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New X-Ray Transient in M31: CXOM31 J004258.6+411527

ATEL # 1115; J.L. Galache, M.R. Garcia, D. Steeghs, M.A.P. Torres, S.S. Murray, F.A. Primini (CfA) and B.F. Williams (U. of Washington)
on 19 Jun 2007; 20:33 UT
Password Certification: Jose Luis Galache (

Subjects: X-ray, Binaries, Black Holes, Transients

We report the discovery of a previously unseen X-ray point source detected in a 10ks observation starting on 2007-06-02.93 UT as part of our ongoing Chandra/HST M31 transient program. The source was detected by ACIS-I at coordinates RA(J2000) 00:42:58.63, Dec(J2000) +41:15:27.00, with centroid position errors of 0.05" and 0.04", respectively (subject to the standard bore-sight correction).

The 0.6-5.0 keV spectrum can be modelled using an absorbed disk blackbody+powerlaw. Fixing nH = 6.67e20 atoms/cm2 (the Galactic value towards M31), we find Tin ~ 0.93 keV and a photon index of 2.6. The luminosity (assuming a distance of 780 kpc to M31) is Lx ~ 5.2e37 erg/s in the 0.5-10 keV range, with a powerlaw contribution of ~25%.

A 5ks follow up observation of the source on 2007-06-18.28 found its luminosity to have decreased to Lx ~ 3.5e37 erg/s in the above energy range. Using the same parameters as in the first observation, the 0.4-3 keV flux is well modelled by a disk blackbody with Tin ~ 0.72 keV and a powerlaw with photon index of 2.5.

The presence of a very soft component in the spectrum and the X-ray luminosity are consistent with the source being a black hole transient in a thermal (high-soft) state. Further optical and X-ray observations of this source are encouraged.

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