Modelling the disc around the primary star of the X-ray binary system: MT91-213

ABSTRACT The viscous decretion disc (VDD) scenario has been used to model the observables of the Be stars. Its capability to predict individual observables has been confirmed for several Be stars. Here, we simultaneously analyse the spectroscopic and $BVI$-band photometric data for the Be star MT91-213 with the Monte Carlo radiative-transfer code hdust to determine the stellar parameters, geometry, and physical conditions for its circumstellar disc. MT91-213 is the primary component of a binary system whose companion is the pulsar PSR J2032+4127. We find that the VDD model can simultaneously reproduce the multiple observables qualitatively, but not quantitatively. We determine the mass of the primary star to be 13.1 ${\rm M}_{\odot }$ which is smaller than reported in the literature. We present a dynamical scenario for the evolving disc density from a diffuse to a dense phase. Also, we determine that the inclination of the disc is about 40$^{\circ }$ which means it is 20$^{\circ }$ tilted from the orbit of the secondary star. Our results indicate that the mass loss rate for MT91-213 is $\sim 10^{-7}$ to $10^{-6} {\rm M}_{\odot }\,\mathrm{yr}^{-1}$ which is in agreement with the suggested values in the literature, required to explain the observed X-ray synchrotron luminosity, $L_\mathrm{x}$, for PSR J2032+4127.