Raptor observations of delayed explosive activity in the high-redshift gamma-ray burst GRB 060206

The Rapid Telescopes for Optical Response (RAPTOR) system at Los Alamos National Laboratory observed GRB 060206 starting 48.1 minutes after gamma-ray emission triggered the Burst Alert Telescope on board the Swift satellite. The afterglow light curve measured by RAPTOR shows a spectacular rebrightening by similar to 1 mag about 1 hr after the trigger and peaks at R similar to 16.4 mag. Shortly after the onset of the explosive rebrightening, the optical transient doubled its flux on a timescale of about 4 minutes. The total R-band fluence received from GRB 060206 during this episode is 2.3 x 10(-9) ergs cm(-2). In the rest frame of the burst (z = 4.045), this yields an isotropic equivalent energy release of E-iso similar to 0.7 x 10(50) ergs in just a narrow UV band, lambda similar or equal to 130 +/- 22 nm. We discuss the implications of RAPTOR observations for untriggered searches for fast optical transients and studies of GRB environments at high redshift.