Identifications of faintChandrasources in the globular cluster M3

Abstract

We report a 30 ks Chandra ACIS-S survey of the globular cluster M3. Sixteen X-ray sources were detected within the half-light radius (2.3 ') with $L_X \gtrsim 2.3 \times 10^{31}~\mathrm{erg~s^{-1}}$⁠. We used Hubble Space Telescope WFC3/UVIS and ACS/WFC images to find 10 plausible optical/UV counterparts. We fit the spectral energy distribution of the known cataclysmic variable (CV) 1E1339.8+2837 with a blue ($T_\mathrm{eff} = 2.10^{+1.96}_{-0.58}\times 10^4~\mathrm{K}$⁠, 90 per cent 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 CV or background active galactic nucleus (AGN) nature. Two (CX6 and CX8) have proper motions (PMs) 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 (UV$_{275}$ and NUV$_{336}$), so do not have PM measurements, and may well be AGNs. One Chandra source can be confidently identified with a red straggler (a star reward of the giant branch). The observed X-ray source population of M3 appears consistent with its predicted stellar interaction rate.