Long Duration Gamma Ray BurstEdit

Long-duration gamma-ray bursts (LGRBs) are among the most luminous and energetic phenomena observed in the cosmos. Emitting intense gamma rays for more than a couple of seconds, these events are visible across vast cosmic distances and often followed by multi-wavelength afterglows that fade over hours to weeks. The radiation is believed to arise from narrowly collimated jets produced when massive stars collapse, a scenario that makes LGRBs powerful probes of star formation, stellar death, and the interstellar environments they inhabit. Observatories such as the Swift Observatory and the Fermi Gamma-ray Space Telescope have been instrumental in detecting and characterizing these bursts, enabling a global effort to map their properties and distribution through time.

Most long-duration bursts are associated with the explosive death of massive stars, a mechanism commonly known as the collapsar model. In this picture, the core of a rapidly rotating star collapses to form a compact object—typically a black hole or, in some variants, a highly magnetized neutron star (a magnetar). Accretion of stellar material onto this central engine launches relativistic jets that bore through the stellar envelope and emerge to produce the observed gamma rays. As the jets interact with the surrounding medium, shocks accelerate particles and produce afterglow radiation at X-ray, optical, and radio wavelengths. In many well-studied cases, a broad-lined, hydrogen-poor supernova of Type Ic (often labeled Ic-BL) accompanies the burst, providing a direct link between LGRBs and a specific class of stellar explosions. For example, the historical association between GRB 980425 and SN 1998bw helped cement the connection between long GRBs and massive star death. The relationship is an active area of research, with ongoing observations refining how often an LGRB is accompanied by a luminous supernova and under what conditions such a connection holds.

Definition and classification

Long-duration gamma-ray bursts are typically defined observationally by a prompt emission duration (T90) longer than about 2 seconds, distinguishing them from short-duration bursts that last less time and are commonly linked to compact object mergers. The broader term used in the field is Long-duration gamma-ray burst or simply LGRB.
The standard model emphasizes a core-collapse origin for most LGRBs, as opposed to the merger scenarios that account for many short-duration bursts.
A subset of events, sometimes called ultra-long gamma-ray bursts, exhibit durations well above 10^3 seconds and may arise from different progenitors—such as blue supergiant stars—or from alternative physical pathways. The exact boundary and taxonomy remain topics of discussion within the literature.

Progenitors and central engines

Progenitor stars: The favored progenitors are rapidly rotating, stripped-envelope massive stars. When such stars collapse, angular momentum can form a compact central engine surrounded by an accretion disk, which then launches the relativistic jets responsible for the prompt gamma rays.
Central engine options: The engine may be a black hole accreting material from the surrounding disk, or, in some models, a nascent magnetar that injects energy into the jets through magnetic processes. Each scenario has distinct implications for jet composition, energetics, and the duration over which energy is supplied to the outflow.
Jet breakout and beaming: The jets must tunnel through the stellar envelope before escaping into space. Once they break out, the gamma-ray emission is highly beamed, so the observed energy depends strongly on the jet opening angle. The typically inferred beaming-corrected energies, once geometry is accounted for, are smaller than the isotropic-equivalent estimates by factors of tens to hundreds.
Alternative or supplementary channels: While the collapsar model explains many LGRBs, some events challenge the universality of a single channel. The potential role of magnetar engines and the possibility of diverse progenitors in different environments are active areas of research.

Observational properties

Prompt emission: The initial gamma-ray flash often exhibits a non-thermal spectrum well described by a Band function, with rapid variability and diverse spectral shapes. The exact spectrum and light curve depend on jet physics, jet composition, and viewing angle.
Afterglow: Following the prompt phase, afterglow emission arises from shocks as the jet decelerates in the circumburst medium. Afterglows are detected across X-ray, UV/optical, and radio bands, enabling localization, distance measurement, and studies of the surrounding environment.
Energetics and beaming: The isotropic-equivalent energy release for LGRBs can reach up to around 10^54 erg in some cases, but the true energy is lower after accounting for beaming. Measurements of jet-opening angles from breaks in afterglow light curves help constrain the total energy budget.
Redshift distribution: LGRBs are seen up to very high redshifts, with some events detected beyond z = 8. This makes them valuable probes of star formation and metal enrichment in the early universe.
Host environments: LGRBs are frequently found in star-forming galaxies and tend to occur in regions of active stellar birth. The host galaxy properties—such as metallicity, mass, and star formation rate—inform discussions about progenitor evolution and the conditions favorable for jet formation.

Host galaxies and environments

Metallicity and star formation: A recurring theme is the tendency for LGRB hosts to have relatively low metallicity, which may influence the evolution of massive stars and the angular momentum retention needed to form jets. However, high-metallicity hosts are not excluded, and there are notable exceptions that challenge a simple metallicity threshold for LGRB production.
Galaxy demographics: LGRBs preferentially locate in galaxies with ongoing star formation, and their distribution over cosmic time tracks, but does not perfectly trace, the global star formation rate. This discrepancy motivates studies of selection effects, beam geometry, and observational biases.
Local environments: The immediate surroundings of LGRBs—gas density, dust, and radiation fields—affect afterglow properties and the visibility of associated supernovae. Dust can obscure optical afterglows in some cases, leading to so-called dark bursts that require radio or infrared follow-up for characterization.

Detection and instrumentation

Space-based observatories: Missions like the Swift Observatory and Fermi Gamma-ray Space Telescope have transformed our ability to detect and localize LGRBs in real time, enabling rapid follow-up across the electromagnetic spectrum.
Ground- and space-based follow-up: Afterglow detections typically involve a network of ground-based optical and radio telescopes to measure redshifts, determine host galaxies, and study jet evolution.
Multi-messenger considerations: Gravitational waves and neutrinos remain a potential, though as yet unconfirmed, complementary channel for long-duration events. Ongoing searches aim to correlate LGRBs with other messengers, which would illuminate the physics of core collapse and jet launching.

Controversies and debates

Progenitor diversity: While the collapsar framework explains many LGRBs, questions persist about whether all long bursts arise from the same mechanism or whether multiple progenitor channels contribute, potentially including magnetar-driven jets in some events.
Central engine debate: Whether black hole accretion systems or magnetar engines dominate LGRB production in various cases remains an area of active research. The relative contributions of each engine type influence expected jet properties and afterglow signatures.
Metallicity bias and selection effects: The apparent preference for low-metallicity hosts could reflect intrinsic physics or observational biases (such as easier detection in certain environments). disentangling nature from selection is a key challenge.
Ultra-long and atypical events: The existence of ultra-long GRBs and GRBs with weak or absent supernova signatures raises questions about whether these bursts constitute a distinct population or represent extremes of a single distribution. This has implications for progenitor models and the end stages of massive stars.
SN associations and exceptions: While many LGRBs are accompanied by Type Ic-BL supernovae, some events lack a clear SN signature. Understanding the conditions that produce or suppress an accompanying SN informs models of energy partition, jet breakout, and explosive nucleosynthesis.
Energy scale and beaming: Inferring true energetics requires careful accounting of jet opening angles. Uncertainties in beaming corrections can lead to divergent conclusions about the intrinsic power of these explosions.
Cosmological role: The relationship between LGRB rates and global star formation, as well as the use of LGRBs as proxies for star-forming activity in the early universe, is debated because of potential biases in progenitor evolution and detectability across redshift.