We present a high time resolution study of the two brightest $gamma$-ray outbursts from a blazar PKS 1222+216 observed by the textit{Fermi} Large Area Telescope (LAT) in 2010. The $gamma$-ray light-curves obtained in four different energy bands: 0.1-
-3, 0.1--0.3, 0.3--1 and 1--3 GeV, with time bin of 6 hr, show asymmetric profiles with a similar rise time in all the bands but a rapid decline during the April flare and a gradual one during the June. The light-curves during the April flare show $sim 2$ days long plateau in 0.1--0.3 GeV emission, erratic variations in 0.3--1 GeV emission, and a daily recurring feature in 1--3 GeV emission until the rapid rise and decline within a day. The June flare shows a monotonic rise until the peak, followed by a gradual decline powered mainly by the multi-peak 0.1--0.3 GeV emission. The peak fluxes during both the flares are similar except in the 1--3 GeV band in April which is twice the corresponding flux during the June flare. Hardness ratios during the April flare indicate spectral hardening in the rising phase followed by softening during the decay. We attribute this behavior to the development of a shock associated with an increase in acceleration efficiency followed by its decay leading to spectral softening. The June flare suggests hardening during the rise followed by a complicated energy dependent behavior during the decay. Observed features during the June flare favor multiple emission regions while the overall flaring episode can be related to jet dynamics.
The $gamma$-ray flare of PKS 1222+216, observed in June 2010, is interpreted as an outcome of jet dynamics at recollimation zone. We obtained the $gamma$-ray light-curves in three different energy bands, namely, 100--300 MeV, 300 MeV--1 GeV and 1--3
GeV from observations by the emph{Fermi} Large Area Telescope (LAT). We also use the emph{Swift}--XRT flux from 0.3--10 keV obtained from archival data. We supplement these with the 0.07--0.4 TeV observations with MAGIC telescope, available in the literature. The detection of source at very high energy (VHE, $E>100$ GeV) with a differential photon spectral index of $2.7pm0.3$ 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 $gamma$-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 broadband 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. The parameters of the model are further constrained by reproducing the broadband 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.
The broadband spectrum of a BL Lac object, OJ 287, from radio to $gamma$-rays obtained during a major $gamma$-ray flare detected by emph{Fermi} in 2009 are studied to understand the high energy emission mechanism during this episode. Using a simple o
ne-zone leptonic model, incorporating synchrotron and inverse Compton emission processes, we show that the explanation of high energy emission from X-rays to $gamma$-rays, by considering a single emission mechanism, namely, synchrotron self-Compton (SSC) or external Compton (EC) requires unlikely physical conditions. However, a combination of both SSC and EC mechanisms can reproduce the observed high energy spectrum satisfactorily. Using these emission mechanisms we extract the physical parameters governing the source and its environment. Our study suggests that the emission region of OJ 287 is surrounded by a warm infrared (IR) emitting region of $sim 250 , K$. Assuming this region as a spherical cloud illuminated by an accretion disk, we obtain the location of the emission region to be $sim 9 pc$. This supports the claim that the $gamma$-ray emission from OJ 287 during the 2009 flare arises from a location far away from the central engine as deduced from millimeter-gamma ray correlation study and very long baseline array images.
