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Chandra Detection of a New Transient X-Ray Source in M31: CXOM31 J004251.5+411701

ATEL # 1978; 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 19 Mar 2009; 16:31 UT
Password Certification: Jose Luis Galache (

Subjects: X-ray, Binaries, Black Holes, Neutron Stars, Pulsars, Transients

We report the discovery of a previously unseen X-ray source detected in a 5ks Chandra/ACIS observation starting on 2009-03-11.59 UT as part of our ongoing Chandra/HST M31 transient program. Its coordinates (subject to the standard boresight correction) are R.A.(J2000) 00:42:51.59, Dec(J2000) +41:17:01.45, with centroiding errors of 0.12" and 0.09", respectively. It is therefore identified as CXOM31 J004251.5+411701.

Its spectrum is well described by an absorbed power law with Γ = 2.0 and nH = 6.67e20 atoms/cm2 (the Galactic absorption towards M31). The luminosity in the 0.3-8 keV range (assuming a 780 kpc distance to M31) is 2.4e37 erg/s, with no photons having been detected above 6 keV.

The soft spectrum suggests that this new transient may be a black hole transient in the hard state, although the power law does not rule out the possibility of it being a neutron star transient.

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