Symbiotic stars in X-rays III: Suzaku observations

TL;DR

This study analyzes Suzaku X-ray observations of five symbiotic stars, revealing high-temperature, accretion-powered emission consistent with boundary layer models, and noting variability in X-ray flux and absorption over years.

Contribution

First detailed Suzaku X-ray spectral analysis of multiple symbiotic stars confirming accretion-driven emission with high plasma temperatures and variability.

Findings

X-ray spectra fit with thermal plasma models indicating boundary layer origin

High plasma temperatures (>3 keV) suggest accretion rather than colliding winds

X-ray flux and absorption vary significantly over years

Abstract

We describe the X-ray emission as observed with Suzaku from five symbiotic stars that we selected for deep Suzaku observations after their initial detection with ROSAT, ASCA and Swift. We find that the X-ray spectra of all five sources can be adequately fit with absorbed, optically thin thermal plasma models, with either single- or multi-temperature plasmas. These models are compatible with the X-ray emission originating in the boundary layer between an accretion disk and a white dwarf. The high plasma temperatures of kT$~>3$ keV for all five targets were greater than expected for colliding winds. Based on these high temperatures, as well as previous measurements of UV variability and UV luminosity, and the large amplitude of X-ray flickering in 4 Dra, we conclude that all five sources are accretion-powered through predominantly optically thick boundary layers. Our X-ray data allow us to observe a small, optically thin portion of the emission from these boundary layers. Given the time between previous observations and these observations, we find that the intrinsic X-ray flux and the intervening absorbing column can vary by factors of three or more on a time scale of years. However, the location of the absorber and the relationship between changes in accretion rate and absorption are still elusive.