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

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

Authors:

Kühnel et al

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-PCA 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 disk around the Be type companion, we are able to qualitatively explain the NH evolution over time. We are able to constrain the Be-disk density to be on the order of 10−11 g cm−3. Our model strengthens the idea of inclined Be disks as origin of double-peaked outbursts as the derived geometry allows accretion twice per orbit under certain conditions.