Fermi Gamma-ray Space Telescope

Gamma-ray light curve and VLBI polarization connection in Mrk 421

Marcello Giroletti
M. Giroletti, F. D'Ammando, M. Orienti, D. Paneque on behalf of the Fermi-LAT collaboration and R. Lico, G. Giovannini, J. Gomez, S. Jorstad, A. Marscher

Abstract:

We present Fermi-LAT and multi-frequency, multi-epoch VLBA data for the TeV blazar Mrk421. We collected the data during a long and intensive multifrequency campaign in 2011. We study the gamma-ray light curve, the photon index evolution and their connection to the radio data on sub-parsec scales, including total intensity, polarized flux density, polarization angle, spectral index, and rotation measure both in the core and the jet region. The VLBA data were obtained at three frequencies for twelve epochs during 2011, thus providing the best temporal and spatial coverage in the radio band ever achieved for a TeV blazar. We provide significant constraints on the jet Doppler factor, the presence of proper motion, the magnetic field configuration, and an intriguing connection between variability in the radio data and the gamma-ray light curve: the total intensity and polarized core emission reach a peak simultaneously to the main gamma-ray peak, followed by a rotation of the polarization angle at low frequency. Opacity-related, long wavelength polarization swings are also detected later in the year, possibly related to secondary peaks in the gamma-ray light curve, setting constraints on the physics of the gamma-ray zone.