Spectral states and transient behaviour of a sample of X-ray bursters observed by BeppoSax

Abstract

During observation campaigns of the Galactic Bulge region, BeppoSAX detected a total of 21 new X-ray bursters in about 6 years of operation. These sources are mostly transient and often feature a hard X-ray spectrum, extending up to ~ 200 keV. A hard spectrum is generally found in weak, relatively short outbursts. On the other hand two sources, SAX J1747.0-2853 and SAX J1750.8-2900, have been seen with soft X-ray transient behaviour. In both low/hard and high/soft state, a thermal component is observed at energies below \~10 keV and the spectra are compatible with the same model consisting of a blackbody or disk blackbody plus a hard or Comptonized component. Light curves are characterized by either weak, short flares or longer, high luminosity eruptions with exponential decay.