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Gamma-ray burst progenitors revisited

This study challenges the traditional dichotomy between gamma-ray burst duration and progenitor type by demonstrating that a significant fraction (31% ± 9%) of nearby long-duration GRBs lack supernova counterparts and instead likely originate from compact object mergers, implying rates comparable to short GRBs with major implications for gravitational-wave astronomy and heavy element enrichment.

Original authors: Andrew J. Levan, Jillian C. Rastinejad, Helena-Margaret S. Grabham, Daniele B. Malesani, Nial R. Tanvir, Eric Burns, Benjamin P. Gompertz, Gavin P. Lamb, Ashley A. Chrimes, Peter G. Jonker, Om Salafia

Published 2026-07-30

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CC BY 4.0

Original authors: Andrew J. Levan, Jillian C. Rastinejad, Helena-Margaret S. Grabham, Daniele B. Malesani, Nial R. Tanvir, Eric Burns, Benjamin P. Gompertz, Gavin P. Lamb, Ashley A. Chrimes, Peter G. Jonker, Om Salafia, Nikhil Sarin, Ilya Mandel, Antonio Martin-Carrillo

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, chaotic fireworks factory. For decades, astronomers have been trying to figure out what kind of "fuse" sets off the biggest, brightest explosions in the sky: Gamma-Ray Bursts (GRBs). These are the most energetic blasts in the cosmos, outshining entire galaxies for a split second. The old rulebook was simple and tidy: if the explosion lasts less than two seconds, it's a "short" burst caused by two dead stars (like neutron stars) smashing into each other. If it lasts longer than two seconds, it's a "long" burst caused by a massive star collapsing in on itself, like a cosmic implosion. It was a clean, two-box system that everyone thought they understood. But recently, some astronomers started finding explosions that didn't fit the boxes. They saw long bursts that didn't leave behind the usual debris of a dying star, and short bursts that seemed to have a different origin story. This paper steps in to ask a big, messy question: Is the old rulebook wrong? Are we missing a whole hidden population of explosions that look like long bursts but are actually caused by star-smashing mergers?

This paper, titled "Gamma-ray burst progenitors revisited," dives deep into the local neighborhood of our universe (specifically, galaxies within about 3 billion light-years) to count and categorize these cosmic fireworks. The authors, led by Andrew Levan and a team of international scientists, gathered a sample of 29 long and short bursts detected by the Swift satellite at low distances. They used a clever trick to sort them: they looked for the "smoke" left behind. Long bursts from collapsing stars usually leave a bright supernova (a massive stellar explosion), while bursts from merging dead stars leave behind a faint, fast-fading glow called a kilonova.

The team found something surprising. In their local sample, the number of long bursts without a supernova was almost exactly the same as the number of long bursts with one. Roughly 31% of the nearby long bursts showed no sign of a dying star, and instead, some of them showed signs of a kilonova or lived in quiet, ancient galaxies where massive stars don't exist. This suggests that a significant chunk of what we thought were "collapsing star" explosions are actually "merging star" explosions. The paper explicitly argues against the idea that all long bursts come from massive stars. Instead, it suggests that about 30% to 70% of nearby long GRBs might actually be the result of compact objects crashing together, just like the short bursts.

The authors also looked at the "personality" of these bursts to confirm their suspicions. They found that the long bursts without supernovae tend to have fainter afterglows (the fading light after the explosion) and live in galaxies that aren't making many new stars. This fits the profile of a merger, which can happen far away from the busy star-forming nurseries where massive stars are born. While they can't prove every single one of these is a merger, the evidence strongly suggests that the "two-box" system is too simple. The universe is messier than we thought, with a hidden population of long-lasting explosions that are actually the result of cosmic collisions. This discovery matters because it changes how we calculate the rate of these mergers, which helps us understand how heavy elements like gold are made and how often we might detect them with gravitational wave detectors. The paper doesn't claim to have solved the mystery completely, but it strongly suggests that the "long" and "short" labels based on time alone are no longer enough to tell us what caused the bang.

Technical Summary: Gamma-ray burst progenitors revisited

Problem Statement
For over four decades, the gamma-ray burst (GRB) population has been dichotomously classified by duration ( $T_{90}$ ), with a dividing line at 2 seconds separating "short" and "long" bursts. This classification has traditionally mapped to distinct progenitor channels: long GRBs to massive star core-collapse (collapsars) and short GRBs to compact object mergers. However, recent observations have challenged this paradigm. Notably, the identification of a supernova in the short-duration GRB 200826A and the association of kilonovae with long-duration bursts (GRBs 211211A and 230307A) suggest significant overlap between these populations. The central question addressed by this work is the relative purity of short and long GRB samples: to what extent do these populations genuinely represent distinct progenitor channels, and what fraction of long GRBs may actually originate from compact object mergers rather than massive stars?

Methodology
The authors construct a uniformly selected sample of low-redshift GRBs to robustly constrain progenitor types. The primary discriminant used is the presence or absence of a supernova counterpart, as supernovae are significantly more luminous and longer-lived than kilonovae, allowing for deep limits to be placed at low redshifts.

Sample Definition: The study focuses on GRBs detected by the Neil Gehrels Swift Observatory at redshifts $z < 0.3$ and $z < 0.5$ . A broader sample including pre-Swift missions and other detectors is also considered for context.
Progenitor Classification: The authors categorize bursts into five groups based on observational constraints:
- Collapsars: Long GRBs with confirmed supernovae.
- L-Merger: Long GRBs ( $T_{90} > 2$ s) with deep limits ruling out supernovae (specifically $f_{1998bw} < 0.26$ , where $f_{1998bw}$ is the luminosity ratio relative to SN 1998bw) and/or located in quiescent/ancient host galaxies.
- L-Weak: Long GRBs with weak supernova constraints ( $f_{1998bw} > 0.26$ ).
- S-Merger: Short GRBs without supernovae and/or in ancient hosts.
- S-Weak: Short GRBs with weak supernova constraints.
Diagnostic Analysis: The study compares sub-samples across multiple parameters:
- Supernova Limits: Using the Redback package and MOSFiT to interpolate SN 1998bw light curves and establish luminosity limits.
- Energetics: Analyzing isotropic-equivalent energy ( $E_{iso}$ ) and luminosity ( $L_{iso}$ ).
- Spectral Properties: Examining the $E_{peak}$ - $E_{iso}$ (Amati) relation and spectral lags.
- Afterglows: Comparing X-ray and optical afterglow brightness relative to prompt fluence.
- Host Galaxies: Investigating stellar mass, specific star formation rate (sSFR), and offsets from the host center.
- Rate Estimation: Calculating volumetric rates ( $R_{GRB}$ ) for on-axis events, accounting for survey duration, sky coverage, and redshift completeness, while acknowledging significant selection biases.

Key Results

Progenitor Fractions: In the $z < 0.3$ sample, the authors find 21 long GRBs. Of these, 9 are identified as Collapsars and 9 as L-Mergers (with 3 L-Weak). This implies that approximately 31% $\pm$ 9% of nearby long GRBs lack supernova counterparts and likely originate from mergers. No short GRBs in this sample show evidence of a supernova.
Distinct Properties: Long GRBs without supernovae (L-Mergers) exhibit distinct properties compared to Collapsars:
- Afterglows: They possess fainter X-ray and optical afterglows for a given prompt fluence.
- Hosts: They reside in host galaxies with lower specific star formation rates and higher stellar masses compared to Collapsar hosts.
- Offsets: They show larger offsets from their host galaxy centers, consistent with natal kicks in merger scenarios.
- Absorption: They tend to have lower X-ray absorption columns.
Energetics and Spectra: While there is no strong difference in bulk energetics, L-Merger and L-Weak bursts tend to lie at the edge of the Amati relation, closer to the short GRB population, supporting a merger origin.
Rates: The estimated on-axis volumetric rates for Collapsars, L-Mergers, and S-Mergers are comparable, ranging from $\sim 0.5$ to $2.5$ Gpc $^{-3}$ yr $^{-1}$ . The authors note that the sample may be only $\sim 50\%$ complete due to the faintness of merger afterglows, suggesting the true merger rate could be higher.
Redshift Evolution: The study suggests that while merger-driven long GRBs are significant in the local universe ( $z < 0.5$ ), the population is likely dominated by Collapsars at higher redshifts due to the strong evolution of the cosmic star formation rate and metallicity biases favoring collapsars at high $z$ .

Significance and Claims
The paper claims that the traditional dichotomy between long and short GRBs is insufficient to describe progenitor channels. The authors assert that a substantial fraction (estimated at 30–70% of local long GRBs) of long-duration bursts likely arise from compact object mergers rather than massive star collapse.

This finding has several implications:

Progenitor Models: It challenges the standard view that jet duration is strictly tied to the mass of accreting material in a collapsar, suggesting alternative merger progenitors can produce long-duration jets.
Heavy Element Enrichment: If a significant portion of long GRBs are mergers, estimates of r-process element enrichment based solely on short GRB rates may require revision.
Star Formation Tracers: Long GRBs are often used as tracers of massive star formation and the star formation rate (SFR). If a fraction of these are mergers (which have delay times), their utility as direct SFR tracers, particularly at high redshift, may be compromised.
Gravitational Wave Coincidence: The results suggest that long GRBs could serve as powerful electromagnetic counterparts for future gravitational wave detectors (like Einstein Telescope and Cosmic Explorer), potentially identifying merger events that are otherwise difficult to localize via kilonovae alone.

The authors maintain a cautious tone regarding the "L-Weak" category and acknowledge that while the merger origin is the most economical explanation for supernova-less long GRBs, alternative progenitors (such as white dwarf-black hole mergers or failed supernovae) cannot be entirely ruled out for specific events. They emphasize that selection effects, particularly the faintness of merger afterglows, likely lead to an underestimation of the merger fraction in current catalogs.

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