X--Ray Spectra from Neutron Stars Accreting at Low Rates

Abstract

The spectral properties of X--ray radiation produced in a static atmosphere around a neutron star accreting at very low rates are investigated. Previous results by Alme \& Wilson (1973) are extended to the range $10^{-7}\le L/L_{Edd}\le 10^{-3}$ to include the typical luminosities, $L\sim 10^{31}-10^{32}\mathrm{ergs}{\mathrm{s}}^{-1}$, expected from isolated neutron stars accreting the interstellar medium. The emergent spectra show an overall hardening with respect to the blackbody at the neutron star effective temperature in addition to a significant excess over the Wien tail. The relevance of present results in connection with the observability of low--luminosity X--ray sources is briefly discussed.

14 pages (3 postscript figures available on request), PlainTex, submitted to ApJ

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