The late-time afterglow of the extremely energetic short burst GRB   090510 revisited
  The discovery of the short GRB 090510 has raised considerable attention mainly because it had a bright optical afterglow and it is among the most energetic events detected so far within the entire GRB population. The afterglow was observed with swift/UVOT and swift/XRT and evidence of a jet break around 1.5 ks after the burst has been reported in the literature, implying that after this break the optical and X-ray light curve should fade with the same decay slope. As noted by several authors, the post-break decay slope seen in the UVOT data is much shallower than the steep decay in the X-ray band, pointing to an excess of optical flux at late times. We reduced and analyzed new afterglow light-curve data obtained with the multichannel imager GROND. Based on the densely sampled data set obtained with GROND, we find that the optical afterglow of GRB 090510 did indeed enter a steep decay phase starting around 22 ks after the burst. During this time the GROND optical light curve is achromatic, and its slope is identical to the slope of the X-ray data. In combination with the UVOT data this implies that a second break must have occurred in the optical light curve around 22 ks post burst, which, however, has no obvious counterpart in the X-ray band, contradicting the interpretation that this could be another jet break. The GROND data provide the missing piece of evidence that the optical afterglow of GRB 090510 did follow a post-jet break evolution at late times. 
A. Nicuesa Guelbenzu, S. Klose, T. Kruehler, J. Greiner, A. Rossi, D.   A. Kann, F. Olivares E., A. Rau, P. M. J. Afonso, J....
astro-ph.HE