An Unexpected Discovery in the Rich Open Cluster NGC 6819 Using XMM-Newton

Abstract

We present the first study of the X-ray population of the intermediate-age rich open cluster NGC 6819 using the XMM-Newton Observatory. In the past decade, Chandra X-ray observations have shown a relationship between the X-ray population of globular clusters and their internal dynamics and encounter frequency. We investigate the role dynamics possibly play in the formation of X-ray sources in NGC 6819, and compare our results with known properties of field and globular cluster X-ray populations. We implement a multi wavelength approach to studying the X-ray sources, utilizing X-ray and UV data from XMM observations along with the wealth of photometry and radial-velocity data from the WIYN Open Cluster Study (WOCS) and the CFHT Open Cluster Survey. Within the cluster half-light radius we detect 12 X-ray sources down to a luminosity of 10^30 erg/s for cluster members. The sources include a candidate quiescent low-mass X-ray binary (qLMXB), a candidate cataclysmic variable, and two active binary systems. The presence of a qLMXB in an open cluster is previously unexpected given the known relationships between luminous X-ray sources and encounter frequency in globular clusters, and most likely has a dynamical origin.

14 pages, 11 figures, accepted for publication in the Astrophysical Journal

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• The hot Gamma-Doradus and Maia stars
• CCD UBV photometry of the open cluster NGC 6819
• A Chandra X-ray study of the interacting binaries in the old open cluster NGC6791
• Barium Surface Abundances of Blue Stragglers in the Open Cluster NGC 6819
• The X-ray Emissivity of Low-Density Stellar Populations
• Chandra X-ray Observatory and Hubble Space Telescope observations of the intermediate-age cluster GLIMPSE-C01
• Time-resolved Spectroscopy of the Polar EU Cancri in the Open Cluster Messier 67
• Discovery of periodicities in two highly variable intermediate polars towards the Galactic Center
• X-ray sources in Galactic globular clusters and old open clusters
• WIYN Open Cluster Study. LXXI. Spectroscopic Membership and Orbits of NGC 6791 Sub-Subgiants
• A Chandra X-ray census of the interacting binaries in old open clusters - Collinder 261
• A Chandra X-ray census of the interacting binaries in old open clusters - NGC188
• Variable Blue Straggler Stars in Open Cluster NGC 6819 Observed in the Kepler 'Superstamp' Field