Astron. Astrophys. 357, 520-526 (2000)

2. Observations

The BeppoSAX platform carries two sets of astrophysical X-ray and -ray devices in space (Boella et al. 1997a). One pertains to two identical Wide Field Cameras (WFCs, Jager et al. 1997) that view the sky with square degrees field-of view in opposite directions with 5´ spatial resolution in the 2 to 26 keV bandpass. The other set includes the Narrow Field Instruments (NFI) that are co-aligned in a direction that is perpendicular to that of both WFCs. The NFI include two imaging instruments that are active below 10 keV, the Low-Energy and the Medium-Energy Concentrator Spectrometer (LECS and MECS respectively, see Parmar et al. 1997 and Boella et al. 1997b respectively), and two non-imaging instruments that have bandpasses of to 300 keV (the Phoswich Detector System or PDS, Frontera et al. 1997) and 4 to 120 keV (the High-Pressure Gas Scintillation Proportional Counter or HP-GSPC, Manzo et al. 1997). The MECS has a photon energy resolution of 8% (full width at half maximum) at 6 keV.

Since mid 1996, the WFCs carry out a long-term program of monitoring observations in the field around the Galactic center. The program consists of campaigns during the spring and fall of each year. Each campaign lasts about two months and typically comprises weekly observations. SAX J1819.3-2525 was detected twice during these campaigns so far, in hourly exposures above a 50 mCrab detection limit, on Feb. 20 and Sep. 10, 1999. The first WFC-detection triggered a target-of-opportunity observation (TOO) with the NFI on March 13.22-14.02, 1999 (this is 186 d before the giant outburst). SAX J1819.3-2525 was strongly detected in three NFI (the HP-GSPC was not turned on), and the average intensity was found to be about 12 mCrab (2-10 keV). The NFI net exposure times on SAX J1819.3-2525 are 10.2 ks for LECS, 27.7 ks for MECS and 14.7 ks for PDS. The LECS and MECS images show a single bright source, the position as determined from the MECS image is R.A. , Decl. = -25^o24´03" (Eq. 2000.0, error radius ) which is from that determined with the WFC (In 't Zand et al. 1999a) and from the optical counterpart V4641 Sgr.

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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