Long gamma-ray bursts (GRBs) and broad-lined Type Ic supernovae (SNe Ic-BL) are two observable outcomes of the same phenomenon — the gravitational collapse of a massive stripped-envelope star — yet the two are wildly asymmetric in frequency: the GRBSN webtool catalogue lists only 61 secure GRB-supernova associations (Finneran, Cotter & Martin-Carrillo 2025), while a much larger population of SNe Ic-BL has been discovered with no detected GRB, and only a minute fraction of all core collapses yields an observable GRB. The source paper states the open question in exactly this form: what makes the progenitors of these events capable of producing a supernova and a GRB when many stars cannot? That question remains open: the physical parameter, or combination of parameters, that decisively distinguishes a collapse producing both an observable long gamma-ray burst and a broad-lined Type Ic supernova from one producing a supernova alone has not been identified.

Background

A gamma-ray burst (GRB) is a brief, extremely luminous flash of high-energy photons; bursts longer than about two seconds ("long" GRBs) occur exclusively in star-forming regions of galaxies, at the positions of massive young stars. A core-collapse supernova results from the gravitational collapse of a massive star's iron core to a neutron star or black hole, which relaunches a shock that unbinds the stellar envelope. Supernovae of Type Ic have lost both their hydrogen and helium envelopes before collapse; the broad-lined subtype (Ic-BL) shows spectroscopic lines broadened by exceptionally fast ejecta, of order one-tenth the speed of light. The discovery of SN 1998bw inside the error box of the nearby long GRB 980425 (Galama et al. 1998) first tied the two phenomena together, and every supernova spatially and temporally coincident with a bona fide long GRB has been of Type Ic-BL. In the collapsar picture (Woosley 1993), the collapsing core forms an accreting black hole that launches an ultra-relativistic, narrowly collimated bipolar jet; if the jet punctures the stellar surface it produces the gamma-rays and an afterglow, while the jet's energy deposition in the star simultaneously powers an energetic supernova. An alternative central engine is a millisecond magnetar — a newborn, rapidly spinning, strongly magnetized neutron star whose rotational spin-down injects a collimated outflow. A jet that is too weak to break out of the star is called choked: all of its energy is deposited in the envelope and no GRB is seen. Because the jets are narrow (opening angles of a few degrees), an otherwise successful GRB viewed far from the jet axis is also missed; such events are searched for as "orphan afterglows", most sensitively at late times in the radio band. The asymmetry at the heart of this problem is old and explicit: as Woosley & Bloom (2006) put it in their Annual Review, "Neither all SNe, nor even all SNe of Type Ibc produce GRBs. The degree of differential rotation in the collapsing iron core of massive stars when they die may be what makes the difference." Differential rotation — the contrast in angular speed between the inner core and outer layers — supplies the rotational energy and centrifugal support that an accreting black hole engine requires, and preserving it until collapse is hard, because angular momentum is transported outward during the star's life. The canonical candidate discriminators that have been investigated since are: the core's angular momentum (set by initial spin, internal transport, and possible spin-up by a binary companion's inspiral or merger); the nature and magnetic field strength of the central engine; pre-explosion mass loss, which scales with metallicity (the fraction of elements heavier than helium in the gas) because line-driven winds of hot luminous stars are weaker at low metallicity, leaving the star compact and rapidly rotating; and the jet power relative to the stellar envelope, which together with the viewing angle decides whether a burst is observed. The problem remained a live cataloguing exercise into the 2020s: the GRBSN webtool (Finneran, Cotter & Martin-Carrillo 2025) maintains the most complete list of associations, and its companion spectroscopic study (Finneran et al. 2025, A&A 700, A200) compares 61 ordinary SNe Ic-BL with 13 GRB-associated ones. The source paper poses the question: "what makes the progenitors of these events capable of producing a supernova and a GRB when many stars cannot?"

Problem Statement

Identify the physical properties of a massive star and its collapse that determine whether the event produces both an observable long gamma-ray burst and a broad-lined Type Ic supernova, or a supernova alone — answering, in the source's own words, 'what makes the progenitors of these events capable of producing a supernova and a GRB when many stars cannot?'. The candidate discriminators to be adjudicated are the angular momentum (differential rotation) of the collapsing core, the nature and magnetic field of the central engine (accreting black hole versus millisecond magnetar), pre-explosion mass loss and metallicity, binary interaction history, and the jet power relative to the stellar envelope, together with viewing geometry. What is sought is which of these (alone or in combination) decides the outcome, linked through an explicit physical mechanism to jet launch, breakout, or choking, and accounting quantitatively for the observed demographics: the 61 secure GRB-supernova associations catalogued by the GRBSN webtool against a much larger discovered population of broad-lined Type Ic supernovae with no detected GRB; the 3-sigma limit of below about 19 per cent on the fraction of SNe Ic-BL as relativistic as SN 1998bw (Corsi et al. 2023); and deep late-time radio limits excluding off-axis jets of classical GRB energy, about 10^51 erg, in most SNe Ic-BL (Schroeder et al. 2025).

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