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T. J. W. Lazio, E. B. Waltman (NRL), F. Ghigo (NRAO), K. J. Johnston (USNO)
Flux densities for 149~sources were monitored with the Green Bank Interferometer for durations ranging from~3 to~15~yrs, covering the interval 1979--1996, with most sources observed for~6~yrs. Observations were at two radio frequencies (approximately 2.5 and~8.2~GHz) and have a typical sampling of one flux density measurement every 2~days. We have used these light curves to conduct various variability analysis of the sources. We find suggestive, though not unambiguous evidence, that these sources have a common, broadband mechanism for intrinsic variations. We also find that the extrinsic variation is more consistent with radio-wave scattering in an extended medium rather than in a thin screen. The primary motivation for this monitoring program was the identification of extreme scattering events. 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.
We present examples of the light curves and variability analyses. Instructions for obtaining the data are also given.
The GBI is a facility of the National Science Foundation and was operated by the National Radio Astronomy Observatory under contract to the USNO and NRL during these observations. A portion of this work was performed while TJWL held a National Research Council-NRL Research Associateship. Basic research in radio astronomy at the NRL is supported by the Office of Naval Research.
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The author(s) of this abstract have provided an email address for comments about the abstract: lazio@rsd.nrl.navy.mil