Chandra and XMM-Newton identify ~50 black hole binary candidates in M31
Abstract
We have identified ~50 X-ray binaries (XBs) containing black hole candidates in M31, falling into two groups. The first group exhibited characteristic "hard state" properties at luminosities that exceed the upper threshold for neutron star (NS) XBs; furthermore, their long-term variability, proximity to a globular cluster, or particularly high flux meant that they were highly unlikely to be background galaxies. The second group consists of bright transient X-ray sources that resemble Galactic black hole binaries with low mass donors. A double thermal (disk blackbody + blackbody) emission model has successfully described the full gamut of NS XB spectra, with two exceptions: the hard state, and the horizontal branch of the "Z-source" subclass of NS XBs. Since many of our BHCs are apparently in the hard state, they exist outside the NS parameter space when fitted with the double thermal model. Furthermore, transient BH XBs often exhibit a "thermally dominated" state in outburst that is never observed in NS XBs. For each of our BHCs we estimate the probabiliy that its emission spectrum is consistent with the NS parameter space: BHCs with probability < 0.27% of being consistent with a NS spectrum (equivalent to 3σ difference) are rated as Strong BHCs, and the others are "plausible" BHCs.
- Publication:
-
American Astronomical Society Meeting Abstracts #223
- Pub Date:
- January 2014
- Bibcode:
- 2014AAS...22342301B