Short-timescale γ -Ray Variability in CTA 102

Abstract

The fl at-spectrum radio quasar 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 2016 December and 2017 May. Fermi -LAT observed a fl ux 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 signi fi cant ( 4.7 σ ) fl ux variations down to timescales of ∼ 5 minutes during this orbit. The measured variability timescale 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-timescale variability is unexpected since the γ -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. The observed fast variability could either indicate the dissipation of magnetic islands or protons in a collimated beam from the base of the jet encountering the turbulent plasma at the end of the magnetic nozzle.