The empirical classiﬁcation of gamma-ray bursts (GRBs) into long and short GRBs based on their durations is already ﬁrmly established. This empirical classiﬁcation is generally linked to the physical classiﬁcation of GRBs originating from compact binary mergers and GRBs originating from massive star collapses, or Type I and II GRBs, with the majority of short GRBs belonging to Type I and the majority of long GRBs belonging to Type II. However, there is a signiﬁcant overlap in the duration distributions of long and short GRBs. Furthermore, some intermingled GRBs, i.e., short-duration Type II and long-duration Type I GRBs, have been reported. A multi-wavelength, multi-parameter classiﬁcation scheme of GRBs is evidently needed. In this paper, we seek to build such a classiﬁcation scheme with supervised machine learning methods, chieﬂy XGBoost. We utilize the GRB Big Table and Greiner’s GRB catalog and divide the input features into three subgroups: prompt emission, afterglow, and host galaxy. We ﬁnd that the prompt emission subgroup performs the best in distinguishing between Type I and II GRBs. We also ﬁnd the most important distinguishing feature in prompt emission to be T 90 , hardness ratio, and ﬂuence. After building the machine learning model, we apply it to the currently unclassiﬁed GRBs to predict their probabilities of being either GRB class, and we assign the most probable class of each GRB to be its possible physical class.

pdf/identifying-the-physical-origin-of-gamma-ray-bursts-with-32vrmtgp.pdf

Identifying the physical origin of gamma-ray bursts with supervised machine learning

Abstract As an interesting subclass of gamma-ray bursts (GRBs), Type IL GRBs (such as GRB 211211A and GRB 230307A) features a long-duration prompt emission but originating from a compact binary merger. The “long duration” emission of Type IL GRBs are dominantly composed of the main burst, rather than the extended emission, differentiating them from the traditional “long-short” GRB (e.g., GRB 060614). A previous study has reported several Type IL GRBs by visual inspection of their light curves. In this work, we established a detailed criterion to identify Type IL GRBs by light curve and then systematically searched the archival Fermi/Gamma-ray Burst Monitor data with this criterion, resulting in a sample of five type IL GRBs from 2014 January 1 to 2024 January 1, i.e., GRB 230307A, GRB 211211A, GRB 200914A, GRB 200311A, and GRB 170228A. Apart from the light-curve pattern, we find that the temporal and spectral properties of these five GRBs also support this classification. Interestingly, we find that the energy ratio between extended emission and main emission is almost constant (∼0.7, with small scattering) for these GRBs, which have strong implications on the mechanism of Type IL burst. We discuss theoretical models to interpret the progenitor, central engine, and extended emission of these Type IL bursts. 

pdf/search-for-type-il-gamma-ray-bursts-criterion-results-ozf021299zoi.pdf

Search for Type IL Gamma-Ray Bursts: Criterion, Results, Verification, and Physical Implication

Abstract GRB 240619A, detected by Fermi-Gamma-ray Burst Monitor with a duration of ∼36.1 s at redshift z = 0.3965, a short, hard main emission (ME), a quiescent interval, and a soft extended emission (EE). We perform a detailed analysis of the temporal and spectral properties of both phases using Bayesian inference and machine learning techniques. Our main results show: (1) the spectral lags are 0.27 ± 0.03 s (ME) and 0.13 ± 0.19 s (EE), with the ME phase following the lag–luminosity relation; (2) The minimum variability timescales are ∼0.04 s (ME) and ∼0.48 s (EE), consistent with short gamma-ray bursts (SGRB) and long gamma-ray bursts (LGRB) characteristics, respectively, suggesting that the radiation in the two phases may originate from different physical mechanisms. (3) the effective amplitude f eff ≈ 3.18 suggests an intrinsic origin consistent with SGRBs; (4) the spectral hardness, peak energy, Amati/Yonetoku relations, and energy–hardness–duration classification indicate that the ME phase is more similar to SGRBs, while the EE phase resembles LGRBs; (5) the local event rate density ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>4.6</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>6</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3.86</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>11.25</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> </mml:math> Gpc −3 yr −1 ) further highlights its peculiarity. Spectral analysis indicates that its nonthermal emission can be well explained by the fast-cooling synchrotron radiation model. Curvature effect analysis suggests jet acceleration in the EE phase, with the jet becoming Poynting-flux dominated. The combined evidence suggests that GRB 240619A is an SGRB with EE, likely originating from a compact binary merger, and exhibiting physical properties similar to those of GRB 211211A. 

GRB 240619A: Evidence for a Compact Star Merger Origin

The central engine that powers gamma-ray bursts (GRBs), the most powerful explosions in the universe, is still not identified. Besides hyper-accreting black holes, rapidly spinning and highly magnetized neutron stars, known as millisecond magnetars, have been suggested to power both long and short GRBs. The presence of a magnetar engine following compact star mergers is of particular interest as it would provide essential constraints on the poorly understood equation of state for neutron stars. Indirect indications of a magnetar engine in these merger sources have been observed in the form of plateau features present in the X-ray afterglow light curves of some short GRBs. Additionally, some X-ray transients lacking gamma-ray bursts (GRB-less) have been identified as potential magnetar candidates originating from compact star mergers. Nevertheless, smoking gun evidence is still lacking for a magnetar engine in short GRBs, and the associated theoretical challenges have been addressed. Here we present a comprehensive analysis of the broad-band prompt emission data of a peculiar, very bright GRB 230307A. Despite its apparently long duration, the prompt emission and host galaxy properties point toward a compact star merger origin, being consistent with its association with a kilonova. More intriguingly, an extended X-ray emission component emerges as the $\gamma$-ray emission dies out, signifying the emergence of a magnetar central engine. We also identify an achromatic temporal break in the high-energy band during the prompt emission phase, which was never observed in previous bursts and reveals a narrow jet with half opening angle of approximately $3.4^\circ$.

pdf/magnetar-emergence-in-a-peculiar-gamma-ray-burst-from-a-3k2nwvxr.pdf

Magnetar emergence in a peculiar gamma-ray burst from a compact star merger

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Magnetar emergence in a peculiar gamma-ray burst from a compact star merger