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Issue A&A
Volume 441, Number 1, October I 2005
Page(s) L1 - L4
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:200500162



A&A 441, L1-L4 (2005)
DOI: 10.1051/0004-6361:200500162

Letter

Chandra observation of the fast X-ray transient IGR J17544-2619: evidence for a neutron star?

J. J. M. in 't Zand1, 2

1  SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
    e-mail: J.J.M.in.t.Zand@sron.nl
2  Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands

(Received 29 June 2005 / Accepted 29 July 2005 )

Abstract
IGR J17544-2619 belongs to a distinct group of at least seven fast X-ray transients that cannot readily be associated with nearby flare stars or pre-main sequence stars and most probably are X-ray binaries with wind accretion. Sofar, the nature of the accretor has been determined in only one case (SAX J1819.3-2525/V4641 Sgr). We carried out a 20 ks Chandra ACIS-S observation of IGR J17544-2619 which shows the source in quiescence going into outburst. The Chandra position confirms the previous tentative identification of the optical counterpart, a blue O9Ib supergiant at 3 to 4 kpc (Pellizza, Chaty & Negueruela, in prep.). This is the first detection of a fast X-ray transient in quiescence. The quiescent spectrum is very soft. The photon index of $5.9\pm1.2$ (90% confidence error margin) is much softer than 6 quiescent black hole candidates that were observed with Chandra ACIS-S (Kong et al. 2002, ApJ, 570, 277; Tomsick et al. 2003, ApJ, 599, L133). Assuming that a significant fraction of the quiescent photons comes from the accretor and not the donor star, we infer that the accretor probably is a neutron star. A fit to the quiescent spectrum of the neutron star atmosphere model developed by Pavlov et al. (1994, A&A, 289, 837) and Zavlin et al. (1996, A&A, 315, 141) implies an unabsorbed quiescent 0.5-10 keV luminosity of $(5.2\pm1.3)\times10^{32}$ erg s-1. We speculate on the nature of the brief outbursts.


Key words: X-rays: binaries -- X-rays: transients -- X-rays: individual: IGR J17544-2619

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