The quiescent x-ray spectrum of the neutron star in centaurus x-4 observed with Chandra/ACIS-S

We report on spectral and intensity variability analysis from a Chandra/ACIS-S observation of the transient, type I X-ray bursting low-mass X-ray binary Cen X-4. The quiescent X-ray spectrum during this observation is statistically identical to one observed previously with BeppoSAX and close, but not identical, to one observed previously with ASCA. The X-ray spectrum is best described as a pure hydrogen atmosphere thermal spectrum plus a power-law component that dominates the spectrum above 2 keV. The best-fit radius of the neutron star is r = 12.9 ± 2.6 (d/1.2 kpc) km if the interstellar absorption is fixed at the value implied by the optical reddening. Allowing the interstellar absorption to be a free parameter yields r = 19⁺⁴⁵_-10 (d/1.2 kpc) km (90% confidence). The thermal spectrum from the neutron star surface is inconsistent with a solar metallicity. We find a 3 σ upper limit of rms variability ≤ 18% (0.2-2.0 keV; 0.0001-1 Hz) during the observation. On the other hand, the 0.5-10.0 keV luminosity decreased by 40 ± 8% in the 4.9 yr between the ASCA and Chandra observations. This variability can be attributed to the power-law component. Moreover, we limit the variation in thermal temperature to ≲ 10% over these 4.9 yr. The stability of the thermal temperature and emission-area radius supports the interpretation that the quiescent thermal emission is caused by the hot neutron star core.