Fluorescent Fe K line emission of γ Cas stars

II. Predictions for magnetic interactions and white-dwarf accretion scenarios

Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, Allée du 6 Août, 19c, Bât B5c, 4000 Liège, Belgium

^★ Corresponding author: g.rauw@uliege.be

Received: 16 July 2025
Accepted: 2 October 2025

Abstract

Context. About 12% of the early-type Be stars, which are not known X-ray binaries, exhibit an unusually hard and bright thermal X-ray emission. The X-ray emission of these so-called γ Cas stars could result from accretion onto a white dwarf companion or from magnetic interactions between the Be star and its decretion disc.

Aims. Exploring the full power of high-resolution X-ray spectroscopy of γ Cas stars requires the comparison of observations of the fluorescent Fe Kα emission lines near ~6.4keV with synthetic lines simulated for both scenarios.

Methods. We computed synthetic profiles of this line complex within the framework of the magnetic interaction and the accreting white dwarf scenarios. For the latter, we further distinguished between accretion onto a non-magnetic and a magnetic white dwarf. The various models account for different reservoirs of reprocessing material: the Be circumstellar decretion disc, the Be photosphere, an accretion disc around the putative white dwarf companion, a magnetically channelled accretion flow, and the white dwarf photosphere.

Results. We find considerably different line properties for the different scenarios. For a non-magnetic accreting white dwarf, the global Fe Kα complex is extremely broad, reaching a full width of 140 eV, whilst it is ~40 eV for the magnetic star–disc interaction and the magnetic accreting white dwarf cases. In the magnetic star-disc interaction, the line centroid is expected to follow the orbital motion of the Be star, whereas it should move along with the white dwarf in the case of an accreting white dwarf. For γ Cas, given the ~15× larger amplitude of the white dwarf orbital motion, the shift in position for an accreting white dwarf should be easily detectable with high-resolution spectrographs such as Resolve on XRISM, but remains essentially undetectable for the magnetic star-disc interaction.

Conclusions. Upcoming high-resolution spectroscopy of the fluorescent Fe Kα emission lines in the X-ray spectra of γ Cas stars will offer important insights into the properties of the primary X-ray source and of the illuminated material, allowing us to distinguish between the competing scenarios.