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Chandra Detection of a New Transient X-Ray Source in M31: CXOM31 J004245.9+411036; and Non-Detection of SWIFT J004320.5+411528

ATel #1859; 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 4 Dec 2008; 01:47 UT
Credential Certification: Jose Luis Galache (jlgalache@cfa.harvard.edu)

Subjects: X-ray, Binary, Black Hole, Neutron Star, Transient, Pulsar

Referred to by ATel #: 1870

We report the discovery of a previously unseen X-ray source detected in a 5ks Chandra/ACIS observation starting on 2008-11-27.72 UT as part of our ongoing Chandra/HST M31 transient program. Its coordinates (subject to the standard boresight correction) are RA(J2000) 00:42:45.96, Dec(J2000) +41:10:36.60, with centroiding errors of 0.18" and 0.21", respectively. It is therefore identified as CXOM31 J004245.9+411036.

Its spectrum is well described by an absorbed power law with Γ = 1.8 and nH = 2.4e21 atoms/cm2. The luminosity in the 0.3-8 keV range is 5.0e37 erg/s, with no photons having been detected above 3 keV.

The presence of a soft component 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.


During this same observation, SWIFT J004320.5+411528 (Pietsch et al. 2008, ATel #1674; Galache et al. 2008, ATel #1693) was in the field of view of ACIS-I. We do not detect it above our limiting luminosity of ~5e35 erg/s. It was last observed to be active on 2008-10-21 (Henze et al. 2008, ATel #1806). We present a table with the known observations of this transient and its X-ray luminosities.

Date (UT) Lx (×1e38 erg/s)
2008-08-21.31 1.8*
2008-08-28.68 2.8
2008-09-01.32 1.8**
2008-09-04.23 2.5
2008-09-06.59 2.1
2008-09-11.48 2.4
2008-09-25.51 1.6
2008-10-13.15 1.6***
2008-10-14.74 1.4
2008-10-15.15 1.2
2008-10-21.63 1.1
2008-11-27.72 <0.005****

Table Notes: Lx values estimated from SWIFT count rates (reported in ATel #1806) via PIMMS assuming an absorbed power law with nH = 8.4e21 atoms/cm2 (ATel #1693) and a canonical Γ = 1.7, except:
*: ATel #1674
**: ATel #1693
***: Chandra Obs.ID. 9524
****: Present ATel