GRBs with no afterglow even in the Swift Era?

Thanks to its great capability Swift succeeds detecting an afterglow more than 93% of the time BAT triggers a new event. In a few cases, mainly Short GRBs or late observations, XRT did not detect any emission. However, for at least seven Long GRBs XRT could not measure any X-afterglow either, even as early as 100 s after the BAT trigger. The detection of this new class of GRBs without any X-ray emission has raised many questions. Are these extreme naked GRB? Is this a completely new class of objects with an uncommon emission release? In this contribution we present a study on these unusual events. We analyse their peculiar behaviour in the Gamma and X-ray bands and discuss their singular origin. [ABSTRACT FROM AUTHOR]