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

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.

arXiv comment16 pages, 5 figures, accepted for publication in Astrophysical Journal

References in corpus (23)

• The chemical composition of the Sun
• On the Absorption of X-rays in the Interstellar Medium
• Validation of the new Hipparcos reduction
• A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
• Hard X-ray Detector (HXD) on Board Suzaku
• Classical Be Stars: Rapidly Rotating B Stars with Viscous Keplerian Decretion Disks
• Two populations of progenitors for type Ia SNe?
• Pulsating White Dwarf Stars and Precision Asteroseismology
• In-Orbit Performance of the Hard X-ray Detector on board Suzaku
• A Binary Origin for Blue Stragglers in Globular Clusters
• On the formation of Be stars through binary interaction
• High variability in Vela X-1: giant flares and off states
• The Thermal Structure of the Circumstellar Disk Surrounding the Classical Be Star gamma Cassiopeia
• Clumped stellar winds in supergiant high-mass X-ray binaries: X-ray variability and photoionization
• CHARA Array K'-band Measurements of the Angular Dimensions of Be Star Disks
• White dwarf masses in intermediate polars observed with the Suzaku satellite
• The X-ray emission of the gamma Cassiopeiae stars
• X-ray and Optical Variations in the Classical Be Star gamma Cas
• High Resolution Chandra Spectroscopy of Gamma Cassiopeia (B0.5IVe)
• The relationship between gamma Cassiopeiae's X-ray emission and its circumstellar environment
• gamma Cassiopeiae: an X-ray Be star with personality
• Properties and nature of Be stars: 29. Orbital and long-term spectral variations of γ Cassiopeiæ
• High-Energy Properties of the Enigmatic Be Star gamma Cassiopeiae

Cited by in corpus (25)

• Prevalence of SED turndown among classical Be stars: Are all Be stars close binaries?
• Detection of the Ultraviolet Spectrum of the Hot Subdwarf Companion of 60 Cygni (B1 Ve) from a Survey of IUE Spectra of Be Stars
• Hot stars observed by XMM-Newton II. A survey of Oe and Be stars
• gamma Cas stars: Normal Be stars with disks impacted by the wind of a helium-star companion?
• A propelling neutron star in the enigmatic Be-star $\gamma$~Cassiopeia
• Three discoveries of gamma Cas analogs from dedicated XMM-Newton observations of Be stars
• Suzaku and NuSTAR X-Ray Spectroscopy of gamma Cas and HD110432
• The X-ray emission of Be+stripped star binaries
• Gamma Cas Stars as Be + White Dwarf Binary Systems
• TESS lightcurves of gamma-Cas stars
• Surprises in the simultaneous X-ray and optical monitoring of pi Aqr
• Is there a propeller neutron star in $\gamma$ Cas?
• pi Aqr is another gamma-Cas object
• Intriguing X-ray and optical variations of the gamma Cas analog HD45314
• The X-ray emission of gamma Cassiopeiae during the 2020-2021 disc eruption
• Ultraviolet Activity as indicator of Small-scale Magnetic Fields in gamma Cassiopeiae
• Short-term variability and mass loss in Be stars VI. Frequency groups in $\gamma$ Cas detected by TESS
• Let there be more variability in two gamma Cas stars
• X-ray and optical spectroscopic study of a gamma Cassiopeiae analog source pi Aquarii
• Soft and hard X-ray dips in the light curves of gamma Cassiopeiae
• X-ray response to disk evolution in two gamma-Cas stars
• A Time-Dependent Spectral Analysis of $\gamma$ Cassiopeiae
• Automated photometry of $\gamma$ Cassiopeiae:the last roundup
• Fluorescent Fe K line emission of gamma Cas stars II. Predictions for magnetic interactions and white-dwarf accretion scenarios
• Irregular emission cycles in the Oe star HD 60848