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

Published 3 Apr 2026 in astro-ph.HE and astro-ph.SR | (2604.03429v1)

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 $α= -0.19 \pm 0.29$ , 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 $L_ν$ -|RM| relation. Including this source yields a scatter of $σΔ= 0.65$ , corresponding to $\hatα|ε| = 1.5 \pm 0.7$ , 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 ( $α\lesssim -1.2$ ) 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 $L_ν$ -|RM| relation supports a nebular origin for the persistent emission.

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Authors (9)

Summary

The paper presents a high-resolution VLBI study confirming the compact, non-thermal nature of the persistent radio source associated with FRB 20190417A.
It measures a flat-to-mildly declining spectrum (α = -0.19 ± 0.29) with precise flux densities at 5 and 1.4 GHz, effectively ruling out star-formation origins.
The refined Lν–|RM| relation supports young, magnetar-driven nebula models and underscores the importance of VLBI in isolating FRB environments.

VLBI Constraints on the Spectrum of the Persistent Radio Source Associated with FRB 20190417A

Introduction

The study presents European VLBI Network (EVN) observations targeting persistent radio sources (PRSs) associated with repeating fast radio bursts (FRBs), with a particular focus on FRB 20190417A. The goals are to confirm the compact, milliarcsecond-scale nature of emission coincident with FRB 20190417A, measure its radio spectrum with very high angular resolution, and place the results in the context of the proposed $L_\nu$ –|RM| relation for PRSs. Additionally, a stringent examination of the candidate PRS linked to FRB 20181030A is provided.

Observational Campaign and Data Analysis

Observations were carried out with the EVN at central frequencies of 5 and 8 GHz. Rigorous amplitude, phase, and polarization calibration procedures were employed to ensure accurate flux density and astrometric measurements. The synthesized beams provided sub-milliarcsecond spatial resolution, isolating emission regions coincident with repeating FRBs with minimal contamination from host galaxy star formation.

Detection and Properties of 20190417A-S1

A clear, unresolved compact radio component was detected at 5 GHz with $S_{5 \mathrm{GHz}} = 150 \pm 45~\mu$ Jy at a precise position coincident with FRB 20190417A. The lack of spatial extension implies a lower limit on the brightness temperature of $T_\mathrm{b} \gtrsim 10^{6-7}$ K, ruling out any thermal or extended star-formation origin, and robustly establishing the non-thermal nature intrinsic to FRB-related engines.

At 8 GHz, only a 5 $\sigma$ upper limit of 250 $\mu$ Jy was achieved. When benchmarked against the VLBI-based 1.4 GHz detection ( $S_{1.4 \mathrm{GHz}} = 191 \pm 39~\mu$ Jy), the spectrum is flat-to-mildly-declining with $\alpha = -0.19 \pm 0.29$ ( $S_\nu \propto \nu^\alpha$ ). This makes FRB 20190417A the second PRS (after FRB 20121102A) to have its spectral index determined solely via VLBI, without host-galaxy contamination.

Upper Limits for 20181030A-S1

No compact emission was detected at either 5 or 8 GHz at the position of the candidate 20181030A-S1. The 5 GHz upper limit ( $<80~\mu$ Jy) imposes a luminosity ceiling, translating to a spectral index constraint of $\alpha \lesssim -1.2$ relative to the VLA 1.5 GHz detection. If the VLA source is compact and persistent, this would indicate an unusually steep spectrum compared to other PRSs, but an alternative hypothesis is that the VLA emission is dominated by diffuse star-forming activity.

Radio Spectral Diagnostics and Physical Models

The measured flat spectral index for 20190417A-S1 is inconsistent with optically thin synchrotron emission from shock-accelerated electrons, which generally produce $S_{5 \mathrm{GHz}} = 150 \pm 45~\mu$ 0 ( $S_{5 \mathrm{GHz}} = 150 \pm 45~\mu$ 1). Instead, the index is consistent with:

A hard ( $S_{5 \mathrm{GHz}} = 150 \pm 45~\mu$ 2) fossil electron population, characteristic of pulsar wind nebulae (PWNe),
The presence of a synchrotron turnover near GHz frequencies, arising either from minimum Lorentz factor electrons or from synchrotron self-absorption.

Constraints on magnetic field, particle density, and source size can be derived under each scenario, but both point toward a nebular origin with high energy density and efficient ongoing particle acceleration.

The $S_{5 \mathrm{GHz}} = 150 \pm 45~\mu$ 3--|RM| Relation

Theoretical models predict that the PRS luminosity and the observed rotation measure (RM) should be physically linked if both originate in a young, magnetized nebula surrounding the compact FRB source. The flux and RM measurements for 20190417A enable recalibration of the prominent $S_{5 \mathrm{GHz}} = 150 \pm 45~\mu$ 4–|RM| relation (see below).

Figure 1: The $S_{5 \mathrm{GHz}} = 150 \pm 45~\mu$ 5–|RM| relation for confirmed PRSs and candidates, showing the placement of FRB 20190417A (black circle), the unit-slope best fit (red line), and the observed scatter ( $S_{5 \mathrm{GHz}} = 150 \pm 45~\mu$ 6).

Inclusion of FRB 20190417A refines the estimated scatter in the relation to $S_{5 \mathrm{GHz}} = 150 \pm 45~\mu$ 7 (in $S_{5 \mathrm{GHz}} = 150 \pm 45~\mu$ 8), yielding an inferred nebular expansion index of $S_{5 \mathrm{GHz}} = 150 \pm 45~\mu$ 9, consistent with expansion scenarios involving forward shocks in the free-expansion phase or nascent, continuously powered PWNe.

Implications and Theoretical Context

The VLBI constraints confirm that the persistent radio emission associated with FRB 20190417A is compact, non-thermal, and nebular. The spectral and $T_\mathrm{b} \gtrsim 10^{6-7}$ 0–|RM| diagnostics support models where the nebula is powered by highly magnetized, young neutron stars—i.e., magnetars—with the persistent emission tracing ongoing outflow/wind activity and/or relic particle populations. The data exclude significant contamination from star formation or diffuse ISM emission and place the PRS squarely within the population of engines associated with luminous, highly-magnetized environments.

The upper limits for 20181030A demonstrate the necessity of high-resolution VLBI to disentangle nuclear persistent emission from host background, and suggest caution when identifying PRSs based solely on lower-resolution imaging.

The methodology, combining multi-frequency VLBI with correlative environment diagnostics ( $T_\mathrm{b} \gtrsim 10^{6-7}$ 1--|RM|), offers a blueprint for scrutinizing and standardizing PRS samples in the future. As sample sizes increase, tighter constraints on evolutionary models, parental channel diversity (e.g., SN vs. merger formation), and possible use of PRSs as cosmological or astrophysical probes will be achievable.

Conclusion

The paper demonstrates that VLBI is essential for robust, contamination-free PRS identification and spectral measurement. The detection of a compact, flat-spectrum PRS for FRB 20190417A and its consistency with nebular, synchrotron-powered emission provides stringent observational support for young magnetar scenarios. The $T_\mathrm{b} \gtrsim 10^{6-7}$ 2–|RM| relation, refined by these results, is consistent with models of forward shock or young PWN expansion. Non-detections for other candidates highlight the necessity for high-angular-resolution follow-up in FRB environment studies. This work strengthens the emerging consensus that at least a subset of repeating FRBs are embedded in young, actively powered, highly magnetized nebulae.