Letter to the Editor

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

Istituto di Astrofisica Spaziale e Fisica Cosmica – Sezione di Milano “G.Occhialini”, CNR, via Bassini 15, 20133 Milano, Italy e-mail: nicola@mi.iasf.cnr.it

Received: 7 October 2004
Accepted: 11 December 2004

Abstract

We report the results of an observation of the galactic black-hole binary 4U 1543-475performed by XMM-Newton on 2002 August 18, about two months after the start of an outburst detected by RXTE. Despite the relatively low flux of the source, corresponding to a luminosity  erg s, 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 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  erg s^-1, i.e. less than 10% of the source total luminosity. Finally, we evaluated that the disk colour temperature must satisfy the condition 0.25 keV in order to obtain an acceptable value for the disk inner radius.