An Occultation Event in Centaurus A and the Clumpy Torus Model

Abstract

We have analyzed 16 months of sustained monitoring observations of Cen A from the Rossi X-ray Timing Explorer to search for changes in the absorbing column in the line of sight to the central nucleus. We present time-resolved spectroscopy which indicates that a discrete clump of material transited the line of sight to the central illuminating source over the course of ~170 days between 2010 August and 2011 February with a maximum increase in the column density of about 8.4 x 10^22 cm^-2. This is the best quality data of such an event that has ever been analyzed with the shape of the ingress and egress clearly seen. Modeling the clump of material as roughly spherical with a linearly decreasing density profile and assuming a distance from the central nucleus commensurate with the dusty torus we found that the clump would have a diameter of 1.4-2.4 x 10^15 cm with a central number density of n_H = 1.8-3.0 x 10^7 cm^-3. This is consistent with previous results for a similar (though possibly much longer) occultation event inferred in this source in 2003-2004 and supports models of the molecular torus as a clumpy medium.