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Astron. Astrophys. 328, L13-L16 (1997)

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Letter to the Editor

Early detection of the Optical Transient following the Gamma-Ray Burst GRB 970228 *

A. Guarnieri 1, C. Bartolini 1, N. Masetti 1, A. Piccioni 1, E. Costa 2, M. Feroci 2, F. Frontera 3, D. Dal Fiume 4, L. Nicastro 4, E. Palazzi 4, A.J. Castro-Tirado 5 and J. Gorosabel 5

1 Dipartimento di Astronomia, Università di Bologna, via Zamboni, 33 I-40126 Bologna, Italy
2 Istituto di Astrofisica Spaziale, CNR, Frascati
3 Dipartimento di Fisica, Università di Ferrara
4 Istituto Tecnologie e Studio Radiazioni Extraterrestri, CNR, Bologna
5 Laboratorio de Astrofísica Espacial y Física Fundamental, INTA, Madrid

Received 13 May 1997 / Accepted 26 June 1997

Abstract

The optical counterpart of the Gamma-Ray Burst GRB 970228, discovered by Groot et al. (1997a), is also detected in the B and R frames obtained [FORMULA] 4 hours earlier at the Bologna Observatory. Our observations indicate a very likely rise of the optical emission within these 4 hours. The R luminosity of the transient at maximum was about 15 times that of an underlying extended object. Follow-up data show that the maximum optical emission was delayed of not less than 0.7 days with respect to the [FORMULA] -ray peak and that no new big flares were seen after the main one. The optical transient became significantly redder once it has reverted to quiescence.

Key words: Gamma rays: bursts – - Methods: observational

* Based on observations obtained at the Osservatorio Astronomico di Loiano, Italy

Send offprint requests to: A. Guarnieri, adriano@astbo3.bo.astro.it

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© European Southern Observatory (ESO) 1997

Online publication: March 26, 1998

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