5,646 research outputs found
Spectroastrometric Reverberation Mapping of Broad-line Regions
Spectroastrometry measures source astrometry as a function of
wavelength/velocity. Reverberations of spectroastrometric signals naturally
arise in broad-line regions (BLRs) of active galactic nuclei as a result of the
continuum variations that drive responses of the broad emission lines with time
delays. Such signals provide a new diagnostic for mapping BLR kinematics and
geometry, complementary to the traditional intensity reverberation mapping (RM)
technique. We present the generic mathematical formulism for spectroastrometric
RM and show that under realistic parameters of a phenomenological BLR model,
the spectroastrometric reverberation signals vary on a level of several to tens
of microarcseconds, depending on the BLR size, continuum variability, and
angular-size distance. We also derive the analytical expressions of
spectroastrometric RM for an inclined ring-like BLR. We develop a Bayesian
framework with a sophisticated Monte-Carlo sampling technique to analyze
spectroastrometic data and infer the BLR properties, including the central
black hole mass and angular-size distance. We demonstrate the potential of
spectroastrometric RM in spatially resolving BLR kinematics and geometry
through a suite of simulation tests. An application to realistic observation
data of 3C~273 obtains tentative, but enlightening results, reinforcing the
practical feasibility of conducting spectroastrometric RM experiments on bright
AGNs with the operating Very Large Telescope Interferometer as well as possibly
with the planned next-generation 30m-class telescopes.Comment: 22 pages, 17 figures, 2 tables; ApJ in press; The code BRAINS
available at https://github.com/LiyrAstroph/BRAIN
A Bayesian Approach to Estimate the Size and Structure of the Broad-line Region in Active Galactic Nuclei Using Reverberation Mapping Data
This is the first paper in a series devoted to systematic study of the size
and structure of the broad-line region (BLR) in active galactic nuclei (AGNs)
using reverberation mapping (RM) data. We employ a recently developed Bayesian
approach that statistically describes the variabibility as a damped random walk
process and delineates the BLR structure using a flexible disk geometry that
can account for a variety of shapes, including disks, rings, shells, and
spheres. We allow for the possibility that the line emission may respond
non-linearly to the continuum, and we detrend the light curves when there is
clear evidence for secular variation. We use a Markov Chain Monte Carlo
implementation based on Bayesian statistics to recover the parameters and
uncertainties for the BLR model. The corresponding transfer function is
obtained self-consistently. We tentatively constrain the virial factor used to
estimate black hole masses; more accurate determinations will have to await
velocity-resolved RM data. Application of our method to RM data with Hbeta
monitoring for about 40 objects shows that the assumed BLR geometry can
reproduce quite well the observed emission-line fluxes from the continuum light
curves. We find that the Hbeta BLR sizes obtained from our method are on
average ~20% larger than those derived from the traditional cross-correlation
method. Nevertheless, we still find a tight BLR size-luminosity relation with a
slope of alpha=0.55\pm0.03 and an intrinsic scatter of ~0.18 dex. In
particular, we demonstrate that our approach yields appropriate BLR sizes for
some objects (such as Mrk 142 and PG 2130+099) where traditional methods
previously encountered difficulties.Comment: 17 pages, 10 figures, 2 tables; minor reversion to match the
published versio
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