A MeerKAT search for persistent radio sources towards twenty-five localised Fast Radio Bursts

The discovery of persistent radio sources (PRSs) associated with repeating fast radio bursts (FRBs) has shed light on the immediate environments and possible progenitors of these FRBs. The confirmed PRSs may support the theory that FRB progenitors are compact central engines, whilst the non-detections suggest diversity of FRB’s local environment. We perform a subarcsecond-resolution MeerKAT search at 1.28 GHz on 25 well-localised FRB positions provided by ASKAP and MeerTRAP. We detect 14 radio sources and provide flux upper limits for 12 non-detections (both these numbers include a source that was detected during two epochs of observation, and not detected during one epoch, adding up to 26). One radio source shows variability as seen in flux variations over three epochs of observation. Archival optical data reveal excesses in the direction of 13 detected radio sources. Similarly for four sources in the X-ray band, with one possibly being a high-energy signature of a radio galaxy core. Since we cannot definitively classify our detected radio sources as PRSs, future high-resolution observations with e-MERLIN will be required to resolve the radio emission and pronounce on the presence of compact PRSs associated with the 14 detected sources presented here.

💡 Research Summary

The paper presents a systematic search for persistent radio sources (PRSs) associated with 25 well‑localized fast radio bursts (FRBs) using the MeerKAT interferometer at 1.28 GHz. The authors selected one‑off FRBs that have arcsecond‑level positions from ASKAP/CRAFT and MeerTRAP, deliberately favoring lower dispersion measures and redshifts to maximise the chance of detecting faint PRSs. Observations were carried out over three years (2021‑2023) with 90 minutes of on‑source integration per field and regular phase calibrator scans. Data reduction employed the oxkat3 pipeline, which performs flagging of low‑gain band edges, the Galactic HI line, and short‑baseline RFI, followed by standard CASA calibration steps and self‑calibration using Cubical. Imaging was performed with WSClean using multi‑scale, wide‑band deconvolution, dividing the full 856‑1712 MHz band into ten 82 MHz sub‑bands and producing a multi‑frequency synthesis (MFS) map centred at 1283 MHz. The final images achieve rms noise levels of ~10–15 µJy beam⁻¹, and a 3σ detection threshold was adopted.

Out of the 25 FRB positions, 14 radio sources were detected at ≥3σ, corresponding to 21 individual field detections when multiple epochs are counted. One source (associated with FRB20220501C) was detected in two of three epochs, indicating possible variability. The detected sources have peak intensities ranging from a few hundred µJy to a few mJy, and their positions lie within 2″–10″ of the FRB coordinates; 13 of the 14 are consistent with the expected localisation uncertainties. The remaining source shows a larger offset, raising doubts about a physical association and suggesting possible centroiding errors due to complex morphology.

For the 11 FRBs with no radio counterpart, the authors provide 3σ upper limits on the flux density, which translate into luminosity limits an order of magnitude below the typical PRS luminosities (10²⁷–10²⁹ erg s⁻¹ Hz⁻¹). This non‑detection does not rule out the presence of very faint PRSs, especially for one‑off bursts.

Cross‑matching with archival optical surveys (e.g., Pan‑STARRS, DES) reveals excess optical emission in the direction of 13 of the detected radio sources, suggesting that many of them could be related to star‑forming regions or host‑galaxy nuclei rather than genuine PRSs. In the X‑ray band, four of the radio detections have counterparts in Chandra or XMM‑Newton catalogs; one of these aligns with a compact radio galaxy core, hinting at an AGN origin.

The authors discuss the implications for FRB progenitor models. The detection of PRS‑like radio emission near one‑off FRBs challenges the notion that PRSs are exclusive to repeating FRBs, but the current angular resolution (≈2″) and sensitivity are insufficient to distinguish compact PRSs from extended star‑formation or AGN emission. They argue that higher‑resolution follow‑up with e‑MERLIN or VLBI, combined with multi‑frequency spectral index measurements and polarization/RM studies, is essential to confirm the nature of these sources. Moreover, the observed variability in one source could be consistent with magnetar wind nebula models, but could also arise from AGN variability.

Statistically, the study finds a 56 % detection rate of radio sources near the FRB positions, but cautions that this figure is heavily influenced by localisation accuracy and sensitivity limits. Consequently, the true fraction of FRBs that host PRSs remains uncertain, and larger, deeper samples are required to place robust constraints on FRB‑PRS associations.

In conclusion, the MeerKAT survey provides the first systematic PRS search around a sizable sample of one‑off FRBs, yielding several candidate radio counterparts but no definitive PRS identifications. The paper emphasizes the need for sub‑arcsecond imaging, spectral diagnostics, and expanded multi‑wavelength coverage to resolve the ambiguity and to advance our understanding of the environments and progenitors of both repeating and non‑repeating fast radio bursts.