Spectral Variability of Radio Sources at Low Frequencies
  Spectral variability of radio sources encodes information about the
conditions of intervening media, source structure, and emission processes. With
new low-frequency radio interferometers observing over wide fractional
bandwidths, studies of spectral variability for a large population of
extragalactic radio sources are now possible. Using two epochs of observations
from the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM)
survey that were taken one year apart, we search for spectral variability
across 100--230 MHz for 21,558 sources. We present methodologies for detecting
variability in the spectrum between epochs and for classifying the type of
variability: either as a change in spectral shape or as a uniform change in
flux density across the bandwidth. We identify 323 sources with significant
spectral variability over a year-long timescale. Of the 323 variable sources,
we classify 51 of these as showing a significant change in spectral shape.
Variability is more prevalent in peaked-spectrum sources, analogous to
gigahertz-peaked spectrum and compact steep-spectrum sources, compared to
typical radio galaxies. We discuss the viability of several potential
explanations of the observed spectral variability, such as interstellar
scintillation and jet evolution. Our results suggest that the radio sky in the
megahertz regime is more dynamic than previously suggested.
