GRB 060206: hints of precession of the central engine?

¹ Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, PR China e-mail: [xwliu;t.lu]@pmo.ac.cn; xfwu@caltech.edu
² National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
³ Joint Center for Particle Nuclear Physics and Cosmology of Purple Mountain Observatory – Nanjing University, Nanjing 210008, PR China
⁴ Theoretical Astrophysics 130-33, California Institute of Technology, Pasadena, California 91125, USA

Received: 3 October 2007
Accepted: 15 May 2008

Abstract

Aims. The high-redshift ( gamma-ray burst GRB 060206 showed unusual behavior, with a significant rebrightening by a factor of ~4 at about 3000 s after the burst. We argue that this rebrightening implies that the central engine became active again after the main burst produced by the first ejecta, then drove another more collimated jet-like ejecta with a larger viewing angle. The two ejecta both interacted with the ambient medium, giving rise to forward shocks that propagated into the ambient medium and reverse shocks that penetrated into the ejecta. The total emission was a combination of the emissions from the reverse- and forward- shocked regions. We discuss how this combined emission accounts for the observed rebrightening.

Methods. We apply numerical models to calculate the light curves from the shocked regions, which include a forward shock originating in the first ejecta and a forward-reverse shock for the second ejecta.

Results. We find evidence that the central engine became active again 2000 s after the main burst. The combined emission produced by interactions of these two ejecta with the ambient medium can describe the properties of the afterglow of this burst. We argue that the rapid rise in brightness at ~3000 s in the afterglow is due to the off-axis emission from the second ejecta. The precession of the torus or accretion disk of the central engine is a natural explanation for the departure of the second ejecta from the line of sight.