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Astron. Astrophys. 330, 181-188 (1998)

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2. IXAE instrument details and observations

The Indian X-ray Astronomy Experiment (IXAE) includes three identical Pointed Proportional Counters (PPCs) and one X-ray Sky Monitor. Each PPC is a multi-cell proportional counter array and has an effective area of 400 cm2. The filling gas is 90% argon and 10% methane at a pressure of 800 mm of Hg. There are 54 cells with a size of 11 mm [FORMULA] 11 mm arranged in 3 layers. The bottom layer and the end cells are joined together to form a veto output for charged particle anti-coincidence. The remaining anode cells in the top two layers form the detection volume and they operate in mutual anti-coincidence. A passive collimator restricts the field of view to [FORMULA]. The operating energy range is between 2 keV and 18 keV. The overall energy resolution is 22% at 6 keV. The gain stability of the detectors is monitored continuously by X-rays from a collimated Cd109 radioactive source irradiating the veto cells.

Each PPC has its own front-end electronics (consisting of amplifiers and command-controllable high voltage unit) and a processing electronics. The processing electronics selects the genuine events based on the pre-determined logic conditions and measures the pulse height spectrum in 64 linear channels. Parallelly, independent counters store the following data i) 2 keV - 6 keV genuine events of top layer, ii) 2 keV - 18 keV genuine events of top layer, iii) 2 keV - 18 keV genuine events of middle layer, iv) [FORMULA] keV counts (ULD counts) for all layers, and v) [FORMULA] keV counts from the veto layer. An 8086 microprocessor based system handles these data and stores them in 4 Mbits of memory. The data storage is done in different modes which can be set by commands. The two available modes are 1) count and spectral mode where the five basic counts are stored in integration time of 0.01, 0.1, 1 or 10 s and 64 channel spectra for three layers separately (top two layers left and right separately) with integration time of 1, 10, 100 or 1000 s. 2) time tagged mode where each event is time tagged to an accuracy of 0.4 msec (for PPC-3) or 0.8 msec (for PPC-1 and PPC-2) and for each event 8 channel linear spectral information and layer information are also stored. The data storage can be stopped and started by the use of time-tagged commands.

The IXAE instrument is a part of the Indian Remote Sensing satellite IRS-P3, which also includes a remote sensing camera and an oceanographic instrument. IRS-P3 was launched using the Polar Satellite Launch Vehicle (PSLV) on 1996 March 21 from Shriharikota Range, India. The satellite is in a circular orbit at an altitude of 830 km and inclination of [FORMULA]. Stellar pointing for any given source is done by inertial pointing by using a star tracker. The pointing accuracy is about [FORMULA]. The observing time for the 3-axes stabilized stellar pointing mode is available for 3 to 4 months in a year. The high inclination and high altitude orbit is found to be very background prone and the useful observation time is limited to the latitude ranges typically from [FORMULA] S to [FORMULA] N. Further, the large extent of the South Atlantic Anomaly (SAA) region restricts the observation to about 5 of the 14 orbits per day. The observations are made in the selected latitude regions by using time-tagged commands, which either reduce the high voltage to the non-operating region and stop data acquisition. The data is down loaded typically twice per day.

The PPCs were first switched-on on 1996 April 30 and Cygnus X-1 was observed in its hard state between April 30 and May 9. The source was again observed, in its soft state, between July 4 and July 10. The log of observation is given in Table 1, for only those observations which are used for the present analysis. The observed count rates as normalized to the Crab flux from a calibration study undertaken in 1996 December, and the binary phase of Cyg X-1 calculated from the ephemeris given by Gies & Bolton (1982), are also given in the table.


Table 1. Log of observation

2.1. Background modeling

All the three PPCs are co-aligned and hence there are no simultaneous background measurements. For each set of observation for a given source (lasting for a few days), background count rates are measured before and after the source observation by pointing the PPCs at regions in the sky close to the source (about [FORMULA]), but free of any known X-ray sources. As mentioned earlier, the IRS P-3 satellite is in a high altitude and high inclination orbit, which results in very high charged particle induced background count rates in high latitude regions and South Atlantic Anomaly region. The good observing regions are generally restricted to latitude ranges from [FORMULA] S to [FORMULA] N.

It is known that the particle background in space environment tracks well with the McIlwain's L parameter and the Earth's magnetic field, B (McIlwain 1977). For the present observations, it is found that the background rates are relatively stable at low magnetic filed values ([FORMULA] G) and low McIlwain's L parameter ([FORMULA]). To further quantify the background value, we tried to correlate the observed background rates in the various channels with the particle indicators like L, B, ULD count rate and veto count rate. We find that for B [FORMULA] G and L [FORMULA] the observed count rates are well correlated with the ULD count rates. In Fig. 1 we show the background count rates obtained in PPC-2 top layer, plotted against the ULD count rates. The integration time for each data point is 100 s. We find a correlation between the two with a correlation coefficient of 0.98. The relationship between the two quantities can be described by a linear relation and the best fit straight line is also shown in the figure. Similar linear relations are found between the observed background counts in different layers of all the PPCs and the following prescription is adopted for background modeling: i) take data only when B [FORMULA] G and L [FORMULA] ; ii) establish a linear relationship between the background counts and the ULD counts and iii) use this relationship for predicting the background at other times (see Agrawal et al. 1997, for details).

[FIGURE] Fig. 1. The observed background counts in PPC2r top layer are plotted against the ULD (upper level discriminator) counts, which is a measure of the particle background. The integration time for the data points is 100 s. The continuous line in the figure is a least square straight line fit.

The background subtracted count rates for each of the PPCs in each observing period of the satellite's orbit are listed in Table 1, after normalizing them to the observed count rates from Crab calibration. The average source flux increased by about a factor of 2 from the hard state to the soft state. The background subtracted count rates in 1 s time resolution is given in Fig. 2 for two of the typical observations in the hard state (top panel) and the soft state (bottom panel) of the source.

[FIGURE] Fig. 2. The background subtracted count rates from PPC-2 obtained during the hard state of Cyg X-1 (1996 May) are shown in the top panel. Count rates from the same detector obtained in 1996 July (during the soft state of Cyg X-1) are shown in the bottom panel. The time resolution is 1 s. The start time is arbitrary.

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Online publication: January 8, 1998