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Astron. Astrophys. 320, L37-L40 (1997)

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2. The IXAE and Observations

The X-ray observations were made with the IXAE which was launched onboard the Indian Remote Sensing satellite-P3 (IRS-P3) using the Polar Satellite Launch Vehicle (PSLV) on March 21, 1996 from Shriharikota Range in India. The satellite is in a circular orbit at an altitude of 830 km and inclination of [FORMULA]. The IXAE includes three collimated Pointed Proportional Counters (PPCs) with an effective area of about 1200 cm2 filled with P-10 gas at a pressure of 800 torr. Each PPC is a multi-anode, multi-layer detector with 54 anode cells of size 11 mm x 11 mm arranged in 3 layers with a wall-less geometry. The end cells of each layer and the third layer are joined together to form a veto layer for rejection of charged particles and background produced by Compton scattering of gamma-rays. The odd and even cells of the first and second layers, which detect X-rays, are connected together and operated in mutual anticoincidence to further reduce the non-cosmic X-ray background. A sandwich of 25 micron thick aluminized mylar and 25 micron thick uncoated polypropylene serves as the X-ray entrance window. The collimators with a field of view of [FORMULA] are made from honeycomb shaped aluminum coated on both sides with a 6 micron thick layer of silver. The detectors and associated electronics are so designed that each PPC has a modular structure with its associated high voltage unit, signal processing electronics, 8086 microprocessor and a memory of 4 MB. In the normal mode of operation count rates are recorded from the first layer in 2-6 keV and 2-18 keV bands, from the second layer in 2-18 keV band and all count in the veto layer above 2 keV with an integration time of 1 sec. The integration time can, however, be changed by command to .01, 0.1 or 10s. The gas gain of each detector is measured by continuously monitoring the pulse height due to 22.2 keV X-rays from a collimated Cadmium-109 radioactive source which irradiates only the end cells of the veto layer. A more detailed description of IXAE has been given by Agrawal et al. (1997 ).

Since the satellite is in a polar orbit and most of the orbits pass through the South Atlantic Anomaly (SAA) region, the useful data are obtained only when the satellite is usually in the latitude range of [FORMULA] N to [FORMULA] S outside the SAA region. The IRS-P3 is a three-axes stabilized satellite with an onboard star tracker which is used for acquisition and pointing at a given X-ray source.

The X-ray instrument was first switched-on on May 2, 1996 and its performance was verified by observing Cyg X-1 during May 2 - 9 period. The observed count rate for Cyg X-1 was about 500 counts per second indicating that it was in a low intensity state. Subsequent pointing at Cyg X-1 during July 4-10, when it had made a transition to a bright state, showed a count rate of about 1000 counts per sec. Preliminary results on Cyg X-1 describing erratic variability on time scales of sub-second to second are described by Agrawal et al. (1996a ). Observation of GRS 1915+105 were conducted during July 20-29, 1996. The target acquisition accuracy was better than [FORMULA]. The background of the PPCs was measured by pointing at a source-free region near GRS1915+105. The total background count rate in 2-18 keV energy band from the three detectors is about 45 counts per second.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998