Radio data challenge the broadband modelling of GRB 160131A afterglow

Abstract

Context. Gamma-ray burst (GRB) afterglows originate from the interaction between the relativistic ejecta and the surrounding medium. Consequently, their properties depend on several aspects: radiation mechanisms, relativistic shock micro-physics, circumburst environment, and the structure and geometry of the relativistic jet. While the standard afterglow model accounts for the overall spectral and temporal evolution for a number of GRBs, its validity limits emerge when the data set is particularly rich and constraining, especially in the radio band. Aims. We aimed to model the afterglow of the long GRB 160131A (redshift z = 0.972), for which we collected a rich, broadband, and accurate data set, spanning from 6 × 108 Hz to 7 × 1017 Hz in frequency, and from 330 s to 160 days post-burst in time. Methods. We modelled the spectral and temporal evolution of this GRB afterglow through two approaches: (1) the adoption of empirical functions to model an optical/X-ray data set, later assessing their compatibility with the radio domain; and (2) the inclusion of the entire multi-frequency data set simultaneously through the Python package named SAGA (Software for AfterGlow Analysis), to obtain an exhaustive and self-consistent description of the micro-physics, geometry, and dynamics of the afterglow. Results. From deep broadband analysis (from radio to X-ray frequencies) of the afterglow light curves, GRB 160131A outflow shows evidence of jetted emission. Moreover, we observe dust extinction in the optical spectra, and energy injection in the optical/X-ray data. Finally, radio spectra are characterised by several peaks that could be due to either interstellar scintillation (ISS) effects or a multi-component structure. Conclusions. The inclusion of radio data in the broadband set of GRB 160131A makes a self-consistent modelling barely attainable within the standard model of GRB afterglows.

Rapid radio brightening of GRB 210702A

“…There are also a few GRBs with radio afterglow evolution that deviates from the expected reverse-forward external shock framework for which ISS may not be the only explanation. A detailed radio analysis of GRB 160131A by Marongiu et al (2022) detected several spectral peaks (the most consistent at 8 GHz) in its radio spectra between 0.8 and 25 days post-burst that could be due to ISS, a two-component jet (e.g van der Horst et al 2014), or a thermally emitting population of electrons (Eichler & Waxman 2005). Such a thermal population would be the fraction of electrons not accelerated into a power law distribution by the shock and may result in an early radio "pre-brightening" (Eichler & Waxman 2005;Giannios & Spitkovsky 2009;Ressler & Laskar 2017).…”

Radio Observations of Six Young Type Ia Supernovae

“…Clearly, ò e < 1, but its value is not entirely certain. Studies of gamma-ray afterglows, for example, find ò e ∼ 0.1 (Panaitescu & Kumar 2002; Yost et al 2003;Chevalier et al 2004;Marongiu et al 2022). Simulations of particle acceleration in shocks also find that about 10% of the post-shock energy is in relativistic ions (Caprioli & Spitkovsky 2014), so, if electrons and ions equilibrate, then ò e ∼ 0.1; however, in low-density winds it is unlikely that they do equilibrate, which would suggest ò e = 0.1.…”