An interpretation of the X-ray spectrum of 4U 1822 - 37

Abstract

White et al. (1981) have suggested that the partial X-ray eclipse observed by them in the binary X-ray source 4U 1822 – 37 is due to an extended scattering region of size |$\sim\,{10}^{10}$| cm surrounding a compact source. We have simulated the observed X-ray spectrum by radiative transfer through a shell of gas and determine that most of that gas must lie within |${10}^{8}\,{L}_{37}$| cm from the source. This is shown to be consistent with simple estimates of an evaporated atmosphere above an accretion disc. The partial eclipse can then be explained in an ad hoc manner by appending an extensive low density envelope to the outer disc. The atmosphere and envelope may encompass much of the binary system in a more luminous source, and so can explain the observed lack of X-ray eclipses in many bright galactic X-ray sources.