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

Abstract

Fast Radio Burst (FRB) 20250316A, detected by CHIME on 2025 March 16 with a fluence of $1.7\pm 0.1\mathrm{Jy}\mathrm{ms}$ and a dispersion measure of $161.3\pm 0.4\mathrm{pc}\mathrm{c}{\mathrm{m}}^{-3}$, 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 $2.8\hat{I}¼\mathrm{Jy}\mathrm{bea}{\mathrm{m}}^{-1}$, yielding a $3\ddot{I}ƒ$ upper limit of $<8.4\hat{I}¼\mathrm{Jy}$ at the FRB position, corresponding to $\hat{I}½L_{\hat{I}}½<2.4\times 10^{35}\mathrm{erg}{\mathrm{s}}^{-1}$. These results represent among the most stringent constraints for a non-repeating FRB, lying $\gtrsim 3$ orders of magnitude below the $\hat{I}½L_{\hat{I}}½$ 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 $|\mathrm{RM}|\lesssim 30\mathrm{rad}{\mathrm{m}}^{-2}$, consistent with a clean magneto-ionic sight line and progenitor channels such as neutron-star mergers or giant flares from older magnetars.

10 pages, 3 figures, two tables. Accepted by ApJL