GRB minimum variability timescales with Fermi/GBM

TL;DR

Gamma-ray burst classification by duration can be misleading for peculiar events, so the study assesses the minimum variability timescale (MVT) as an additional diagnostic. Using MEPSA to measure the shortest significant pulse ($FWHM_{min}$) across ~3350 Fermi/GBM GRBs, the authors compare multiple MVT definitions, update the $MVT-T_{90}$ plane, and analyze populations including SEE-GRBs, short GRBs with extended emission, and extragalactic magnetar giant flares. Key findings show SEE-GRBs and SGRBs share similar MVT ranges, MGFs are even shorter, and a few long-duration merger candidates exhibit very short MVTs, while LGRBs show a robust $L_p$–$FWHM_{min}$ anti-correlation and a linkage to the Lorentz factor. The results highlight the value of MVT when combined with other diagnostics to flag mergers and distinguish MGFs from typical SGRBs, and they offer insights into emission radii and jet physics consistent with ICMART-like scenarios in many bursts.

Abstract

Context. Gamma-ray bursts (GRBs) have traditionally been classified by duration into long (LGRBs) and short (SGRBs), with the former believed to originate from massive star collapses and the latter from compact binary mergers. However, events such as the SGRB 200826A (coming from a collapsar) and the LGRBs 211211A and 230307A (associated with a merger) suggest that duration-based classification could be sometimes misleading. Recently, the minimum variability timescale (MVT) has emerged as a key metric for classifying GRBs. Aims. We calculate the MVT, defined as the full width at half maximum (FWHM) of the narrowest pulse in the light curve, using an independent dataset from Fermi/GBM and we compare our results with other MVT definitions. We update the MVT-T90 plane and analyse peculiar events like long-duration merger candidates 211211A, 230307A, and other short GRBs with extended emission (SEE-GRBs). We also examine extragalactic magnetar giant flares (MGFs) and explore possible new correlations with peak energy. Methods. We used the MEPSA algorithm to identify the shortest pulse in each GRB light curve and measure its FWHM. We calculated the MVT for around 3700 GRBs, 177 of which with spectroscopically known redshift. Results. SEE-GRBs and SGRBs share similar MVTs (from few tens to a few hundreds of ms), indicating a common progenitor, while extragalactic MGFs exhibit even shorter values (from few ms to few tens of ms). Our MVT estimation method consistently yields higher values than another existing technique, the latter aligning with the pulse rise time. For LGRBs, we confirmed the correlations of MVT with peak luminosity and Lorentz factor. Conclusions. We confirmed that, although MVT alone cannot determine the GRB progenitor, it is a valuable tool when combined with other indicators, helping to flag long-duration mergers and distinguish MGFs from typical SGRBs.

Figures (18)

• Figure 1: This plot represents $\Delta t_{\rm{min}}$ versus $\rm{FWHM_{min}}$, for the GRBs in common. Red points show GBM data, where $\Delta t_{\rm{min}}$ was taken from Golkhou15 and Veres23, while blue points are BAT data, $\Delta t_{\rm{min}}$ being taken from Golkhou14. Equality is shown with a solid line, while dashed lines show $\pm 1$ dex.
• Figure 2: $\Delta t_{\rm min}$ is the MVT estimate from Golkhou15 and Veres23 obtained with GBM, while $\Delta t_{\rm det}$ is the detection timescale found with mepsa. Solid and dashed lines have the same meaning as in Figure \ref{['fig:FWHM_min_vs_dt_min']}.
• Figure 3: $\Delta t_{\rm{min}}$ vs. the rise time $t_r$ of the fitted pulse, for the samples of GRBs defined in Appendix \ref{['sec:appendix']}. Red points show GBM data, where $\Delta t_{\rm{min}}$ was taken from Golkhou15 and Veres23, while blue points are BAT data, $\Delta t_{\rm{min}}$ being taken from Golkhou14. Solid and dashed lines have the same meaning as in Figure \ref{['fig:FWHM_min_vs_dt_min']}.
• Figure 4: Scatter plot of $\rm{FWHM}_{\rm{min}}$ and $\rm{T}_{\rm{90}}$ for the Fermi/GBM sample, along with the corresponding marginal distributions. Blue (red) points represent short (long) GRBs. Gold points represents SN-associated GRBs. Magenta, lime, and cyan points represent SEE-GRBs from Lien16Lan20Kaneko15, respectively. Three extragalactic MGFs candidates, 180128A, 200415A, and 231115A are shown in brown. SEE-GRBs from the three samples considered are shown altogether in grey in the top and right panel. We also showed with a black star the two peculiar LGRBs 211211A and 230307A associated with a KN event, and 191019A wich can be a short GRB that exploded in a dense environment. We also highlighted the peculiar short collapsar GRB 200826A associated with a SN.
• Figure 5: Top panels: LC of 211211A (left) and of 230307A (right), using the 8-1000 keV range. Bottom panels, left to right: LC of 080807, 090720B, and 090832A, respectively (same energy range as top panels). The yellow window includes the initial short spike, while the blue one includes the extended emission. The inset of each panel is a close-in on the narrowest pulse. The black point indicates the detection timescale $\Delta t_{\rm det}$ of the narrowest pulse, while the orange region shows the window encompassing $\rm{FWHM_{min}}$.