Gamma-ray Burst Optical Afterglow

Regardless of the progenitor and central engine, the gamma-ray burst (GRB) afterglows are produced by the synchrotron emission external forward shock. Swift and the ground-based telescopes provide a rich early afterglow data which revealed many unexpected and interesting features. Based on the statistics of a large GRB sample, this paper gives a brief introduction of the GRB optical afterglow, including observations, emission components and the afterglow puzzle ―achromatic or chromatic?‖. The afterglows provide a very important window between the afterglows and prompt emission to reveal the veil of the progenitor, central engine, ejecta composition and radiation mechanism. GRB 140323A is a good case interpreted with circumburst medium transition from a stellar wind to a homogenous density medium in the external shock model. GRB 140419A and 150910A are good cases for a magnetar spin down to a stable neutron star and the collapse in black hole, respectively.