2005/08/10 by J. J. M. in 't Zand · 148 citations
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysical Phenomena and Observations #Black hole (networking) #Luminosity #Neutron star #Photon #Pulsars and Gravitational Waves Research #Spectral line #Stars #Supergiant #Transient (computer programming) #astro-ph
paper · pdf · doi:10.1051/0004-6361:200500162
published in Astronomy and Astrophysics 441(1), L1-L4 (EDP Sciences) · accepted for publication in Astronomy & Astrophysics
arxiv created 2005/08/10 · openalex publication_date 2005/09/13 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
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.