Long-term monitoring of repeating FRB 20220912A with the uGMRT at low radio frequencies

TL;DR

This study presents nearly two years of low-frequency (300–750 MHz) monitoring of the hyperactive repeating FRB 20220912A with uGMRT, discovering 643 bursts and analyzing their fluence, energy distributions, and activity modulation. The bursts largely follow a power-law energy distribution, with a characteristic break around a few ×10^-29 erg Hz^-1 that persists across epochs and frequencies, suggesting robust emission mechanisms. No robust short-timescale periodicity is detected, though the source exhibits extended high activity followed by prolonged quiescence, reinforcing magnetar-based progenitor scenarios with broad rotation-phase coverage. Energy-budget estimates indicate the radio output is a small fraction of a magnetar’s available magnetic energy, consistent with a young, highly magnetized neutron star powering the emission over long timescales.

Abstract

Some repeating FRBs exhibit occasional extreme repetition rates, but very few show a sustained high activity level. One such hyperactive repeater is FRB 20220912A, which was discovered by CHIME/FRB Collaboration on 2022 September 12. Here, we present results from a long-term monitoring campaign of FRB 20220912A using the upgraded Giant Metrewave Radio Telescope (uGMRT) in the frequency range from 300 to 750 MHz. Over the course of nearly two years, we detected a total of 643 bursts in this frequency range. The source exhibited extreme activity for a few months after its discovery and sustained its active phase for over 500 days, with unsystematic modulations in the activity during this phase. The cumulative energy distributions in both bands show a break, consistent with other active repeaters like FRB 20121102A, FRB 202011124A, etc., suggesting common underlying emission mechanisms. Moreover, we show that the energy distribution shape for FRB 20220912A remains broadly same across a large range of frequencies and over time. Overall, the extended high activity, estimated total energy output, persistent power-law tails in the energy distributions, and the lack of detectable short timescale periodicity favor progenitor models invoking young dynamic magnetars, potentially emitting pulses across large rotation phase ranges.

Figures (16)

• Figure 1: Dynamic Spectra of some of the bursts detected in band-3 (300-500 MHz) and band-4 (550-750 MHz) during the observing campaign.
• Figure 2: Completeness function for band-3 (left panel) and band-4 (right panel) at different widths, determined by injections of bursts in raw unprocessed data taken on 2022 November 24.
• Figure 3: Burst rate as a function of time during the observation campaign from 2022 November 22 to 2024 September 27 is shown for both band-3 (300-500 MHz) and band-4 (550-750 MHz). Top: the burst rate calculated based on all the bursts detected above the respective 90% completeness thresholds for different widths, as explained in the text. Bottom: The burst rate at each epoch is calculated based on the highest fluence threshold among all widths and both the bands, to provide a meaningful comparison of the rates at the two bands. The single band observations at band-3 and band-4 on 2022 November 22 are excluded.
• Figure 4: Lomb Scargle periodogram obtained using the number of bursts detected by CHIME/FRB in each day from FRB 20220912A during the period from MJD 59833 to MJD 60269. The red dashed line indicates the 1% false alarm probability.
• Figure 5: Cumulative distribution of fluences for all the bursts above the completeness threshold detected during the observing campaign at band-3 (left) and band-4 (right) are shown as black points. Black lines indicate the fitted broken power law. The yellow points indicate the cumulative distribution of fluences for all the detected bursts.