The expansion of the GRB 221009A afterglow

¹ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
² INAF Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
³ INAF Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate, Italy
⁴ INFN – sezione di Milano-Bicocca, Piazza della Scienza 3, I-20126 Milano, (MI), Italy
⁵ Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
⁶ ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD, Dwingeloo The Netherlands
⁷ Joint Institute for VLBI ERIC, Oude Hoogeveensedijk 4, 7991 PD, Dwingeloo The Netherlands
⁸ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, SE-439 92 Onsala, Sweden
⁹ Shanghai Astronomical Observatory, Chinese Academy of Sciences, Nandan Road 80, Shanghai 200030, China
¹⁰ Key Laboratory of Radio Astronomy and Technology, Chinese Academy of Sciences, A20 Datun Road, Beijing 100101, PR China
¹¹ International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987, Perth WA 6845, Australia
¹² SETI Institute, 339 Bernardo Ave, Suite 200, Mountain View CA 94043, USA
¹³ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
¹⁴ David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario M5S 3H4, Canada
¹⁵ Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario M5S 3H4, Canada
¹⁶ Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
¹⁷ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, E-18008 Granada, Spain
¹⁸ Facultad de Ciencias, Universidad de Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spain
¹⁹ School of Sciences, European University Cyprus, Diogenes street, Engomi, 1516 Nicosia, Cyprus
²⁰ Department of Physics, George Washington University, 725 21st St NW, Washington, DC 20052, USA
²¹ Berkeley SETI Research Centre, University of California, Berkeley CA 94720, USA

Received: 9 November 2023
Accepted: 3 July 2024

Abstract

We observed γ-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 > 4σ-equivalent significance, and confirm its relativistic nature by revealing an apparently superluminal expansion rate. Fitting a power law expansion model, s ∝ 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.