A search for long-timescale, low-frequency radio transients
  We present a search for transient and highly variable sources at low radio
frequencies (150-200 MHz) that explores long timescales of 1-3 years. We
conducted this search by comparing the TIFR GMRT Sky Survey Alternative Data
Release 1 (TGSS ADR1) and the GaLactic and Extragalactic All-sky Murchison
Widefield Array (GLEAM) survey catalogues. To account for the different
completeness thresholds in the individual surveys, we searched for compact
GLEAM sources above a flux density limit of 100 mJy that were not present in
the TGSS ADR1; and also for compact TGSS ADR1 sources above a flux density
limit of 200 mJy that had no counterpart in GLEAM. From a total sample of 234
333 GLEAM sources and 275 612 TGSS ADR1 sources in the overlap region between
the two surveys, there were 99658 GLEAM sources and 38 978 TGSS ADR sources
that passed our flux density cutoff and compactness criteria. Analysis of these
sources resulted in three candidate transient sources. Further analysis ruled
out two candidates as imaging artefacts. We analyse the third candidate and
show it is likely to be real, with a flux density of 182 +/- 26 mJy at 147.5
MHz. This gives a transient surface density of rho = (6.2 +/- 6) x 10-5 deg-2 .
We present initial follow-up observations and discuss possible causes for this
candidate. The small number of spurious sources from this search demonstrates
the high reliability of these two new low-frequency radio catalogues.
