The thermal radiation of the isolated neutron star RX J1856.5-3754 observed with Chandra and XMM-Newton

We present results of the analysis of data collected in 57-ks XMM-Newton and 505-ks Chandra observations of the nearby ( =~ 120 pc) isolated neutron star RX J1856.5-3754. We confirm most of the statements made by Burwitz et al. (\cite{Buretal01}) 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^\infty =~ 63.5 eV and R_bb,X^\infty =~ 4.4 (d/120 pc) km for the hot X-ray emitting region, and kT_bb,opt^\infty < 33 eV and R_bb,opt^\infty > 17 (d/120 pc) 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 2sigma confidence level) in the 10^-3-50 Hz frequency range. In an attempt to explain this small variability, we discuss an one-component model with kT_bb^infty =~ 63 eV and R_bb^infty =~ 12.3 (d/120 pc) 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.