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ATEL # 1736; Michael T. Wolff(NRL), Paul S. Ray(NRL), Kent S. Wood(NRL)
on 24 Sep 2008; 14:12 UT
Password Certification: Michael T. Wolff (Michael.Wolff@nrl.navy.mil)
Subjects: X-ray, Binaries, Neutron Stars
We regularly monitor the X-ray eclipses of the LMXB source EXO0748-676 with the Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array (PCA). EXO0748-676 was discovered by the EXOSAT satellite in February 1985 (Parmar et al. 1985, IAUC # 4039 ). The compact object is known to be a neutron star since it shows X-ray bursts (Gottwald et al. 1986, ApJ, v.308, p.213). Since the beginning of the RXTE mission in 1996 this source has maintained a low but consistent flux (but also with occasional flares) near 8 mcrab in the 2-20 keV energy range. Recent measurements (ObsIDs 92040-05-01-00 - 92040-05-04-00, August 12, 2008) of the X-ray flux from EXO0748-676 by the RXTE PCA, however, show that EXO0748-676 is currently at a very low flux level. During these observations, the PCA count rate in the post-egress 2-20 keV energy range (layer 1 only) was roughly 7.5 counts/s/PCU. Past PCA count rates are usually above 15 counts/s/PCU in this energy band. Fits to the post-egress X-ray spectrum show the flux level is 6.83E-11 ergs/cm^2/s or about 3.4 mcrab in the 3-12 keV energy band in mid-August 2008. During September 2008, the flux level appears to be near the detection threshold of the RXTE All Sky Monitor.
If EXO0748-676 ceases X-ray activity after being a steady accreting X-ray binary since February 1985 then it is essential to look at the source both as it turns off and in the period after it ceases X-ray activity. Past observations of EXO0748-676 have yielded a result for the neutron star mass near 2.1 solar masses (Ozel 2006, Nature, v.441, p.1115) based on spectra from combined X-ray burst spectra (Cottam et al. 2002, Nature, v.420, p.51) and results from Eddington-limited X-ray burst measurements (Wolff et al. 2005, ApJ, v.632, p.1099). This high mass estimate should be confirmed in as many independent ways as possible. The cessation of accretion onto the neutron star in this system may make possible the observation of the neutron star surface directly, which, in turn, will possibly constrain the equation of state of neutron star matter.