We report the detection of repeat bursts from FRB 171019, one of the brightest fast radio bursts (FRBs) detected in the Australian Square Kilometre Array Pathfinder (ASKAP) fly's eye survey. Two bursts from the source were detected with the Green Bank Telescope in observations centered at 820 MHz. The repetitions are a factor of ∼590 times fainter than the ASKAP-discovered burst. All the three bursts from this source have consistent pulse widths and evidence for steep spectra. They also show strong spectral modulation, whose spectral characteristics are inconsistent with diffractive interstellar scintillation. The two repetitions were the only ones found in an observing campaign for this FRB totaling 1000 hr which also included ASKAP and the 64-m Parkes radio telescope, over a range of frequencies (720-2000 MHz) and at epochs spanning two years. The inferred scaling of repetition rate with fluence of this source agrees with the other repeating source, FRB 121102. The detection of faint pulses from FRB 171019 shows that at least some single-burst FRBs will repeat if follow-up observations are conducted with more sensitive telescopes.

The fast radio burst (FRB) population is observationally divided into sources that have been observed to repeat and those that have not. There is tentative evidence that the bursts from repeating sources have different properties than the non-repeating ones. In order to determine the occurrence rate of repeating sources and characterise the nature of repeat emission, we have been conducting sensitive searches for repetitions from bursts detected with the Australian Square Kilometre Array Pathfinder (ASKAP) with the 64-m Parkes radio telescope, using the recently commissioned Ultra-wideband Low (UWL) receiver system, over a band spanning 0.7–4.0 GHz. We report the detection of a repeat burst from the source of FRB 20190711A. The detected burst is 1 ms wide and has a bandwidth of just 65 MHz. We find no evidence of any emission in the remaining part of the 3.3 GHz UWL band. While the emission bandwidths of the ASKAP and UWL bursts show ν−4 scaling consistent with a propagation effect, the spectral occupancy is inconsistent with diffractive scintillation. This detection rules out models predicting broad-band emission from the FRB 20190711A source and puts stringent constraints on the emission mechanism. The low spectral occupancy highlights the importance of sub-banded search methods in detecting FRBs.

We present the Australian Square Kilometre Array Pathfinder localization and follow-up observations of the host galaxy of the repeating fast radio burst (FRB) source, FRB 20201124A, the fifth such extragalactic repeating FRB with an identified host. From spectroscopic observations using the 6.5 m MMT Observatory, we derive a redshift z = 0.0979 ± 0.0001, a star formation rate inferred from Hα emission SFR(Hα) ≈ 2.1 M ⊙ yr−1, and a gas-phase metallicity of 12+log(O/H) ≈ 9.0. By jointly modeling the 12 filter optical−mid-infrared (MIR) photometry and spectroscopy of the host, we infer a median stellar mass of ∼2 × 1010 M ⊙, internal dust extinction A V ≈ 1–1.5 mag, and a mass-weighted stellar population age of ∼5–6 Gyr. Connecting these data to the radio and X-ray observations, we cannot reconcile the broadband behavior with strong active galactic nucleus activity and instead attribute the dominant source of persistent radio emission to star formation, likely originating from the circumnuclear region of the host. The modeling also indicates a hot dust component contributing to the MIR luminosity at a level of ∼10%–30%. We model the host galaxy’s star formation and mass assembly histories, finding that the host assembled >90% of its mass by 1 Gyr ago and exhibited a fairly constant SFR for most of its existence, with no clear evidence of past starburst activity.

The mechanism that produces fast radio burst (FRB) emission is poorly understood. Targeted monitoring of repeating FRB sources provides the opportunity to fully characterise the emission properties in a manner impossible with one-off bursts. Here we report observations of the source of FRB},20201124A, with the Australian Square Kilometre Array Pathfinder (ASKAP) and the Ultra-wideband Low (UWL) receiver at the Parkes 64-m radio telescope (Murriyang). The source entered a period of emitting bright bursts during early April 2021. We have detected 16 bursts from this source. One of the bursts detected with ASKAP is the brightest burst ever observed from a repeating FRB source with an inferred fluence of 640 ± 70 Jy ms. Of the five bursts detected with the Parkes UWL, none display any emission in the range 1.1–4 GHz. All UWL bursts are highly polarized, with their Faraday rotation measures (RMs) showing apparent variations. We obtain an average RM of −614 rad m−2 for this FRB source with a standard deviation of 16 rad m−2 in the UWL bursts. In one of the UWL bursts, we see evidence of significant circularly polarized emission with a fractional extent of 47 ± 1 per cent. Such a high degree of circular polarisation has never been seen before in bursts from repeating FRB sources. We also see evidence for significant variation in the linear polarization position angle in the pulse profile of this UWL repeat burst. Models for repeat burst emission will need to account for the increasing diversity in the burst polarization properties.

A handful of fast radio bursts (FRBs) are now known to repeat. However, the question remains – do they all? We report on an extensive observational campaign with the Australian Square Kilometre Array Pathfinder (ASKAP), Parkes, and Robert C. Byrd Green Bank Telescope, searching for repeat bursts from FRBs detected by the Commensal Real-time ASKAP Fast Transients survey. In 383.2 h of follow-up observations covering 27 FRBs initially detected as single bursts, only two repeat bursts from a single FRB, FRB 171019, were detected, which have been previously reported by Kumar et al. We use simulations of repeating FRBs that allow for clustering in burst arrival times to calculate new estimates for the repetition rate of FRB 171019, finding only slight evidence for incompatibility with the properties of FRB 121102. Our lack of repeat bursts from the remaining FRBs set limits on the model of all bursts being attributable to repeating FRBs. Assuming a reasonable range of repetition behaviour, at most 60 per cent (90 per cent confidence limit) of these FRBs have an intrinsic burst distribution similar to FRB 121102. This result is shown to be robust against different assumptions on the nature of repeating FRB behaviour, and indicates that if indeed all FRBs repeat, the majority must do so very rarely.