[Paper Review] Short Gamma Ray Bursts and their Afterglow Signatures in Dense Stellar Systems

Rion K. Parsons, E. Ramírez-Ruiz+1 more|ArXiv.org|Apr 11, 2009

Gamma-ray bursts and supernovae2 references3 citations

TL;DR

This paper proposes that short gamma-ray bursts (SGRBs) originating in dense stellar systems like globular clusters can produce afterglows shaped by interactions with red giant winds, resolving the paradox of observed dense environments despite expected low-density intergalactic media. Hydrodynamic simulations show that stellar wind density and cluster core structure significantly distort blast waves, leading to non-spherical, jet-like remnants that depend on energy and wind properties, providing a key diagnostic for progenitor environments.

ABSTRACT

The hypothesis that short GRBs arise from the coalescence of binary compact stars has recently gained support. With this comes the expectation that the afterglow should bear the characteristic signature of a tenuous intergalactic medium (IGM). However, fits to the observational data suggest that some detected afterglows arise in relatively dense gaseous environments rather than in the low density IGM. Here we show that considering the effect of red giant winds in the core of a star cluster may resolve this paradox if short GRB progenitors are contained in such an environment and close encounters rather than pure gravitational wave emission brings the compact objects together. Clear confirmation is provided here of the important notion that the morphology and visibility of short gamma-ray burst remnants are determined largely by the state of the gas in the cluster's core.

Motivation & Objective

• To resolve the discrepancy between observed dense afterglow environments and the expected low-density intergalactic medium for short gamma-ray bursts (SGRBs).
• To investigate how stellar winds from red giants in globular cluster cores influence the morphology and observability of SGRB afterglows.
• To determine whether dynamically formed compact binaries in dense stellar systems can produce afterglows consistent with observed high-density environments.
• To assess the role of cluster core gas density and wind properties in shaping the evolution of SGRB remnants.

Proposed method

• Modeling the circumburst medium in globular cluster cores using steady-state, spherically symmetric wind solutions for red giants.
• Estimating electron density in the cluster core using mass-loss rates ($\dot{M}_{\rm w}$), wind velocities ($v_{\rm w}$), and stellar number density ($N_\ast$) via equation (3).
• Performing 3D hydrodynamic simulations of GRB ejecta expansion into the dense, inhomogeneous medium of a globular cluster core.
• Tracking the evolution of blast waves, including shock formation, deceleration, and interaction with density discontinuities from individual stars.
• Comparing the dynamics of SGRB remnants in dense stellar environments to classical Sedov-Taylor supernova remnants and isolated IGM scenarios.
• Analyzing pressure contours and morphology evolution to assess how energy content and wind properties affect afterglow visibility and structure.

Experimental results

Research questions

• RQ1 Can the observed high-density afterglow environments of short gamma-ray bursts be explained by dense stellar systems rather than the intergalactic medium?
• RQ2 How do red giant winds in globular cluster cores affect the morphology and evolution of SGRB blast waves?
• RQ3 To what extent does the energy content of the GRB ejecta determine the degree of distortion and non-spherical structure in the remnant?
• RQ4 What role does the spatial distribution and density of stellar winds play in shaping the observable afterglow light curve?
• RQ5 Can the detection of absorption signatures in SGRB afterglows provide evidence for dynamically formed compact binary progenitors?

Key findings

• The electron density in the core of a globular cluster can reach $n_\perp \sim 80\, N_{\ast,2}^{2/3} r_{\rm c,-1}^{-2} v_{\rm w,1}^{-1} \dot{M}_{\rm w,-7} \mu_e^{-1} \, \text{cm}^{-3}$, significantly exceeding the intergalactic medium density.
• Blast waves in dense cluster cores suffer pronounced distortions, including dimples and wrapping around stars, due to inhomogeneous wind densities, even for adiabatic, isotropic ejecta.
• The remnant morphology remains highly non-spherical until the two expanding shells collide, with the collision time scaling with energy content: higher energy leads to later collision and prolonged non-spherical evolution.
• In low-energy cases, the remnant decelerates rapidly and approaches a spherical shape by $\sim 2.3 \times 10^8$ seconds, while high-energy cases maintain non-spherical structure deeper into the core.
• The presence of multiple density discontinuities from red giants causes the blast wave to wrap around stars, creating complex pressure contours that wash out initial ejecta asymmetries.
• The simulations confirm that the visibility and morphology of SGRB remnants are dominated by the state of the gas in the cluster core, making afterglow observations a key probe of progenitor environment.