Probing particle acceleration at trans-relativistic shocks with off-axis gamma-ray burst afterglows

Kazuya Takahashi, Kunihito Ioka, Yutaka Ohira, Hendrik Van Eerten

Research output: Contribution to journalArticlepeer-review


Particle acceleration is expected to be different between relativistic and non-relativistic collisionless shocks. We show that electromagnetic counterparts to gravitational waves (GWs), gamma-ray burst (GRB) afterglows, are ideal targets for observing trans-relativistic evolution of accelerated electron distribution because the GWs spot nearby GRBs with off-axis jets, otherwise missed in gamma-ray observations. We find that the relativistic spectral slope begins to change steeply near the peak time of the light curve and approaches the non-relativistic limit in about 10 times the peak time. The trans-relativistic evolution of the afterglow synchrotron spectrum is consistent with GRB 170817A observations within errors, and will be measurable in similar but more distant events at a GW horizon ∼200 Mpc in a denser environment. We roughly estimate that such events represent a fraction of 10-50 per cent of the GRB 170817A-like off-axis short GRBs. We also find that the spectral evolution does not depend on the jet structure if their light curves are similar to each other.
Original languageEnglish
Pages (from-to)5541–5559
Number of pages19
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Early online date22 Oct 2022
Publication statusPublished - 1 Dec 2022


  • gamma-ray bursts
  • methods: analytical

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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