Refractive interstellar scintillation of radio sources

The phenomenon of refractive interstellar scintillation (RISS) has recently been established as the cause of week- to month-long fluctuations in the meter-wave amplitude of pulsars. A simplified theory for the RISS of extended radio sources is presented and examined as an explanation for the variations observed from many radio sources. An approximate criterion for RISS is an intrinsic source diameter somewhat less than the scattered diameter. Low-frequency variables can indeed be explained as the RISS of compact (about 10 mas) extragalactic sources. The amplitude of the variations increases with wavelength as expected up to a wavelength of 1 m; the approximate constancy of amplitude observed at longer wavelengths then requires that the source diameters increase with wavelength, as is typical in many models of flat-spectrum compact sources. The centimeter wavelength Galactic plane variables can be explained as compact extragalactic sources showing RISS due to the enhanced scattering now recognized in the inner Galaxy. A list of such sources is presented.