X-ray Obscuration

X-ray eclipses in AGN

An additional way to probe the innermost regions of AGN is the study of X-ray eclipses by broad-line regions (BLR) clouds orbiting the black hole. As a cloud passes through the line of sight, it shades various regions of the accretion disc, which are affected differently by special and general relativity (Doppler and gravitational shifts). The detection of such events with the next generation of X-ray observatories would allow us to perform `disk tomography' and improve our understanding the close environment of Supermassive black holes, testing the strong-gravity regime in their vicinity. This could be achieved thanks to the emerging field of X-ray polarimetry which will open a new window in X-ray astronomy, as it addresses several key questions that could not be answered using currently applied techniques.


The figure shows expected polarization, in the 2–8 keV range, throughout the accretion disc (projected in the observer’s sky) for inclinations of 30° (left) and 60° (right). The black curves depict the direction of the polarization at the detector. The color scale represents the polarized flux (in log scale). The lower panels show a zoomed-in snapshot of the innermost regions of the disc, where the relativistic effects dominate.

Thus, in a similar way to the energy spectrum, one can also look at the polarized light of the accretion disc getting obscured as the cloud passes through our line of sight. Here are animations for the evolution in Polarization degree and Polarization angle during the eclipsing event.


Read more here: Kammoun et al. (2018c)

The obscuration variability of NGC 4395

The dwarf Seyfert 1.8 galaxy NGC 4395 is one of the most X-ray variable non-jetted active galactic nuclei. By studying its XMM-Newton and NuSTAR archival data I could identify changes in the obscuration state of this source. I also confirmed through timing analysis the conclusion of Nardini & Risaliti (2011) about the possible presence of eclipsing events in one of the XMM-Newton observations.

Future missions such as XRISM, Athena and eXTP will allow us to study with much more details the properties of this source.

Read more here: Kammoun et al. (2019c)

NGC 5347: a Compton-thick AGN sitting in our backyard

The most rapid black hole growth by accretion likely occurs in Compton-thick quasars (CTQSOs) at moderate redshifts. In these sources, massive black holes may be accreting at potentially super-Eddington rates. CTQSOs are therefore particularly important targets for our understanding of black hole and galaxy co-evolution. The local population of Compton-thick Seyfert-2 AGN (CTAGN) represents nearby, a lower-mass proxy for CTQSOs.

While XMM-Newton enables accurate measures of the obscuring NH. When combined with new spectral models that properly account for scattering and re-emission, and allow for different geometrical configurations, studies of local CTAGN have the potential to reveal the geometry of (potentially super-Eddington) accretion onto black holes in these special, previously-invisible settings.

Thanks to NuSTAR, allowing us to extend our studies of obscured sources up to ~30 keV, I was able to identify the presence of Compton-thick AGN at the center of NGC 5347 located at a distance of 35.5 Mpc only!

Also in this case, Athena/X-IFU will play a major role in unveiling the secrets of the obscuring material and the torus in NGC 5347. The inset in the this figure shows clearly that both Fe Kα1,2 lines would be separated and resolved, in addition to the corresponding Compton shoulder. This will enable a better identification and characterization of faint CT sources, with high-quality spectra. It will also allow us to study and identify the various spectral components in these sources, the geometry and covering fraction of the obscuring material.

A hard look at local, optically-selected, obscured Seyfert galaxies

Multi-wavelength studies suggest that a large fraction (20-30%) of AGN are CT. The values of the CT fraction, quoted from X-ray background synthesis models, range between 9-35% in different models and as a function of redshift. Recent cosmic X-ray background modeling by predicts that ~50% of AGN within z = 0.1 are CT. These fractions are higher than the ones observed in hard X-ray surveys (being below 20%). A major difficulty in identifying CT sources is the attenuation of the direct emission produced by the central engine (in the soft X-rays, ultraviolet, and optical) by the obscuring material. The hard X-rays (~15 keV) and the mid-infrared (5-50 micron) are the only spectral bands where this material is optically thin up to high column densities.

I studied the X-ray spectra of a sample of 19 obscured, optically-selected Seyfert galaxies (Seyfert 1.8, 1.9 and 2) in the local universe (d<175 Mpc), drawn from the CfA Seyfert sample. This analysis is driven by the high sensitivity of NuSTAR in the hard X-rays, coupled with soft X-ray spectra using XMM-Newton, Chandra, Suzaku, and Swift/XRT. The optical spectra of these sources enabled a accurate estimates of BH masses and Eddington fractions. I employed four different models to analyze the X-ray spectra of these sources, which all result in consistent results.

As a result, I found that 79-90% of the sources are heavily obscured with line-of-sight column density NH>10^23 cm^-2. I also found a Compton-thick fraction of 37-53%. These results are consistent with previous estimates based on multi-wavelength analyses. In addition the fraction of reprocessed to intrinsic emission is positively correlated with NH and negatively correlated with the intrinsic, unabsorbed, X-ray luminosity (in agreement with the Iwasawa-Taniguchi effect). These results support the hypothesis that radiation pressure regulates the distribution of the circumnuclear material.

Read more here: Kammoun et al. (2020a)