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

TL;DR

This study links the X-ray spectral diversity of symbiotic binaries to the physical state of the accretion disc by coupling SPH hydrodynamics (PHANTOM) with radiative-transfer calculations (SKIRT). By varying disc density structure, inclination $i$, boundary-layer temperature $kT$, and the presence of extended emission, the authors reproduce the α, β, δ, and β/δ spectral classes and explain state transitions such as δ-to-β and δ-to-β/δ through disc mass changes and jet/outflow contributions. Key findings include: (i) δ-type spectra require high $kT$ and large $N_{\mathrm{H,disc}}}$, (ii) β-type can arise across a range of $kT$ and $i$, (iii) β/δ requires a softer, extended component with ISM-like absorption, and (iv) α-type arises from low $kT$ and/or low absorption/viewing geometry, with super-soft emission often linked to jets. The framework provides predictive capability for future X-ray monitoring and clarifies how accretion-disc physics maps onto observed spectral states, while cautioning against using the soft component alone to infer mass accretion rates.

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 $α$, $β$, $δ$, and $β/δ$ 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 $δ$ to $β$ states, ii) $δ$-types that become $β/δ$ 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 $α$ 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.

Figures (12)

• Figure 1: Density maps of the phantom simulation of a binary system in a circular orbit (see description in Section \ref{['sec:phantom']}) at an arbitrary time. The top panel shows the binary components in the central $x-y$ plane, with the mass donor star shown with a black circle and the companion is surrounded by the accretion disc. The cross shows the position of the centre of mass. The bottom shows cuts in the $x-z$ and $x - y$ planes that correspond to face-on and edge-on views of the companion to highlight the details of the accretion disc. The arrows point towards the direction of the mass donor star.
• Figure 2: Same as Fig. \ref{['fig:den_maps']} but for the accretion disc during maximum of accretion (left; $t/P = 0.18$) and apastron passage (right; $t/P = 0.5$) in the eccentric ($e=0.45$) orbit simulation. Note the differences in the scale bars.
• Figure 3: Temporal evolution of the mass accretion rate $\dot{M}_\mathrm{acc}$ for the simulation with an eccentric system over one orbital period. $t/P=0.0$ represents the periastron position while $t/P=0.5$ indicates the system's apastron. The star symbols mark the moments of maximum accretion ($t/P=0.18$) and that of the apastron passage ($t/P=0.50$).
• Figure 4: Examples of apec model spectra in the 0.3--50.0 keV energy range used as input in the radiative transfer simulations.
• Figure 5: A schematic representation of the source of X-ray photons inside the accretion disc. The inclination angle is denoted with $i$ and is measured from the symmetry axis.