[Paper Review] The Structure of Gamma Ray Burst Jets

O. S. Salafia, G. Ghirlanda|arXiv (Cornell University)|Jun 22, 2022

Gamma-ray bursts and supernovae4 citations

TL;DR

This review synthesizes two decades of research on the structure of relativistic jets in gamma-ray bursts (GRBs), emphasizing how jet geometry—shaped by central engine activity and interaction with progenitor remnants—affects prompt and afterglow emission. It demonstrates that structured jets, particularly in short GRBs like GW170817A, leave distinct observational imprints such as jet breaks and off-axis emission, enabling constraints on jet opening angles, Lorentz factors, and viewing geometry.

ABSTRACT

Due to relativistic bulk motion, the structure and orientation of gamma-ray burst jets have a fundamental role in determining how they appear. The recent discovery of the GW170817 binary neutron star merger and the associated GRB boosted the interest in the modelling and search of signatures of the presence of a (possibly quasi-universal) jet structure in long and short GRBs. In this review, following a pedagogical approach, we summarize the history of GRB jet structure research over the last two decades, from the inception of the idea of a universal jet structure to the current understanding of the complex processes that shape the structure, that involve the central engine that powers the jet and the interaction of the latter with the progenitor vestige. We put some emphasis on the observable imprints of jet structure on prompt and afterglow emission and on the luminosity function, favoring intuitive reasoning over technical explanations.

Motivation & Objective

• To provide a pedagogical overview of the evolution of jet structure modeling in GRBs over the past 20 years.
• To clarify how the interplay between the central engine and external medium shapes the jet's lateral structure.
• To explain the observable imprints of jet structure on prompt emission, afterglow light curves, and luminosity functions.
• To assess the implications of the GW170817 neutron star merger for validating the existence of a quasi-universal jet structure in short GRBs.
• To guide new researchers in understanding the physical intuition behind jet modeling with minimal technical formalism.

Proposed method

• Tracing the historical development of jet structure models from the 2001–2002 seminal works to current understanding.
• Using intuitive physical reasoning to link jet geometry to observable features such as jet breaks in afterglow light curves.
• Analyzing observational data from GRB 030329, GRB 170817A, and other key events to infer jet parameters like opening angle and Lorentz factor.
• Applying relativistic beaming and compactness arguments to constrain emission regions and bulk Lorentz factors.
• Evaluating the role of afterglow modeling with multi-wavelength datasets to break degeneracies in jet parameter estimation.
• Highlighting the importance of high-cadence, multi-wavelength observations—especially VLBI—for improving jet structure constraints.

Experimental results

Research questions

• RQ1 How does the structure of GRB jets influence the observed prompt and afterglow emission light curves?
• RQ2 What observational signatures, such as jet breaks or off-axis emission, can be used to infer jet opening angles and viewing geometry?
• RQ3 To what extent is the jet structure in short GRBs, like GW170817A, consistent with a quasi-universal model?
• RQ4 How do central engine properties and interaction with the progenitor vestige shape the lateral profile of GRB jets?
• RQ5 What role do future gravitational wave detections and multi-messenger observations play in validating jet structure models?

Key findings

• Jet breaks in afterglow light curves—caused by the relativistic beaming angle becoming comparable to the jet opening angle—provide a robust method to estimate $ heta_{ m j} \approx 5^\circ$ for many GRBs.
• The observed emission from GW170817A, with a viewing angle $\theta_{\rm v} \gtrsim 15^\circ$ and $\Gamma \gtrsim 3$, is inconsistent with emission from the jet edge, suggesting emission from off-jet regions like a cocoon or off-axis jet core.
• Lorentz factors $\Gamma \gtrsim 3$ inferred from compactness limits for GRB170817A are consistent with a mildly relativistic outflow, challenging standard high-$\Gamma$ jet models.
• The lack of a clear jet break in GW170817A’s afterglow suggests a broader or structured jet, supporting the idea of a quasi-universal jet structure across short GRBs.
• High-cadence, multi-wavelength observations—including VLBI—are essential to break degeneracies in afterglow modeling and to constrain jet parameters with higher precision.
• Future detections of binary neutron star and black hole–neutron star mergers with associated GRBs will provide golden opportunities to test the universality of jet structure and refine progenitor population models.