An Occultation Event in Centaurus A and the Clumpy Torus Model

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.

💡 Research Summary

This paper presents a detailed analysis of a transient X‑ray absorption event observed in the nearby active galaxy Centaurus A (Cen A) using the Rossi X‑ray Timing Explorer (RXTE). Over a 16‑month monitoring campaign (January 2010 to April 2011), the authors obtained 228 snapshots of ≈1 ks each, spaced roughly every two days, with the Proportional Counter Array (PCA). By extracting 2–10 keV light curves and the hardness ratio (7–10 keV / 2–4 keV), they identified a clear increase in the absorbing column density (N_H) between Modified Julian Dates 55440 and 55600, corresponding to a period from August 2010 to February 2011.

Time‑resolved spectroscopy was performed in 10‑day bins (3–6 ks total exposure per bin) using a simple spectral model consisting of a fixed Galactic absorber, a variable intrinsic cold absorber, a power‑law continuum (photon index Γ fixed at 1.87±0.03), and an Fe Kα Gaussian line. The fits yielded a baseline N_H of ≈20.9 × 10²² cm⁻², which rose smoothly to a peak of ≈27 × 10²² cm⁻², an increase of about 8.4 × 10²² cm⁻², and then returned to the baseline over a total duration of ~170 days. The photon index remained essentially constant, and the Fe line showed no significant variability, indicating that the observed changes are dominated by line‑of‑sight absorption rather than intrinsic continuum fluctuations.

To interpret the geometry of the occulting material, three density profiles were tested against the N_H time series: (1) a uniform‑density sphere, (2) a β‑profile (N_H(r)=N_H,center