Abstract
ABSTRACT The mJy variable extragalactic radio sky is known to be broadly non-changing with approximately $3{{\ \rm per\ cent}}$ of persistent radio sources exhibiting variability that is largely active galactic nucleus-related (AGN). In the faint (<mJy) flux density regime, it is widely accepted that the radio source population begins to change from AGN dominated to star formation dominated, together with an emergent radio-quiet AGN component. Very little is known about the variable source component in this sub-mJy regime. In this paper, we provide the first insight into the μJy variable sky by performing a careful analysis using the deep VLA data in the well-studied GOODS-N field. Using five epochs spread across 22 yr, we investigate approximately 480 radio sources finding 10 that show signs of variability. We attribute this variability to the presence of an AGN in these systems. We confirm and extend the results of previous surveys, finding that variability in the faint radio sky is rather modest with only ≤2 per cent of sources exhibiting significant variability between any two epochs. We find that 70 per cent of variable sources show variability on time-scales of a few days while on longer decadal time-scales, the fraction of variable sources decreases to $\lt 1{{\ \rm per\ cent}}$. This suggests that the radio variability peaks on shorter time-scales as suggested by other studies. We find that 80 per cent of variable sources have VLBI counterparts, and we use multiwavelength data to infer that these may well be core-dominated FR-I sources as postulated by the wide-field VLBI surveys and semi-empirical simulations.
Highlights
Deep radio observations of extragalactic fields have revealed the nature of the persistent sub-100 μJy radio source population
5.1 Comparison to other variability surveys. While this survey does not have the advantage of an ultra-wide field of view, which is advantageous for finding transients and variables, it does have the considerable advantage of probing deeper than other surveys across extremely long decadal time-scales
We have conducted a study on the variability in the GOODS-N field using five epochs of the JVLA and Very Large Array (VLA) data spread across 22 yr
Summary
Deep radio observations of extragalactic fields have revealed the nature of the persistent sub-100 μJy radio source population. It is generally agreed that at μJy flux densities the radio source population is dominated by moderate to high-redshift starforming galaxies and radio-quiet active galactic nuclei (AGNs). This is in contrast to the radio source counts at flux densities of ∼mJy and above, which are dominated by radio galaxies and quasars Healy et al 2017), while the transient and variable extragalactic radio sky includes AGNs, tidal disruption events, supernovae, gamma-ray bursts, fast radio bursts, and the recently observed binary neutron star mergers Of the slower transients (on time-scales of minutes or longer), AGNs are by far the most common extragalactic radio variable sources ( 60 per deg above 0.3 mJy) and are found to
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