Fast radio burst -- persistent radio source systems. I. A 1.2 GHz search and catalog definition

Abstract

Fast radio bursts (FRBs) are millisecond-duration radio transients observed at extragalactic distances, whose origin remains uncertain. Given their extremely short duration and high implied luminosities, magnetars are commonly identified as sources of FRBs. A small (∼ 2%) fraction of FRBs are known to repeat, but it is still unclear whether repeaters represent a distinct population. Four repeating FRBs present a co-spatial persistent radio source (PRS) that is compact at parsec scales, and is interpreted as a dense and highly magnetized synchrotron-emitting nebula. We investigate the connection between FRBs and PRSs, in particular the PRS occurrence in a sample of localized FRBs. We carried out new 1.26 GHz observations of 24 FRBs (9 repeaters and 11 one-offs), conducted with the upgraded Giant Meter Wave Radio Telescope (uGMRT) at arc-second angular scale resolution, and combined them with literature data to construct an extended sample of 75 sources with either persistent luminosity measurements or upper limits. Our catalog is complete down to a 10^29 erg s -1 $ Hz$^ -1 luminosity threshold, which corresponds to the value of the first-discovered PRS, associated with FRB 20121102. We detect compact radio emission in six FRB fields, two of which are previously known PRSs and four are new candidates. The four new candidates are associated with three new repeating FRBs and one apparently one-off source. We computed the PRS occurrence using the whole catalog of 75 FRBs and found that PRSs more luminous than 10^29 erg s -1 $ Hz^-1 are rare. Unless otherwise stated, all uncertainties and upper limits are reported at the 95% confidence level (CL). If we only consider confirmed PRSs, we find an occurrence of f_ all = 0.06^ +0.09 _ -0.03 , while including PRS candidates as well -- i.e., sources for which parsec-scale constraints are not yet available -- yields $f_ all = 0.14^ +0.22 _ -0.08 . We also compute the PRS occurrence in the two classes of FRBs: repeating and non-repeating. In the conservative case, i.e., when none of the new PRS candidates are considered to be associated with their FRBs, we find no preferred association between the PRSs and FRB class, with an occurrence of f_ ̊m r = 0.14^ +0.20 _ -0.09 for the sample of repeating FRBs and f_ ̊m nr łeq 0.12 for the sample of one-off sources. In the inclusive case, i.e., when all the new candidates are considered confirmed PRSs, we find f_ r = 0.27^ +0.21 _ -0.14 and f_ nr = 0.03^ +0.14 _ -0.02 , with marginal (90% CL) evidence that PRSs are preferably associated with repeating FRBs.