GRB minimum variability timescale with Insight-HXMT and Swift

Abstract

Context. There has been significant technological and scientific progress in our ability to detect, monitor and model the physics of gamma-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 discriminate 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 half maximum (FWHM) duration. We then applied this framework to two sets of GRBs:

Flares, Jets, and Quasiperiodic Outbursts from Neutron Star Merger Remnants

“…Such a (dominant) subkilohertz QPO frequency has not been inferred from GRB910711. Further evidence for the origin of subkilohertz variability may also come from population statistics (Camisasca et al 2023).…”

The Minimum Variation Timescales of X-Ray Bursts from SGR J1935+2154

“…Therefore, Golkhou & Butler (2014); Golkhou et al (2015) defined the MVT as the transition between correlated and uncorrelated variability, which is approximately the same as the rise time of the shortest pulse in a light curve. Recently, Camisasca et al (2023) defined MVT as the FWHM duration of the shortest pulse in the time domain and reported a very strong correlation between their results and those of Golkhou & Butler (2014), but they are not completely consistent; in particular, some differ by more than a decade. In addition, some authors found that half of the shortest Bayesian block (BB) is consistent with the MVT obtained by the wavelet analysis of individual GRBs (e.g., Ackermann et al 2014;Vianello et al 2018), but to our knowledge, the MVT estimated by BB has not been systematically compared with the results by other methods to prove its robustness, which will be demonstrated in this work.…”

Evidence for Two Distinct Populations of Kilonova-associated Gamma-Ray Bursts

“…The GRB prompt-emission light curves are of paramount importance in understanding the characteristics of the central engine. The T 90 duration describes the timescale over which the central engine is active, and the minimum variability timescale (t mvts ) gives an idea about the compactness of the emitting region (Sari & Piran 1997;Golkhou et al 2015;Sonbas et al 2015; see also Camisasca et al 2023 for a recent work that studied the correlation of t mvts of GRBs with quantities such as peak luminosity and Lorentz factor). We, therefore, explored the effect of these two parameters on the clusters of KNassociated GRBs.…”