Gamma-ray flare of PKS 1222+216 in 2010: effect of jet dynamics at the recollimation zone

Abstract

The γ-ray flare of PKS 1222+216, observed in 2010 June, is interpreted as an outcome of jet dynamics at recollimation zone. We obtained the γ-ray light curves in three different energy bands, namely 100–300 MeV, 300 MeV–1 GeV and 1–3 GeV from observations by the Fermi-LAT (Large Area Telescope). We also use the Swift-XRT (X-Ray Telescope) flux from 0.3–10 keV obtained from archival data. We supplement these with the 0.07–0.4 TeV observations with MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescope, available in the literature. The detection of source at very high energy (E > 100 GeV) with a differential photon spectral index of 2.7 ± 0.3 and the rapid variability associated with it suggests that the emission arises from a compact region located beyond the broad line emitting region. The plausible γ-ray emission mechanism can then be inverse Compton scattering of IR photons from obscuring torus. Further, the decay time of LAT flare cannot be explained by considering simple radiative loss mechanisms. Hence, to interpret the LAT light curves, we develop a model where the broad-band emission originates from a compact region, arising plausibly from the compression of jet matter at the recollimation zone. The flare is then expressed as an outcome of jet deceleration probably associated with this focusing effect. Based on this model, the rise of the LAT flare is attributed to the opening of emission cone followed by the decay resulting from jet deceleration. The parameters of the model are further constrained by reproducing the broad-band spectral energy distribution of the source obtained during the flare episode. Our study suggests that the particle energy density exceeds magnetic energy density by a large factor which in turn may cause rapid expansion of the emission region. However, near equipartition can be achieved towards the end of LAT flare during which the compact emission region would have expanded to the size of jet cross-section.