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 with 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 (PNs) 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 PNs is only ∼1%, suggesting that there is some connection between these [O III] emitters (possibly PNs) 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 a low-mass X-ray binary 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.