Why are X-ray sources in the M31 Bulge so close to Planetary Nebulae?

Abstract

We compare a deep (37 ks) Chandra ACIS-S image of the M31 bulge to deep [O III] Local Group Survey data of the same region. Through precision image alignment using globular cluster X-ray sources, we are able to improve constraints on possible optical/X-ray associations suggested by previous surveys. Our image registration allows us to rule out several emission-line objects, previously suggested to be the optical counterparts of X-ray sources, as true counterparts. At the same time, we find six X-ray sources peculiarly close to strong [O III] emission-line sources, classified as planetary nebulae (PNe) by previous optical surveys. Our study shows that, while the X-rays are not coming from the same gas as the optical line emission, the chances of these six X-ray sources lying so close to cataloged PNe is only ~1%, suggesting that there is some connection between these [O III] emitters (possibly PNe) and the X-ray sources. We discuss the possibility that these nebulae are misidentified supernova remnants, and we rule out the possibility that the X-ray sources are ejected X-ray binaries. There is a possibility that some cases involve a PN and an LMXB that occupy the same undetected star cluster. Beyond this unconfirmed possibility, and the statistically unlikely one that the associations are spatial coincidences, we are unable to explain these [O III]/X-ray associations.