Black hole hunting in the Andromeda Galaxy

We present a new technique for identifying stellar mass black holes in low mass X-ray binaries (LMXBs), and apply it to XMM-Newton observations of M31. We examine X-ray time series variability seeking power density spectra (PDS) typical of LMXBs accreting at a low accretion rate (which we refer to as Type A PDS); these are very similar for black hole and neutron star LMXBs. Galactic neutron star LMXBs exhibit Type A PDS at low luminosities (10 36-1037 erg/s) while black hole LMXBs can exhibit them at luminosities >1038 erg s-1. We propose that Type A PDS are confined to luminosities below a critical fraction of the Eddington limit, lc that is constant for all LMXBs; we have examined a sample of black hole and neutron star LMXBs and find they are all consistent with lc ∼ 0.1 in the 0.3-10 keV band. We present luminosity and PDS data from 167 observations of X-ray binaries in M31 that provide strong support for our hypothesis. Since the theoretical maximum mass for a neutron star is 3.1 M ⊙, we therefore assert that any LMXB that exhibits a Type A PDS at a 0.3-10 keV luminosity greater than 4×1037 erg s -1 is likely to contain a black hole primary.