A Short Hard X-ray Flare from the Blazar NRAO 530 Observed by INTEGRAL
  We report about a short flare from the blazar NRAO 530 occurred on 17 February 2004 and detected serendipitously by the IBIS/ISGRI detector on board INTEGRAL. In the 20-40 keV energy range, the source, that is otherwise below the detection limit, is detected at a level of ~2 x 10^-10 erg cm^-2 s^-1 during a time interval of less than 2000 s, which is about a factor 2 above the detection threshold. At other wavelengths, only nearly-simultaneous radio data are available (1 observation at 2 cm on 11 February 2004), indicating a moderate increase of the polarization. This appears to be the shortest time variability episode ever detected in a high luminosity blazar at hard X-rays, unless the blazar is contaminated by the presence of an unknown unresolved rapidly varying source. 
L. Foschini, E. Pian, L. Maraschi, C.M. Raiteri, F. Tavecchio, G.   Ghisellini, G. Tosti, G. Malaguti, G. Di Cocco