An activity transition in FRB 20201124A: Methodological rigor, detection of frequency-dependent cessation, and a geometric magnetar model

¹ ASTRON, The Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
² Independent researcher, Zwolle, The Netherlands
³ National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Pune, 411007 Maharashtra, India
⁴ Anton Pannekoek Institute, University of Amsterdam, Postbus 94249 1090 GE Amsterdam, The Netherlands
⁵ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
⁶ Netherlands eScience Center, Science Park 402, 1098 XH Amsterdam, The Netherlands
⁷ Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, United Kingdom

^⋆ Corresponding author; hanna.bilous@gmail.com

Received: 7 July 2024
Accepted: 15 November 2024

Abstract

We report detections of fast radio bursts (FRBs) from the repeating source FRB 20201124A with Apertif/WSRT and GMRT, and measurements of basic burst properties, especially the dispersion measure (DM) and fluence. Based on comparisons of these properties with previously published larger samples, we argue that the excess DM reported earlier for pulses with integrated signal-to-noise ratios ≲1000 is due to incompletely accounting for what is known as the sad trombone effect, even when using structure-maximizing DM algorithms. Our investigations of fluence distributions next lead us to advise against formal power-law fitting; we especially caution against the use of the least-squares method, and we demonstrate the large biases involved. A maximum likelihood estimator (MLE) provides a much more accurate estimate of the power law, and we provide accessible code for direct inclusion in future research. Our GMRT observations were fortuitously scheduled around the end of the Spring 2021 activity window as recorded by FAST. We detected several bursts (one of them very strong) at 400/600 MHz, a few hours after sensitive FAST non-detections already showed the 1.3 GHz FRB emission to have ceased. After FRB 20180916B, this is a second example of a frequency-dependent activity window identified in a repeating FRB source. Since numerous efforts have so far failed to determine a spin period for FRB 20201124A, we conjecture that it is an ultra-long-period magnetar, with a period on the scale of months, and with a very wide, highly irregular duty cycle. Assuming the emission comes from closed field lines, we used radius-to-frequency mapping and polarization information from other studies to constrain the magnetospheric geometry and location of the emission region. Our initial findings are consistent with a possible connection between FRBs and crustal motion events.