Other
Scientific paper
Jan 1979
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1979aj.....84....1c&link_type=abstract
Astronomical Journal, vol. 84, Jan. 1979, p. 1-11.
Other
59
Extragalactic Radio Sources, Frequency Distribution, Electron Radiation, Flux Density, Optical Thickness, Red Shift, Relativistic Effects, Synchrotron Radiation
Scientific paper
The 318-MHz flux densities of two samples of strong extragalactic radio sources, one complete at 1400 MHz and the other at 5000 MHz, have been remeasured. These flux densities are compared with those measured between 1970 and 1973 to establish variability on time scales of several years. The results obtained demonstrate that flat-spectrum sources (spectral index less than +0.5) are frequently variable, while steep-spectrum sources (spectral index at least +0.5) are not, and that at least 1/4 of the flat-spectrum sources in the complete sample exhibited low-frequency variations. Evidence is presented confirming Hunstead's (1972) report of 408-MHz variability in 3C 454.3 on a time scale of only a few years. It is suggested that the most plausible physical mechanism for apparently superluminal flux variations is free-free absorption in a plasma shell or in filaments of very high optical depth.
Condon James J.
Dennison Brian
Ledden J. E.
Odell S. L.
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