The Quiescent X‐Ray Spectrum of the Neutron Star in Centaurus X‐4 Observed withChandra/ACIS‐S

We report on spectral and intensity variability analysis from aChandra/ACIS-S observation of the transient, type-I X-ray bursting low-massX-ray binary Cen X-4. The quiescent X-ray spectrum during this observation isstatistically identical to one observed previously with Beppo/SAX, and close,but not identical, to one observed previously with ASCA. The X-ray spectrum isbest described as a pure Hydrogen atmosphere thermal spectrum plus a power-lawcomponent that dominates the spectrum above 2 keV. The best-fit radius of theneutron star is r=12.9+/-2.6 (d/1.2 kpc) km if the interstellar absorption isfixed at the value implied by the optical reddening. Allowing the interstellarabsorption 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 inconsistentwith a solar metallicity. We find a 3sigma upper-limit of root-mean-squarevariability <18% (0.2-2.0 keV; 0.0001-1 Hz) during the observation. On theother hand, the 0.5-10.0 keV luminosity decreased by 40+/-8% in the 4.9 yearsbetween the Asca and Chandra observations. This variability can be attributedto the power-law component. Moreover, we limit the variation in thermaltemperature to <10% over these 4.9 years. The stability of the thermaltemperature and emission area radius supports the interpretation that thequiescent thermal emission is due to the hot neutron star core.Comment: 25 pages, 1 figure; ApJ, Accepted for April 10 2001 Issu