Raptor Observations of Delayed Explosive Activity in the High-redshift Γ-ray Burst

The RAPid Telescopes for Optical Response (RAPTOR) system at Los Alamos National Laboratory observed GRB 060206 starting 48.1 minutes after γ-ray emission triggered the Burst Alert Telescope (BAT) on-board the Swift satellite. The afterglow light curve measured by RAPTOR shows a spectacular re-brightening by ∼1 mag about 1 h after the trigger and peaks at R ∼ 16.4 mag. Shortly after the onset of the explosive re-brightening the OT doubled its flux on a time-scale of about 4 minutes. The total R-band fluence received from GRB 060206 during this episode is 2.3 × 10 erg cm. In the rest frame of the burst (z = 4.045) this yields an isotropic equivalent energy release of Eiso ∼ 0.7 × 10 50 erg in just a narrow UV band λ ≃ 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. Subject headings: gamma rays: bursts – cosmology: observations – shock waves