The Chandra LETGS high resolution X-ray spectrum of the isolated neutron star RX J1856.5-3754

We present the Chandra LETGS X-ray spectrum of the nearby (${\simeq} 60$ pc) neutron star RXJ1856.5-3754. Detailed spectral analysis of the combined X-ray and optical data rules out the nonmagnetic neutron star atmosphere models with hydrogen, helium, iron and solar compositions. We also conclude that strongly magnetized atmosphere models are unable to represent the data. The data can be explained with a two-component blackbody model. The harder component with temperature of $kT_{\rm bb}^\infty \simeq 63$ eV and a radius $R_{\rm bb}^\infty\simeq 2.2$ km of the emitting region well fits the X-ray data and can be interpreted as radiation from a hot region on the star's surface