Fast radio burst - persistent radio source systems II. A faint PRS associated with the nearby FRB 20181030A?

Persistent radio sources (PRSs) are the continuum counterparts of fast radio bursts (FRBs), the latter being extragalactic transients of millisecond duration and Jy-level flux density. An FRB-PRS system is thought to be a flaring magnetar surrounded by an highly magnetized, baryon-loaded nebula. We aim to constrain the size of 20181030A-S1, a new PRS candidate, potentially associated with the repeating FRB 20181030A. The latter is localized with about 1' uncertainties in the outskirts of NGC 3252, which is a spiral galaxy at a luminosity distance of 20 Mpc. We report very long baseline interferometric (VLBI) observations using the European VLBI Network at 1.7 GHz of this PRS candidate at an angular resolution of 20 milliarcseconds. Our observations reveal the presence of an unresolved radio source (20181030A-S1) at the position of the PRS candidate, confirming its compactness at milli-arcsecond angular scales. A fit to the position of the point-source yields a peak flux density of 280 plus or minus 30 μ Jy and a transverse physical size constrained to be R < 0.5 pc at 68% confidence level (CL). This flux density converts to a spectral luminosity of (9 plus or minus 1) times 10 25 erg s-1 Hz-1, about 3 orders of magnitude lower than confirmed PRSs, making 20181030A-S1 the closest and faintest PRS candidate known. Its low luminosity and modest rotation measure are consistent with the L ν- rotation measure (RM) relation followed by confirmed FRB--PRS systems, supporting a common physical origin in magnetar-powered nebulae. We show how a magnetized wind nebula powered by an initially weak (B 10 15 G) and young (t age 15 - 150 yrs) magnetar can account for both the observed spectral luminosity and RM of the system. Other possible origin scenarios for 20181030A-S1, in the case in which it is unrelated to the FRB source, are also discussed.