2003A&A...399.1109B

We present results of the analysis of data collected in 57-ks XMM-Newton and 505-ks Chandra observations of the nearby (≃120pc) isolated neutron star RXJ1856.5-3754. We confirm most of the statements made by Burwitz et al. (2001A&A...379L..35B) who discussed the original 55-ks Chandra data. Detailed spectral analysis of the combined X-ray and optical data rules out the currently available nonmagnetic light and heavy element neutron star atmosphere (LTE) models with hydrogen, helium, iron and solar compositions. We find that strongly magnetized atmosphere models also are unable to represent the data. The X-ray and optical data show no spectral features and are best fitted with a two-component blackbody model with kT_bb,X^∞≃63.5eV and R_bb,X^∞≃4.4(d/120pc)km for the hot X-ray emitting region, and kT_bb,opt^∞<33eV and R_bb,opt^∞>17(d/120pc)km for the rest of the neutron star surface responsible for the optical flux. The large number of counts collected with XMM-Newton allows us to reduce the upper limit on periodic variation in the X-ray range down to 1.3% (at a 2σ confidence level) in the 10^–3-50Hz frequency range. In an attempt to explain this small variability, we discuss an one-component model with kT_bb^∞≃63eV and R_bb^∞≃12.3(d/120pc)km. This model requires a low radiative efficiency in the X-ray domain, which may be expected if the neutron star has a condensed matter surface.