Fermi Gamma-ray Space Telescope

Modeling the broadband emission of blazar S50716+714 during its brigthest outburst

Marina Manganaro
(Bindu Rani, Giovanna Pedaletti and Elina Lindfors for the Fermi-LAT and MAGIC Collaborations)


In January 2015 an unprecedented outburst of the BL Lac object S5 0716+714 was registered in all energy bands, from low frequency radio to the VHE (VHE, E > 100 GeV). The dataset collected was impressive and for the first time included simultaneous data of the source from Fermi-LAT and MAGIC, allowing a deep investigation of the Spectral Energy Distribution (SED) in the HE (HE, 0.1 GeV > E > 100 GeV) and VHE band and a complete modeling of the object covering the entire electromagnetic spectrum. The broadband flaring activity coincides with the passage of a moving feature through a stationary one in the VLBI radio map of the source. We have found a very fast change of the electric vector position angle (EVPA): we explain the > 400 degree swing in the optical EVPA as the passage of a superluminal knot through a stationary feature near the radio core. The VHE emission is found originating in the entrance and exit of a superluminal knot in and out a recollimation shock in the inner jet. This suggests that shock-shock interaction in the jet can be responsible for the observed flares and EVPA swing. The jet behaviour suggests a connection between jet kinematics and the observed broadband flaring activity. The gamma-ray emission in the HE and VHE bands is attributed to a shock in the helical jet downstream of the core, closely followed by an optical and X-ray outburst in the core. A strong indication that the VHE gamma-ray emission is associated to a component entering and exiting the core region consist in the separation between the two VHE gamma-ray emission peaks: the first one taking place ∼2 days after the very fast EVPA rotation, while the second one ∼18 days after the new knot has been emerged from the VLBA core. The broadband SED could not be described by a simple one-zone model. Instead we used a two-zone model, where two spherical blobs are co-spatial and provide seed photons to each other. This modeling setup provides an acceptable description of the spectral energy distributions, even if it is certainly an over-simplified presentation of the true physical processes taking place when superluminal knots enter and exit the recollimation shock region.