We report the search for low-mass X-ray binaries in quiescence ( qLMXBs ) in the globular cluster ( GC ) NGC 6304 using XMM-Newton observations .
We present the spectral analysis leading to the identification of three candidate qLMXBs within the field of this 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 5–20 { km } .
One ( XMMU 171433 - 292747 , with \mbox { $R _ { \infty } $ } = 11.6 \mbox { $ { } ^ { +6.3 } _ { -4.6 } $ } ( D/5.97 { kpc } ) { km } and \mbox { $kT _ { eff } $ } = 122 \mbox { $ { } ^ { +31 } _ { -45 } $ } \mbox { $ { eV } $ } 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 0.5–10 { keV } flux .
A second one ( XMMU 171411 - 293159 with \mbox { $R _ { \infty } $ } = 10.7 \mbox { $ { } ^ { +6.3 } _ { -3.1 } $ } ( D/5.97 kpc ) km and \mbox { $kT _ { eff } $ } = 115 \mbox { $ { } ^ { +21 } _ { -16 } $ } eV ) is found well outside the optical core ( at \sim 32 r _ { 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 \mbox { $R _ { \infty } $ } = 23 \mbox { $ { } ^ { +69 } _ { -10 } $ } ( D/5.97 { kpc } ) { km } and \mbox { $kT _ { eff } $ } = 70 \mbox { $ { } ^ { +28 } _ { -20 } $ } \mbox { $ { eV } $ } ) is a low signal-to-noise 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 source with an unusually hard photon index ( \alpha = -2.0 \mbox { $ { } ^ { +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 .