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

作者：Guillot Sebastien^*; Rutledge Robert E; Bildsten Lars; Brown Edward F; Pavlov George G; Zavlin Vyacheslav E

来源：Monthly Notices of the Royal Astronomical Society, 2009, 392(2): 665-681.

DOI：10.1111/j.1365-2966.2008.14076.x

• stars: neutron
• globular clusters: individual: NGC 6304
• X-rays: binaries

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 infinity = 11.6(-4.6)( 6.3) (D/5.97 kpc) km and kT(eff) = 122(-45)( 31) 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 infinity = 10.7(-3.1)( 6.3) (D/5.97 kpc) km and kT(eff) = 115(-16)( 21) eV) is found well outside the optical core (at similar to 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 infinity = 23(-10)( 69) (D/5.97 kpc) km and kT(eff) = 70(-20)( 28) 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 (alpha= -2.0(-2.2)( 1.2)). 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.

• 出版日期2009-1-11
• 单位中国科学院理论物理研究所