Part Of Active Galactic Nuclei Research Articles

Properties of the Obscuring Torus in NGC 1052 from Multiepoch Broadband X-Ray Spectroscopy

Abstract Obscuration of the innermost parts of active galactic nuclei (AGN) is observed in the majority of the population both in the nearby universe and at high redshift. However, the nature of the structures causing obscuration, especially in low-luminosity AGN, is poorly understood at present. We present a novel approach to multiepoch broadband X-ray spectroscopy, anchored in the long-term average spectrum in the hard X-ray band, applied to the nearby, X-ray-bright AGN in the galaxy NGC 1052. From spectral features due to X-ray reprocessing in the circumnuclear material, based on a simple, uniform-density torus X-ray reprocessing model, we find a covering factor of 80%–100% and a globally averaged column density in the range (1−2) × 1023 cm−2. This closely matches the independently measured variable line-of-sight column density range, leading to a straightforward and self-consistent picture of the obscuring torus in NGC 1052, similar to several other AGN in recent literature. Comparing this X-ray-constrained torus model with measurements of spatially resolved subparsec absorption from radio observations, we find that it may be possible to account for both X-ray and radio data with a torus model featuring a steep density gradient along the axis of the relativistic jets. This provides a valuable direction for the development of improved physical models for the circumnuclear environment in NGC 1052 and potentially in a wider class of AGN.

Unveiling Sub-pc Supermassive Black Hole Binary Candidates in Active Galactic Nuclei

Abstract The elusive supermassive black hole binaries (SMBHBs) are thought to be the penultimate stage of galaxy mergers, preceding a final coalescence phase. SMBHBs are sources of continuous gravitational waves, possibly detectable by pulsar timing arrays; the identification of candidates could help in performing targeted gravitational wave searches. Due to SMBHBs’ origin in the innermost parts of active galactic nuclei (AGN), X-rays are a promising tool for unveiling their presence, by means of either double Fe Kα emission lines or periodicity in their light curve. Here we report on a new method for selecting SMBHBs by means of the presence of a periodic signal in their Swift Burst Alert Telescope (BAT) 105 month light curves. Our technique is based on Fisher’s exact g-test and takes into account the possible presence of colored noise. Among the 553 AGN selected for our investigation, only the Seyfert 1.5 galaxy Mrk 915 emerges as a candidate SMBHB; from subsequent analysis of its light curve we find a period P 0 = 35 ± 2 months, and the null hypothesis is rejected at the 3.7σ confidence level. We also present a detailed analysis of the BAT light curve of the only previously X-ray-selected binary candidate source in the literature, the Seyfert 2 galaxy MCG+11-11-032. We find P 0 = 26.3 ± 0.6 months, consistent with the one inferred from previously reported double Fe Kα emission lines.

Comparison between the Influence of Outflows and Supermassive Binary Black Holes in Active Galactic Nuclei on the Polarization Angle Profiles

Optical polarization signal coming from the innermost part of active galactic nuclei (AGNs) is highly sensitive on the geometry and kinematics of the central engine. Due to the compact size of the AGN central region, which is spatially unresolved with current observing facilities, we rely on spectropolarimetry which can provide us insight in their hidden physics. We model equatorial scattering for various broad line region (BLR) configurations using radiative transfer code STOKES. We analyze the polarization position angle () profiles for four supermassive binary black holes (SMBBHs) models and compare them with the profiles found for a unified model in AGNs with a single supermassive black hole (SMBH) and with notable outflowing velocity component of the BLR.. We find that the profiles for SMBBHs are axis-symmetric, while the profiles for a single SMBHs are point-symmetric and that there is a clear distinction between the two cases. Our conclussion is that spectropolarimetry might play a key role in the search for the SMBBHs by inspecting the polarization angle profiles.

Modeling optical and UV polarization of AGNs

We model the spectropolarimetric signature resulting from the radiative coupling between the innermost parts of active galactic nuclei (AGNs). We use a new public version of STOKES, a Monte Carlo radiative transfer code presented in the first paper of this series. The code has been significantly improved for computational speed and polarization imaging has been implemented. We couple continuum sources with equatorial scattering regions, polar outflows, and toroidal obscuring dust and we study the resulting polarization. We compute a grid of thermal AGN models for different half-opening angles of the torus and polar winds. We also consider a range of optical depths for equatorial and polar electron scattering and investigate how the model geometry influences the type-1/type-2 polarization dichotomy for thermal AGNs. We put new constrains on the inflowing medium within the inner walls of the torus. The inflow should be confined to the common equatorial plane of the torus and the accretion disc and have a radial optical depth of 1 < tau < 3. Our modeling of type-1 AGNs also indicates that the torus is more likely to have a large (~ 60{\deg}) half-opening angle. Polarization perpendicular to the axis of the torus may arise at a type-1 viewing angle for a torus half-opening angle of 30{\deg}- 45{\deg} or polar outflows with an optical depth near unity. Our modeling suggests that most Seyfert-2 AGN must have a half-opening angle > 60{\deg} to match the level of perpendicular polarization expected. If outflows are collimated by the torus inner walls, they must not be optically thick (tau < 1) in order to preserve the polarization dichotomy. The wind's optical depth is found not to play a critical role for the degree of polarization of type-2 thermal AGNs but it has a significant impact on the type-1/type-2 polarization dichotomy when the optical depth exceeds tau = 0.3.

Some characteristic timescales in a numerical model of the central parts of active galactic nuclei

A realistic number of particles in the simulations of the cental parts of active galactic nuclei (AGNs) remains a computational challenge. For this reason, we describe the timescales of the dynamic processes of our AGN model as a function of the number of particles.