English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

We observed gamma-ray burst (GRB) 221009A using Very Long BaselineInterferomety (VLBI) with the European VLBI Network (EVN) and the Very LongBaseline Array (VLBA), over a period spanning from 40 to 262 days after theinitial 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 thesurrounding medium. Our observations support the expansion of the shock with a3.6 $\sigma$-equivalent significance, and confirm its relativistic nature byrevealing an apparently superluminal expansion rate. Fitting a power lawexpansion model, $s\propto t^a$, to the observed size evolution, we find aslope $a=1.9_{-0.6}^{+0.7}$, which is steeper than expected from either aforward shock (FS) or reverse shock (RS) model, implying an apparentacceleration of the expansion. 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 RS plus FS inthe case of a wind-like circum-burst medium, provided that the two shocksdominate the emission at different frequencies and, possibly, at differenttimes.

The expansion of the GRB 221009A afterglow