circumburst

The circumburst region is the material surrounding the site of a gamma-ray burst (GRB) and includes interstellar and circumstellar gas and dust that the GRB jet encounters as it expands. The properties of this circumburst medium influence the GRB afterglow produced when the relativistic jet decelerates in the surrounding material.

Two main progenitor classes are associated with different circumburst environments. Long-duration GRBs, linked to the core collapse of massive stars, tend to occur in star-forming galaxies and often show a wind-like circumburst medium, where the density falls off roughly as n ∝ r^-2 due to the progenitor’s stellar wind. Short-duration GRBs, arising from compact binary mergers, typically encounter a more uniform interstellar medium, though local structures and winds can modify this.

The circumburst density profile is commonly described by n(r) ∝ r^-k, with k ≈ 0 for uniform ISM and k ≈ 2 for a steady wind. The density and composition affect the afterglow's light curves and spectra through the forward shock, which accelerates electrons that emit synchrotron radiation across X-ray, optical, and radio bands. The strength of the reverse shock and the spectral break evolution depend on microphysical parameters and the circumburst environment.

Observationally, afterglow spectra reveal absorption lines and extinction that constrain metallicity, dust content, and ionization, providing clues about the circumburst environment. Complex structures such as wind termination shocks, clumps, or cavities can imprint features on the afterglow.

The circumburst medium thus provides essential context for interpreting GRB energetics, jet physics, and the environments of their host galaxies.