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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
Password Certification: Jose Luis Galache (firstname.lastname@example.org)
Subjects: Optical, X-ray, Request for Observations, Binaries, Black Holes, Neutron Stars, Pulsars, Transients
Referred to by ATEL #: 1820
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 nH = 6.67e20 atoms/cm2 (the Galactic value towards M31), Γ = 1.7 and a distance of 780 kpc.
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.
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 Lx = 1.4e36 erg/s and peaked on 24 September 2006 at Lx = 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:
nH = 2.4e21 atoms/cm2
Γ = 1.8
Tin = 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 Lx 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.