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Authors: Sebastien Guillot, Robert E. Rutledge, Edward F. Brown

Date: 14 Jul 2010

Abstract: This paper reports the spectral and timing analyses of the quiescent low-mass X-ray binary U24 observed during five archived Chandra-ACIS exposures of the nearby globular cluster NGC 6397, for a total of 350 ksec. We find that the X-ray flux and the parameters of the hydrogen atmosphere spectral model are consistent with those previously published. Following the timing analysis, we find no evidence of short or long-term intensity variability. We also report the improved neutron star physical radius measurements, with statistical accuracy of the order of ~10%: R_ns = 8.9(+0.9)(-0.6) km for M_ns = 1.4 Msun. Alternatively, we provide the best-fit projected radius R_infinity= 11.9(+2.2)(-2.5)km, as seen by an observer at infinity. The best-fit effective temperature, kTeff = 80(+4)(-5) eV, is used to estimate the neutron star core temperature which falls in the range T_core = (3.0 - 9.8) x10 7 K, depending on the atmosphere model considered. This makes U24 the fourth most precisely measured neutron star radius among qLMXBs, after those in OmCen, in M13 and the qLMXB 47Tuc X7.

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