A multi-wavelength polarimetric study of the blazar CTA 102 during a Gamma-ray flare in 2012
astro-ph.HE
/ Authors
Carolina Casadio, José L. Gómez, Svetlana G. Jorstad, Alan P. Marscher, Valeri M. Larionov, Paul S. Smith, Mark A. Gurwell, Anne Lähteenmäki, Iván Agudo, Sol N. Molina
and 21 more authors
Vishal Bala, Manasvita Joshi, Brian Taylor, Karen E. Williamson, Arkady A. Arkharov, Dmitry A. Blinov, George A. Borman, Andrea Di Paola, Tatiana S. Grishina, Vladimir A. Hagen-Thorn, Ryosuke Itoh, Evgenia N. Kopatskaya, Elena G. Larionova, Liudmila V. Larionova, Daria A. Morozova, Elizaveta Rastorgueva-Foi, Sergey G. Sergeev, Merja Tornikoski, Ivan S. Troitsky, Clemens Thum, Helmut Wiesemeyer
/ Abstract
We perform a multi-wavelength polarimetric study of the quasar CTA 102 during an extraordinarily bright $γ$-ray outburst detected by the {\it Fermi} Large Area Telescope in September-October 2012 when the source reached a flux of F$_{>100~\mathrm{MeV}} =5.2\pm0.4\times10^{-6}$ photons cm$^{-2}$ s$^{-1}$. At the same time the source displayed an unprecedented optical and NIR outburst. We study the evolution of the parsec scale jet with ultra-high angular resolution through a sequence of 80 total and polarized intensity Very Long Baseline Array images at 43 GHz, covering the observing period from June 2007 to June 2014. We find that the $γ$-ray outburst is coincident with flares at all the other frequencies and is related to the passage of a new superluminal knot through the radio core. The powerful $γ$-ray emission is associated with a change in direction of the jet, which became oriented more closely to our line of sight ($θ\sim$1.2$^{\circ}$) during the ejection of the knot and the $γ$-ray outburst. During the flare, the optical polarized emission displays intra-day variability and a clear clockwise rotation of EVPAs, which we associate with the path followed by the knot as it moves along helical magnetic field lines, although a random walk of the EVPA caused by a turbulent magnetic field cannot be ruled out. We locate the $γ$-ray outburst a short distance downstream of the radio core, parsecs from the black hole. This suggests that synchrotron self-Compton scattering of near-infrared to ultraviolet photons is the probable mechanism for the $γ$-ray production.