The VLBI spectrum of the persistent radio source associated with FRB 20190417A

TL;DR

This study uses VLBI observations to confirm the compact, non-thermal nature of a persistent radio source associated with FRB 20190417A, constraining its spectral index and supporting a nebular origin.

Contribution

First VLBI spectral constraint on FRB 20190417A's PRS, confirming its compactness and non-thermal emission, and analyzing its relation to the $L_{ u}$-|RM| correlation.

Findings

Detected a compact source at 5 GHz with flux density 150±45 μJy.

Derived a nearly flat spectral index α = -0.19 ± 0.29 for FRB 20190417A.

Supported a nebular origin for the persistent emission based on spectral and luminosity analysis.

Abstract

We aim to confirm the compact nature and constrain the radio spectra of candidate persistent radio sources (PRSs) associated with repeating fast radio bursts (FRBs). We performed European VLBI Network (EVN) observations at 5 and 8 GHz targeting two candidates identified in a recent VLA survey. We measured flux densities and upper limits at milliarcsecond resolution and combined them with published VLBI data at lower frequencies to derive spectral constraints. We detect a compact source associated with FRB 20190417A at 5 GHz with a flux density of $150\pm45$ uJy, while no detection is obtained at 8 GHz. The source is unresolved and has a brightness temperature $T_{\rm b} \gtrsim 10^{6-7}$ K, confirming its non-thermal nature. Combining our measurement with VLBI data at 1.4 GHz, we derive a spectral index $\alpha = -0.19 \pm 0.29$, consistent with a nearly flat spectrum. This makes FRB 20190417A only the second PRS with a spectral index constrained using VLBI data. The inferred luminosity places the source on the proposed $L_{\nu}$-|RM| relation. Including this source yields a scatter of $\sigma_\Delta = 0.65$, corresponding to $\hat{\alpha}|\epsilon| = 1.5 \pm 0.7$, consistent with forward shocks in the free-expansion phase or young pulsar wind nebulae. For the candidate PRS associated with FRB 20181030A, we report upper limits of 80 uJy at 5 GHz and 150 uJy at 8 GHz, corresponding to $L_{5\,\mathrm{GHz}} \lesssim 3.8 \times 10^{25}\ {\rm erg\ s^{-1}\ Hz^{-1}}$, and implying a steep spectral index ($\alpha \lesssim -1.2$) if the VLA emission arises from a compact component. Our results highlight the importance of VLBI in isolating compact emission from FRB engines and provide one of the few spectral constraints for PRSs at milliarcsecond resolution. The consistency of FRB 20190417A with the $L_{\nu}$-|RM| relation supports a nebular origin for the persistent emission.