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

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 J1819.3-25252525/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 +/- 1.2 (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 +/- 1.3) x 10(32) erg s(-1). We. speculate on the nature of the brief outbursts.