Description |
Blazars are well known to show very frequent variations with time-scales ranging from less
than an hour to few years, over the entire EM spectrum. The understanding of blazar
emission and the corresponding physical processes, has been completely changed with the
six years of unprecedented contribution by Fermi, a space-based γ-ray observatory, launched
in June, 2008. The high energy (MeV-GeV) radiative output is supposed to be attributed to
the inverse Compton scattering (IC) of low energy seed photons. The origin of seed photons
may be either the synchrotron emission in the jet or the external photons form
disk/torus/line-regions etc. Generally, the high energy flares in blazars are associated with
the formation of shock in the jet. However, the origin and nature of the shock and role of
magnetic field are not yet well established. Simultaneous study of observables (lightcurves,
SEDs and polarization), corresponding to a particular flaring activity, is essential in order to
understand the underlying mechanism.
BL Lac object S5 0716+714 (z~0.31), a highly variable blazar, was extensively observed
during its historical brightened state during the early 2015. A high cadence polarization
observations covering the complete duration were also performed. The dual γ-ray flares,
seen within 3 days apart, were witnessed to be closely followed at X-rays and UV/optical
bands. A significant variations in degree of polarization (DP) and two successive rotation of
180o in position angle (PA) was also observed during the second flare. We modelled the
observed lightcurves, SED and polarization (DP & PA) using the Helical Magnetic Field
Model (HMFM), proposed by Haocheng, et al., (2015), to understand the underlying physics
In this seminar, I will summarize the observations and results obtained from this study.
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