Astron. Astrophys. 325, L13-L16 (1997)

2. Data reduction

The X-ray satellite BeppoSAX (Boella et al. 1997a), a program of the Italian space agency (ASI) with participation of the Netherlands agency for Aereospace Program (NIVR), includes four co-aligned Narrow Field Instruments: a Low Energy Concentrator Spectrometer (LECS), three Medium Energy Concentrator Spectrometers (MECS), a High Pressure Gas Scintillation Proportional Counter (HPGSPC), and a Phoswich Detector System (PDS). The LECS and MECS have imaging capabilities and cover the 0.1-10 keV and 1.3-10 keV energy ranges respectively; in the overlapping band the total effective area of the MECS (which is 150 cm² at 6 keV) is about three times that of the LECS. The energy resolution is 8% and the angular resolution is 1.2 arcmin (Half Power radius) at 6 keV for both instruments. The HPGSPC and the PDS are collimated instruments covering the 4-120 keV and the 15-300 keV energy ranges respectively. In the overlapping band the PDS is more sensitive, the best characteristic of the HPGSPC being instead the good energy resolution.

BeppoSAX observed the source from December 30 1996 to January 3 1997 for about 110 ks effective time. In this paper we present only MECS (Boella et al. 1997b) and PDS (Frontera et al. 1997) data, because of our choice to restrict the analysis to energies greater than 4 keV (see next section).

MECS spectra have been extracted from a 4 arcmin radius region around the centroid of the source image; the spectra from the three units have been equalized to the MECS1 energy-PI relationship and added together. Since the MECS background is very low and stable ², data selection is straightforward; we used all data acquired with an angle, with respect to the Earth limb, higher than . The background subtraction has been performed using blank sky spectra extracted from the same region of the detector field of view.

The PDS consists of four units, and was operated in collimator rocking mode, with a pair of units pointings to the source and the other pair pointings arcmin away, the two pairs switching on and off source every 96 seconds. The net source spectra have been obtained by subtracting the 'off' to the 'on' counts. Then the spectra of the four crystals have been summed together after performing gain equalization. A slight improvement in the S/N has been obtained by optimizing the Rise Time selection: only events in the range 5-133 (digital units; see Fig. 2 of Frontera et al. 1997) have been used. This choice guarantees no reduction in the detection efficiency due to pulse shape analysis. Isolated spikes, likely due to short particle "bursts", have been removed. Data within 5 minutes after any South Atlantic Anomaly passage have been discarded to avoid gain problems due to high voltage recovery after instrument switch-off. The total effective on-source time is =65000 s. The net count rate (CR) is cts/s, corresponding to a 20-200 keV flux 1.3 mCrab if a simple power-law with =2 is assumed (note that this flux is well below the OSSE and SIGMA upper limits, McNaron-Brown et al., in preparation, Jourdain et al. 1994).

As a further check, we have divided the whole observation in three temporal segments; the net count rates are all consistent one another: cts/s ( 22400 s), cts/s ( 20300 s), cts/s ( 21800 s).

Deep observations of blank fields show that the systematic residuals in the background subtraction procedure are at most 0.25 mCrab in the 20-200 keV energy range where the PDS response matrix is currently well calibrated. Even if this maximum value were subtracted from the observed count rate, the S/N would remain .

We have also checked for the presence of contaminating sources in the PDS field of view. No source at a flux level 0.03 mCrab in the 2-10 keV band is reported in X-ray catalogs. In particular, no source is present neither in the PDS field of view nor in an annulus at angle arcmin from the pointing position in the hard X-ray extragalactic sources catalog of Malizia & Bassani (1996).

© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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