National Aeronautics and Space Administration

The payload consists of three instruments:

• the Wide Field Scanner WiFS, provided by ISRO
• the Modular Optoelectronic Scanner MOS, provided by DLR
• an X-Ray astronomy experiment by ISRO.

The principle objective of the X-ray Astronomy Instrument is to carry out timing studies of X-ray pulsars, X-ray binaries, and other rapidly varying X-ray sources. The XSM detects transient X-ray sources and monitors the light intensity of bright X-ray binaries.

The in-orbit performance checks of the X-ray Instrument was carried out May 1 - May 9 1996 by pointing the PPCs towards Cyg X-1. All instruments have been checked out. Due to the polar orbit of IRS-P3, most orbits pass through the South Atlantic Anamoly and regions of high intensity charged particles while the satellite's latitude is more than 50 degrees from the equator (North or South). To protect the instrumentation, the high voltage levels are reduced during SAA passages and when the latitude is +/- 50 or higher/lower. This reduces the useful duty cycle to roughly 15% of the satellite's orbit. The summed background of the 3 PPCs in the 2 - 18 keV range is about 45 counts per second when no X-ray point source is in the field-of-view.

Since the primary objective of the satellite was to carry out remote sensing, the time for stellar mode observation was limited to 2 months in a year. The stellar mode observations were made using a star tracker and gas jets for moving the satellite from earth pointing to stellar pointing mode and for correction due to drift of gyros. The satellite and the x-ray instrument is still healthy and working. However the x-ray observations came to an end on June 30, 2000 because the gas necessary to carry out any pointed observations was almost depleted. The satellite is therefore, carrying out only remote sensing observations of earth since June 2000.

Cyg X-1 was found to show pronounced variability on time scales of 100 ms, with some lesser variability on 10 ms time scales. QPOs of 0.22 Hz were also clearly detected. The 121 second X-ray pulsar GX 1+4 failed to show X-ray pulsations when observed, contrary to expectations. The 437 second X-ray pulsar 4U 1907+09 was then observed to verify that the lack of pulsations was not due to instrument failures. 4U 1907+09 clearly showed the expected pulse periods. GRS 1915+105 observations produced similar results to Cyg X-1.