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 cm ^ { -2 } s ^ { -1 } for several days .
The peak flux was reached on 2011 October 19 , with a value of \sim 4.4 \times 10 ^ { -5 } ph cm ^ { -2 } s ^ { -1 } , which in turn corresponds to a luminosity of \sim 2 \times 10 ^ { 49 } erg s ^ { -1 } .
A very short timescale variability was measured .
Particularly on 2011 October 18 , the flux-doubling time was as short as \sim 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 .