Volume 441, Number 1, October I 2005
|Page(s)||L1 - L4|
|Published online||13 September 2005|
Letter to the Editor
Chandra observation of the fast X-ray transient IGR J17544-2619: evidence for a neutron star?
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
Accepted: 29 July 2005
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 (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 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
© ESO, 2005
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.