The Symbiotic System SS73 17 Seen with Suzaku

TL;DR

This study presents Suzaku observations of the symbiotic star SS73 17, revealing a highly-absorbed, hard X-ray spectrum with strong iron lines, suggesting it is an extreme member of a small class of hard X-ray emitting symbiotic systems.

Contribution

First detailed Suzaku X-ray analysis of SS73 17, highlighting its unique spectral features and potential classification as an extreme hard X-ray emitting symbiotic star.

Findings

Highly absorbed X-ray spectrum with NH > 10^23 cm^-2

Presence of strong, narrow iron lines including Fe K, Fe xxv, Fe xxvi

Likely a white dwarf with complex reflection and absorption features

Abstract

We observed with Suzaku the symbiotic star SS73 17, motivated by the discovery by the INTEGRAL satellite and the Swift BAT survey that it emits hard X-rays. Our observations showed a highly-absorbed X-ray spectrum with NH > 10^23 cm-2, equivalent to A_V > 26, although the source has B magnitude 11.3 and is also bright in UV. The source also shows strong, narrow iron lines including fluorescent Fe K as well as Fe xxv and Fe xxvi. The X-ray spectrum can be fit with a thermal model including an absorption component that partially covers the source. Most of the equivalent width of the iron fluorescent line in this model can be explained as a combination of reprocessing in a dense absorber plus reflection off a white dwarf surface, but it is likely that the continuum is partially seen in reflection as well. Unlike other symbiotic systems that show hard X-ray emission (CH Cyg, RT Cru, T CrB, GX1+4), SS73 17 is not known to have shown nova-like optical variability, X-ray flashes, or pulsations, and has always shown faint soft X-ray emission. As a result, although it is likely a white dwarf, the nature of the compact object in SS73 17 is still uncertain. SS73 17 is probably an extreme example of the recently discovered and relatively small class of hard X-ray emitting symbiotic systems.