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

Sebastien Guillot, Robert E. Rutledge, Lars Bildsten, Edward F. Brown, George G. Pavlov, Vyacheslav E. Zavlin

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22 Scopus citations


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 (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 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 ∼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/5.97 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.

Original languageEnglish (US)
Pages (from-to)665-681
Number of pages17
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - Jan 2009

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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