R. J. Leacock

Optical observations of 22 violently variable extragalactic sources - 1968-1986

Broadband photographic observations of 22 optically violent variable (OVV) active galactic nuclei are presented. Over 3100 observations made between 1968 and 1986 at Rosemary Hill Observatory are tabulated and displayed graphically. The majority of the observations were made in either the Johnson B system or the international photographic (PG) system. Multicolor data are presented for a few objects. Descriptions of the light curves include the assignment of each OVV to an arbitrary variability subclass. The light curves, some extending over 18 yr, are analyzed for linear trends and underlying structure using linear regression and unequal-interval Fourier transform techniques. The results of the analysis for each of the 22 objects are given, and models of the light variations of 3C 120, 3C 345, and 3C 446 are presented. The models of these light curves show underlying structure with rapid variations superimposed. The time scales seen in the light curves of 3C 120, 3C 345, and 3C 446 are compared with characteristic time scales found in massive-accretion-disk models. The time scales most likely to be responsible for the optical behavior are either the viscous or the thermal time scales. 50 references.

The longer optical time scales of a large sample of quasars

Of a carefully studied sample of 60 quasars, 75% displayed evidence of long-term changes in the base levels of their optical fluxes. Internally consistent light curves were analyzed by three different methods in an effort to assign time scales to these changes. The familiar rapid, large-amplitude flares were specifically excluded from the study. The majority of the observed time scales range from 3 to 15 yr, but when they are translated into the rest frames of the objects, there is a strong concentration between 2 and 6 yr.

Long-term optical variations of 20 violently variable extragalactic radio sources

Photometric data for 20 optically violent variable extragalactic radio sources have been obtained during a continuous 11-year monitoring program. Light curves and photometric data, in some instances in more than one color, are presented and discussed. Evidence of temporal changes in the variability characteristics of particular sources is examined.

Correlated radio and optical variations in a sample of AGN

Radio light curves of 20 sources from the Metsahovi monitoring program are compared with optical light curves collected from the literature. The Discrete Correlation Function analysis is applied to the data sets. A new qualitative method to study correlations is introduced, where the radio light curves are replaced by model light curves consisting of exponential outbursts. The optical flux level is compared to the phase and flux level of the model flares. Seven sources show clear correlations using the DCF analysis with six more showing a possible correlation between optical and radio events with time lags from zero to several hundred days. For twelve sources at least one simultaneous optical and radio event is seen. For eleven sources, when comparing optical flux levels with the phase of the model radio outbursts, the optical flux levels were high at the peak of the model radio outbursts. For sixteen sources, when comparing the optical flux level with the radio model flare flux level, the optical flux level was high when radio flux level was high, as well.

Optical behavior of 20 violently variable extragalactic radio sources

A 7-yr photometric study of nearly 200 extragalactic radio sources has confirmed 20 of these objects as violent optical variables. Light curves and photometric data are given for these 20 sources, which are placed in one of four categories according to the type of variablitity displayed. (AIP)

Search for correlated radio and optical events in long-term studies of extragalactic sources

For the first time, long-term records of radio and optical fluxes of a large sample of variable extragalactic sources have been assembled and compared, with linear cross-correlation analysis being used to reinforce the visual comparisons. Only in the case of the BL Lac object OJ 287 is the correlation between radio and optical records strong. In the majority of cases there is no evidence of significant correlation, although nine sources show limited or weak evidence of correlation. The results do not support naive extrapolation of the expanding source model. The general absence of strong correlation between the radio and optical regions has important implications for the energetics of events occurring in such sources. (AIP)

THE MULTIFREQUENCY SPECTRAL EVOLUTION OF BLAZAR 3C 345 DURING THE 1991 OUTBURST

The blazar 3C 345 underwent a 2.5 mag optical outburst between 1990 November and 1991 May. We have obtained 10 nearly simultaneous multifrequency spectra during the course of the outburst in order to study the multifrequency spectral variations of 3C 345 as a function of time. Although our observations were not sampled frequently enough to completely resolve the variations in every frequency band, the general rise and decline of the outburst were seen in the UV through radio with differing rise times. Simulations of an electron distribution injected into a tangled magnetic field show a relationship between frequency and characteristic timescale that was also observed in the radio variations of 3C 345. The two X-ray observations made during the monitoring period showed no evidence of variability. The multifrequency spectrum was modeled with two major components: a relativistic jet and a relativistic thermal accretion disk. Models calculated for each spectrum indicate that the outburst can be explained in terms of these models by varying the high-energy cutoff of the injected electron distribution in the jet model, while also varying the mass accretion rate in the disk model. There is marginal evidence that the inferred accretion rate varies with the jet luminosity.

Multifrequency observation of the optically violent variable quasar 3C 446

Extensive optical and radio monitoring data and seven multifrequency spectra were obtained of the violently variable quasar 3C 446. The monitoring data suggest a correlation between the radio and optical outbursts, with the optical flare preceding the radio activity by 400-600 days. A difference in the statistical behavior of the optical and radio variability indicates that considerable processing occurs to the optical emitting plasma before it becomes radio emitting plasma. Within the radio band, outbursts proceed from high to low frequencies. An outburst in 1983 showed greater and more rapid variation in the optical than in the near-IR region. The 10-100 μm fluxes did not follow the higher frequency variation, suggesting a time delay between these spectral domains. During another time, the X-ray emission varied on a time scale of days and more rapidly than the UV or optical emission. On a time scale of weeks-months, the X-ray fluxes are well correlated with the UV-IR fluxes but not with the radio fluxes. The multifrequency data show that the flat radio spectrum turns over at 3-10 x 10^(11) Hz and the continuum steepens with frequency; ɑ(IR) = 1.1, ɑ(opt-UV) = -2 to -3. The X-ray emission lies an order of magnitude above an extrapolation of the optical-UV spectrum and has a harder spectrum. The power is primarily concentrated in the submillimeter and infrared region. When the source is faint, a blue bump may be present. The flux in the Lycx line is proportional to the UV continuum flux density when the source is bright but is independent of the continuum level when the source is faint. The data suggest that the X-rays are produced by the inverse Compton process from an emitting region (10^(16) cm) smaller than but related to the synchrotron emitting UV-IR region. The characteristic size of the emitting region increases with decreasing frequency from 10^(16) (X-ray region) to 1-3 x 10^(17) cm (far IR-submillimeter region) to 10^(19)-10^(20) cm (radio region). Plasma conditions are best constrained at the frequency when the source becomes transparent, the far IR-submillimeter band, where B ≈ 3-100 G, n ≈ 40-100 cm^(-3); and the Doppler boosting factor δ ≈ 1-5.

Four-color photometry of OJ 287 during its recent three-magnitude decline

Results are discussed for photographic UBV observations of OJ 287 to determine whether its brightness changes are accompanied by variations in color. A total of 99 sets of observations were obtained over a three-year declining phase, and a few IR observations were also made. The results of the four-color observations are graphed together with least-squares parabolic curves fitted to the UBV observations through April 1974, and they reveal random short-term fluctuations of small amplitude superimposed on a general decline of about three magnitudes. Short- and long-term (U-B) and (B-V) color indices are computed, and all indicate no significant color trend during the three-magnitude decline.

Two multifrequency observations of 3C 371

We present observations of 3C 371, made at frequencies from the radio to the ultraviolet, which were coordinated during two short time intervals separated by 3 months. We also present 1 keV X-ray flux densities measured at a different time. The multifrequency measurements indicate spectral steepening at visual wavelengths, and that an extrapolation of the ultraviolet continuum falls below the X-ray data. We explain the infrared through X-ray data as relativistically beamed synchrotron self-Compton emission and derive source parameters for two possible models. Our ultraviolet spectra both show strong Lyɑ emission at the same redshift as weak optical emission lines reported previously. We favor production of these lines by recombination of gas after its ionization by the ultraviolet to X-ray continuum radiation. We tentatively identify C IV and N v absorption lines in one of our ultraviolet spectra, which, if real, suggest the presence of a hot (~ 3 x 10^5 K) gaseous halo in 3C 371.