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ATEL # 1812; Michael Wolff, Paul Ray, Kent Wood (NRL), Rudy Wijnands (University of Amsterdam)
on 24 Oct 2008; 20:32 UT
Password Certification: Michael T. Wolff (Michael.Wolff@nrl.navy.mil)
Subjects: X-ray, Binaries, Neutron Stars, Transients
Referred to by ATEL #: 1816
EXO0748-676, a low mass X-ray binary system that shows regular total X-ray eclipses with a period of 3.82 hours, until August 2008 maintained a relatively steady X-ray flux of ~8 mcrab implying a luminosity of ~1.3E36 ergs/s (2-10 keV) at a distance of 7.7 kpc (Wolff et al. 2005, ApJ, v.632, p.1099). It began to fade in brightness during August-September 2008 (Wolff et al. Atel#1736). A Swift X-Ray Telescope (XRT) observation done on September 28, 2008 found a faint, soft X-ray source at the position of EXO0748-676 with a flux of approximately 1.2E-12 ergs/cm^2/s (0.5-5.0 keV ) corresponding to a luminosity of approximately 8.5E33 ergs/s at 7.7 kpc. A new set of observations done on October 5, 2008, using the Proportional Counter Array (PCA) on the Rossi X-ray Timing Explorer (RXTE) satellite failed to detect EXO0748-676. A series of monitoring observations of eclipse cycles N=54181-54185 [predicted mid-eclipse times: MJD(UT)=54744.15676-54744.79411] in the numbering system of Parmar et al. (1991, ApJ, v.366, p.253) failed to detect any signs of the ingress or egress transitions. An off-eclipse observation between cycles N=54181-54182 resulted in a background-subtracted 2-20 keV count rate consistent with zero (0.2 +/- 3.2 counts/s, 2 PCUs, all layers) and also consistent with the Swift/XRT result.
Mass accretion onto the neutron star in the EXO0748-676 system appears to have stopped and the system has gone into quiescence. We encourage observations in optical and infrared bands if at all possible. If the lack of X-ray activity has been brought about by the cessation of mass transfer between the binary components, the secondary star may be visible without any interfering emission from the accretion disk surrounding the neutron star for the first time since this source was discovered in 1985. Direct observations of the secondary star may make possible an independent determination its properties, the value of the system mass function, and the masses of the individual stellar components.