A repeating fast radio burst associated with a persistent radio source

Fast radio bursts (FRBs) exhibit dispersion measures (DMs) dominated by extragalactic ionized gas and have been used to probe the intergalactic medium (IGM). However, host-galaxy and near-source environments can contribute significantly in some cases, complicating DM-based redshift inferences. This study reports the detection, precise localization, and multiwavelength host identification of the repeating FRB 20190520B, assesses its dispersion budget, and evaluates implications for using FRB DMs as redshift proxies. The work aims to understand the role of host-galaxy and local environments—particularly in repeaters associated with persistent radio sources (PRSs)—in shaping observed FRB properties, with consequences for cosmological applications and FRB progenitor models.

Prior studies have established FRBs as probes of the cosmic baryon content via the DM–redshift relation (for example, Macquart et al. 2020). While many FRBs appear to have modest host contributions, at least one well-studied repeater (FRB 121102) shows an extreme magneto-ionic environment with a compact PRS. Models and measurements of Galactic DM (NE2001, YMW16) and IGM DM as a function of redshift enable DM budget decompositions. Previous localizations show diversity: some repeaters (e.g., FRB 121102) have compact PRSs, others (e.g., FRB 20201124A) show extended radio consistent with star formation, and nearby repeaters (e.g., FRB 20200120E) can lack detectable PRSs and reside in old stellar populations. This diversity motivates investigating whether repeating and non-repeating FRBs represent different subclasses or evolutionary stages and how host/near-source media impact observed DMs, RMs, scattering, and burst rates.

- Discovery and radio follow-up: FRB 20190520B was discovered with FAST in 2019 in drift-scan mode (1.05–1.45 GHz). Four bursts occurred in the 24-s discovery scan. Monthly tracking between April–September 2020 (18.5 h total) yielded 75 additional bursts. Mean DM = 1,204.7 ± 4.0 pc cm−3; mean burst width 13.5 ± 1.2 ms; mean scattering timescale 10 ± 2 ms at 1.25 GHz. Burst waiting times modeled with a Weibull distribution to infer a fluence-limited burst rate R = 4.5(+1.3/−1.2) h−1 (for 9 mJy ms and 1 ms width). No linear polarization detected in FAST band; RM detected at higher radio frequencies in separate work. - Interferometric localization and PRS detection: VLA observations in 2020 (L/S/C bands) with the realfast system detected bursts and localized the source at RA 16:02:04.272, Dec −11:17:17.32 (J2000) with 1σ uncertainties (0.10″, 0.08″). Deep imaging revealed a compact persistent radio counterpart at RA 16:02:04.261, Dec −11:17:17.35 with 1σ uncertainties (0.10″, 0.05″). The counterpart is unresolved (<0.36″ ≈ <1.4 kpc) with flux density 202 ± 8 µJy at 3.0 GHz, averaged over ~2 months (30 Aug–16 Nov 2020). A power-law fit across sub-bands yields spectral index −0.41 ± 0.04. - Optical/NIR imaging and host identification: CFHT/MegaCam R′-band imaging and Subaru/MOIRCS J-band imaging identified galaxy J160204.31−111718.5 at the FRB location. The FRB is offset by ~1.3″ from the optical light peak. Chance spatial coincidence probability estimated at 0.8%, supporting association. The J-band magnitude is 22.1 ± 0.1 AB, with FRB and PRS on the galaxy periphery. - Optical spectroscopy and host properties: Palomar 200-inch (Double Spectrograph) spectrum at the FRB position detected Hα, [O III] 4959, 5007 Å at z = 0.241 ± 0.001. Keck I/LRIS confirmed [O III]-dominated R′-band structure at the same redshift. After extinction correction, L(Hα) = (7.4 ± 0.2) × 10^40 erg s−1 implies SFR ≈ 0.41 M⊙ yr−1. From J-band magnitude, stellar mass ≈ 6 × 10^8 M⊙. At luminosity distance 1,218 Mpc, the PRS has L_3GHz = 3 × 10^29 erg s−1 Hz−1. - DM budget and modeling: Adopt Galactic DM contributions DM_MW,disk ≈ 60 pc cm−3 (NE2001) and halo 25–80 pc cm−3 (nominal 50 pc cm−3). Using IGM DM expectations at z = 0.241 with cosmic variance (68% interval) and ionized baryon fraction ~0.85 (range 0.6–1 explored), the inferred host-frame DM contribution is DM_host ≈ 903 pc cm−3 (range ~745–1,020 pc cm−3 over broader ionized fractions). - Foreground-host test via scattering and emission measure: The observed Hα implies an EM consistent with a DM contribution of ~230–650 pc cm−3 (observer frame) for gas temperatures 0.5×10^4–5×10^4 K. If the identified galaxy were foreground, geometric leverage would predict scattering times of ~0.6–20 s at 1.25 GHz, far exceeding the observed 10 ± 2 ms, disfavouring a foreground configuration. - PRS origin assessment: The radio luminosity would imply SFR ~10 M⊙ yr−1 if due to star formation—25× the measured SFR and above typical values for such low-mass galaxies—indicating the PRS is a compact non-star-formation source physically connected to the FRB.

- Discovery and activity: FRB 20190520B is a highly active repeater. FAST detected 75 bursts in 18.5 h (plus 4 in discovery), with a modeled burst rate R = 4.5(+1.3/−1.2) h−1 above 9 mJy ms (1 ms width). Mean DM = 1,204.7 ± 4.0 pc cm−3; mean burst width 13.5 ± 1.2 ms; mean scattering time 10 ± 2 ms at 1.25 GHz. - Precise localization and PRS: VLA localized bursts to RA 16:02:04.272, Dec −11:17:17.32 (J2000). A compact, co-located persistent radio source was detected at RA 16:02:04.261, Dec −11:17:17.35, unresolved (<0.36″) with 3 GHz flux density 202 ± 8 µJy and spectral index −0.41 ± 0.04. The PRS luminosity is L_3GHz = 3 × 10^29 erg s−1 Hz−1. - Host galaxy: A dwarf galaxy (J160204.31−111718.5) at z = 0.241 ± 0.001 is identified with 0.8% chance coincidence probability. Stellar mass ≈ 6 × 10^8 M⊙, SFR ≈ 0.41 M⊙ yr−1 (from L(Hα) = 7.4 × 10^40 ± 0.2 × 10^40 erg s−1), J = 22.1 ± 0.1 AB, with the FRB/PRS on the periphery. Luminosity distance 1,218 Mpc. - DM budget: The host contribution is large, DM_host ≈ 903 pc cm−3 (posterior; range ~745–1,020 pc cm−3 for IGM ionized fraction 0.6–1), greatly exceeding the IGM contribution at z = 0.241. This shows that host/near-source media can dominate the DM for some repeaters. - Foreground scenario disfavoured: Expected scattering for a foreground galaxy with similar EM would be orders of magnitude larger (0.6–20 s at 1.25 GHz) than observed (10 ± 2 ms), supporting that the identified galaxy is the true host. - PRS origin: The PRS radio luminosity cannot be explained by star formation (would imply SFR ~10 M⊙ yr−1, ~25× measured); it is a compact source physically connected to the FRB, analogous to FRB 20121102A. - Implications: DM-based redshift estimates without host identifications can be severely biased high; searches should allow for large DM_host and potentially increased scattering. FRB 20190520B strengthens the link between high repetition, large DM_host, and the presence of a compact PRS in at least some repeaters.

The results address whether host and near-source environments significantly affect observed FRB DMs and whether a compact PRS is linked to repeating behavior. FRB 20190520B exhibits a very large host DM (~903 pc cm−3), a compact PRS, a high repetition rate, and likely strong magneto-ionic conditions—features reminiscent of FRB 20121102A. This indicates that for at least some repeaters, the local environment dominates the dispersion budget, undermining the reliability of using DM alone to infer redshift. The scarcity of compact PRSs among other localized repeaters and the diversity of environments suggest either multiple progenitor channels or evolutionary effects: young sources embedded in dense, magnetized media could show high DM_host and PRSs that fade with age, as repetition rates decline and surrounding plasma disperses. Consequently, population studies must account for a broad and possibly skewed distribution of DM_host, and survey strategies should include high-DM searches and sensitivity to scattering. A larger, precisely localized sample with host redshifts, detailed radio/optical diagnostics, and VLBI size constraints is needed to map the relationship between PRSs, host/near-source media, and repetition.

This work discovers and localizes the highly active repeater FRB 20190520B, firmly associates it with a compact persistent radio source and a star-forming dwarf host at z = 0.241, and demonstrates that the host/near-source environment can dominate the dispersion measure. The PRS luminosity and offset from the optical light peak indicate a compact non-star-formation origin physically connected to the FRB, analogous to FRB 20121102A. The findings caution against using DM alone to estimate FRB redshifts and motivate searches and analyses that incorporate large host DMs and scattering. Future work should: (i) expand the sample of precisely localized FRBs with host redshifts; (ii) obtain VLBI constraints on PRS sizes and structure; (iii) measure time-variable DM and RM to probe local media; and (iv) model the demographics and evolution of PRSs and host DMs across repeating and non-repeating populations.

- Ambiguity in DM_host origin: Current data cannot determine the fraction of DM_host arising from the immediate circum-source medium versus the broader interstellar medium of the host. VLBI and further polarimetric monitoring are needed. - PRS diversity: Many repeaters lack detectable compact PRSs, complicating simple associations between repetition, magneto-ionic activity, and PRS luminosity. - Model assumptions: DM budget decomposition depends on Galactic electron density models (e.g., NE2001), halo contributions, IGM ionized fraction, and cosmic variance. - Scattering estimates for the foreground test rely on uncertain parameters (gas temperature, path length, turbulence), though observed millisecond scattering disfavors a foreground host. - Sensitivity limits: Burst rate and fluence statistics are threshold-dependent; polarization constraints are limited in the FAST band.