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ATEL # 1693; J.L. Galache, M.R. Garcia, M.A.P. Torres, S.S. Murray (CfA), D. Steeghs (U. of Warwick/CfA) and B.F. Williams (U. of Washington)
on 3 Sep 2008; 22:55 UT
Password Certification: Jose Luis Galache (firstname.lastname@example.org)
Subjects: Optical, X-ray, Binaries, Black Holes, Neutron Stars, Pulsars, Transients
Referred to by ATEL #: 1698
As part of our ongoing Chandra/HST M31 transient program we report on a Chandra ACIS observation of M31 containing the new transient source reported by Pietsch et al. (2008, ATel #1674). A single 5ks observation was performed on 2008-09-01.32 UT. The Chandra coordinates for this source (subject to the boresight correction) are RA(J2000) 00:43:20.710, Dec(J2000) +41:15:31.31.
Although we cannot rule out an absorbed power law, an absorbed disk black body with nH = 8.4e20 atoms/cm2 and Tin = 0.7 keV represents the best physically motivated model that fits the data. The unabsorbed (0.5-5 keV) luminosity (assuming a distance of 780 kpc to M31) is 1.8e38 erg/s. If nH and Tin are fixed to the above values and a power law is incorporated into the spectral model, its contribution to the source luminosity would be no greater than 15%.
Despite slightly different values of the spectral parameters, this luminosity is exactly the same as that reported in ATel #1674 from an observation made 11 days prior. This could imply that the compact object is accreting at the Eddington luminosity. If so, the compact object would have a mass of ~1.38 solar masses, close to the canonical mass of neutron stars. However, given the weak or nonexistent power law component to the spectrum, it is possible that this is a black hole in a thermal (high soft) state accreting below its Eddington limit.