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 \sim 10 % : \mbox { $R _ { NS } $ } = 8.9 \mbox { $ { } ^ { +0.9 } _ { -0.6 } $ } \hbox { $ { km } $ } for \mbox { $M _ { NS } $ } = 1.4 \mbox { $ M _ { \odot } $ } .
Alternatively , we provide the confidence regions in mass–radius space as well as the best-fit projected radius \mbox { $R _ { \infty } $ } = 11.9 \mbox { $ { } ^ { +1.0 } _ { -0.8 } $ } \hbox { $ { km } $ } , as seen by an observer at infinity .
The best-fit effective temperature , \mbox { $kT _ { eff } $ } = 80 \mbox { $ { } ^ { +4 } _ { -5 } $ } \mbox { $ { eV } $ } , is used to estimate the neutron star core temperature which falls in the range T _ { core } = \left ( 3.0 - 9.8 \right ) \mbox { $ \times 10 ^ { 7 } $ } \mbox { $ 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 in M13 .