ISDC Data Centre for Astrophysics

Reflection in Seyfert galaxies and the unified model of AGN

From an extremely long hard X-ray look at AGN we found unexpected differences amongst different classes of objects. These differences could be explained by supermassive black holes living in different environments, and might help shading light on the origin of the peak of the cosmic X-ray background.

A&A 532, 102 (2011) | Press Release

ABSTRACT

We present a deep study of the average hard X-ray spectra of Seyfert galaxies. We aim to test the unified model of active galactic nuclei, and constrain differences and similarities between different classes of objects. We analyzed all public INTEGRAL IBIS/ISGRI data available on all the 165 Seyfert galaxies detected at z < 0.2. Our final sample consists of 44 Seyfert 1s, 29 Seyfert 1.5s, 78 Seyfert 2s, and 14 narrow-line Seyfert 1s. For each subsample, we stacked all the images, and derived their average hard X-ray spectra in the 17–250 keV energy range. We performed a detailed spectral analysis using both a model-independent and a model-dependent approach. All classes of Seyfert galaxies show on average the same nuclear continuum, as foreseen by the zeroth order unified model, with a cutoff energy of E_c ≥ 200 keV, and a photon index of Γ ≃ 1.8. The average optical depth of the Comptonizing medium is consistent for the different classes (τ ≃ 0.8). Compton-thin Seyfert 2s show a reflection component stronger than Seyfert 1s and Seyfert 1.5s. Most of this reflection is due to mildly obscured (10²³ cm^-2 ≤ N_H < 10²⁴ cm^-2) Seyfert 2s, which have a significantly stronger reflection component (R≃ 2) than Seyfert 1s (R ≤ 0.4), Seyfert 1.5s (R ≤ 0.4), and lightly obscured (N_H < 10²³ cm^-2) Seyfert 2s (R ≤ 0.5). This cannot be explained easily by the unified model. The absorber/reflector in mildly obscured Seyfert 2s might cover a large fraction of the X-ray source, and contain clumps of Compton-thick material. The large reflection found in the spectrum of mildly obscured Seyfert 2s reduces the amount of Compton-thick objects needed to explain the peak of the cosmic X-ray background. Our results are consistent with the fraction of Compton-thick sources being ~10%. The spectra of Seyfert 2s with and without polarized broad lines do not show significant differences, the only difference between the two samples being the higher hard X-ray and bolometric luminosity of Seyfert 2s with polarized broad lines. The average hard X-ray spectrum of narrow-line Seyfert 1s is steeper than those of Seyfert 1s and Seyfert 1.5s, probably due to a lower energy of the cutoff.

Artist's impression of the different environments surrounding the supermassive black hole in different classes of objects
This image illustrates two different configurations of the absorbing material surrounding Active Galactic Nuclei (AGN) of different types.

In the left of the two upper panels, the black hole and the accretion disc feeding it are surrounded by a patchy and sparse distribution of gas clouds. The clouds absorb only slightly the radiation emitted by the central source and give rise to little reflection (or none) at hard X-ray wavelengths.

In the right of the two upper panels, instead, the black hole and accretion disc are embedded in a much denser environment of clouds. This different morphology results in both stronger reflection at hard X-ray wavelengths and stronger absorption at shorter wavelengths.

Such a scenario could explain the excess of hard X-ray emission detected in a sample of AGN observed with INTEGRAL. The excess emission characterises the AGN affected by stronger absorption at shorter wavelengths (from the infrared to the soft X-ray bands).

The left upper panel corresponds to the active nucleus of a Lightly Obscured (LOB) Seyfert 2 galaxy, such as NGC 4258 (also known as M 106) which is shown in the lower panel to the left; the right upper panel corresponds to the active nucleus of a Mildly Obscured (MOB) Seyfert 2 galaxy, such as NGC 4941 which is shown in the lower panel to the right.

Credits: ESA / AOES Medialab and Adrian Zsilavec / Michelle Qualls / Adam Block / NOAO / AURA / NSF (NGC 4258); George Seitz / Adam Block / NOAO / AURA / NSF (NGC 4941)

Ratio between the average spectra of Seyfert 2s and Seyfert 1/1.5s
Ratio between the normalized average hard X-ray spectra of Seyfert 1/1.5s and Seyfert 2s. The average spectrum of Seyfert 2s shows a larger reflection component than that of Seyfert 1/1.5s. This cannot be easily explained by the unified model of AGN, which predicts that at these energies all the classes have the same average emission.
Credits: ISDC/C. Ricci

Ratio between the average spectra of MOB Sy2s and Sy1s
Ratio between the normalized average hard X-ray spectra of mildly obscured (MOB) Seyfert 2s and Seyfert 1s. Most of the reflection observed in the average spectrum of Seyfert 2s is due to the contribution of MOB Seyfert 2s, while less obscured objects have an average emission consistent with that that of Seyfert 1s and Seyfert 1.5s.
Credits: ISDC/C. Ricci

Fit to the spectrum of the Cosmic X-ray Background
Fit to the spectrum of the Cosmic X-ray Background (CXB) measured by INTEGRAL and Swift/BAT. The model used is a combination of our best-fit models for the average spectra of the different classes of objects, normalizing their ratio using the column density distribution of Treister et al. (2009). The large reflection of MOB Seyfert 2s reproduces well the peak at ~30 keV of the CXB.
Credits: ISDC/C. Ricci