IN-ORBIT PERFORMANCE OF THE GAS IMAGING SPECTROMETER ONBOARD ASCA

The in-orbit performance and calibration of the Gas Imaging Spectromet er (GIS), located on the focal plane of the X-ray astronomy satellite ASCA, are described. An extensive in-orbit calibration has confirmed i ts 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 ke V. The in-orbit non X-ray background of the GIS has been confirmed to be as low as (5-9) x 10(-4) cs(-1) cm(-2) keV(-1) over the 1-10 keV ra nge. The long-term detector gain has been stable within a few ro for n early 3 years. Extensive observations of the Crab Nebula and other sou rces have provided accurate calibrations of the position response, pho tometric capability, dead time, and timing accuracy of the GIS. Furthe rmore, the overall energy response, including the temporal and positio nal gain variations and the absolute gain scale, has been calibrated t o similar 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 ph otometry With a flux dynamic range covering more than 5 orders of magn itude, and fast X-ray photometry with a time resolution up to 60 mu s.