The expansion of the GRB 221009A afterglow

TL;DR

This paper presents high-resolution VLBI observations of GRB 221009A's afterglow, measuring its relativistic expansion over time, revealing frequency-dependent behavior and supporting a model with reverse and forward shocks dominating emission at different stages.

Contribution

First direct measurement of the relativistic shock expansion in GRB 221009A using VLBI, confirming superluminal expansion and frequency-dependent size evolution.

Findings

Confirmed relativistic, superluminal expansion with >4σ significance.

Found size evolution follows a power law with index ~0.69.

Observed different expansion rates at different frequencies.

Abstract

We observed $\gamma$-ray burst (GRB) 221009A using very long baseline interferomety (VLBI) with the European VLBI Network (EVN) and the Very Long Baseline Array (VLBA), over a period spanning from 40 to 262 days after the initial GRB. The high angular resolution (mas) of our observations allowed us, for the second time ever, after GRB 030329, to measure the projected size, $s$, of the relativistic shock caused by the expansion of the GRB ejecta into the surrounding medium. Our observations support the expansion of the shock with a $>4\sigma$-equivalent significance, and confirm its relativistic nature by revealing an apparently superluminal expansion rate. Fitting a power law expansion model, $s\propto t^a$, to the observed size evolution, we find a slope $a=0.69^{+0.13}_{-0.14}$. Fitting the data at each frequency separately, we find different expansion rates, pointing to a frequency-dependent behaviour. We show that the observed size evolution can be reconciled with a reverse shock plus forward shock, provided that the two shocks dominate the emission at different frequencies and, possibly, at different times.