3. Observations and data reduction
A description of the BeppoSAX observatory is given in Boella et al. (1997a). The payload instruments include four co-aligned narrow field instruments: a Low Energy Concentrator Spectrometer (LECS, Parmar et al. 1997), three Medium Energy Concentrator Spectrometers (MECS, Boella et al. 1997b), a High Pressure Gas Scintillation Proportional Counter (HPGSPC, Manzo et al. 1997) and a Phoswich Detector System (PDS, Frontera et al. 1997). Both LECS and MECS spectrometers have imaging capabilities (angular resolution 1.2 arcmin, FWHM) and cover the 0.1-10 keV and 1.5-10 keV spectral bands, respectively (in the overlapping spectral region the MECS are three times more sensitive than LECS). Their energy resolution is about 8% at 6 keV. HPGSPC and PDS operate in the 4-120 keV and 15-300 keV spectral bands, respectively. In the overlapping region the PDS is more sensitive (by a factor of 4-8), while the HPGSPC has superior energy resolution.
Table 1 lists the sources observed in our program so far, along with the on-source total integration time and net count rate (i.e. background subtracted) for each instrument. One of the MECS units stopped working in May 1997, as a consequence MCG-05-18-002 was observed with two MECS units only.
Table 1. Observations log
For all sources, but NGC 5643, LECS and MECS spectra were extracted from an aperture of 8´ and 4´ in radius, since these apertures were found to optimize the signal-to-noise ratios for these faint sources. For NGC 5643 we chose an extraction radius of 1´, since outside of this radius the observed flux is affected by the emission of a nearby galaxy cluster (see Sect. 4.4). Another exception is NGC 1386, where we used for the LECS the same extraction aperture used for the MECS (i.e. 4´ in radius) to avoid different contributions to the soft X-ray flux from the surrounding Fornax cluster. The background was extracted by using the same apertures in blank sky observation files.
The PDS consists of four phoswich units, two pointing at the source and the other two 210´ away. The two pairs switch on and off source every 96 seconds. The net count rate is obtained by subtracting the "off" counts from the "on" counts.
We rebinned the spectra to have a minimum of 10 counts per channel (source + background), so that statistics can be treated in the Poissonian limit and the energy resolution is not affected significantly.
We did not detect short term variability in any of our sources. However, the signal-to-noise is too low to provide stringent constraints since these sources are weak and the integration times relatively short.
© European Southern Observatory (ESO) 1998
Online publication: September 17, 1998