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

Abstract

About 12 percent of the early-type Be stars, so-called gamma Cas stars, exhibit an unusually hard and bright thermal X-ray emission that could result from accretion onto a white dwarf companion or from magnetic interactions between the Be star and its decretion disk. Exploring the full power of high-resolution X-ray spectroscopy of gamma Cas stars requires comparison of observations of the fluorescent Fe Kalpha emission lines near 6.4 keV with synthetic profiles of this line complex computed in the framework of the magnetic interaction and the accreting WD scenarios. For the latter, we further distinguish between accretion onto a non-magnetic and a magnetic WD. Our models account for different reservoirs of reprocessing material: the Be circumstellar decretion disk, the Be photosphere, an accretion disk around the WD companion, a magnetically channelled accretion flow and the WD photosphere. We find considerably different line properties for the different scenarios. For a non-magnetic accreting WD, the global Fe Kalpha complex is extremely broad, reaching a full width of 140 eV, whilst it is ~ 40 eV for the magnetic star-disk interaction and the magnetic accreting WD cases. In the magnetic star-disk interaction, the line centroid follows the orbital motion of the Be star, whereas it moves along with the WD in the case of an accreting WD. For gamma Cas, given the 15 times larger amplitude of the WD orbital motion, the shift in position for an accreting WD should be easily detectable with high-resolution spectrographs such as Resolve on XRISM, but remains essentially undetectable for the magnetic star-disk interaction. Upcoming high-resolution spectroscopy of the fluorescent Fe Kalpha emission lines in the X-ray spectra of gamma Cas stars will thus allow to distinguish between the competing scenarios.

Accepted for publication in A&A

References in corpus (19)

• A Grid of NLTE Line-Blanketed Model Atmospheres of Early B-type Stars
• On the formation of Be stars through binary interaction
• Velocity monitoring of gamma-Cas stars reveals their binarity status
• New Gamma Cas-like objects: X-ray and optical observations of SAO49725 and HD161103
• 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
• Discovery of Rapidly Moving Partial X-ray Absorbers within gamma Cassiopeiae
• Swift J011511.0-725611: Discovery of a rare Be Star / White Dwarf binary system in the SMC
• The X-ray emission of Be+stripped star binaries
• Time-dependent modeling of extended thin decretion disks of critically rotating stars
• Gamma Cas Stars as Be + White Dwarf Binary Systems
• X-ray reflection from cold white dwarfs in magnetic cataclysmic variables
• Is there a propeller neutron star in $\gamma$ Cas?
• 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
• X-ray and optical spectroscopic study of a gamma Cassiopeiae analog source pi Aquarii
• Application of a new X-ray reflection model to V1223 Sagittarii
• Fluorescent Fe K line emission of gamma Cas stars I. Do gamma Cas stars host propelling neutron stars?
• High-spatial-resolution simulations of Be star disks in binary systems: I. Structure and kinematics of coplanar disks