On the origin of the γ -ray emission from the flaring blazar PKS 1222+216 (original) (raw)

2011, Astronomy & Astrophysics

The flat spectrum radio quasar PKS 1222+216 (4C+21.35, z = 0.432) was detected in the very high energy γ-ray band by MAGIC during a highly active γ-ray phase following an alert by the Large Area Telescope (LAT) onboard Fermi. Its relatively hard spectrum (70-400 GeV photon index Γ = 2.7 ± 0.3) without a cut off, together with the observed variability on timescale of ∼ 10 min challenges standard emission models. In particular, if the emission originates in a portion of the relativistic jet located inside the broad line region (BLR), severe absorption of γ rays above few tens of GeV is expected due to the γγ → e ± process. We study a possible scenario for the observed high energy emission in the framework of models advanced to explain ultra-fast events in other TeV blazars. A single compact emission zone is disfavored by a very demanding energetics. We study the combined emission of a very compact (R b ∼ 5 × 10 14 cm) and very fast blob located far beyond the BLR radius (to avoid absorption), responsible for the rapidly varying high energy flux, plus a "standard" large emission region responsible for the "quiescent" flux. The radiative feedback between the two emitting zones is unimportant for the formation of the high energy flux since the radiation field inside the blob is dominated by the IR thermal radiation of the dusty torus.