2. Beppo SAX observations and data preparation
The observations were performed with the Beppo SAX Narrow Field Instruments, LECS (0.1-10 keV, Parmar et al. 1997), MECS (1.3-10 keV, Boella et al. 1997), HPGSPC (4-60 keV, Manzo et al. 1997) and PDS (13-200 keV, Frontera et al. 1997). We report here the analysis of the imaging instruments data (LECS and MECS). PDS and HPGSPC are collimated instruments with field of view of about 1.5 degrees (FWHM) and have a rather large and structured background which makes them not sensitive enough for very faint sources.
At launch the MECS was composed of three identical units. Unfortunately on 1997 May 6th the unit MECS1 had a technical failure. All observations after this date were performed with two units (MECS2 and MECS3). The LECS is operated during spacecraft dark time only, therefore LECS exposure times are usually smaller than MECS exposures by a factor 1.5-3. The MECS energy resolution is about 8% at 6 keV, while the LECS energy resolution is about 11% at 3 keV.
Table 2 gives the journal of observations for the two high redshift clusters. For both clusters we have acquired fewer photons than anticipated (because of the unavailability of one of the three MECS in the case of MS, and because of the shorter effective exposure in the case of RXJ 0152.7-1357).
Table 2. Beppo SAX data observation journal
Standard data reduction was performed using the software package "SAXDAS" (see http://www.sdc.asi.it/software and Fiore et al. 1999). In particular, data are linearized and cleaned from Earth occultation periods and unwanted periods of high particle background (satellite passages through the South Atlantic Anomaly). We accumulated data for Earth elevation angles degrees and magnetic cut-off rigidity . Data from the two MECS units have been merged after gain equalization and single MECS spectra have been accumulated.
Both MECS and LECS source counts have been extracted from a circular region of 4 arcmin radius to maximize the statistics and the signal-to-noise (S/N) ratio.
LECS and MECS internal backgrounds depend on the position of the target in the detector (see Chiappetti et al. 1998, the Beppo SAX Cookbook, http://www.sdc.asi.it/software/cookbook and Parmar et al. 1999). Accordingly, background spectra were extracted from high Galactic latitude "blank" fields from the same source extraction region, in detector coordinates. The mean level of the background in the "blank fields" was compared with that of our observations using source-free regions at various positions in the detectors. The level of the "blank fields" background is consistent with the "local" background in the two cluster observations.
Spectral fits were performed using the XSPEC 9.0 software package and the September 1997 version of the calibration files (Ancillary Response Files and Redistribution Matrix Files). In the spectra analysis only the 1.65-10 keV band counts for the MECS (channels 37 - 220) and the 0.12-4 keV counts for the LECS (channels 11 - 400) were used as suggested by the Beppo SAX Cookbook (Fiore et al. 1999, v. 1.2). The background subtracted count rates () in both LECS (0.12-4 keV) and MECS (1.65-10 keV) are given in Table 2. Source counts were binned in order to have a S/N 3 in each energy bin. The LECS and MECS data were fitted jointly, and the model normalization for each data set were allowed to be an independent parameter, in order to take into account differences in the absolute calibration of the two instruments. If not explicitly quoted all the errors reported in this paper represent the confidence levels for 1 interesting parameter ( = 1.0).
© European Southern Observatory (ESO) 2000
Online publication: December 17, 1999