Two New X-Ray Sources in M31 and M31GC J004234+405709 Observed by Chandra

ATel #1815; 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 27 Oct 2008; 22:23 UT
Credential Certification: Jose Luis Galache (jlgalache@cfa.harvard.edu)

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We report the discovery of two previously unseen X-ray sources detected in a 5ks Chandra/ACIS observation starting on 2008-10-13.15 UT as part of our ongoing Chandra/HST M31 transient program. Due to the low number of counts, no detailed spectral analysis was possible. The spectra were modelled with an absorbed power law of n_H = 6.67e20 atoms/cm² (the Galactic value towards M31), Γ = 1.7 and a distance of 780 kpc.


CXOM31 J004243.8+411612

This source was detected on the ACIS-I chip at coordinates RA(J2000) 00:42:43.80, Dec(J2000) +41:16:12.54, with a centroid position error of 0.09" (subject to the standard bore-sight correction). This places the source ~7" NW of the centre of M31. The estimated unabsorbed luminosity in the 0.3-8 keV range is 1.2e37 erg/s. There is not enough information to determine whether this source is a neutron star or a black hole in outburst. We note this source was not in outburst on 2008-09-01 at the time of our previous observation of this field with Chandra.


CXOM31 J004243.9+411610

This source was detected on the ACIS-I chip at coordinates RA(J2000) 00:42:43.93, Dec(J2000) +41:16:10.59, with centroid position errors of 0.14" and 0.12", respectively (subject to the standard bore-sight correction). This places the source ~5" NW of the centre of M31. Studying older ACIS-I observations we find that this source first appeared on 31 July 2006 at L_x = 1.4e36 erg/s and peaked on 24 September 2006 at L_x = 1.8e37 erg/s, after which it has remained at luminosities in the (4-7)e36 erg/s range. This behaviour and luminosity is consistent with the source being a black hole or a neutron star X-ray binary.

We note that Swift observed this same field on the two subsequent days after our Chandra observations (Henze et al. 2008, ATel #1806). We inspected the Swift data and conclude that their non-detection of these sources is due to the brevity of the Swift observations coupled with a larger PSF that wouldn't resolve these sources from each other, as they are ~2.4" apart.


M31GC J004234+405709 (Bol 117)

Within the same observation, we observed the M31 globular cluster Bol 117 with the ACIS-S3 chip. Its spectrum is well described by an absorbed power law plus disk black body whose parameters are:

n_H = 2.4e21 atoms/cm²
Γ = 1.8
T_in = 2.2 keV

The luminosity in the 0.3-8 keV range is 1.9e38 erg/s (assuming a distance of 780 kpc to M31). The spectrum is dominated by the power law component, with the disk contributing only 10% to the total flux. While these characteristics are consistent with a black hole transient in a hard state (as defined by McClintock & Remillard 2005), we cannot rule out the possibility that a number of neutron star X-ray binaries are in outburst simultaneously. Given the high L_x and the number of NS XRBs it would take to reach this luminosity, and the fact that past observations of this globular cluster have shown transient X-ray activity at the ~3e37 erg/s level (Williams et al. 2006, ApJ 643, 356, object s1-1), it is likely that we are observing emission from a single source. The persistent X-ray luminosity from this GC is <1e36 erg/s (Williams et al. 2006).

Further observations of these sources are encouraged.