NEUTRON STAR RADIUS MEASUREMENT WITH THE QUIESCENT LOW-MASS X-RAY BINARY U24 IN NGC 6397

This paper reports the spectral and timing analyses of the quiescent low-mass X-ray binary (qLMXB) U24 observed during five archived Chandra/ACIS exposures of the nearby globular cluster NGC 6397, for a total of 350 ks. We find that the X-ray flux and the parameters of the hydrogen atmosphere spectral model are consistent with those previously published for this source. On short timescales, we find no evidence of aperiodic intensity variability, with 90% confidence upper limits during five observations ranging between < 8.6% rms and < 19% rms, in the 0.0001-0.1 Hz frequency range (0.5-8.0 keV); and no evidence of periodic variability, with maximum observed powers in this frequency range having a chance probability of occurrence from a Poisson-deviated light curve in excess of 10%. We also report the improved neutron star (NS) physical radius measurement, with statistical accuracy of the order of similar to 10%: R-NS = 8.9(-0.6)(+0.9) km for M-NS = 1.4M(circle dot). Alternatively, we provide the confidence regions in mass-radius space as well as the best-fit projected radius R-infinity = 11.9(-0.8)(+1.0) km, as seen by an observer at infinity. The best-fit effective temperature, kT(eff) = 80(-5)(+4) eV, is used to estimate the NS core temperature which falls in the range T-core = (3.0-9.8) x 10(7) K, depending on the atmosphere model considered. This makes U24 the third most precisely measured NS radius among qLMXBs, after those in omega Cen and M13.