The Indian Remote Sensing Satellite IRS-P3 was launched with the third test
launch of the Indian Polar Satellite Launch Vehicle PSLV at March 21st 1996.
It is an experimental mission mainly oriented on earth remote sensing. The
launcher and the satellite bus was provided by the Indian Space Research
Organization ISRO, a part of the payload has been provided by German Aerospace
Research Establishment DLR in the frames of the Indian-German Agreement on
scientific and technological cooperation.
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.
||polar, sun-synchronous circular|
The Indian X-ray Astronomy Instrument aboard the satellite consists of two
instruments. The Pointed-mode Proportional Counters (PPC) consists of three
identical, coaligned, multilayer proportional counters collimated to a
2 deg x 2 deg field-of-view, with each with an effective area of 400 cm2.
The proportional counters are sensitive to 2 - 18 keV photons. The X-ray Sky
Monitor (XSM) is a a pinhole camera with 1 cm2 opening placed above a position
sensitive proportional counter. The instrument has a 90 deg x 90 deg
field-of-view and is sensitive to 3 - 8 keV photons. The viewing axes of the
PPCs is aligned with the roll axis of the satellite while the XSM is offset by
35 degrees in the roll-pitch plane.
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 were carried
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.
IRS-P3 observed Cyg X-1 during system checks, plus a follow-up observation
in July 4-9 1996 after the source underwent a transition to its bright state.
Other sources observed include GRS 1915+105, GX 1+4, and 4U 1907+09.
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.