XMM-Newton observation of 4U 1543-475: the X-ray spectrum of a stellar-mass black-hole at low luminosity

We report the results of an observation of the galactic black-hole binary 4U 1543-475 performed by XMM-Newton on 2002 August 18, about two months after the start of an outburst detected by Rossi-XTE. Despite the relatively low flux of the source, corresponding to a luminosity Lx ~ 4x10^34 erg/s (i.e. about 10^(-5) times the Eddington luminosity), we could obtain a good quality spectrum thanks to the high throughput of the XMM-Newton EPIC instrument. The spectrum is well fit by a power law with photon index 1.9-2 without any evidence for iron emission lines or for thermal emission from an accretion disk. We could estimate an upper-limit on the disk bolometric luminosity as a function of the colour temperature: it is always lower than ~10^33 erg/s, i.e. less than 10 % of the source total luminosity. Finally, we evaluated that the disk colour temperature must satisy the condition kTcol<0.25 keV in order to obtain an acceptable value for the disk inner radius.