Multi-Frequency Study of FRB20201124A with the uGMRT

TL;DR

This work presents a comprehensive multi-frequency study of FRB 20201124A using uGMRT across 300–1460 MHz during its May–June 2021 active phase, detecting 141 Band-4 bursts and 5 Band-5 bursts. The analysis uncovers a rich phenomenology: bimodal waiting-time and energy distributions, a broken power-law fluence distribution, and frequency-dependent activity with high-frequency emission fading earlier than lower frequencies, alongside patches of rapid, sub-second repetition. Drift-rate measurements reveal systematic downward frequency drifts in several bursts, and multi-band temporal offsets indicate non-simultaneous, frequency-dependent emission across bands. Together, these results support a magnetar-based emission scenario in a dynamic magneto-ionic environment and emphasize the need for simultaneous wideband monitoring to capture patchy, frequency-dependent FRB activity.

Abstract

We present results from multi-epoch observations of the repeating fast radio burst FRB 20201124A with the upgraded Giant Metrewave Radio Telescope (uGMRT) during its active phase between 8 May and 28 May 2021. The bursts exhibit significant morphological diversity, including multiple sub-bursts, downward frequency drifts, and intrinsic widths ranging from 1.032 - 32.159 ms. Bursts were detected in both Band 4 (550 - 950 MHz) and Band 5 (1060 - 1460 MHz), with the last Band 5 burst occurring on 24 May, while Band 4 activity persisted until 28 May, indicating a frequency-dependent decline. Consecutive bursts were observed with separations of 16.7 - 291.5 ms, revealing short repetition intervals or potential sub-second quasi-periodicity. The waiting-time and energy distributions are bimodal, suggesting at least two distinct emission timescales and energy modes. Burst fluence ranges from 1.72 - 78.47 Jy ms, and the cumulative fluence distribution follows a broken power law. Multi-frequency analysis further shows closely spaced burst pairs across Band 4 and Band 5, with sub-second offsets of 1.08 - 1.15 s, and no strict simultaneity with contemporaneous FAST detections. These findings demonstrate that FRB 20201124A exhibits closely spaced, patchy, multi-frequency emission with frequency-dependent activity, highlighting the complex and dynamic nature of repeating FRBs.

Figures (11)

• Figure 1: A gallery of bursts detected in our observations, displaying dynamic spectra (lower panels) and their dedispersed time profiles (upper panels). The examples highlight a wide range of temporal and spectral structures, including narrowband and broadband emission, multi-component, sub-components and closely spaced independent components, faint and bright bursts, short- and long-duration events, highly scattered and less scattered profiles, as well as frequency-drifting features.
• Figure 2: Summary of detected bursts from FRB20201124A during May–June 2021. The bottom x-axis shows Modified Julian Dates (MJDs), while the top x-axis shows corresponding calendar dates. The bar plots represent the number of detected bursts in Band-4 (blue, hatched) and Band-5 (orange, hatched) for each observing session. The purple dashed line (right y-axis) shows the total observation time per session. The grey shaded region highlights the period from 12 to 15 June, during which no bursts were detected despite significant observing hours (totaling 11.63 hrs). Total detected bursts: 141 in Band-4 and 5 in Band-5.
• Figure 3: Distributions of key burst properties. Top left: Structure-optimized dispersion measures (DM), showing a concentration around 412–415 pc/cc. Top right: Scattering widths, indicating that most bursts exhibit minimal to moderate scattering. Bottom left: Intrinsic burst durations, with the majority under 10 ms. Bottom right: Burst flux densities, peaking below 2 Jy.
• Figure 4: Histogram of waiting times fitted with a two-component log-normal distribution. The bimodal structure includes a short-time peak around 48 ms, associated with closely spaced sub-burst groups, and a long-time peak around 122 s, corresponding to independent burst events.
• Figure 5: Cumulative burst rate above a fluence threshold for fluence-complete bursts (fluence $>3.05$ Jy ms), plotted in log-log scale. A broken power-law fit to the differential distribution shows a slope change at $F_\mathrm{break} = 16.89$ Jy ms. The cumulative power-law indices are $\alpha_1 = 0.76 \pm 0.06$ and $\alpha_2 = 2.89 \pm 0.64$, corresponding to differential slopes $dN/dF \propto F^{-1.76}$ and $dN/dF \propto F^{-3.89}$.