A Deep VLA Search for a Persistent Radio Counterpart to the One-off FRB 20250316A

TL;DR

This study used deep VLA radio observations to search for a persistent radio source associated with the bright, one-off FRB 20250316A, setting stringent upper limits that challenge certain progenitor models.

Contribution

First deep radio follow-up of a bright, one-off FRB providing the most restrictive constraints on persistent sources, informing models of FRB environments and progenitors.

Findings

No persistent radio source detected at the FRB location.

Upper limit on radio luminosity constrains magnetar-nebula scenarios.

Results favor low-density, weakly magnetized environments for this FRB.

Abstract

Fast Radio Burst (FRB) 20250316A, detected by CHIME on 2025 March 16 with a fluence of $1.7\pm0.1~\mathrm{Jy\,ms}$ and a dispersion measure of $161.3\pm0.4~\mathrm{pc\,cm^{-3}}$, ranks among the brightest extragalactic FRBs at $\sim 40$ Mpc. We obtained deep Karl G. Jansky Very Large Array follow-up at 15~GHz on 2025 April 5 and 9 and find no persistent radio source (PRS). Our best image reaches an rms of $2.8~\mu\mathrm{Jy\,beam^{-1}}$, yielding a $3\sigma$ upper limit of $<8.4~\mu\mathrm{Jy}$ at the FRB position, corresponding to $\nu L_\nu < 2.4\times10^{35}~\mathrm{erg\,s^{-1}}$. These results represent among the most stringent constraints for a non-repeating FRB, lying $\gtrsim 3$ orders of magnitude below the $\nu L_\nu$ of compact persistent radio sources around well-studied repeaters, thereby disfavoring bright magnetar-nebula scenarios and pointing to low-density, weakly magnetized environments. Interpreting our limit through pulsar-/magnetar-wind synchrotron frameworks places joint constraints on ambient density and engine power. If the empirical PRS--rotation-measure trend reported for repeaters extends to one-off sources, our limit implies $\vert \mathrm{RM} \vert \lesssim 30~\mathrm{rad\,m^{-2}}$, consistent with a clean magneto-ionic sight line and progenitor channels such as neutron-star mergers or giant flares from older magnetars.