RESULTS OF A 5-YEAR PROGRAM OF MULTIFREQUENCY MONITORING OF LOW-FREQUENCY VARIABLE RADIO-SOURCES

We present the results of a detailed multifrequency monitoring program of low-frequency variable radio sources. This consists of light curve s at 318, 430, 606, 880, and 1400 MHz over a 5 year period. The observ ations were carried out with the Arecibo 305 m radiotelescope and the Green Bank 91 m radio telescope. The spectral characteristics of the v ariations confirm the general picture that at least two mechanisms are responsible. The first is clearly intrinsic evolution of synchrotron- emitting components as it is manifested by variations that appear firs t and most strongly at high frequencies, subsequently drifting to lowe r frequencies with diminished amplitude. The more common type of low-f requency variability, however, dominates at frequencies below approxim ately 800 MHz, while the variations near 1 GHz are often quite weak. T his spectral property is strong evidence that these variations are int erstellar refractive scintillation.