A precessing Be disc as a possible model for occultation events in GX 304-1

Abstract

We report on the RXTE detection of a sudden increase in the absorption column density, NH, during the 2011 May outburst of GX 304-1. The NH increased up to ~16 × 1022 atoms cm-2, which is a factor of 3-4 larger than what is usually measured during the outbursts of GX 304-1 as covered by RXTE. Additionally, an increase in the variability of the hardness ratio as calculated from the energy resolved RXTE-Proportional Counter Array light curves is measured during this time range. We interpret these facts as an occultation event of the neutron star by material in the line of sight. Using a simple 3D model of an inclined and precessing Be disc around the Be-type companion, we are able to qualitatively explain the NH evolution over time. We are able to constrain the Be disc density to be of the order of 10-11 g cm-3. Our model strengthens the idea of inclined Be discs as origin of double-peaked outbursts as the derived geometry allows accretion twice per orbit under certain conditions.