A precessing Be disk 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, $N_{\mathrm{H}}$, during the 2011 May outburst of GX 304-1. The $N_{\mathrm{H}}$ increased up to $\sim 16\times 10^{22}$ 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 $N_{\mathrm{H}}$ 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.

12 pages, 8 figures, 3 tables. Accepted for publication in the Monthly Notices of the Royal Astronomical Society

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