2. Instrument and observations

The Indian X-ray Astronomy Experiment (IXAE) consists of three pointed proportional counters (PPCs) and one X-ray sky monitor (XSM) (see Agrawal et al. 1997 for more details). The Indian satellite IRS-P3 carrying these instruments along with two remote sensing payloads, was launched with the Polar Satellite Launch Vehicle (PSLV) from Shriharikota (India), on 1996, March 21. The satellite orbits at an altitude of 830 km with an inclination of with the equator. Observations of X-ray sources are carried out only in those parts of the orbits which do not go through the South Atlantic Anomaly (SAA) region. The satellite orbit being nearly polar the charged particle background in the detectors increases near the poles and the good observation band is defined to be within the latitude band of S to N. The operating region is chosen to be in this latitude range where the detector background is found to remain constant.

The PPCs have an energy range of  keV with 60% efficiency at 6 keV. The typical energy resolution of a PPC averaged over the entire detector is 22% at 6 keV. Total photon collecting area of the three detectors is 1200 cm². The detectors gain can be controlled by changing the high voltage supply in discrete steps and hence the effective energy range can be kept within the desired value. The X-rays detected in each PPC are analyzed in processing electronics units (one for each detector). The processed data, which contain the pulse height histograms and the count rate profiles, are stored in the onboard memories. The timing resolution is different for different modes of observations which is driven by scientific need for the source being observed. The spectral response and detection efficiency of the PPCs has been calibrated by observing the bright X-ray source Crab nebula.

The galactic superluminal X-ray source GRS 1915+105 was observed during 1996 July, 23-27 with the PPCs for a total duration of 8850 s. Observations were limited to the satellite position between the latitude and to avoid high background regions induced by the charged particles. All but one observations were done in a mode with timing resolution of 100 ms, and 64 channel energy spectrum of the source was stored every 10 s. Observation stretches were at most about 20 min long and in some cases of smaller duration.