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.

arXiv commentAccepted for publication in A&A

References in corpus (44)

• The chemical composition of the Sun
• Collisional Plasma Models with APEC/APED: Emission Line Diagnostics of Hydrogen-like and Helium-like Ions
• A Grid of NLTE Line-Blanketed Model Atmospheres of Early B-type Stars
• Classical Be Stars: Rapidly Rotating B Stars with Viscous Keplerian Decretion Disks
• X-ray reflected spectra from accretion disk models. III. A complete grid of ionized reflection calculations
• X-ray reflected spectra from accretion disk models. I. Constant density atmospheres
• Photoionization Modeling and the K Lines of Iron
• A natural explanation for periodic X-ray outbursts in Be/X-ray binaries
• X-ray Emissions from Accreting White Dwarfs: a Review
• Viscous Effects on the Interaction between the Coplanar Decretion Disc and the Neutron Star in Be/X-Ray Binaries
• On the formation of Be stars through binary interaction
• Modeling of iron K lines: radiative and Auger decay data for Fe II-Fe IX
• Magnetic spots on hot massive stars
• 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
• Deriving an X-ray luminosity function of dwarf novae based on parallax measurements
• Evidence for intermediate polars as the origin of the Galactic Center hard X-ray emission
• Be discs in binary systems I. Coplanar orbits
• Fe Line Diagnostics of Cataclysmic Variables and Galactic Ridge X-ray Emission
• The virtual observatory service TheoSSA: Establishing a database of synthetic stellar flux standards. I. NLTE spectral analysis of the DA-type white dwarf G 191-B2B
• 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?
• Velocity monitoring of gamma-Cas stars reveals their binarity status
• New Gamma Cas-like objects: X-ray and optical observations of SAO49725 and HD161103
• 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
• 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 Remarkable Be Star HD110432
• The X-ray emission of Be+stripped star binaries
• Gamma Cas Stars as Be + White Dwarf Binary Systems
• Time-dependent modeling of extended thin decretion disks of critically rotating stars
• X-ray reflection from cold white dwarfs in magnetic cataclysmic variables
• Is there a propeller neutron star in $\gamma$ Cas?
• XMM-Newton observations of the eclipsing polar V2301 Oph
• First XMM-Newton observations of strongly magnetic cataclysmic variables I: spectral studies of DP Leo and WW Hor
• 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
• Developing the physical understanding of intermediate polars: an X-ray study of TV Col and V2731 Oph
• Irregular Mass Transfer in the Polars VV Puppis and V393 Pavonis during the Low State
• 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