Symbiotic stars in X-rays

G. J.M. Luna, J. L. Sokoloski, K. Mukai, T. Nelson

Research output: Contribution to journalArticlepeer-review

20 Scopus citations

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 stars, we modified and extended the α/β/γ classification scheme for symbiotic-star X-ray spectra that was introduced by Muerset et al. based upon observations with the ROSAT satellite, to include a new δ classification for sources with hard X-ray emission from the innermost accretion region. Because we have identified the elusive accretion component in the emission from a sample of symbiotic stars, our results have implications for the understanding of wind-fed mass transfer in wide binaries, and the accretion rate in one class of candidate progenitors of type Ia supernovae.

Original languageEnglish (US)
Article numberA6
JournalAstronomy and Astrophysics
Volume559
DOIs
StatePublished - 2013

Bibliographical note

Funding Information:
We thank the anonymous referee for comments and suggestions which improved the final quality of this article. We acknowledge the Swift team for planning these observations. G. J. M. Luna is a member of the CIC-CONICET (Argentina) and acknowledges support from grants PIP-Conicet/2011 #D4598 and FONCyT/PICT/2011 #269. J. L. Sokoloski acknowledges support from grants SAO GO1-12041A, NASA NNX10AK31G, and NASA NNX11AD77G. We thank N. Masetti and P. Evans for useful comments that helped to improve the manuscript.

Keywords

  • Accretion, accretion disks
  • Binaries: symbiotic
  • X-rays: binaries

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