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 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 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 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
confidence regions in mass-radius space as well as the best-fit projected
radius R_infinity= 11.9(+1.0)(-0.8)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 third
most precisely measured neutron star radius among qLMXBs, after those in OmCen
and in M13.
