Discovery of Rapidly Moving Partial X-ray Absorbers within gamma Cassiopeiae

TL;DR

This study reports rapid, partial X-ray absorption events in gamma Cassiopeiae, revealing potential binary interactions and localized absorbing material near the star or its companion, advancing understanding of its complex X-ray behavior.

Contribution

First detection of rapid partial X-ray absorbers in gamma Cas, suggesting the presence of localized absorbing blobs and a possible white dwarf companion, offering new insights into its X-ray emission mechanisms.

Findings

Detected six rapid softness dips indicating partial absorption.

Absorbers are likely neutral hydrogen clouds with 2-8e21 cm^-2.

Evidence for two distinct X-ray emitting regions.

Abstract

Gamma Cassiopeiae is an enigmatic Be star with unusually strong hard X-ray emission. The Suzaku observatory detected six rapid X-ray spectral hardening events called "softness dips" in a ~100 ksec duration observation in 2011. All the softness dip events show symmetric softness ratio variations, and some of them have flat bottoms apparently due to saturation. The softness dip spectra are best described by either ~40% or ~70% partial covering absorption to kT ~12 keV plasma emission by matter with a neutral hydrogen column density of ~2-8e21 cm-2, while the spectrum outside of these dips is almost free of absorption. This result suggests the presence of two distinct X-ray emitting spots in the gamma Cas system, perhaps on a white dwarf companion with dipole mass accretion. The partial covering absorbers may be blobs in the Be stellar wind, the Be disk, or rotating around the white dwarf companion. Weak correlations of the softness ratios to the hard X-ray flux suggest the presence of stable plasmas at kT ~0.9 and 5 keV, which may originate from the Be or white dwarf winds. The formation of a Be star and white dwarf binary system requires mass transfer between two stars; gamma Cas may have experienced such activity in the past.