An insight into the extragalactic transient and variable microJy radio sky across multiple decades

Abstract

The mJy variable extragalactic radio sky is known to be broadly non-changing with approximately 3% of persistent radio sources exhibiting variability which is largely AGN-related. 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 deep VLA data in the well studied GOODS-N field. Using five epochs spread across 22 years, we investigate approximately 480 radio sources finding 10 which 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% of sources exhibiting significant variability between any two epochs. We find that 70% of variable sources show variability on timescales of a few days whilst on longer decadal time-scales, the fraction of variable sources decreases to < 1%. This suggests that the radio variability peaks on shorter timescales as suggested by other studies. We find that 80% of variable sources have VLBI counterparts, and we use multi-wavelength data to infer that these may well be core-dominated FR-I sources as postulated by wide-field VLBI surveys and semiempirical simulations.

The JAGWAR Prowls LIGO/Virgo O3 Paper I: Radio Search of a Possible Multimessenger Counterpart of the Binary Black Hole Merger Candidate S191216ap

“…This level of variability is typical for the radio sky (e.g. Carilli et al 2003;Bell et al 2015;Mooley et al 2016;Hancock et al 2016), and at these frequencies it is attributed to normal activity from active galactic nuclei (AGN; Radcliffe et al 2019).…”

Nowhere to hide: Radio-faint AGN in the GOODS-N field

“…Of the remaining 4 variable sources not identified by VLBI, two are below the VLBI detection threshold, while one is a 6.7σ detection in the EVN observations and may be a supernovae (Radcliffe et al 2019a). The remaining variable source is undetected by the EVN observations presented here, but is detected by Chi et al (2013) and has changed dramatically in VLBI measured flux densities between 2004 and 2014 (from ∼350 µJy to <60 µJy).…”

“…In the GOODS-N field, Radcliffe et al (2019a) compared the flux densities of 5 epochs of 1.5 GHz VLA data over 22 years to investigate the variability of approximately 480 radio sources to a limiting detection threshold of ∼30 µJy beam −1 . In this study, a total of 10 sources were found to show significant variability.…”

“…A27, page 13 of 26 A&A 649, A27 (2021) Radcliffe et al (2019a). ( 7) -Radio excess parameter using Spitzer MIPS 24 µm emission.…”

The e-MERGE Survey (e-MERLIN Galaxy Evolution Survey): overview and survey description

“…Previous studies have shown that the fraction of sub-100μJy variable radio sources is low (a few per cent, e.g. Mooley et al 2016; Radcliffe et al 2019). However, relatively small levels of intrinsic flux density variability of sources in the field, along with any small discrepancies in the relative flux density scale assigned to each epoch, will result in errors in the final combined image if not properly accounted for.…”