Fermi LAT detection of extraordinary variability in the gamma-ray emission of the blazar PKS 1510-089

We have reanalyzed the giant outburst of the blazar PKS 1510-089 (z=0.36) that occurred on 2011 October-November. The gamma-ray flux in the 0.1-100 GeV energy range exceeded the value of 10^-5 ph/cm2/s for several days. The peak flux was reached on 2011 October 19, with a value of 4.4 x 10^-5 ph/cm2/s, which in turn corresponds to a luminosity of 2 x 10^49 erg/s. A very short timescale variability was measured. Particularly on 2011 October 18, the flux-doubling time was as short as 20 minutes. This is the shortest variability ever detected in the MeV-GeV energy band. We compared our analysis with two other outbursts observed in 2009 March and 2012 February-March, when the blazar was also detected by HESS and MAGIC to infer information about the emission at hundreds of GeV.