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Register a new userReverberation Mapping of Active Galactic Nuclei
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Added by
Bradley M. Peterson
Publication date
2004
fields
Physics
and research's language is
English
Authors
B.M. Peterson
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Reverberation mapping is a proven technique that is used to measure the size of the broad emission-line region and central black hole mass in active galactic nuclei. More ambitious reverberation mapping programs that are well within the capabilities of Hubble Space Telescope could allow us to determine the nature and flow of line-emitting gas in active nuclei and to assess accurately the systematic uncertainties in reverberation-based black hole mass measurements.
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The central engines of Active Galactic Nuclei (AGNs) are powered by accreting supermassive black holes, and while AGNs are known to play an important role in galaxy evolution, the key physical processes occur on scales that are too small to be resolved spatially (aside from a few exceptional cases). Reverberation mapping is a powerful technique that overcomes this limitation by using echoes of light to determine the geometry and kinematics of the central regions. Variable ionizing radiation from close to the black hole drives correlated variability in surrounding gas/dust, but with a time delay due to the light travel time between the regions, allowing reverberation mapping to effectively replace spatial resolution with time resolution. Reverberation mapping is used to measure black hole masses and to probe the innermost X-ray emitting region, the UV/optical accretion disk, the broad emission line region and the dusty torus. In this article we provide an overview of the technique and its varied applications.
We present HI 21 cm spectroscopy from the GBT for the host galaxies of 31 nearby AGNs with direct M$_{textrm{BH}}$ measurements from reverberation mapping. These are the first published HI detections for 12 galaxies, and the spectral quality is generally an improvement over archival data for the remainder of the sample. We present measurements of emission-line fluxes, velocity widths, and recessional velocities from which we derive HI mass, total gas mass, and redshifts. Combining M$_{textrm{GAS}}$ with constraints on M$_{textrm{STARS}}$ allows exploration of the baryonic content of these galaxies. We find a typical M$_{textrm{GAS}}$/M$_{textrm{STARS}}$ fraction of 10%, with a few reaching $sim$30-50%. We also examined several relationships between M$_{textrm{STARS}}$, M$_{textrm{GAS}}$, M$_{textrm{BH}}$, baryonic mass, and morphological type. We find a weak preference for galaxies with larger M$_{textrm{GAS}}$ to host more massive black holes. We also find gas-to-stellar fractions to weakly correlate with later types in unbarred spirals, with an approximately constant fraction for barred spirals. Consistent with previous studies, we find declining M$_{textrm{GAS}}$/M$_{textrm{STARS}}$ with increasing M$_{textrm{STARS}}$, with a slope suggesting the gas reservoirs have been replenished. Finally, we find a clear relationship for M$_{textrm{BH}}$-M$_{textrm{BARY}}$ with a similar slope as M$_{textrm{BH}}$-M$_{textrm{STARS}}$ reported by Bentz & Manne-Nicholas (2018). The dwarf Seyfert NGC 4395 appears to follow this relationship as well, even though it has a significantly higher gas fraction and smaller M$_{textrm{BH}}$ than the remainder of our sample.
Close binaries of supermassive black holes (CB-SMBHs) with separations of $lesssim 0.1$pc as the final stage of galaxy mergers are sources of low frequency gravitational waves (GW), however, they are still elusive observationally because they are not spatially resolved. Fortunately, reverberation as echoes of broad emission lines to ionizing continuum conveys invaluable information of the dynamics of broad-line regions (BLRs) governed by supermassive black holes in the central regions of active galactic nuclei (AGNs). In this paper, we demonstrate how to composite the hybrid 2-dimensional transfer functions of binary BLRs around the CB-SMBHs in AGNs, providing an opportunity of identifying them from reverberation mapping (RM) data. It is found that there are variation-coupling effects in the transfer functions, arising from the coupling of CB-SMBH light curves in the Fourier space. We provide semi-analytical formulations of the transfer functions for kinematic maps of the gas. For cases with the simplest variation-coupling effects, we make calculations for several BLR models and reveal significant distinctions from those of single active black holes. In principle, the difference is caused by the orbital motion of the CB-SMBH systems. In order to search for CB-SMBHs in time-domain space, selection of target candidates should focus on local AGNs with H$beta$ double-peaked profiles and weaker near-infrared emission. High-fidelity RM-campaigns of monitoring the targets in future will provide opportunities to reveal these kinematic signatures of the CB-SMBHs and hence for measurements of their orbital parameters.
The changes of broad emission lines should be a crucial issue to understanding the physical properties of changing-look active galactic nucleus (CL-AGN). Here, we present the results of an intensive and homogeneous 6-month long reverberation mapping (RM) monitoring campaign during a low-activity state of the CL-AGN Seyfert galaxy NGC 3516. Photometric and spectroscopic monitoring was carried out during 2018--2019 with the Lijiang 2.4 m telescope. The sampling is 2 days in most nights, and the average sampling is $sim$3 days. The rest frame time lags of H$alpha$ and H$beta$ are $tau_{rm{H}alpha}=7.56^{+4.42}_{-2.10}$ days and $tau_{rm{H}beta}=7.50^{+2.05}_{-0.77}$ days, respectively. From a RMS H$beta$ line dispersion of $sigma_{rm{line}} = 1713.3 pm 46.7$ $rm{km}$ $rm{s^{-1}}$ and a virial factor of $f_{sigma}$ = 5.5, the central black hole mass of NGC 3516 is estimated to be $M_{rm{BH}}= 2.4^{+0.7}_{-0.3} times 10^{7} M_{odot}$, which is in agreement with previous estimates. The velocity-resolved delays show that the time lags increase towards negative velocity for both H$alpha$ and H$beta$. The velocity-resolved RM of H$alpha$ is done for the first time. These RM results are consistent with other observations before the spectral type change, indicating a basically constant BLR structure during the changing-look process. The CL model of changes of accretion rate seems to be favored by long-term H$beta$ variability and RM observations of NGC 3516.
X-ray reverberation in Active Galactic Nuclei, believed to be the result of the reprocessing of coronal photons by the underlying accretion disc, has allowed us to probe the properties of the inner-most regions of the accretion flow and the central black hole. Our current model (KYNREFREV) computes the time-dependent reflection spectra of the disc as a response to a flash of primary power-law radiation from a point source corona located on the axis of the black hole accretion disc (lamp-post geometry). Full relativistic effects are taken into account. The ionization of the disc is set for each radius according to the amount of the incident primary flux and the density of the accretion disc. We detect wavy residuals around the best-fit reverberation model time lags at high frequencies. This result suggests that the simple lamp-post geometry does not fully explain the X-ray source/disc configuration in Active Galactic Nuclei. There has been a noticeable progress into the development of codes for extended coronae (Wilkins+16, Chainakun & Young 2017, Taylor & Reynolds 2018a,b). Indeed, the model from Chainakun & Young (2017), consisting of two axial point sources illuminating an accretion disc that produce the reverberation lags is able to reproduce the observed time-lag versus frequency spectra. The goal of this paper is to observationally justify the need for an extended corona in order to provide (in the near future) with a mathematical formulation of a model for an extended corona in its simplest form.
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