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 20 km. One (XMMU 171433−292747, with R∞= 11.6+6.3−4.6 (D/5.97 kpc) km and kTeff= 122+31−45 eV is located within one core radius (rc) of the centre of NGC 6304. This candidate also presents a spectral power-law component contributing 49 per cent of the 0.5–10 keV flux. A second one (XMMU 171411−293159 with R∞= 10.7+6.3−3.1 (D/5.97 kpc) km and kTeff= 115+21−16 eV) is found well outside the optical core (at ∼32rc) 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/5.97 kpc) km and kTeff= 70+28−20 eV) 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.