EDP Sciences
Free access
Volume 430, Number 3, February II 2005
Page(s) L53 - L56
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:200400123

A&A 430, L53-L56 (2005)
DOI: 10.1051/0004-6361:200400123


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

N. La Palombara and S. Mereghetti

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)

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 $L_{\rm X}\sim 4\times 10^{34}$ erg s $^{-1}\sim 10^{-5} L_{\rm EDD}$, 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 $\Gamma=1.9{-}2$ 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 ${\sim}10^{33}$ 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 $kT_{\rm col}<$ 0.25 keV in order to obtain an acceptable value for the disk inner radius.

Key words: accretion, accretion disks -- black hole physics -- stars: individual: 4U 1543-475, Il Lupi -- X-rays: binaries

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