Fe Kα line in hard X-ray emitting symbiotic stars

Abstract

The 6.4 keV iron emission line is typically created by irradiation of the neutral (or low ionized) iron by a hard X-ray source. Whereas the 6.7 and 7.0 keV emission lines are mainly produced by photoionization and collisional excitation in hot plasma, the 6.4 keV fluorescence line is typically a signature of either reflection from an accretion disc or absorption. We have surveyed the emission using a collection of Suzaku observations of hard X-ray emitting symbiotic stars (hSSs) to better understand the geometry of these systems. We find that they do not seem to have a single geometry, and that while absorption-induced fluorescence leads to some emission in three of the hSSs in our study, CH Cyg, T CrB and RT Cru, there are strong hints that significant 6.4 keV emission arises in the accretion disc irradiated by the hard X-rays from the boundary layer between the accretion disc and hot white dwarf in one of our sources, SS73 17. The 6.7 and 7.0 keV lines, however, are largely produced by collisional excitation in the vicinity of the compact white dwarf.