Gamma: A new method for modelling relativistic hydrodynamics and non-thermal emission on a moving mesh

Eliot Ayache, Hendrik Van Eerten, Rupert W Eardley

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In recent years, dynamical relativistic jet simulation techniques have progressed to a point where it is becoming possible to fully numerically resolve gamma-ray burst (GRB) blast-wave evolution across scales. However, the modelling of emission is currently lagging behind and limits our efforts to fully interpret the physics of GRBs. In this work we combine recent developments in moving-mesh relativistic dynamics with a local treatment of non-thermal emission in a new code: gamma. The code involves an arbitrary Lagrangian-Eulerian approach only in the dominant direction of fluid motion that avoids mesh entanglement and associated computational costs. Shock detection, particle injection, and local calculation of their evolution including radiative cooling are done at runtime. Even though gamma has been designed with GRB physics applications in mind, it is modular such that new solvers and geometries can be implemented easily with a wide range of potential applications. In this paper, we demonstrate the validity of our approach and compute accurate broad-band GRB afterglow radiation from early to late times. Our results show that the spectral cooling break shifts by a factor of ∼40 compared to existing methods. Its temporal behaviour also significantly changes from the previously calculated temporary steep increase after the jet break. Instead, we find that the cooling break does not shift with time between the relativistic and Newtonian asymptotes when computed from our local algorithm. gamma is publicly available at:

Original languageEnglish
Pages (from-to)1315-1330
Number of pages16
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Early online date2 Dec 2021
Publication statusPublished - 28 Feb 2022

Bibliographical note

We thank the anonymous referee for a constructive report. This work used the Isambard 2 UK National Tier-2 HPC Service ( operated by GW4 and the UK Met Office, and funded by EPSRC (EP/T022078/1). This research made use of the Balena High Performance Computing (HPC) Service at the University of Bath. HJv acknowledges partial support by the European Union Horizon 2020 Programme under the AHEAD2020 project (grant agreement number 871158).

The data underlying this paper will be shared on reasonable request to the corresponding author


  • gamma-ray bursts
  • hydrodynamics
  • methods: numerical
  • radiation mechanisms: non-thermal
  • shock waves
  • software: simulations

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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