Constraining the GRB collimation with a survey for orphan afterglows

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany e-mail: arau@mpe.mpg.de
² Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Received: 7 October 2005
Accepted: 22 November 2005

Abstract

Gamma-ray bursts are believed to be produced in highly-relativistic collimated outflows. Support for this comes among others from the association of the times of detected breaks in the decay of afterglow light curves with the collimation angle of the jets. An alternative approach to estimate a limit on the collimation angle uses GRB  afterglows without detected prompt-emission counterparts. Here we report on the analysis of a dedicated survey for the search of these orphan afterglows using the Wide Field Imager at the 2.2 m MPI/ESO telescope at La Silla, Chile. We monitored ~12 deg² in up to 25 nights typically spaced by one to two nights with a limiting magnitude of . Four previously unknown optical transients were discovered and three of these associated with a flare star, a cataclysmic variable and a dwarf nova. The fourth source shows indications for an extragalactic origin but the sparse sampling of the light curve prevents a reliable classification. We discuss the results in the context of the collimation of GRBs.