Identification of X‐Ray Point Sources and a Study of the Nature of 62 X‐Ray Globular Cluster Candidates in M31

Abstract

ABSTRACTThis paper includes two parts. The first presents the spectral energy distributions (SEDs) of 49 globular cluster (GC) X‐ray sources in 13 BATC (Beijing‐Arizona‐Taiwan‐Connecticut) intermediate‐band filters from 3800 to 10000 Å, and identifies 8 previously unidentified X‐ray sources in M31. Using X‐ray data from Einstein observations from 1979 to 1980, ROSAT High Resolution Imager observations in 1990, Chandra High Resolution Channel and ACIS‐I observations from 1999 to 2001, and the BATC optical survey from 1995 to 1999, we find 49 GC X‐ray sources and 8 new X‐ray sources in the BATC M31 field. By analyzing SEDs and FWHMs, we determine that 4 of the 8 X‐ray sources may be GC candidates. The second part presents some statistical relationships between 62 GC X‐ray sources, of which 58 are already known and 4 are identified in this paper. The distribution of M31's GC X‐ray sources’ V magnitudes is bimodal, with peaks at mv = 15.65 and 17.89, which is different from the distribution of GC candidates. The distribution of B - V color shows that the GC X‐ray sources seem to be associated preferentially with the redder GCs, in agreement with previous results. Kolmogorov‐Smirnov test shows that the maximum value of the absolute difference of B - V distributions of GC X‐ray sources and GCs is Dmax = 0.181, and the probability P = 0.068, which means we can reject the hypothesis that the two distributions are the same at the 90.0% confidence level. Finally, we study the correlation between X‐ray luminosity (0.3–10 keV) and optical luminosity (in the V band) of the GC X‐ray sources in M31, and find that there exits a weak relationship with the linear correlation coefficient r = 0.36 at a confidence level of 98.0%.