X-ray spectral identification of three candidate quiescent low-mass X-ray binaries in the globular cluster NGC 6304

Abstract

We report the search for low-mass X-ray binaries in quiescence (qLMXBs) in the globular cluster NGC 6304 using XMM-Newton observations. We present the spectral analysis leading to the identification of three candidate qLMXBs within the field of this globular cluster (GC), each consistent with the X-ray spectral properties of previously identified qLMXBs in the field and in other GCs - specifically, with a hydrogen atmosphere neutron star with radius between 5 and 20km. One (XMMU 171433-292747, with R_∞ = 11.6^+6.3_-4.6(D/5.97kpc)km and kT_eff = 122⁺³¹_-45eV is located within one core radius (r_c) of the centre of NGC 6304. This candidate also presents a spectral power-law component contributing 49 per cent of the 0.5-10keV flux. A second one (XMMU 171411-293159 with R_∞ = 10.7^+6.3_-3.1(D/5.97kpc)km and kT_eff = 115⁺²¹_-16eV) is found well outside the optical core (at ~32r_c) but still within the tidal radius. From spatial coincidence, we identify a bright 2MASS infrared counterpart which, at the distance of NGC 6304, seems to be a post-asymptotic giant branch star. The third qLMXB (XMMU 171421-292917 with R_∞ = 23⁺⁶⁹_-10(D/5.97kpc)km and kT_eff = 70⁺²⁸_-20eV) is a low signal-to-noise ratio candidate for which we also identify from spatial coincidence a bright 2MASS infrared counterpart, with 99.916 per cent confidence. Three qLMXBs from this GC is marginally consistent with that expected from the encounter rate of NGC 6304. We also report a low signal-to-noise ratio source with an unusually hard photon index (α = -2.0^+1.2_-2.2). Finally, we present an updated catalogue of the X-ray sources lying in the field of NGC 6304, and compare this with the previous catalogue compiled from ROSAT observations.