Multi-wavelength study of flaring activity in BL Lac object S5 0716+714 during 2015 outburst

TL;DR

This study analyzes a major flaring event of the BL Lac object S5 0716+714 in 2015, using multi-wavelength observations and a 3D jet model to understand polarization changes and emission mechanisms.

Contribution

It presents a detailed multi-wavelength analysis of the 2015 flare and applies a time-dependent helical magnetic field jet model to interpret polarization and emission variations.

Findings

Detection of TeV emission during the flare

Significant polarization degree and angle changes

Evidence of magnetic reconnection in the jet

Abstract

We present a detailed investigation of the flaring activity observed from a BL Lac object, S5 0716+714 , during its brightest ever optical state in the second half of January 2015. Observed almost simultaneously in the optical, X-rays and {\gamma}-rays, a significant change in the degree of optical polarization (PD) and a swing in the position angle (PA) of polarization were recorded. A detection in the TeV (VHE) was also reported by the MAGIC consortium during this flaring episode. Two prominent sub-flares, peaking about 5-days apart, were seen in almost all the energy bands. The multi-wavelength light-curves, spectral energy distribution (SED) and polarization are modeled using the time-dependent code developed by Zhang et al. (2014). This model assumes a straight jet threaded by large scale helical magnetic fields taking into account the light travel time effects, incorporating synchrotron flux and polarization in 3D geometry. The rapid variation in PD and rotation in PA are most likely due to re-connections happening in the emission region in the jet, as suggested by the change in the ratio of toroidal to poloidal components of magnetic field during quiescent and flaring states.