Two spectral states of the transient X-ray burster SAX J1747.0-2853

Dept. of Physics and Astronomy, University of Southampton, Hampshire SO17 1BJ, U.K.Received; Accepted 04/12/2003Abstract. The neutron star binary SAX J1747.0-2853, located in the Galactic Center region at about 0.5 deg from Sgr A* andat a distance of ∼ 9 kpc, has been observed in outburst four times (1998, 1999, 2000 and 2001) by BeppoSAX and RossiXTE.At the time of its discovery in 1998 the source was observed in a low/hard state, showing a hard tail with a high energy cutoffof ∼ 70 keV. About two years later the source reappeared about one order of magnitude brighter in the X-rays (0.5-10 keV) andwith a significantly steeper spectrum. As was the case for the low state, the data could be fitted by an input model based on tw ocontinuum primary components: a) a soft thermal excess, which is ∼ 4 times more luminous than the one found in hard state;b) a non-thermal component which is compatible with either apower-law or a comptonization spectrum. The soft componentis equally well described by pure blackbody or multi-color disk emission, with significantly higher temperature than ob servedin low state (∼ 1.3 vs. the ∼ 0.5 keV assuming pure blackbody). For this model, the flux of the non-thermal component below∼ 10 keV is a significant fraction of the total X-ray flux, i.e. gr eater than ∼ 50% in the 2-10 keV band.Key words. binaries: close, individual (SAX J1747.0-2853) — X-rays: bursts