Letter to the Editor

Optical and NIR observations of the afterglow of GRB 020813^*

¹ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
² INAF, Osservatorio Astronomico di Roma, via Frascati 33, Monteporzio Catone (Roma), Italy
³ Central Astronomical Observatory at Pulkovo, Pulkovskoe shosse 65, 196140 Saint Petersburg, Russia
⁴ INAF, Telescopio Nazionale Galileo, Roque de Los Muchachos, PO box 565, 38700 Santa Cruz de La Palma, Spain
⁵ St. Petersburg University, St. Petersburg, Petrodvorets, Universitetsky pr. 28, 198504 St. Petersburg, Russia
⁶ Isaac Newton Institute of Chile, St. Petersburg Branch
⁷ Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA Cambridge, UK
⁸ Istituto di Radioastronomia, CNR, largo E. Fermi 5, 50125 Firenze, Italy
⁹ Istituto di Astrofisica Spaziale e Fisica Cosmica, via Gobetti 101, 40129 Bologna, Italy
¹⁰ INAF, Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, 35122 Padova, Italy
¹¹ Instituto de Astrofísica de Andalucía, CSIC, PO Box 03004, 18080 Granada, Spain
¹² INAF, Osservatorio Astrofisico di Arcetri, largo E. Fermi 5, 50125 Firenze, Italy
¹³ Dipartimento di Fisica, Università di Ferrara, Via Paradiso 12, 44100 Ferrara, Italy
¹⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁵ Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101-1292, USA
¹⁶ Departamento de Astronomía y Astrofísica, PUCA, Casilla 306, Santiago 22, Chile
¹⁷ Princeton University Observatory, Princeton, NJ 08544-1001, USA
¹⁸ Thüringer Landessternwarte Tautenburg, Karl-Schwarzschild-Observatorium, Sternwarte 5, 07778 Tautenburg, Germany
¹⁹ INAF, Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy

Corresponding author: S. Covino, covino@mi.astro.it

Received: 10 October 2002
Accepted: 5 April 2003

Abstract

We present optical and near-infrared (NIR) photometry of the bright afterglow of GRB 020813. Our data span from 3 hours to 4 days after the GRB event. A rather sharp achromatic break is present in the light curve, 14 hours after the trigger. In the framework of jetted fireballs, this break corresponds to a jet half-opening angle of , the smallest value ever inferred for a GRB. We discuss our results in the framework of currently available models, and find that they have problems in explaining the joint temporal and spectral properties, and in particular the slow decay before the break.