Abstract
We present a new grid of LTE model atmospheres for weakly magnetic (B < 10 10 G) neutron stars, using opacity and equation of state data from the OPAL project and employing a fully frequency- and angle-dependent radiation transfer. We discuss the dierences from earlier models, including a comparison with a detailed NLTE calculation. We suggest heating of the outer layers of the neutron star atmosphere as an explanation for the featureless X-ray spectra of RX J1856.5{3754 and RX J0720.4{3125 recently observed with Chandra and XMM.
Highlights
Modern X-ray observatories detect the thermal surface emission of a number of neutron stars
Heating at τ ∼ 1 strongly supresses the ≤ keV absorption features; surface heating drives the line features on the Rayleigh-Jeans tail into emission. We stress that this illustrative approach is by no means a self-consistent model of a heated neutron star atmosphere, as we include neither any physical assumption on the actual heating mechanims, e.g., there are currently no plausbile physical processes known which are capable of heating such shallow layers, nor a proper energy balance of the heating/cooling processes
We have presented a set of angle-dependant low field neutron star atmosphere spectra
Summary
Modern X-ray observatories detect the thermal surface emission of a number of neutron stars. In analogy to the classic model atmosphere analysis of normal stars, such observations permit the direct measurement of fundamental neutron star properties, such as their effective temperatures, atmospheric abundances and surface gravities. The latter point is especially of great importance, as an accurate measurement of the surface gravity of a neutron star is (with the distance known) equivalent to a measurement of its mass/radius ratio. Rajagopal & Romani (1996, RR96) computed hydrogen, solar abundance, and iron model atmospheres for low-field neutron stars, using improved opacity and equation of state data from the OPAL project. We present new model atmosphere grids for low-field neutron stars that overcome a number of shortcomings and errors in the RR96 and ZPS96 calculations and that are made available to the community
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