Ti\={a}ngu\={a}n ($\zeta$ Tau) as a binary system consisting of a Be-star and an accreting White Dwarf: opening a gate to understanding enigmatic $\gamma$ Cas analogues

Abstract

The analogues of $\gamma$ Cassiopea are binary early type Be stars which are X-ray bright with hard thermal spectra. The nature of companions in these stars and mechanisms of their X-ray emission remain enigmatic. Among the proposed ideas is the presence of an accretion disc around a white dwarf (WD) companion to the Be star donor. We use radiative transfer models including reflection physics in order to calculate the synthetic spectra of such systems, and assume that the hottest plasma is thermal and is located in the accretion disc boundary layer. The models are used to analyse the XMM-Newton observations of the $\gamma$ Cas analogue $\zeta$ Tau (a.k.a. Ti\={a}ngu\={a}n). Comparisons with X-ray-emitting symbiotic systems, particularly $\delta$- and $\beta/\delta$-type systems, support the idea that the hard X-ray emission in $\zeta$ Tau is best explained by a WD accreting material expelled from the Be star. The plasma temperature and luminosity of the boundary layer associated with the accretion disc are used to estimate a mass accretion rate of $\dot{M}_\mathrm{acc} \approx 4\times 10^{-10}$ M$_\odot$ yr$^{-1}$, implying a nova recurrence time above 10$^{5}$ yr. Our analysis advances the understanding the production of hard X-ray emission in $\gamma$ Cas analogues, further supporting the idea of accreting WDs as companions of Be-stars in these systems.