Identifications of Faint Chandra Sources in the Globular Cluster M3

Abstract

We report a $30\mathrm{ks}$ $Chandra$ ACIS-S survey of the globular cluster M3. Sixteen X-ray sources were detected within the half-light radius ($2.3^{\prime }$) with $L_X\gtrsim 2.3\times 10^{31}\mathrm{erg}{\mathrm{s}}^{-1}$. We used $HubbleSpaceTelescope$ WFC3/UVIS and ACS/WFC images to find 10 plausible optical/UV counterparts. We fit the spectral energy distribution of the known cataclysmic variable 1E1339.8+2837 with a blue ($T_{\mathrm{eff}}=2.10_{-0.58}^{+1.96}\times 10^4\mathrm{K}$, 90% conf.) spectral component from an accretion disc, plus a red component ($T_{\mathrm{eff}}=3.75_{-0.15}^{+1.05}\times 10^3\mathrm{K}$) potentially from a subgiant donor. The second brightest source (CX2) has a soft blackbody-like spectrum suggesting a quiescent low-mass X-ray binary (qLMXB) containing a neutron star. Six new counterparts have obvious UV and/or blue excesses, suggesting a cataclysmic variable (CV) or background active galactic nucleus (AGN) nature. Two (CX6 and CX8) have proper motions indicating cluster membership, suggesting a CV nature. CX6 is blue in UV filters but red in V-I, which is difficult to interpret. Two CV candidates, CX7 and CX13, show blue excesses in B-V colour but were not detected in the UV. The other two CV candidates were only detected in the two UV bands ($\mathrm{U}{\mathrm{V}}_{275}$ and $\mathrm{NU}{\mathrm{V}}_{336}$), so do not have proper motion measurements, and may well be AGNs. One $Chandra$ source can be confidently identified with a red straggler (a star redward of the giant branch). The observed X-ray source population of M3 appears consistent with its predicted stellar interaction rate.

19 pages, 13 figures

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