Short-Time-Scale Gamma Ray Variability in CTA 102

The Flat Spectrum Radio Quasar (FSRQ) CTA 102 experienced a prolonged state of enhanced activity across the entire observed electromagnetic spectrum during 2016-2017, most pronounced during a major outburst between December 2016 and May 2017. Large Area Telescope (LAT) which is a primary instrument on-board Fermi observed a flux of 2.2 ± 0.2×10-5  photons cm-2  s-1  at energies above 100 MeV on 2017 April 19 during a single orbit. We report here the detection of significant (4.7 σ) flux variations down to time scales of ~ 5 minutes during this orbit. The measured variability time scale is much shorter than the light-travel time across the central black hole ( ~ 70 minutes) indicating a very compact emission region within the jet, similar to that seen in IC 310, Mrk 501 or PKS 1222+21 from MAGIC observations. This short-time scale variability is unexpected since the gamma-ray spectrum shows no sign of attenuation due to pair creation in interactions with photons from the broad emission line region, and therefore must be assumed to originate far from the black hole. We propose that the observed fast variability indicates dissipation of magnetic islands from the base of the jet when they encounter the turbulent plasma at the end of the magnetic nozzle where the jet collimation breaks down.