6,325 research outputs found
The Lick AGN Monitoring Project: Velocity-Delay Maps from the Maximum-Entropy Method for Arp 151
We present velocity-delay maps for optical H I, He I, and He II recombination
lines in Arp 151, recovered by fitting a reverberation model to
spectrophotometric monitoring data using the maximum-entropy method. H I
response is detected over the range 0-15 days, with the response confined
within the virial envelope. The Balmer-line maps have similar morphologies but
exhibit radial stratification, with progressively longer delays for Hgamma to
Hbeta to Halpha. The He I and He II response is confined within 1-2 days. There
is a deficit of prompt response in the Balmer-line cores but strong prompt
response in the red wings. Comparison with simple models identifies two classes
that reproduce these features: freefalling gas, and a half-illuminated disk
with a hotspot at small radius on the receding lune. Symmetrically illuminated
models with gas orbiting in an inclined disk or an isotropic distribution of
randomly inclined circular orbits can reproduce the virial structure but not
the observed asymmetry. Radial outflows are also largely ruled out by the
observed asymmetry. A warped-disk geometry provides a physically plausible
mechanism for the asymmetric illumination and hotspot features. Simple
estimates show that a disk in the broad-line region of Arp 151 could be
unstable to warping induced by radiation pressure. Our results demonstrate the
potential power of detailed modeling combined with monitoring campaigns at
higher cadence to characterize the gas kinematics and physical processes that
give rise to the broad emission lines in active galactic nuclei.Comment: 7 pages, 5 figures, accepted for publication in the Astrophysical
Journal Letter
An Alternative Approach To Measuring Reverberation Lags in Active Galactic Nuclei
Motivated by recent progress in the statistical modeling of quasar
variability, we develop a new approach to measuring emission-line reverberation
lags to estimate the size of broad-line regions (BLRs) in active galactic
nuclei. Assuming that all emission-line light curves are scaled, smoothed, and
displaced versions of the continuum, this alternative approach fits the light
curves directly using a damped random walk model and aligns them to recover the
time lag and its statistical confidence limits. We introduce the mathematical
formalism of this approach and demonstrate its ability to cope with some of the
problems for traditional methods, such as irregular sampling, correlated
errors, and seasonal gaps. We redetermine the lags for 87 emission lines in 31
quasars and reassess the BLR size--luminosity relationship using 60 H-beta
lags. We confirm the general results from the traditional cross-correlation
methods, with a few exceptions. Our method, however, also supports a broad
range of extensions. In particular, it can simultaneously fit multiple lines
and continuum light curves which improves the lag estimate for the lines and
provides estimates of the error correlations between them. Determining these
correlations is of particular importance for interpreting emission-line
velocity--delay maps. We can also include parameters for luminosity-dependent
lags or line responses. We use this to detect the scaling of the BLR size with
continuum luminosity in NGC 5548.Comment: match to the version accepted to ApJ. New code release available at
http://www.astronomy.ohio-state.edu/~yingzu/spear.htm
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