R. L. Fiedler

Daily observations of compact extragalactic radio sources at 2695 and 8085 MHz, 1979-1985

Radio observations of 33 compact extragalactic sources, obtained at 2.695 and 8.085 GHz using the Green Bank interferometer as part of a regular monitoring program during 1979-1985, are reported. The calibration and data-analysis procedures are described, and the results are presented in extensive tables and graphs. Although the light-curve variations are found to be dominated by intrinsic changes, some refractive interstellar scintillation effects and anomalous light-curve dropouts (tentatively attributed to occultation events) are also observed. An unexpected linear dependence of the rms variation at 2.695 GHz compared with that at 8.085 GHz was found, indicating that the onset and evolution of flares must be similar in different sources. 39 references.

A Dual-Frequency, Multiyear Monitoring Program of Compact Radio Sources

We present light curves for 149 sources monitored with the Green Bank Interferometer. The light curves are at two radio frequencies (approximately 2.5 and 8.2 GHz) and range from 3 to 15 yr in length, covering the interval 1979-1996, and have a typical sampling of one flux density measurement every 2 days. We have used these light curves to conduct various variability analysis (rms flux density variations and autoregressive, integrated, moving average modeling) of these sources. We find suggestive though not unambiguous evidence that these sources have a common, broadband mechanism for intrinsic variations, in agreement with previous studies of a subset of these sources. We also find that the sources generally display a short-term variability (~10 days) that arises from radio-wave scattering in an extended medium. These conclusions extend those of Fiedler et al., who used a subsample of these data. The primary motivation for this monitoring program was the identification of extreme scattering events (ESEs). In an effort to identify ESEs in a systematic manner, we have taken the wavelet transform of the light curves. We find 15 events in the light curves of 12 sources that we classify as probable ESEs. However, we also find that five ESEs previously identified from these data do not survive our wavelet selection criteria. Future identification of ESEs will probably continue to rely on both visual and systematic methods. Instructions for obtaining the data are also presented.

Daily observations of compact radio sources at 2.7 and 8.1 GHz - 1979-1987

2.7 and 8.1 GHz radio light curves are presented for 46 compact radio sources observed with the Green Bank interferometer during the period 1979-1987. Flux densities for the period 1979-1985 were reported in a previous paper. The flux densities reported here extend the daily monitoring an additional 2 years. The interferometer is currently dedicated to searching for extreme scattering events (ESEs). ESEs may be found in the radio light curves of 0300+470, 0333+321, 0954+658, 1502+106, 1611+343, 1821+107, and 2352+495 during the period 1979-1987, including three events observed in 0300+470 and 0333+321 during the additional 2 years reported here

The Extreme Scattering Event toward PKS 1741–038: VLBI Images

We report multiepoch VLBI observations of the source PKS 1741-038 (OT 068) as it underwent an extreme scattering event (ESE). Observations at four epochs were obtained, and images were produced at three of these. One of these three epochs was when the source was near the minimum flux density of the ESE, the other two were as the flux density of the source was returning to its nominal value. The fourth epoch was at the maximum flux density during the egress from the ESE, but the VLBI observations had too few stations to produce an image. During the event the source consisted of a dominant, compact component, essentially identical to the structure seen outside the event. However, the sources diameter increased slightly at 13 cm, from near 0.6 mas outside the ESE to near 1 mas during the ESE. An increase in the sources diameter is inconsistent with a simple refractive model in which a smooth refractive lens drifted across the line of sight to PKS 1741-038. We also see no evidence for ESE-induced substructure within the source or the formation of multiple images, as would occur in a strongly refractive lens. However, a model in which the decrease in flux density during the ESE occurs solely because of stochastic broadening within the lens requires a larger broadening diameter during the event than is observed. Thus, the ESE toward PKS 1741-038 involved both stochastic broadening and refractive defocusing within the lens. If the structure responsible for the ESE has a size of order 1 AU, the level of scattering within an ESE lens may be a factor of 107 larger than that in the ambient medium. A filamentary structure could reduce the difference between the strength of scattering in the lens and ambient medium, but there is no evidence for a refractively induced elongation of the source. We conclude that, if ESEs arise from filamentary structures, they occur when the filamentary structures are seen lengthwise. We are able to predict the amount of pulse broadening that would result from a comparable lens passing in front of a pulsar. The pulse broadening would be no more than 1.1 μs, consistent with the lack of pulse broadening detected during ESEs toward the pulsars PSR B1937+21 and PSR J1643-1224. The line of sight toward PKS 1741-038 is consistent with a turbulent origin for the structures responsible for ESEs. The source PKS 1741-038 lies near the radio Loop I and is seen through a local minimum in 100 μm emission.

Observations of SS 433 at 2695 and 8085 MHz, 1979-1985

Flux densities of SS 433 at 2695 and 8085 MHz spanning the period July 1979-December 1985 are presented. The variations may be characterized as a clustering of flare events separated by periods of quiescent emission. A harmonic analysis reveals no significant periodicities in either the entire 7 yr data set or in subsets corresponding to active and quiescent time frames. There is, however, broadband power from roughly 50 to 200 days which fades during 1985. Moreover, it is found that the temporal power distribution of SS 433 at both frequencies strongly resembles l/f noise. Finally, a comparison with 408 MHz data (Bonsignori-Facondi, et al. 1986) shows variations in the light curve to lag those at 2695 and 8085 MHz by approximately 4 days. 20 references.

The Extreme Scattering Event toward 1741–038: H I Absorption

We report multi-epoch Very Large Array H I absorption observations of the source 1741-038 (OT -068) before and during an extreme scattering event (ESE). Observations at four epochs, three during the ESE, were obtained. We find no changes in the equivalent width, maximum optical depth, or velocity of maximum optical depth during the ESE, but we do find a secular trend of decreasing maximum optical depth between our observations and ones by other observers a decade prior. The resulting limit on the H I column density change during the ESE for a structure with a spin temperature Ts is 6.4 × 1017 cm-2(Ts/10 K). Tiny-scale atomic structures (TSAS), with NH ~ 3 × 1018 cm-2, are ruled out marginally by this limit, though geometric arguments may allow this limit to be relaxed. Galactic halo molecular clouds that are opaque in the H I line cannot be excluded from causing the ESE because the observed velocity range covers only 25% of their allowed velocity range.

Extreme scattering events

Daily flux density measurements of compact extragalactic radio sources using the Green Bank interferometer (36 sources)1 and the NRAO 300ft antenna (340 sources)2 reveal several unusual variations in the light curves that do not follow typical source variations. Unusual variations have been identified in the light curves of eight quasars. Refractive effects involving small‐scale inhomogeneities in an ionized structure in the interstellar medium seem to most likely explanation for the unusual variations observed.