Why are X-Ray Sources in the M31 Bulge So Close to Planetary Nebulae?

We compare a deep (37 ks) Chandra ACIS-S image of the M31 bulge to deep [OIII] Local Group Survey data of the same region. Through precision imagealignment using globular cluster X-ray sources, we are able to improveconstraints on possible optical/X-ray associations suggested by previoussurveys. Our image registration allows us to rule out several emission-lineobjects, previously suggested to be the optical counterparts of X-ray sources,as true counterparts. At the same time, we find six X-ray sources peculiarlyclose to strong [O III] emission-line sources, classified as planetary nebulae(PNe) by previous optical surveys. Our study shows that, while the X-rays arenot coming from the same gas as the optical line emission, the chances of thesesix X-ray sources lying so close to cataloged PNe is only ~1%, suggesting thatthere is some connection between these [O III] emitters (possibly PNe) and theX-ray sources. We discuss the possibility that these nebulae are misidentifiedsupernova remnants, and we rule out the possibility that the X-ray sources areejected X-ray binaries. There is a possibility that some cases involve a PN andan LMXB that occupy the same undetected star cluster. Beyond this unconfirmedpossibility, and the statistically unlikely one that the associations arespatial coincidences, we are unable to explain these [O III]/X-rayassociations.Comment: 21 pages, 3 figures, 2 tables, accepted for publication in A