Symbiotic stars in X-rays III: long term variability

TL;DR

This study analyzes long-term X-ray variability in five symbiotic stars, revealing that their X-ray emission originates from accretion processes in the boundary layer, with flux and absorption varying significantly over time.

Contribution

It provides new insights into the long-term X-ray variability and accretion mechanisms of symbiotic stars using Suzaku observations.

Findings

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

Intrinsic X-ray flux and absorption vary by factors of three or more.

All sources are accretion-powered through optically thick boundary layers.

Abstract

We study the X-ray emission from five symbiotic stars observed with Suzaku. These objects were selected for deeper observations with Suzaku after their first detection with ROSAT and Swift. We found that the X-ray spectra can be adequately fit with absorbed optically thin thermal plasma models, either single or multi-temperature. Such a model is compatible with the X-ray emission being originated in the innermost region of the accretion disk, i.e. a boundary layer. Based on the large flickering amplitude (only detected in 4 Dra), the high plasma temperature and previous measurements of UV variability and luminosity, we conclude that all five sources are accretion-powered through predominantly opticall thick boundary layer. Given the time lapse between previous and these observations, we were able to study the long term variability of their X-ray emission and found that the intrinsic X-ray flux and the intervening absorption column can vary by factors of three or more. However, it is still elusive the location of the absorber and how the changes in the accretion rate and absorption are related.