High energy emission powered by accreting companions of Be/$\gamma $ Cas stars

[en] The origin of the hard, bright X-ray emission that defines the gamma Cas analog class of Be stars remains an outstanding question in Be star literature. This work explores the possibility that the X-ray flux is produced by accretion onto a white dwarf companion. We use three-dimensional smoothed particle hydrodynamics simulations to model the prototype gamma Cas system assuming a white dwarf companion and investigate the accretion of the circumstellar material by the secondary star. We contrast these results to a model for 59 Cyg, a non- gamma Cas Be star system with a stripped companion. We find that the secondary stars in both systems form disk-like accretion structures with Keplerian characteristics, similar to those seen in the Be decretion disks. We also find that white dwarf accretion can produce X-ray fluxes that are consistent with the observed values for gamma Cas, while the predicted X-ray luminosities are significantly lower for the non-degenerate companion in 59 Cyg. In addition, using the three-dimensional radiative transfer code, hdust, we find that these models produce Halpha emission consistent with the observations for both gamma Cas and 59 Cyg, and that the predicted polarization degrees across optical and UV wavelengths are at detectable levels. Finally, we discuss the impact that future UV spectropolarimetry missions could have on our understanding of these systems.

Research Center/Unit :

STAR - Space sciences, Technologies and Astrophysics Research - ULiège

Disciplines :

Space science, astronomy & astrophysics

Author, co-author :

Rast, Rina G.

Nazé, Yaël ; Université de Liège - ULiège > Département d'astrophysique, géophysique et océanographie (AGO) > Groupe d'astrophysique des hautes énergies (GAPHE)

Labadie-Bartz, Jonathan

Jones, Carol E.

Erba, Christiana

Gayley, Ken

ud-Doula, Asif

Neiner, Coralie

Drake, Jeremy J.

Language :

English

Title :

High energy emission powered by accreting companions of Be/$\gamma $ Cas stars

Publication date :

October 2025

Journal title :

Astrophysics and Space Science

ISSN :

0004-640X

eISSN :

1572-946X

Publisher :

Springer Science and Business Media LLC

Volume :

370

Issue :

Pages :

id 108(20p)

Peer reviewed :

Peer Reviewed verified by ORBi

Additional URL :

https://link.springer.com/content/pdf/10.1007/s10509-025-04497-6.pdf

Available on ORBi :

since 20 October 2025

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