The equatorial disc of the Be star X Persei

Abstract

We study the long-term behaviour of the equatorial disc of the Be/X-ray binary X Persei (X Per), combining new low-resolution IUE spectra and IR photometry with UV, optical and IR observations collected from the literature. We find that the near-UV continuum level of X Per varies along with the optical brightness. From the UV observations, we also find that during optical high states the flux excess due to the intrinsic stellar variability and/or electron scattering in the disc is at most 15-20 per cent of the photospheric flux. From the data taken in discless and near-discless states (optical low states) we show that the stellar photosphere can be modelled with T_eff=31000K and log g=4. With this model we derive E(B-V)=0.39, and estimate the distance to X Per as 950+/-200 pc (assuming R^*=9 Rsolar). We fit the (quasi-)simultaneous optical and IR photometry with a simple disc model including free-bound and free-free radiation. We find that the density of the disc at the photosphere of the star varies along with the brightness of X Per, and that in optical high states the disc in X Per is among the densest of all Be stars: rho_0=(1.5+/-0.3)x10^-10 g cm^-3. The disc density at the photosphere varies by a factor of at least 20 from optical high to low states. During disc build-up and break-down phases, and also in phases when the disc is relatively stable, we find a very steep radial density gradient of the disc of X Per. This may reflect the limitations of some of the assumptions in our model. We find that in a disc-loss event the disc loses mass at a rate of about 5x10^-9 Msolar yr^-1. For a disc build-up phase we find a disc-mass growth rate of about 4x10^-9 Msolar yr^-1. This growth rate is consistent with a model that feeds the disc from the `ordinary' mass-loss of the star, but we cannot exclude the possibility that other phenomena contribute to the disc growth as well.