The puzzling clustering and bimodality of long GRBs optical afterglow
  luminosities
  The study of the rest frame properties of long Gamma-Ray bursts (GRBs)
afterglows is a fundamental aspect for a better understanding of the nature of
these powerful explosions. The launch of the Swift satellite (November 2004)
marked a strong improvement of the observational capabilities of X-rays and
optical afterglows. We studied the intrinsic optical afterglows of a sample of
long GRBs finding an unexpected clustering and a hint of bimodality of the
optical luminosity distribution (at 12h after the trigger). Through a
Montecarlo simulation we proved that both the observed clustering and
bimodality are not simply due to selection effects but should hide important
information in the understanding of the nature of the afterglow emission. These
findings can shed also light on the nature of the large fraction of optically
dark GRBs.
