Brightest Fermi-LAT Flares of PKS 1222+216: Implications on Emission and Acceleration Processes

TL;DR

This study analyzes two intense gamma-ray flares from blazar PKS 1222+216 observed by Fermi-LAT, revealing detailed temporal and spectral behaviors that inform models of emission regions and particle acceleration in jets.

Contribution

It provides high time resolution analysis of gamma-ray flares, highlighting complex spectral and temporal features, and discusses implications for emission mechanisms and jet dynamics.

Findings

Asymmetric flare profiles with rapid decline in April and gradual in June

Spectral hardening during flare rise and softening during decay in April

Evidence for multiple emission regions during the June flare

Abstract

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.