Optical afterglows of gamma-ray bursts: peaks, plateaus and possibilities

The optical light curves of gamma-ray burst (GRB) afterglows display either peaks or plateaus. We identify 16 afterglows of the former type, 17 of the latter and four with broad peaks, which could be of either type. The optical energy release of these two classes is similar and is correlated with the GRB output, the correlation being stronger for peaky afterglows, which suggests that the burst and afterglow emissions of peaky afterglows are from the same relativistic ejecta and that the optical emission of afterglows with plateaus arises more often from ejecta that did not produce the burst emission. Consequently, we propose that peaky optical afterglows are from impulsive ejecta releases and that plateau optical afterglows originate from long-lived engines, the break in the optical light curve (peak or plateau end) marking the onset of the entire outflow deceleration. In the peak luminosity-peak time plane, the distribution of peaky afterglows displays an edge with L-p proportional to t(p)(-3), which we attribute to variations (among afterglows) in the ambient medium density. The fluxes and epochs of optical plateau breaks follow an L-p proportional to t(b)(-1) anticorrelation. Sixty per cent of 25 afterglows that were well monitored in the optical and X-rays show light curves with comparable power-law decay indices and achromatic breaks. The other 40 per cent display three types of decoupled behaviours: (1) chromatic optical light-curve breaks (perhaps due to the peak of the synchrotron spectrum crossing the optical), (2) X-ray flux decays faster than in the optical (suggesting that the X-ray emission is from local inverse-Compton scattering) and (3) chromatic X-ray light-curve breaks (indicating that the X-ray emission is from external upscattering).