Letter to the Editor

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

¹ Istituto Nazionale di Astrofisica – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
² South African Radio Astronomy Observatory, Cape Town, 7700, South Africa
³ Centre for Radio Astronomy Techniques and Technologies (RATT), Department of Physics and Electronics, Rhodes University, Makhanda 6140, South Africa
⁴ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Cagliari, Via della Scienza 5, I-09047 Selargius (CA), Italy
⁵ Dipartimento di Fisica e Astronomia – Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
⁶ Scuola Universitaria Superiore IUSS Pavia, Palazzo del Broletto, piazza della Vittoria 15, I-27100 Pavia, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 23 March 2026
Accepted: 13 April 2026

Abstract

Context. Persistent radio sources (PRSs) are (sub-)parsec-scale compact non-thermal continuum sources associated with some repeating fast radio bursts (FRBs). Their nature is debated, but their properties provide insights into the FRB environment and progenitors.

Aims. We measure the spectrum of the recently confirmed PRS associated with FRB 20190417A. Spectral features such as the self-absorption and cooling break can be used to constrain the age and size of PRSs and test theoretical models.

Methods. We present observations made at 1.26 GHz with the upgraded Giant Metrewave Radio Telescope (uGMRT) and observations at 6 GHz made with the Karl Jansky Very Large Array (VLA). With complementary archival data and the LOw Frequency ARray Two Meter Sky Survey (LoTSS), we characterise the spectrum of the PRS between 144 MHz and 6 GHz.

Results. The spectrum follows a power-law behaviour at gigahertz frequencies. The source is not detected at 144 MHz down to a 2σ = 170 μJy sensitivity. We modelled the spectrum with a broken power law, obtaining a spectral index α = 0.20 ± 0.05 between 1–6 GHz. We placed a lower limit on the turn-over frequency of > 370 MHz (95% confidence).

Conclusions. The flat spectrum and low-frequency turn-over of the target are consistent with the spectral properties predicted for magneto-ionic nebulae, inflated behind the supernova ejecta by a flaring young magnetar. Considering the multi-zone magnetar wind nebula scenario, we estimate an age of t < 250 yr and a radius of R < 0.4 pc for the target, which would thus be slightly older than the PRSs associated with FRB 20121102A and FRB 20190520B.