The Influence of the Accretion Disc Structure on X-ray Spectral States in Symbiotic Binaries

Abstract

Symbiotic stars are binary systems where a white dwarf (WD) accretes material from the wind of an evolved, late-type companion. X-ray-emitting symbiotic systems are classified into $\alpha$, $\beta$, $\delta$, and $\beta/\delta$ types, attributed to distinct physical mechanisms such as thermonuclear burning, wind interactions, and accretion-driven boundary layers. We present synthetic X-ray spectra derived from hydrodynamics simulations using the PHANTOM code, coupled with radiative-transfer calculations from SKIRT. We reproduce all X-ray spectral types by exploring different density structure of the accretion disc, the viewing angle, the plasma temperature of the boundary layer, and/or the presence of extended emission. The synthetic X-ray spectra consist of both absorbed and reflected components. In systems with massive, high-column density discs and viewing angles close to edge-on, the reflected continuum can dominate the X-ray emission. This effect is less pronounced in systems with low-mass, lower-column density discs. We explore i) systems going from $\delta$ to $\beta$ states, ii) $\delta$-types that become $\beta/\delta$ sources, iii) the variability of the three Fe emission lines in the 6.0-7.0 energy range, and iv) the possible physical processes behind the $\alpha$ sources. The observations from iconic symbiotic systems are discussed in line of the present models. Our framework offers predictive power for future X-ray monitoring and provides a path toward connecting accretion disc physics with observed spectral states in symbiotic binaries with accreting WDs.