A Chandra High-Resolution Camera Observation of X-Ray Point Sources in M31

Abstract

We present results from a 47 ks observation of the Andromeda galaxy, M31, using the High-Resolution Camera of the Chandra X-Ray Observatory. We detect 142 point sources spanning three orders of magnitude in luminosity, from $L_X = 2\times 10^{35} \rm erg s^{-1}$ to $L_X = 2\times 10^{38} \rm erg s^{-1}$ in the 0.1-10 keV band. The X-ray source location accuracy is better than $1\arcsec$ in the central regions of the galaxy. One source lies within $1.3\arcsec$ of SN 1885 but does not coincide with the UV absorption feature identified as the supernova remnant. However, there is an optical transient, which is likely an optical nova, at the location of the X-ray source. There is a weak source, $L_X \sim 4 \times 10^{36} \rm erg cm^{2} s^{-1}$, coincident with the nucleus of M31, and 14 sources coincident with globular clusters. Our observation has very high efficiency down to luminosities of $1.5\times 10^{36} \rm erg s^{-1}$ for sources within $5\arcmin$ of the nucleus. Comparing with a ROSAT observation made 11 years earlier, we find that $0.46 \pm 0.26$ of the sources with $L_X > 5 \times 10^{36} \rm erg s^{-1}$ are variable. We find no evidence for X-ray pulsars in this region, indicating that the population is likely dominated by low-mass X-ray binaries. The source density radial profile follows a powerlaw distribution with an exponent of $1.25 \pm 0.10$ and is inconsistent with the optical surface brightness profile. The x-ray point source luminosity function is well fitted by a differential broken powerlaw with a break at a luminosity of $(4.5^{+1.1}_{-2.2}) \times 10^{37} \rm erg s^{-1}$. The luminosity function is consistent with a model of an aging population of X-ray binaries.