The expansion of the GRB 221009A afterglow

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 $a=0.69_{-0.14}^{+0.13}$. 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.

Accepted version for publication

References in corpus (17)

• GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral
• Gravitational Waves and Gamma-rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A
• A tera-electronvolt afterglow from a narrow jet in an extremely bright gamma-ray burst 221009A
• GRB 221009A, The BOAT
• Fermi-GBM Discovery of GRB 221009A: An Extraordinarily Bright GRB from Onset to Afterglow
• The SFXC software correlator for Very Long Baseline Interferometry: Algorithms and Implementation
• Very high energy gamma-ray emission beyond 10 TeV from GRB 221009A
• The Radio to GeV Afterglow of GRB 221009A
• GRB 221009A Afterglow from a Shallow Angular Structured Jet
• Two-component jet model for multiwavelength afterglow emission of the extremely energetic burst GRB 221009A
• Multi-wavelength view of the close-by GRB 190829A sheds light on gamma-ray burst physics
• The first JWST spectrum of a GRB afterglow: No bright supernova in observations of the brightest GRB of all time, GRB 221009A
• The Possibility of Modeling the Very High Energy Afterglow of GRB 221009A in a Wind Environment
• A Narrow Uniform Core with a Wide Structured Wing: Modeling the TeV and Multi-wavelength Afterglows of GRB 221009A
• Resolving the blazar CGRaBS J0809+5341 in the presence of telescope systematics
• VLBI observations of GRB 201015A, a relatively faint GRB with a hint of Very High Energy gamma-ray emission
• Analytic Expressions for the Surface Brightness Profile of GRB Afterglow Images

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