Current Projects

The Clemson Compton-Thick AGN Project

The goal of the Clemson-INAF Compton thick AGN project is to have a complete census of the heavily obscured active galactic nuclei (AGN) in the local Universe observed between 15 and 150 keV. The motivation behind this project is the discrepancy between the fraction of Compton-thick AGN (with large obscuring hydrogen column densities) observed in surveys and the fraction predicted by AGN population synthesis models. A much larger fraction than is currently observed is necessary to explain the Cosmic X-ray Background (CXB).

We use X-ray observations to characterize local AGN, within a redshift small enough for even dim, obscured AGN to be detectable. Current x-ray models such as MYTorus, borus02 or UXCLUMPY, allow us to model torus properties (e.g. average column density, covering factor, inclination angle) based on the features present in the X-ray spectrum.

Our final objective is to obtain the true fraction of Compton-thick AGN in the local Universe, and to characterize the properties of the AGN torus.

Related Publications: CI-CTAGN Project

Obscuration variability in AGN

The AGN torus was originally described as homogeneous, but recent observations, both in infrared and X-rays, point out to its material being clumpy instead. In this scenario, the torus would be a collection of clouds of varying sizes and densities, more or less confined to a plane around the supermassive black hole. This new picture of the AGN obscurer explains both infrared emission, and varying obscuring column densities in X-rays (e.g. eclipsing events).

While this picture agrees with recent observations, we still lack a complete model of the cloud sizes and distribution around AGN. To better understand the problem, we monitor AGN and analyze their multiepoch x-ray spectra, from which we derive the column densities in our line of sight.

Past Projects

Diffuse X-ray emission in LIRGs

Luminous Infrared Galaxies have huge bursts of star formation, born from the large quantities of gas funneled toward their inner regions. These galaxies emit intensely in infrared, hence their name, but also in X-rays. That is due both to thermal emission from hot gas, and to the abundance of end-products of star formation (SNRs, X-ray binaries). Often, the large quantities of gas also trigger AGN activity, which can be observed in X-rays despite the heavy obscuration.

We analyze the X-ray data of these galaxies to understand their energetics, and to detect obscured AGN. This research is part of the larger GOALS project, a multiwavelength effort to characterize local LIRGs and study their similarities to high-redshift star-forming galaxies.

The image shows Chandra (0.5-7 keV) contours overlaid on HST, both showing complex, disrupted morphologies.

Related publications: C-GOALS

Interaction between AGN jets and obstacles

AGN can launch powerful, relativistic, collimated outflows, or jets. Since they are launched from the inner regions of the galaxy, as they cross their host, they are bound to interact with a variety of objects (e.g. gas clouds, stars). This interaction can have two major effects:

1) The strong collision can result in particle acceleration, which may produce detectable gamma-rays. This emission can be observed with ground-based Cherenkov telescopes (like CTA, shown in the picture) or satellites such as Fermi.

2) The matter introduced into the jet during the interaction can affect it dynamically, potentially slowing it down in the exchange of momentum. This could explain the morphological (and other) differences between Fanaroff-Riley type I and II jets.

Related publications: AGN jet interactions