EXOSAT measurements of the quiescent X-ray spectrum of the burster 2S1636-536 : evidence for Comptonization in a hot corona.

Abstract

Measurement of the X-ray spectrum of the quiescent flux from the burster 2S 1636-536 with EXOSAT on 1983 July 17 - 18 gave as a best-fitting model a power law with a high-energy cut-off. The flux (2 - 10 keV) was seen to increase from 1.6×10^-9erg cm^-2s^-1 to 2.5×10^-9erg cm^-2s^-1 during the observation and this increase was accompanied by a steepening power-law index (1.7 to 2.1 and a decreasing high-energy cut-off). The observed power-law spectrum is consistent with a model where soft photons emitted from the surface of the neutron star are Comptonized in a hot plasma surrounding the star (neutron star corona). The increasing power-law index observed as the intensity rises can be interpreted as Compton cooling of the corona when the flux of photons from the neutron star rises in response to increasing accretion rate.