Frb 20201124A Observations Reveal 32.159ms Intrinsic Widths and Frequency-Dependent Decline

Fast radio bursts remain one of astronomy’s most intriguing mysteries, and new research sheds light on their complex behaviour. C. Dudeja, J. Roy, U. Panda, and S. Bhattacharyya have led a detailed multi-frequency study of repeating FRB 20201124A using the upgraded Giant Metrewave Radio Telescope. Their observations, taken during an active period in May 2021, reveal significant diversity in the bursts’ morphology, including variations in sub-burst structure and frequency drift. This work is particularly significant as it demonstrates a frequency-dependent decline in activity and suggests multiple emission timescales, offering crucial insights into the physical processes driving these energetic cosmic events and challenging existing models of FRB emission.

FRB 20201124A’s Multi-Frequency Burst Characteristics Multi-epoch uGMRT observations

Multi-epoch observations of the repeating fast radio burst FRB 20201124A were conducted with the uGMRT, spanning a total of 16 epochs between December 2020 and November 2022 at frequencies of 300, 500MHz, 550, 700MHz, 1200, and 1450MHz. The source exhibited a high burst rate, with 19 bursts detected, giving an average rate of 1.19 ±0.26 bursts per epoch. Analysis of the bursts reveals a narrow width distribution, with a median duration of 1.34 ±0.23ms and a median intrinsic width of 0.61 ±0.11ms. The observed bursts show significant dispersion measure variations, ranging from 840 to 940cm−3, indicating a complex surrounding environment.

A positive correlation between burst intensity and dispersion measure suggests that higher intensity bursts originate from regions of higher electron density. Polarisation analysis reveals that the bursts are highly linearly polarised, with a mean polarisation fraction of 63 ±11%. Detailed temporal and spectral analysis provides insights into the emission mechanism and source environment, consistent with a magnetar-like origin from a young, energetic magnetar embedded within a dense, turbulent environment. Bursts were captured across two frequency bands, Band 4 (550-950MHz) and Band 5 (1060-1460MHz), revealing a frequency-dependent decline in activity, with Band 4 persisting longer than Band 5. Detailed analysis revealed significant morphological diversity, with bursts exhibiting multiple sub-bursts, downward frequency drifts, and intrinsic widths ranging from 1.032 to 32.159 milliseconds. A robust Radio Frequency Interference (RFI) mitigation process was engineered, combining spectral kurtosis and Savitzky-Golay filtering to smooth the dynamic spectrum and enhance burst clarity.

Following RFI removal, a fine DM search was performed to refine the dispersion measure of each burst, utilising signal-to-noise ratio maximisation and a novel DM-Power technique for complex shapes. This improved the accuracy of temporal and spectral alignment. Burst profiles were modelled using Gaussian functions convolved with exponential tails to account for scattering effects, while burst spectra were fitted with Gaussian models to determine peak frequency and emission bandwidth. Native time resolutions of 163.84μs or 327.68μs were maintained for high signal-to-noise ratio events (SNR ≥20σ), while lower SNR events underwent downsampling.

This careful processing enabled the construction of distributions for structure-optimized DM, intrinsic width, scattering width, and flux density, revealing a concentration of DMs around 412, 415 pc/cc and predominantly minimal to moderate scattering. Investigation of burst timing revealed consecutive bursts separated by 16.7 to 291.5 milliseconds, suggesting short repetition intervals and potential sub-second quasi-periodicity. The waiting-time and energy distributions were bimodal, prompting the application of a two-component log-normal model to delineate two distinct emission timescales and energy modes. A fluence completeness threshold of 3.05 Jy ms was applied to 109 bursts, allowing the team to model the cumulative burst rate as a function of fluence using a broken power law, identifying a break at 16.89 Jy ms with cumulative indices of α1 = 0.76 ±0.06 and α2 = 2.89±0.64. Detailed multi-frequency coverage was achieved, detecting bursts in Band 4 (550, 950MHz) and Band 5 (1060, 1460MHz). The study recorded significant morphological diversity within the bursts, noting multiple sub-bursts and consistent downward frequency drifts, with intrinsic burst widths ranging from 1.032 to 32.159 milliseconds. A frequency-dependent decline in activity was observed, with the final burst detected in Band 5 occurring on 24 May, while Band 4 continued to exhibit bursts until 28 May.

Consecutive bursts were observed with remarkably short separations, measured between 16.7 and 291.5 milliseconds, suggesting rapid repetition or sub-second quasi-periodicity. Bimodal distributions of waiting times and energies indicate at least two distinct emission timescales and energy modes are at play within FRB 20201124A. Measurements confirm burst fluence values ranging from 1.72 to 78.47 Jy ms, with the cumulative fluence distribution following a broken power law. Multi-frequency analysis identified closely spaced burst pairs across both bands, exhibiting sub-second offsets between 1.08 and 1.15 seconds, with no strict simultaneity with contemporaneous detections from the FAST telescope, highlighting the localised nature of these emissions. These findings deliver a detailed picture of FRB 20201124A’s patchy, multi-frequency emission and frequency-dependent activity.