X-ray spectral evolution of SAX J1747.0-2853 during outburst activity and confirmation of its transient nature

¹ Faculty of Sciences, P. J. Safarik University, Moyzesova 16, 040 01 Kosice, Slovak Republic
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 Istituto di Astrofisica Spaziale (CNR), Area Ricerca Roma Tor Vergata, via del Fosso del Cavaliere, 00133 Roma, Italy
⁵ NASA Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA
⁶ Dept. of Astronomy, University of Maryland, College Park, MD 20742, USA
⁷ Dept. of Physics and Astronomy, University of Southampton, Hampshire SO17 1BJ, UK

Corresponding author: N. Werner, werner@ta3.sk

Received: 2 September 2003
Accepted: 29 November 2003

Abstract

SAX J1747.0-2853 is an X-ray transient which exhibited X-ray outbursts yearly between 1998 and 2001, and most probably also in 1976. The outburst of 2000 was the longest and brightest. We have analyzed X-ray data sets that focus on the 2000 outburst and were obtained with BeppoSAX, XMM-Newton and RXTE. The data cover unabsorbed 2–10 keV fluxes between 0.1 and  erg s^-1 cm^-2. The equivalent luminosity range is to  erg s^-1. The 0.3–10 keV spectrum is well described by a combination of a multi-temperature disk blackbody, a hot Comptonization component and a narrow Fe–K emission line at 6.5 to 6.8 keV with an equivalent width of up to 285 eV. The hydrogen column density in the line of sight is  cm^-2. The most conspicuous spectral changes in this model are represented by variations of the temperature and radius of the inner edge of the accretion disk, and a jump of the equivalent width of the Fe–K line in one observation. Furthermore, 45 type-I X-ray bursts were unambiguously detected between 1998 and 2001 which all occurred during or close to outbursts. We derive a distance of  kpc which is consistent with previous determinations. Our failure to detect bursts for prolonged periods outside outbursts provides indirect evidence that the source returns to quiescence between outbursts and is a true transient.