BeppoSAX observations of the nearby low-mass X-ray binary and fast transient SAX J1819.3-2525

SAX J1819.3-2525 is a nearby X-ray transient which exhibited a fast and lar ge X-ray outburst on Sep. 15, 1999 (Smith et al. 1999). The Wide Field Came ras and the Narrow Field Instruments (NFI) on board BeppoSAX observed SAX J 1819.3-2525 at various stages of its activity before that, in the spring an d fall of 1999. The fluxes range between 0.012 and 0.3 Crab units (2-10 keV ). The NFI observation is unique because it is the longest semi-continuous observation of SAX J1819.3-2525 so far, and it offers a study of the spectr um at a relatively high resolution of 8% full width at half maximum at 6 ke V. We discuss the observations with emphasis on the X-ray spectrum. A stron g Fe-K emission line was detected in SAX J1819.3-2525 with an equivalent wi dth between 0.3 and I keV. The line energy is up to 6.85+/-0.02 keV and sug gests the presence of highly ionized iron. We identify this as fluorescent emission from a photo-ionized plasma. The continuum spectrum is typical for a low-mass X-ray binary in which mission from an accretion disk corona pla ys an important role. There is no sign of an eclipse or periodic signal due to the binary orbit in this exposure, despite the fact that the twin jets seen at radio wavelengths suggest a high inclination angle.