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The Physics of Gamma-Ray Bursts

Tsvi Piran

arXiv:astro-ph/0405503v1astro-ph

TL;DR

The review addresses how GRBs and their afterglows arise and synthesizes observational and theoretical work through the fireball internal-external shocks model. It reports broad observational coverage, including GRB redshifts reaching 4.5, while noting unresolved questions about dark GRBs, density profiles, progenitors, and GRB-associated supernovae.

  • Problem

    The review addresses the physical origin of GRBs, their afterglows, dark bursts, progenitors, and associated supernovae.

  • Method

    The paper reviews GRB theory and focuses on the fireball internal-external shocks model and related physical processes.

  • Results

    The reviewed observations include a reported GRB redshift record of 4.5 and findings on GRB populations, density profiles, and GRB-associated supernovae.

  • Takeaways & Limitations

    The review presents the fireball model as the framework used to organize GRB prompt-emission, afterglow, population, and progenitor evidence.

Abstract

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Gamma-Ray Bursts (GRBs), short and intense pulses of low energy gamma-rays, have fascinated astronomers and astrophysicists since their unexpected discovery in the late sixties. During the last decade, several space missions: BATSE (Burst and Transient Source Experiment) on Compton Gamma-Ray Observatory, BeppoSAX and now HETE II (High-Energy Transient Explorer), together with ground optical, infrared and radio observatories have revolutionized our understanding of GRBs showing that they are cosmological, that they are accompanied by long lasting afterglows and that they are associated with core collapse Supernovae. At the same time a theoretical understanding has emerged in the form of the fireball internal-external shocks model. According to this model GRBs are produced when the kinetic energy of an ultra-relativistic flow is dissipated in internal collisions. The afterglow arises when the flow is slowed down by shocks with the surrounding circum-burst matter. This model has numerous successful predictions like the prediction of the afterglow itself, the prediction of jet breaks in the afterglow light curve and of an optical flash that accompanies the GRBs themselves. In this review I focus on theoretical aspects and on physical processes believed to take place in GRBs.

X. Open Questions and Future Prospects

This review surveys GRB observations and the fireball internal-external shocks model, emphasizing prompt emission, afterglows, host galaxies, progenitors, and unresolved questions. Observations broadly agree with the model, but the inner engine and several spectral and population properties remain incompletely understood.

  • Observational setting: GRBs are cosmological, span redshifts from 0.16 to 4.5, and are associated with host galaxies and star-forming regions.
  • Fireball model: The fireball model attributes prompt GRB emission to internal dissipation and afterglows to external shocks in circum-burst matter.
  • Afterglows and open questions: The review reports optical flashes, long-lived afterglows, luminosity-related variability and lag correlations, while noting that the inner engine remains unresolved.
  • Prompt emission: Prompt emission is nonthermal, often peaks at a few hundred keV, can extend to GeV energies, and sometimes includes delayed high-energy components.
  • Prompt emission: The Band function provides an excellent fit to most observed spectra, while spectral correlations lack a particular theoretical model that predicts them.

3. Populations

GRB observations distinguish long and short bursts, while X-ray flashes form a lower-energy population with evidence for continuity across relevant distributions. Afterglow detections, prompt optical flashes, and polarization measurements further constrain GRB populations and their observational differences.

  • Long and Short Bursts: Short bursts are typically harder than long bursts, lack soft short events, and show a different spatial distribution.The BATSE short-burst sample has higher ⟨V/Vmax⟩ than the long-burst sample, although detector sensitivity makes short bursts harder to detect.
  • Long and Short Bursts: ⟨V/Vmax⟩long = 0.282 and ⟨V/Vmax⟩long = 0.390, implying observed short bursts are nearer on average than observed long bursts.This difference is not necessarily a difference in physical locations because triggering is less sensitive to short bursts.
  • Long and Short Bursts: Afterglow has been detected only from long bursts, while the well-localized short hard burst GRB020531 had an X-ray afterglow at least 100–300 times weaker than comparable long-burst afterglows.Because afterglows enable host and redshift identification, the progenitors and environments of short bursts remain unknown.
  • X-ray Flashes: X-ray flashes have GRB-like temporal structure but lower energies; their peak energy is below 40 keV and most of their flux is in X-rays.In six years of BeppoSAX observations, 32 X-ray flashes were observed, approximately half the GRB frequency, and their distributions show continuity with GRBs.
  • Prompt Optical Flashes: A 9th-magnitude optical flash from GRB 990123 peaked separately from the γ-ray emission, indicating that the optical signal was not simply its low-energy tail.Rapid optical detections from later bursts suggest that more such prompt signals may be found.
  • Afterglow Observations: The X-ray afterglow follows fν(t) ∝ν−βt−α with α ∼1.4 and β ∼0.9, while optical-counterpart bursts have X-ray afterglows five times brighter than dark GRBs.The overall X-ray-afterglow energy is generally a few percent of the GRB energy, and beaming-corrected luminosity is approximately constant within a factor of 2.

2. Optical and IR afterglow

Optical and infrared afterglows are detected for only about half of well-localized GRBs, but rapid localization has enabled earlier and more complete monitoring. Their light curves show power-law fading, achromatic breaks, late red bumps associated with supernovae, and diverse multiwavelength behavior.

  • About 50% of well-localized GRBs show optical or infrared afterglows, while X-ray counterparts occur in roughly 90% of cases.
  • Optical afterglows initially decay approximately as t^-α with typical α ≈1.2, while many later show achromatic breaks to steeper declines with α ≈2.
  • Rapid HETE II localization enabled an almost complete optical light curve for GRB021004 beginning 193 seconds after the trigger.
  • Late red bumps are generally interpreted as underlying supernovae, with GRB030329 providing a particularly strong signature.
  • Dark GRBs are heterogeneous: most have broadly consistent optical-to-X-ray ratios, whereas a remaining fraction is 4–10 times optically weaker or shows optical-band suppression.
  • Radio afterglows occur in about half of well-localized bursts, and scintillation measurements provided direct evidence for relativistic expansion at scales around 10^17 cm.

4. Association with Supernovae

Multiple observations connect long GRBs with unusually energetic Type Ic supernovae and massive-star environments. The association is strongest for GRB030329/SN2003dh, while incomplete late-time coverage limits conclusions about the full GRB–supernova population.

  • The GRB980425/SN1998bw association became stronger after XMM resolved the suspected alternative source S2 into multiple faint sources, undermining its afterglow interpretation.
  • Most supernova-bump searches provide only upper limits, and the faintest non-detection probes only the brightest roughly 40% of local Type Ib/Ic supernovae.
  • SN bumps in GRB afterglows, especially the GRB030329/SN2003dh event, provide the field’s conclusive link between long GRBs and supernovae.
  • GRB-associated supernovae resemble SN1998bw, although SN2003dh had somewhat larger expansion velocity and a brighter X-ray signal that could include afterglow emission.
  • Beaming-corrected GRB energies cluster near 10^51 ergs, with dispersion about 0.35 dex—roughly three orders of magnitude narrower than isotropic-equivalent energies.

III. THE GLOBAL PICTURE - GENERALLY ACCEPTED INGREDIENTS

The review presents a broadly accepted GRB picture involving ultra-relativistic motion, shock dissipation, synchrotron emission, jet collimation, and links to massive-star death. Important uncertainties remain about the central engine, flow composition, prompt-emission radiation, and short-burst origins.

  • A (Newborn) Compact Object: Beaming-corrected total GRB energy is estimated near 10^51 ergs, comparable to supernova energy and implying a compact central object.
  • Dissipation: Internal shocks are generally favored for prompt dissipation, whereas external shocks with circumburst matter produce the afterglow.
  • Synchrotron Radiation: Synchrotron emission from shock-accelerated electrons agrees reasonably with afterglow observations and polarization, but prompt-emission spectral slopes remain problematic.
  • Jets and Collimation: Achromatic afterglow breaks are interpreted as jet breaks or universal-structured-jet effects, implying beaming and lower true energies than isotropic estimates.
  • Association with Star Formation and SNe: Long GRBs arise in star-forming regions and are associated with massive-star collapse, whereas the origin of short GRBs remains much less established.
  • Relativistic Motion: GRBs require ultra-relativistic motion, with Γ ≳100 needed to make the source optically thin and resolve the compactness problem.

V. PHYSICAL PROCESSES

GRB emission is modeled through collisionless internal and external shocks that accelerate particles, amplify magnetic fields, and produce radiation. Relativistic shock conditions are robust, but uncertain microphysics and turbulence limit detailed spectral predictions.

  • Shock physics: Relativistic shocks convert ordered upstream kinetic energy into comparable downstream random energy and compress the matter by a factor Γ.
  • Microphysics: The energy fractions in electrons and magnetic fields, ǫe and ǫB, parameterize poorly known microphysics and are commonly assumed constant through the burst.
  • Microphysics: A constant-equipartition-parameter model cannot reproduce the observed prompt spectrum, motivating models in which ǫe,B depend on physical conditions.
  • Shock physics: Collisionless shock jump conditions follow conservation laws and are therefore expected to remain valid despite uncertain microscopic shock structure.
  • Particle acceleration: Diffuse shock acceleration repeatedly scatters particles across relativistic shocks, producing a power-law spectrum and a first-crossing energy gain larger than later cycles.

2. The Optically thin Synchrotron Spectrum

The optically thin synchrotron spectrum follows from a power-law electron population whose characteristic frequencies are set by γe,min and γe,c. Fast- and slow-cooling regimes produce distinct broken power laws, while low-energy spectral observations expose tensions for the simplest model.

  • A power-law electron distribution produces synchrotron spectra that fit several observed GRB spectra.
  • The minimum Lorentz factor γe,min sets the typical electron energy and characteristic synchrotron frequency νm.
  • About 1/5 of bursts show spectra steeper than the synchrotron low-energy limit, although HETE spectra remain within the expected range and BATSE resolution may contribute.
  • The cooling regime is determined by γe,c: fast cooling has γe,c < γe,min, whereas slow cooling has γe,c > γe,min.
  • Slow cooling: Slow cooling extends Fν ∝ν1/3 to νm and gives Fν ∝ν−p/2 above νc, with the peak flux at νm and peak emitted energy at νc.
  • Inverse Compton can add an ultra-high-energy component and accelerate cooling when Y > 1, even when it does not produce the observed γ-ray photons.

E. Quasi-Thermal Comptonization

Quasi-thermal Comptonization uses pair-rich, optically thick internal shocks in which self-absorbed synchrotron photons seed inverse Compton emission. The broader polarization analysis shows that relativistic geometry and broken symmetry can produce substantial observed polarization, with efficiency–polarization trade-offs.

  • E. Quasi-Thermal Comptonization: The quasi-thermal model places internal shocks at optical depth of order unity, where pair production and self-absorbed synchrotron seed inverse Compton emission.
  • E. Quasi-Thermal Comptonization: The resulting pairs act as a thermostat, producing a flat spectrum Fν ∝ν0 in the 30-300kev range.
  • F. Polarization from Relativistically Moving Sources: Local synchrotron polarization can reach 75% for 2 < p < 3, but integration over the relativistically emitting region reduces the global polarization.
  • F. Polarization from Relativistically Moving Sources: For α = 1, relativistic integration gives about 45% overall polarization, while exact fast-cooling parameters and p = 2.5 give about 50%.
  • F. Polarization from Relativistically Moving Sources: Random magnetic fields can yield net polarization when jet geometry or angular emission breaks spherical symmetry.
  • F. Polarization from Relativistically Moving Sources: Sharp-edged jets can exceed 40% polarization, but efficiency decreases sharply as polarization increases.
  • 1. General Considerations: Internal shocks reproduce the source variability, whereas external shocks produce only smooth bursts because shell spreading limits their variability.

2. Caveats and Complications

The review examines alternatives intended to evade the low-efficiency problem for variable external-shock emission, but finds persistent conflicts with observed GRB variability and pulse correlations. Internal shocks and relativistic turbulence avoid some objections, while their efficiencies and physical conditions remain constrained.

  • Clumpy external medium: Although small clumps can produce angular times d/cΓ < tR, the model struggles with bursts having δt/T ∼10^-3, nearly 100% modulation, or millisecond variability.It also predicts earlier pulses narrower than later ones, contrary to observed bursts.
  • Clumpy external medium: The clumpy external-medium scenario has a covering factor δt/T ≪1, so its γ-ray conversion efficiency can fall below 10^-2 for typical variable bursts.The covering factor follows from the number and size of clumps relative to the observed cone, and the efficiency is smaller still.
  • Clumpy media and bullets: Observed pulse-width and interval correlations challenge models in which pulse widths depend on clump geometry while intervals are set independently by the source or inner engine.The review notes that separate clumps or bullets provide no reason for these distributions to correlate.
  • Narrow bullets: Shotgun-like narrow bullets can reproduce observed variability, but their pulse widths are controlled by angular or hydrodynamic times while burst duration follows inner-engine activity.This separation preserves the light curve’s source-activity imprint but leaves the observed pulse correlations unexplained.
  • Relativistic turbulence: Relativistic turbulence may generate variability while external interaction slows the flow, but it is not described by regular shocks and leaves pulse evolution and variability mechanisms open.The review explicitly identifies whether the observed pulse correlations and the absence of late pulse spreading can be produced as open questions.
  • Internal shocks: Internal shocks naturally reproduce pulse-width/interval similarity, interval–subsequent-pulse correlations, and source-like light curves, with simulations reporting more than 80% efficient energy conversion in one case.Too-large initial Lorentz factors can instead place external shocks first and limit the relevant Lorentz-factor range and variability of Ep.

2. Synchrotron Spectrum from External Shocks

The external-shock synchrotron model depends strongly on magnetic-field amplification and shock conditions, producing different forward- and reverse-shock spectra. It predicts high-energy early emission, a short-lived reverse-shock optical flash, and a possible transition from hard GRB emission to a softer combined GRB–afterglow signal.

  • Magnetic fields: Pure shock compression would give ǫB ≪1, inconsistent with GRB observations, so turbulent instabilities are considered as a route toward near-equipartition magnetic fields.The field evolution remains uncertain, and the review adopts the same ǫB for both shocks for simplicity.
  • Forward shock: Forward-shock electrons are fast cooling, with early photons spanning the low γ-ray to X-ray range, although the regime depends strongly on model parameters.For canonical parameters νm < νc, but a slightly larger Lorentz factor than 100 can reverse this ordering.
  • Reverse shock: Reverse-shock emission is typically lower in energy and peaks in the infrared, though parameter changes can shift its characteristic frequency into the optical.The reverse shock generally has νm < νc and is in the slow-cooling regime.
  • Reverse shock: The short-lived reverse shock is expected to produce a powerful optical flash coincident with the late part of the GRB.This prediction is based on the reverse shock’s synchrotron conditions during its brief existence.
  • GRB–afterglow overlap: For long bursts, the afterglow can begin while internal shocks continue, producing a transition from hard GRB emission to a softer, smoother combined GRB–afterglow signal.Early forward-shock emission peaks in high X-rays and can contribute to the observed γ-ray flux.
  • Polarization: High polarization does not uniquely establish synchrotron emission or a uniform, Poynting-dominated magnetic field because random shock-plane fields can produce comparably high polarization in narrow jets.The review concludes that the tentative polarization detection provides limited information without further data from other bursts.

VII. THE AFTERGLOW

The afterglow is modeled as emission from relativistic ejecta decelerated by the surrounding medium, evolving from possible early radiative behavior to a predominantly adiabatic blast wave. The framework connects hydrodynamic evolution, geometry, and synchrotron break frequencies to observable light curves across cooling regimes.

  • Afterglow dynamics: The afterglow begins when ejecta energy is transferred to shocked external material, with long-burst afterglows potentially starting before the prompt burst ends.The early phase may be radiative, while later radiation losses become minor and the hydrodynamics become adiabatic.
  • Hydrodynamics and geometry: Jet geometry changes the evolution when lateral expansion alters the solid angle, while the flow eventually becomes Newtonian after the relativistic blast-wave stage.For a conical double-sided jet, Ω ≈2πθ^2; the Blandford–McKee solution breaks down near R ∼l.
  • Hydrodynamics and geometry: The Blandford–McKee solution provides a self-similar relativistic blast-wave description, but varying solid angle with radius or Lorentz factor changes the resulting R(t) and Γ(t) dependences.The model assumes Γ ≫1 and spherical or nearly spherical evolution when Ω is constant.
  • Synchrotron light curves: The synchrotron light curve is obtained by combining R(t) and Γ(t) with νc, νm, νsa, and Fν,max; its peak flux remains constant while moving to lower frequencies.The model transitions from fast cooling at early times to slow cooling at late times when νc crosses νm.
  • Radiative evolution: The adiabatic approximation applies for most of the afterglow, whereas the first hour or longer in a wind profile can be radiative or partially radiative when ǫe ≈1.Additional energy injection and density profiles ρ = ρ0(R/R0)^−k provide further model variations.
  • Cooling regimes: At early times νc < νm and the spectrum is fast cooling, while at late times νc > νm and it is slow cooling; the transition occurs at νc = νm.Power-law temporal and spectral segments are separated by evolving break frequencies.

D. Light Curve During the Newtonian transition

The review extends afterglow light-curve calculations from homogeneous media to Newtonian evolution, variable density, variable energy, refreshed shocks, and inhomogeneous environments.

  • D. Light Curve During the Newtonian transition: The afterglow reaches the Newtonian Sedov-Taylor phase at t ≈ t_NR, where late-time emission enables a calorimetric estimate of its energy.The radio flux can also support searches for orphan radio afterglows because it remains relatively large and varies over several months.
  • Variable circumburst density: For a wind profile, the initially low cooling frequency increases with time, producing temporal relations that differ from the constant-density case.The wind and constant-density models nevertheless become spectrally and temporally similar when ν_m and ν_c are both below the observing frequency.
  • Refreshed shocks: Refreshed shocks produce step-wise light curves above the continuing power-law decline, while post-jet-break spreading can yield faster transitions with δt ∼ t_jet < t.The step-wise behavior was seen in GRB 030329, where transitions were faster than the typical δt ∼ t expectation.
  • Inhomogeneous density: Analytic inhomogeneous-density models approximate the light curve as emission from instantaneous Blandford–McKee solutions with different external densities.The approximation fails when there is a sharp density increase over a radial interval ΔR.

H. Generalizations: IV. Jets

Jet geometry changes afterglow light curves through relativistic beaming, sideways evolution, and viewing angle, producing jet breaks and orphan afterglows.

  • IV. Jets: Jet breaks depend on viewing angle: on-axis breaks are sharper and earlier, while off-axis observers can see orphan afterglows as the beaming cone widens.For ν > ν_m, the post-break temporal slope can reach α = 2.85 for p = 2.5; lower-frequency radio emission steepens more gradually before a later ν_m passage.
  • IV. Jets: The constants governing jet-break onset are uncertain, and their values strongly affect the inferred opening angle and total GRB energy.This uncertainty reflects the relation between Lorentz factor and observing time, and between the break onset and jet opening angle.
  • IV. Jets: After a jet break, the cooling frequency becomes constant, leaving the high-frequency optical and X-ray spectrum unchanged while the radio spectrum continues to vary.In a wind profile, the jet-break transition may last up to four decades in time, making it difficult to observe.
  • IV. Jets: Hydrodynamic simulations find much less sideways spreading than simple one-dimensional models, with most emission remaining within the initial opening angle and concentrated near the jet front.The shock front is egg-shaped, the emissivity-weighted opening angle is nearly constant, and side material contains substantial shocked mass but is less luminous.
  • V. Angular Dependent Jets and the Structured Jet Model: Structured-jet light curves can resemble one another despite different physical assumptions, but some angular profiles predict rising or flattening behavior inconsistent with most observed afterglows.Model 1 is especially useful at early times, whereas model 2 better approximates late-time jet dynamics.

J. Afterglow Polarization - a tool that distinguished between the different jet models

Afterglow polarization and orphan-afterglow searches provide observational diagnostics of jet geometry, viewing angle, and circumburst interaction.

  • Afterglow polarization: Uniform jets predict three polarization peaks, with the central position angle rotated by 90°; observers near the jet center instead see one polarization direction.The sequence follows changing portions of the emitting ring as the jet decelerates and expands sideways.
  • Afterglow polarization: Structured jets predict polarization near 20% around the jet break without position-angle jumps, distinguishing them from uniform-jet predictions.The polarization maximum occurs when θ_obs ∼ Γ^-1 and the observed beam reaches the emissivity center.
  • Patchy shell model: Patchy-shell hot spots produce light-curve and polarization fluctuations whose polarization-angle jumps are random, sharp, and accompanied by changes in polarization amplitude.The first light-curve bump occurs when Γ^-1 ∼ θ_f, and later bumps decrease statistically in amplitude.
  • Optical Orphan Afterglow: Orphan afterglows arise when observers outside the prompt-emission cone see the decelerating ejecta after Γ^-1 reaches their viewing angle, but no orphan afterglow had yet been securely detected.X-ray orphans may appear within hours to days, while optical candidates can be confused with other transients; one candidate was identified but later classified as a variable AGN.
  • Optical Orphan Afterglow: The post-jet-break afterglow is predicted to be universal, giving a detection angle θ_max(z,m) independent of θ_j when θ_j < θ_max.This makes orphan-afterglow rates and upper limits useful for constraining GRB beaming, while radio nondetections provide a lower limit.
  • Pre-acceleration: Radiation-front pre-acceleration can clear a cavity, delaying deceleration until R_gap and producing a sharp pulse when the ejecta encounters substantial ambient material.The ambient Lorentz factor approaches unity near R_acc ≈ 3R_gap before the delayed interaction.

2. Neutron decoupling and decay

Neutron decoupling can alter GRB dynamics and generate delayed shocks, early brightening, high-energy photons, and neutrinos, while propagation effects limit detectability.

  • Neutron decay dynamics: Neutron decay affects the fireball according to the ordering of R_decay and R_ext: when R_decay < R_ext, decay products mix with protons without significantly changing evolution.Their main additional effect is adding energy to the adiabatic fireball.
  • Neutron decay dynamics: Low-Γ_n neutron decoupling produces a delayed shock when decay products catch the slowing proton shell.This scenario is discussed by Pruet and Dalal.
  • Neutron decay dynamics: For Γ_n ≈ Γ_0, decay products move ahead, interact with ambient matter, and produce triple-shell interactions and very early brightening.The interaction occurs at radii of a few times R_decay and observed times of a few seconds.
  • Particle signatures: Inelastic proton-neutron collisions can produce ν_μ near 10 GeV and ν_e near 5 GeV, with fluxes corresponding to roughly 7 events per year in km3 detectors.The associated high-energy photon signal is unlikely to be detectable.

D. Gravitational Radiation

GRBs can be accompanied by gravitational radiation from compact-object formation, jet acceleration, or relativistic ejecta, but detectability depends strongly on geometry, distance, and source model.

  • Signal geometry: GRB-associated gravitational radiation is generally indirect, while the jet-acceleration signal is weak and emitted perpendicular to the GRB direction.The direct relativistic-ejecta signal is therefore difficult to observe despite the expected association of gravitational waves with GRBs.
  • Compact-object mergers: Neutron-star mergers associated with short GRBs could yield about one gravitational-wave event per year within LIGO II sensitivity, assuming a high short-GRB rate.The same events would be only marginally detectable by LIGO I.
  • Source models: In the Supranova model, the first-collapse gravitational waves precede the GRB by weeks or months, whereas a second-collapse component should coincide with it.This timing distinguishes the two components observationally.
  • Relativistic ejecta: Instantaneous acceleration yields an unphysical frequency divergence, replaced in realistic acceleration by a cutoff ω_max ≈ 2π/δt with δt of order 0.01 sec.The idealized angular pattern is also modified when finite-width blobs and multiple emission directions are included.
  • Detectability: For E = MΓ = 10^51 ergs, δt = 0.01 sec, and d = 500 Mpc, the direct signal has h ≈ 0.5·10^-25 and remains below planned-detector sensitivity even ten times nearer.The maximal characteristic frequency is f_max ≈ 100 Hz.

C. Rotating black holes and the Blandford Znajek mechanism

GRB inner-engine models center on accretion-powered relativistic jets from compact objects, with the Collapsar model linking long bursts to massive-star collapse and competing models differing in timing and environment.

  • Collapsar model: In the Collapsar model, a rapidly rotating massive star forms a black hole and accretion disk, while polar energy deposition drives jets through the stellar envelope.The jets can be powered by neutrino annihilation [242] or the Blandford-Znajek mechanism.
  • Jet collimation: Jet propagation through the stellar mantle collimates initially broad outflows: simulations find half-angles around 5° and terminal Lorentz factors near 150.More than 80% of the jet energy is initially internal before conversion into kinetic energy.
  • Burst duration: Collapsar accretion and envelope-propagation timescales imply long GRBs, while instabilities can provide the Lorentz-factor variability needed for internal shocks.The combined core-collapse, accretion, and propagation processes take approximately 10 sec.
  • Alternative engines: The Supranova model separates the supernova and GRB by weeks or months, producing different afterglow and iron-line expectations from the Collapsar model.The association of GRB 030329 with SN 2003dh is described as incompatible with the Supranova model, though proponents allow a delay distribution.
  • Compact-object mergers: Merger simulations produce about 5 × 10^53 ergs, mostly in low-energy neutrinos and gravitational waves, while retaining enough energy to power a GRB.Afterglow modeling and observations support a slowing relativistic flow, synchrotron emission, and collimated jets.

Figures

The figures survey GRB prompt and afterglow observations, supernova associations, energetics, polarization, and the internal–external shock framework. Together they connect observed light curves and spectra with the fireball model’s predicted emission processes.

  • Observations: GRB observations span prompt light curves, spectral hardness, X-ray, optical, and radio afterglows across multiple timescales.The figures include burst variability, hardness-duration behavior, afterglow fluxes, multiband coverage, and late-time radio emission.
  • Afterglow breaks: Many afterglow light curves show an achromatic break to a steeper decline with α ≈2.The GRB990510 optical fit gives α1 = 0.82 ± 0.02, α2 = 2.18 ± 0.05, and t∗ = 1.2 ± 0.08 days.
  • GRB–supernova connection: GRB-SN comparisons and spectroscopy link long-duration bursts with supernova-like emission, including broad spectral peaks emerging several days after GRB 030329.The GRB-SN brightness comparison indicates that observed nondetections probe only part of the local Type Ib/Ic population, so the GRB-SNe population may be incomplete.
  • Polarization: Polarization depends on magnetic-field structure and viewing geometry, reaching a maximum perpendicular to a uniform field and vanishing in the other direction near the Γ^-1 circle.The figures contrast uniform-field polarization with random-field and jet-viewing configurations.
  • Shock model: The internal–external shock framework represents prompt emission as collisions among relativistic shells and afterglows as shocks propagating into circum-burst material.The shock structure includes forward and reverse shocks separated by a contact discontinuity, with synchrotron spectra determined by evolving break frequencies.
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