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

¹ CNR-Istituto di Astrofisica Spaziale e Fisica Cosmica, Area Ricerca Roma 2/Tor Vergata, via del Fosso del Cavaliere 100, 00133 Roma, Italy
² SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
³ Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
⁴ Dept. of Physics and Astronomy, University of Southampton, Hampshire SO17 1BJ, UK

Corresponding author: L. Natalucci, lorenzo@rm.iasf.cnr.it

Received: 29 October 2003
Accepted: 4 December 2003

Abstract

The neutron star binary SAX J1747.0–2853, located in the Galactic Center region at about 0.5 deg from Sgr A* and at 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 cutoff of ~70 keV. About two years later the source reappeared about one order of magnitude brighter in the X-rays (0.5–10 keV) and with a significantly steeper spectrum. As was the case for the low state, the data could be fitted by an input model based on two continuum 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 a power-law or a comptonization spectrum. The soft component is equally well described by pure blackbody or multi-color disk emission, with significantly higher temperature than observed in 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. greater than ~50% in the 2–10 keV band.