Reflection physics in X-ray-emitting symbiotic stars

Abstract

ABSTRACT X-ray-emitting symbiotic stars exhibit a variety of spectral shapes classified as α, β, γ, δ, and β/δ types, which have been attributed to different phenomena such as thermonuclear burning on the surface of the white dwarf component, shocks between winds and jets with the red giant companion’s extended atmosphere, the presence of heavily extinguished hot plasma from the inner region from an accretion disc, and/or a combination of these. However, there is observational evidence that this classification scheme is not definite and, for example, some sources change from one type to another within months or years. In this work, it is proposed that a simple disc-like model can be used to explain the X-ray properties observed from reflection-dominated symbiotic stars. For this purpose, we use the Stellar Kinematics Including Radiative Transfer (skirt) code, which has been recently upgraded to include radiative transfer from X-ray photons. It is found that the properties of the accretion disc (geometry and density) in combination with the viewing angle can be invoked to explain the spectral properties of β, δ, and β/δ X-ray-emitting symbiotic stars. Spectral variations and type swaps observed for some X-ray-emitting sources can also be explained by variations in the disc properties.