Intriguing Sources from the XMM-Newton Survey of Rich Open Clusters
American Astronomical Society, AAS Meeting #220, #121.05
Published in May 2012
We present results of our uniform survey of low-luminosity (〈10^33 erg/s) X-ray sources in open clusters using XMM-Newton. Previous Chandra X-ray observations have shown a relationship between the number of X-ray sources in globular clusters and the cluster dynamical frequency. We investigate if a similar link exists in open clusters, utilizing X-ray, UV, and optical photometry and radial-velocity data. From 8 open clusters in our study we find a total of 170 X-ray sources, with 60-65 expected background sources. In our sample, 13 sources have luminosities above 2x10^31 erg/s. Six of the 13 have soft spectra, as shown through their X-ray color, and luminosities similar to quiescent LMXBs in globular clusters, including the previously detected quiescent LMXB candidate in NGC 6819. We also find 7 sources with hard spectra, whose X-ray colors and luminosities are similar to cataclysmic variables. We present the open cluster survey X-ray CMD, and discuss the possibility that the soft sources are quiescent LMXB candidates and may be a new class of X-ray objects to be found in open cluster environments. The presence of quiescent LMXBs in open clusters, if formed dynamically, may have important implications for understanding cluster dynamical environments.This work is supported by National Science Foundation grant AST-0908082.
(c) 2012: American Astronomical Society