1,331 research outputs found
Ground-based studies of emission-line variability: Recent results for NGC 5548 and future plans
Ever since emission-line variability was first detected in AGNs, it has been clearly understood that light-travel time effects in the broadline region (BLR) afford a tool for studying the structure of these spatially unresolved regions. However, only within the last few years has the observational problem become well defined in terms of the sampling rates and quality of data necessary to address the problem correctly. The amount of data required to extract structural information about the BLR from the continuum and emission-line light curves is so considerable that the most promising approach seemed to be to combine the observational efforts. The goal was to work together to produce a large, high-quality database which would then be released to individual investigators for more complete analysis. The cornerstone of this effort, was a joint NASA/ESA/SERC program to monitor the spectrum of NGC 5548 every four days with IUE from December 1988 through August 1988. A concurrent ground-based program was organized in an effort to enhance the scientific return on the project by extending the wavelength coverage and providing higher spectral resolution and signal to noise ratios than would be possible with IUE. It also turned out that the temporal resolution of the ground-based program was somewhat better than the temporal resolution of the IUE program, and the temporal baseline is longer and continues to grow. The database and initial results are presented by Peterson et al., (1991). The important results of the ground-based program are summarized, some additional applications of the existing data are described, and a few comments are made on what can be done in the future to expand on this work
Toward Precision Measurement of Central Black Hole Masses
We review briefly direct and indirect methods of measuring the masses of
black holes in galactic nuclei, and then focus attention on supermassive black
holes in active nuclei, with special attention to results from reverberation
mapping and their limitations. We find that the intrinsic scatter in the
relationship between the AGN luminosity and the broad-line region size is very
small, ~0.11 dex, comparable to the uncertainties in the better reverberation
measurements. We also find that the relationship between reverberation-based
black hole masses and host-galaxy bulge luminosities also seems to have
surprisingly little intrinsic scatter, ~0.17 dex. We note, however, that there
are still potential systematics that could affect the overall mass calibration
at the level of a factor of a few.Comment: 10 pages, 2 figures. To be published in the Proceedings of IAU
Symposium 267 "Co-Evolution of Central Black Holes and Galaxies
Determining Central Black Hole Masses in Distant Active Galaxies and Quasars. II. Improved Optical and UV Scaling Relationships
We present four improved empirical relationships useful for estimating the
central black hole mass in nearby AGNs and distant luminous quasars alike using
either optical or UV single-epoch spectroscopy. These mass-scaling
relationships between line widths and luminosity are based on recently improved
empirical relationships between the broad-line region size and luminosities in
various energy bands and are calibrated to the improved mass measurements of
nearby AGNs based on emission-line reverberation mapping. The mass-scaling
relationship based on the Hbeta line luminosity allows mass estimates for
low-redshift sources with strong contamination of the optical continuum
luminosity by stellar or non-thermal emission, while that based on the C IV
lambda 1549 line dispersion allows mass estimates in cases where only the line
dispersion (as opposed to the FWHM) can be reliably determined. We estimate
that the absolute uncertainties in masses given by these mass-scaling
relationships are typically around a factor of 4. We include in an Appendix
mass estimates for all the Bright Quasar Survey (PG) quasars for which direct
reverberation-based mass measurements are not available.Comment: 48 pages including 12 figures and 7 tables. Accepted by Ap
Black Hole Masses from Reverberation Measurements
We have reanalyzed in a consistent way existing reverberation data for 35
AGNs for the purpose of refining the black hole masses derived from these data.
We find that the precision (or random component of the error) of
reverberation-based black hole mass measurements is typically around 30%,
comparable to the precision attained in measurement of black hole masses in
quiescent galaxies by gas or stellar dynamical methods. As discussed in this
volume by Onken et al., we have established an absolute calibration for AGN
reverberation-based masses by assuming that AGNs and quiescent galaxies follow
an identical relationship between black hole mass and host-galaxy bulge
velocity dispersion. The scatter around this relationship implies that the
typical systematic uncertainties in reverberation-based black hole masses are
smaller than a factor of three. We present a preliminary version of a
mass-luminosity relationship that is much better defined than any previous
attempt. Scatter about the mass-luminosity relationship for these AGNs appears
to be real and could be correlated with either Eddington ratio or source
inclination.Comment: To appear in "The Interplay Among Black Holes, Stars, and ISM in
Galactic Nuclei," IAU Coll. 222, eds. Storchi-Bergmann, Ho, and Schmitt. 6
pages, 2 figures. Higher resolution versions of the figures are available at
http://www.astronomy.ohio-state.edu/~peterson/Astroph
Variability of Active Galactic Nuclei
Continuum and emission-line variability of active galactic nuclei provides a
powerful probe of microarcsecond scale structures in the central regions of
these sources. In this contribution, we review basic concepts and methodologies
used in analyzing AGN variability. We develop from first principles the basics
of reverberation mapping, and pay special attention to emission-line transfer
functions. We discuss application of cross-correlation analysis to AGN light
curves. Finally, we provide a short review of recent important results in the
field.Comment: 63 pages, 41 figures. The larger figures can be retrieved at higher
resolution from http://www.astronomy.ohio-state.edu/~peterson/INAOE
An Intrinsic Baldwin Effect in the H-beta Broad Emission Line in the Spectrum of NGC 5548
We investigate the possibility of an intrinsic Baldwin Effect (i.e.,nonlinear
emission-line response to continuum variations) in the broad H-beta emission
line of the active galaxy NGC 5548 using cross-correlation techniques to remove
light travel-time effects from the data. We find a nonlinear relationship
between the H-beta emission line and continuum fluxes that is in good agreement
with theoretical predictions. We suggest that similar analysis of multiple
lines might provide a useful diagnostic of physical conditions in the
broad-line region.Comment: 15 pages, 4 figures. Accepted for publication in The Astrophysical
Journa
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