Fluorescent Fe K line emission of gamma Cas stars I. Do gamma Cas stars host propelling neutron stars?

Abstract

Gamma~Cas stars are early-type Be stars that exhibit an unusually hard and bright thermal X-ray emission. One of the proposed scenarios to explain these properties postulates the existence of a neutron star companion in the propeller stage, during which the magnetosphere of a rapidly rotating neutron star repels infalling material. To test this model, we examined the fluorescent Fe K$\alpha$ emission line at $\sim 6.4$\,keV in the X-ray spectra of $\gamma$~Cas stars, which offers a powerful diagnostic of both the primary source of hard X-rays and the reprocessing material. We computed synthetic line profiles of the fluorescent Fe K$\alpha$ emission line in the framework of the propelling neutron star scenario. Two reservoirs of material contribute to the fluorescence in this case: the Be circumstellar decretion disk and a shell of cool material that surrounds the shell of X-ray-emitting plasma around the putative propelling neutron star. We analysed the synthetic line profiles and expected equivalent widths of the lines for three well-studied $\gamma$~Cas stars. The predicted line strengths fall short of the observed values by at least an order of magnitude. Pushing the model parameters to reproduce the observed line strengths led to column densities towards the primary X-ray source that exceed the observationally determined values by typically a factor of 20, and would further imply a higher X-ray luminosity than observed. The strengths of the observed Fe K$\alpha$ fluorescent emission lines in $\gamma$~Cas stars are inconsistent with the expected properties of a propeller scenario as proposed in the literature.

Accepted for publication in Astronomy & Astrophysics

References in corpus (48)

• The chemical composition of the Sun
• Collisional Plasma Models with APEC/APED: Emission Line Diagnostics of Hydrogen-like and Helium-like Ions
• Constraining Black Hole Spin Via X-ray Spectroscopy
• Photoionization Modeling and the K Lines of Iron
• A natural explanation for periodic X-ray outbursts in Be/X-ray binaries
• Effects of gravitational darkening on the determination of fundamental parameters in fast rotating B-type stars
• Relativistic iron emission lines in neutron star low-mass X-ray binaries as probes of neutron star radii
• Modeling of iron K lines: radiative and Auger decay data for Fe II-Fe IX
• A Chandra survey of fluorescence Fe lines in X-ray Binaries at high resolution
• The Thermal Structure of the Circumstellar Disk Surrounding the Classical Be Star gamma Cassiopeia
• CHARA Array K'-band Measurements of the Angular Dimensions of Be Star Disks
• Estimating Be Star Disk Radii using H-alpha Emission Equivalent Widths
• The X-ray emission of the gamma Cassiopeiae stars
• New Evidence for Efficient Collisionless Heating of Electrons at the Reverse Shock of a Young Supernova Remnant
• A Study of Pi Aquarii During a Quasi-normal Star Phase: Refined Fundamental Parameters and Evidence for Binarity
• Prevalence of SED turndown among classical Be stars: Are all Be stars close binaries?
• An XMM-Newton view of FeKα in HMXBs
• Revealing the structure of the outer disks of Be stars
• Fe Line Diagnostics of Cataclysmic Variables and Galactic Ridge X-ray Emission
• Hot stars observed by XMM-Newton II. A survey of Oe and Be stars
• High Resolution Chandra Spectroscopy of Gamma Cassiopeia (B0.5IVe)
• New Gamma Cas-like objects: X-ray and optical observations of SAO49725 and HD161103
• The relationship between gamma Cassiopeiae's X-ray emission and its circumstellar environment
• Velocity monitoring of gamma-Cas stars reveals their binarity status
• gamma Cas stars: Normal Be stars with disks impacted by the wind of a helium-star companion?
• Three discoveries of gamma Cas analogs from dedicated XMM-Newton observations of Be stars
• The Infrared Continuum Sizes of Be Star Disks
• Stellar parameters and H$\alpha$ line profile variability of Be stars in the BeSOS survey
• gamma Cassiopeiae: an X-ray Be star with personality
• High-resolution X-ray spectroscopy reveals the special nature of Wolf-Rayet star winds
• X-ray Fluorescent Fe Kalpha Lines from Stellar Photospheres
• Evidence for Different Disk Mass Distributions Between Early and Late-Type Be Stars in the BeSOS Survey
• Doppler tomography of the circumstellar disk of π Aquarii
• The X-ray emission of Be+stripped star binaries
• Spectroscopic atlas of Halpha and Hbeta in a sample of northern Be stars
• Time-dependent modeling of extended thin decretion disks of critically rotating stars
• Gamma Cas Stars as Be + White Dwarf Binary Systems
• Spectro-interferometric observations of a sample of Be stars. Setting limits to the geometry and kinematics of stable Be disks
• Spectral Energy Distributions of Be and Other Massive 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
• The X-ray emission of gamma Cassiopeiae during the 2020-2021 disc eruption
• X-ray and optical spectroscopic study of a gamma Cassiopeiae analog source pi Aquarii
• Comparing Be Star Inclination Angles Determined from H$\alpha$ Fitting and Gravitational Darkening
• Stellar Inclination Angles from Be Star H$\alpha$ Emission-Line Profiles
• Chandra and XMM-Newton X-ray Observations of the Hyperactive T Tauri Star RY Tau
• Evolution of the disk of pi Aqr: from near-disappearance to a strong maximum

Cited by in corpus (1)

• Fluorescent Fe K line emission of gamma Cas stars II. Predictions for magnetic interactions and white-dwarf accretion scenarios