Engineering topics

Peterson, B. M.

Publications and source records attributed to Peterson, B. M..

22 records · Page 2

NASA NTRS1992

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. III - Further observations of NGC 5548 at optical wavelengths

The results of the second year of an intensive ground-based spectroscopic and photometric study of variability in the bright Seyfert 1 galaxy NGC 5548 are reported in order to study the relationship between continuum and emission-line variability. Relative to the first year of the monitoring program, the nucleus of NGC 5548 was considerably fainter and the continuum variations slower during the second year, but the continuum H-beta cross-correlation results for the two years are nearly identical. The variations in the broad H-beta emission-line lag behind those in the continuum by somewhat less than 20 days, as concluded from the first year's data.

Peterson, B. M.↗

NASA NTRS1992

Anisotropic line emission and the geometry of the broad-line region in active galactic nuclei

Attention is given to an escape probability-based formalism, including the interaction with a background opacity, which is used to predict line-beaming factors for broad-line region (BLR) clouds of AGN. Several computational details necessary to model gas exposed to an intense nonthermal continuum are outlined. It is found that all hydrogen lines are strongly inwardly beamed, and that C IV 1549 is moderately beamed. The implications of this for the geometry of the emission-line regions of AGN are discussed. Beaming is found to cause the source-cloud separation to be overestimated by variability studies (so that the radiation field at the BLR is even more intense than previously thought); it can even cause spherical geometries to have little instantaneous response to continuum changes.

Ferland, G. J.↗

NASA NTRS1991

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. II - An intensive study of NGC 5548 at optical wavelengths

A large, international program of ground-based optical spectroscopy and photometry of the variable Seyfert 1 galaxy NGC 5548 undertaken in support of an IUE monitoring campaign is described. This contribution presents the data base and describes the methods used to correct for systematic differences between spectra from different sources. Optical continuum and H-beta emission-line light curves are derived from the spectra. The behavior of the optical continuum is qualitatively the same as the behavior of the ultraviolet continuum. Cross-correlation of the ultraviolet and optical continuum measurements does not reveal any significant lag between them. The h-beta emission-line variations show the same basic pattern as seen in the continuum and ultraviolet emission lines, with H-beta lagging behind the continuum by about 20 days. This is significantly larger than the about 10 day lag deduced for Ly-alpha.

Peterson, B. M.↗

NASA NTRS1986

Evidence for a low-density component in the broad-line region of Seyfert 1 galaxies

It is found that the forbidden 4959-5007 A broad O III lines, good candidates for the excess shelf emission, are found in the vicinity of H-beta in Seyfert 1 galaxy spectra. The shelf-study method used to remove Fe II assumes that the profiles of the contributors to the shelf are known, and that subtraction of these contributors in the correct amount will result in a fairly smooth H-beta profile. Data were obtained with the Ohio State University Image-Dissector Scanner with a 600 line/mm grating, on a 1.8-m reflector. The estimated cloud electron density of 1 billion/sq cm is at the low end of those used for BLR photoionization models. The presence of a broad residual emission feature in the neighborhood of 5050 A after subtraction of the shelf is thought to be the result of a weak broad emission in this range from yet another species.

Crenshaw, D. M.↗