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Astron. Astrophys. 348, L1-L4 (1999)

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Letter to the Editor

GRB 990510: linearly polarized radiation from a fireball *

Stefano Covino 1, Davide Lazzati 1,2, Gabriele Ghisellini 1, Paolo Saracco 1, Sergio Campana 1, Guido Chincarini 1,2, Sperello Di Serego 3, Andrea Cimatti 3, Leonardo Vanzi 4, Luca Pasquini 4, Francesco Haardt 5, Gian Luca Israel 6, Luigi Stella 6 and Mario Vietri 7

1 Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate (Lc), Italy
2 Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, I-20133 Milano, Italy
3 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy
4 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany
5 Università dell'Insubria, via Lucini 3, I-22100 Como, Italy
6 Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monteporzio Catone, Roma, Italy
7 Università di Roma III, via della Vasca Navale 84, I-00147 Roma, Italy

Received 1 June 1999 / Accepted 21 June 1999


Models for gamma-ray burst afterglows envisage an hyper-relativistic fireball that is decelerated in the ambient medium around the explosion site. This interaction produces a shock wave which amplifies the magnetic field and accelerates electrons to relativistic energies, setting the conditions for an efficient production of synchrotron photons. If produced in a region of large-scale ordered magnetic field, synchrotron radiation can be highly polarized. The optical transient associated with GRB 990510 was observed [FORMULA] hr after the event and linear polarization in the R band was measured at a level of [FORMULA]. This is the first detection of linear polarization in the optical afterglow of a gamma-ray burst. We exclude that this polarization is due to dust in the interstellar material, either in our Galaxy or in the host galaxy of the gamma-ray burst. These results provide important new evidence in favor of the synchrotron origin of the afterglow emission, and constrains the geometry of the fireball and/or magnetic field lines.

Key words: gamma rays: bursts – polarization – radiation mechanisms: non-thermal

* Based on ESO VLT-Antu (UT1) observations (63.H-0233). Raw data are available upon request.

Send offprint requests to: S. Covino

Correspondence to: covino@merate.mi.astro.it

© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999