2. Data reduction
The Italian-Dutch satellite BeppoSAX carries four co-aligned Narrow Field Instruments. Two of them are gas scintillation proportional counters with imaging capabilities: the Low Energy Concentrator Spectrometer (LECS, 0.1-10 keV, Parmar et al. 1997) and the Medium Energy Concentrator Spectrometer (MECS, 1.8-10.5 keV, Boella et al. 1997b). The other two instruments are direct-view detectors, seen through a rocking collimator to achieve a continuous monitoring of the background: the High Pressure Gas Scintillator Proportional Counter (HPGSPC, 4-120 keV, Manzo et al. 1997) and the Phoswitch Detector System (PDS, 13-200 keV, Frontera et al. 1997). The HPGSPC is tuned for spectroscopy of very bright sources with good energy resolution, while the PDS possesses an unprecedented sensitivity in its energy bandpass. Only LECS, MECS and PDS data will be considered in this paper . The HPGSPC data points could not in fact provide significant constraints on the spectral shape of NGC 4593.
NGC4593 was observed by BeppoSAX from 1997 December 31 05:30:18 UT to 1998 January 2 07:10:17 UT. Data were telemetred in direct modes for all instruments, which provide full information about the arrival time, energy, burst length/rise time (RT) and, when available, position for each photon. Standard reduction procedures and screening criteria have been adopted to produce linearized and equalized event files. In particular, time intervals have been excluded from the scientific product accumulation when: the angle between the pointing direction and the Earth's limb was ; the momentum associated to Geomagnetic Cutoff Rigidity was GeV/c; the satellite passed through the South Atlantic Geomagnetic Anomaly (SAGA). The PDS data have been further screened by eliminating 5 minutes after any SAGA passage to avoid gain instabilities due to the recovery to the nominal voltage value after instrumental switch-on. The RT selection has been performed using crystal temperature dependent thresholds (instead of the fixed thresholds in the standard processing). Total exposure times were 31426 s, 84195 s, and 38467 s for the LECS, MECS and PDS, respectively.
Spectra of the imaging instruments have been extracted from circular regions of radius 8´ and 6´ around the apparent centroid of the source for the LECS and MECS, respectively. The background subtraction has been performed using spectra extracted from blank sky event files in the same region as the source. The background contributes 5% and 3% to the LECS and MECS count rates in the relevant energy band passes. PDS background-subtracted products (spectra and light curves) have been produced by plain subtraction of the "off-" from the "on-source" products. Total net count rates were s-1, s-1 and s-1 in the LECS, MECS and PDS, respectively.
In this paper : errors are quoted at 90% level of confidence for two interesting parameters (, Lampton et al. 1976); energies are in the source rest frame; the cosmological parameters and km s-1 Mpc-1 are assumed, unless otherwise specified.
© European Southern Observatory (ESO) 1999
Online publication: May 21, 1999