Recent results fitting ATM atmosphere models to Chandra spectra of thermally radiating neutron stars

Abstract

We present new results fitting Chandra ACIS and LETG/HRC-S spectra of the Cas A X-ray point source (XPS) and the isolated neutron star RX J1856.5-3754 to H, H–He, Si–ash, and Fe atmosphere models calculated with our compact atmosphere model code. In the case of the Cas A XPS, we find our atmosphere models fit significantly better than blackbody models, although a wide range of radii is formally permitted. The best fit H model is consistent with a neutron star of radius ∼7.6 km, but with a radiating area scale size of only ∼3 km, consistent with a hot spot geometry (discussed by, e.g. [Astrophys. J. Lett. 531 (2000) L53]). We also show the effects of the Cas A supernova remnant material on the XPS spectrum. In the case of RX J1856.5-3754 we find H and Fe spectra provide unacceptable fits to the X-ray and optical data, but we do find physically plausible results with a Si–ash model for a rapidly rotating neutron star.