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3 New X-Ray Transients in M31

ATEL # 1171; J.L. Galache, M.R. Garcia, D. Steeghs, M.A.P. Torres, S.S. Murray (CfA) and B.F. Williams (U. of Washington)
on 7 Aug 2007; 22:18 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Jose Luis Galache (jlgalache@cfa.harvard.edu)

Subjects: X-ray, Request for Observations, Binaries, Black Holes, Transients

We report the discovery of 3 previously unseen X-ray point sources detected in a 5ks Chandra/ACIS observation starting on 2007-07-31.14 UT as part of our ongoing Chandra/HST M31 transient program. The sources were detected at the edge of the FoV, on the ACIS-S chips, which explains the larger than usual positional errors.

Notes:
- All coordinates are subject to the standard bore-sight correction, which in our case is less than our centroiding errors.
- Distance to M31 assumed to be 780 kpc.
- Galactic absorption towards M31 assumed to be 6.67e20 atoms/cm2.

CXOM31 J004100+411531.5
Coordinates: RA(J2000) 00:41:00.0, Dec(J2000) +41:15:31.5, with centroid position errors of 4.0" and 5.0", respectively.
The spectrum is best modeled using an absorbed disk blackbody (after ascertaining a negligible contribution from a powerlaw to the flux). We find nH = 7.9e20 atoms/cm2 (note this is slightly higher than the Galactic value towards M31) and Tin ~ 0.56 keV. The luminosity at the distance of M31 is Lx ~ 2.2e37 erg/s in the 0.5-10 keV range.

CXOM31 J004145+411108.8
Coordinates: RA(J2000) 00:41:45.1, Dec(J2000) +41:11:08.8, with centroid position errors of 3.6" and 3.9", respectively.
Spectral fitting was not performed on this source given the low number of counts available (< 100). Using the method described in Prestwitch et al. (2003, ApJ 595, 719) we find its hardness ratios to be HR1 = 0.174 and HR2 = -0.453. These values place the source near the edge of the LMXB region defined by Prestwitch et al. Assuming an absorbed power law spectrum with photon index of 2, at the distance of M31, the unabsorbed luminosity of this source would be Lx ~ 8e36 erg/s.

CXOM31 J004210+410434.1
Coordinates: RA(J2000) 00:42:10.9, Dec(J2000) +41:04:34.1, with centroid position errors of 2.7" and 2.0", respectively.
The spectrum is best modeled using an absorbed disk blackbody with nH fixed at the Galactic value towards M31 (after ascertaining a negligible contribution from a powerlaw to the flux). We find Tin ~ 1.5 keV and Lx ~ 2.5e37 erg/s in the 0.5-10 keV range at the distance of M31.

The presence of very soft components in the spectrum and the X-ray luminosity are consistent with these sources being black hole transients in a thermal (high-soft) state. Further optical and X-ray observations of these sources are encouraged.


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