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Astron. Astrophys. 319, 507-510 (1997)

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2. Observations

The X-ray pulsar GX 1+4 was observed with a large area xenon-filled multi-anode proportional counter (XMPC) telescope in the 20-100 keV band in a balloon flight experiment on 22 March 1995. The balloon reached a ceiling altitude of 41.5 km corresponding to a residual atmosphere of 2.5 gm cm-2.

The XMPC telescope consists of two identical xenon-filled proportional counters with a total effective area of 2400 cm2 and has a [FORMULA] [FORMULA] [FORMULA] field of view defined by a passive tin-copper graded collimator. The telescope, mounted on an orientation platform, can be pre-programmed to track a given source by an onboard automated tracking system. For details of the X-ray telescope refer to Rao et al. (1991). Observations of the source were carried out by alternately looking at the source and a nearby source-free background region in the tracking mode. Background observations were also made in a tracking mode in a region about [FORMULA] away from the source and free of any other known bright X-ray sources.

During the first and the last 10 minutes of the 240 minutes observations of GX 1+4, some noise was present in the lower energy channels in one of the proportional counters. All data from that detector during that time were discarded. Data in those lower channels for the entire duration of the observation were also discarded to remove any ambiguity. The background count rate was found to be constant during the entire balloon flight; the source count rate varied with the zenith distance because of absorption along the varying residual atmosphere along the line of sight. The constancy of the background count rate was checked by fitting a straight line and a reduced [FORMULA] of 1.1 was obtained.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998