Symbiotic stars in X-rays

TL;DR

This study used Swift/XRT to discover nine new X-ray emitting symbiotic stars, revealing diverse spectral types and linking X-ray properties with UV flickering, thus expanding understanding of their accretion and shock processes.

Contribution

It reports the first systematic X-ray survey of symbiotic stars, identifying three spectral groups and connecting X-ray emission features with accretion and shock mechanisms.

Findings

Nine new X-ray symbiotic stars discovered, increasing known sources by 30%.

X-ray spectra fall into three groups: hard, soft, and combined components.

Hard X-ray sources show strong UV flickering, indicating accretion disk activity.

Abstract

Until recently, symbiotic binary systems in which a white dwarf accretes from a red giant were thought to be mainly a soft X-ray population. Here we describe the detection with the X-ray Telescope (XRT) on the Swift satellite of nine white dwarf symbiotics that were not previously known to be X-ray sources and one that had previously been detected as a supersoft X-ray source. The nine new X-ray detections were the result of a survey of 41 symbiotic stars, and they increase the number of symbiotic stars known to be X-ray sources by approximately 30%. The Swift/XRT telescope detected all of the new X-ray sources at energies greater than 2 keV. Their X-ray spectra are consistent with thermal emission and fall naturally into three distinct groups. The first group contains those sources with a single, highly absorbed hard component that we identify as probably coming from an accretion-disk boundary layer. The second group is composed of those sources with a single, soft X-ray spectral component that probably originates in a region where low-velocity shocks produce X-ray emission, i.e., a colliding-wind region. The third group consists of those sources with both hard and soft X-ray spectral components. We also find that unlike in the optical, where rapid, stochastic brightness variations from the accretion disk typically are not seen, detectable UV flickering is a common property of symbiotic stars. Supporting our physical interpretation of the two X-ray spectral components, simultaneous Swift UV photometry shows that symbiotic stars with harder X-ray emission tend to have stronger UV flickering, which is usually associated with accretion through a disk. To place these new observations in the context of previous work on X-ray emission from symbiotic star.............