GIS experiment. Performances of imaging GSPCs obtained with pre-flight models are given in Ohashi et al (1991), and preliminary results from the ground and the in-orbit calibrations of the GIS system are reported in Kohmura et al (1993) The focal plane of the X-ray astronomy satellite ASCA (Tanaka et al 1994) is equipped with imaging gas scintillation proportional counters (Gas Imaging Spectrometer: GIS) for the first time, along with X-ray sensitive CCDs (Solid-state Imaging Specrometer: SIS).…”