GRB minimum variability timescale with Insight-HXMT and Swift: Implications for progenitor models, dissipation physics, and GRB classifications

A. E. Camisasca, C. Guidorzi, L. Amati, F. Frontera, X. Y. Song, S. Xiao, S. L. Xiong, S. N. Zhang, R. Margutti, S. Kobayashi, C. G. Mundell, M. Y. Ge, A. Gomboc, S. M. Jia, N. Jordana-Mitjans, C. K. Li, X. B. Li, R. MacCary, M. Shrestha, W. C. XueS. Zhang

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Context. There has been significant technological and scientific progress in our ability to detect, monitor, and model the physics of γ-ray bursts (GRBs) over the 50 years since their first discovery. However, the dissipation process thought to be responsible for their defining prompt emission is still unknown. Recent efforts have focused on investigating how the ultrarelativistic jet of the GRB propagates through the progenitor's stellar envelope for different initial composition shapes, jet structures, magnetisation, and, consequently, possible energy dissipation processes. Study of the temporal variability -in particular the shortest duration of an independent emission episode within a GRB -may provide a unique way to distinguish the imprint of the inner engine activity from geometry and propagation related effects. The advent of new high-energy detectors with exquisite time resolution now makes this possible. Aims. We aim to characterise the minimum variability timescale (MVT) defined as the shortest duration of individual pulses that shape a light curve for a sample of GRBs in the keV-MeV energy range and test correlations with other key observables such as the peak luminosity, the Lorentz factor, and the jet opening angle. We compare these correlations with predictions from recent numerical simulations for a relativistic structured -possibly wobbling -jet and assess the value of temporal variability studies as probes of prompt-emission dissipation physics. Methods. We used the peak detection algorithm MEPSA to identify the shortest pulse within a GRB time history and preliminarily calibrated MEPSA to estimate the full width at half maximum duration. We then applied this framework to two sets of GRBs: Swift GRBs (from 2005 to July 2022) and Insight Hard Modulation X-ray Telescope (Insight-HXMT) GRBs (from June 2017 to July 2021, including the exceptional 221009A). We then selected 401 GRBs with measured redshift to test for correlations. Results. We confirm that, on average, short GRBs have significantly shorter MVTs than long GRBs. The MVT distribution of short GRBs with extended emission such as 060614 and 211211A is compatible only with that of short GRBs. This is important because it provides a new clue concerning the progenitor's nature. The MVT for long GRBs with measured redshift anti-correlates with peak luminosity; our analysis includes careful evaluation of selection effects. We confirm the anti-correlation with the Lorentz factor and find a correlation with the jet opening angle as estimated from the afterglow light curve, along with an inverse correlation with the number of pulses. Conclusions. The MVT can identify the emerging putative new class of long GRBs that are suggested to be produced by compact binary mergers. For otherwise typical long GRBs, the different correlations between MVT and peak luminosity, Lorentz factor, jet opening angle, and number of pulses can be explained within the context of structured, possibly wobbling, weakly magnetised relativistic jets.

Original languageEnglish
Article numberA112
JournalAstronomy and Astrophysics
Early online date9 Mar 2023
Publication statusPublished - 31 Mar 2023

Bibliographical note

Funding Information:
We are grateful to the Referee for their useful comments which helped us to improve the paper. A.E.C. and C.G. acknowledge financial support from FIRD 2022 of UNIFE Dept. Physics and Earth Science under the project “Caratterizzazione e simulazioni di curve di luce di Gamma-Ray Burst come processi stocastici e validazione attraverso tecniche di machine learning” (PI: CG). C.G.M. and N.J.M. acknowledge support from Hiroko and Jim Sherwin. This work is supported by the National Program on Key Research and Development Project (2021YFA0718500) and the National Natural Science Foundation of China under grants 11733009, U1838201 and U1838202. This work made use of data from the Insight-HXMT mission, a project funded by China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS).


  • Gamma-ray burst: general
  • Radiation mechanisms: non-thermal
  • Relativistic processes
  • Stars: jets

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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