Speaker: Laura Vega Garcia
Title: Space VLBI studies of internal structure and physical processes in extragalactic relativistic jets
0836+710 is a high redshift quasar that was observed with extremely high resolution with space VLBI using RadioAstron at 1.6~GHz, and 22~GHz between October 2013 and January 2014. A spectral analysis, together with a kinematic analysis using the 43 GHz Boston University Blazar program data, reveals the presence of a recollimation shock in the jet, at a distance of 0.15 mas from the jet origin. We observed that moving components interacting with this recollimation shock might be causing the gamma ray flares observed in the source. We use the core shift analysis and synchrotron spectrum fitting to estimate the magnetic field strength in the jet core, with both methods providing a consistent estimate of B_core ~ 0.03 G at 5 GHz. We use the spectral index images to estimate a value of the magnetic power of the jet at distances of 400 pc from the central engine.
We also use the 1.6 GHz data (ground and space VLBI) to determine the ridgeline of the jet and use it for investigating the development of Kelvin-Helmholtz instability in the flow. We identify several distinct modes of the instability and determine their wavelengths and growth lengths. This study allows us to derive relevant physical parameters such as the jet classical Mach number and density ratio of the jet with respect to the ambient medium. At last, the space VLBI image of 0836+710 at 22 GHz reveals a rich, asymmetric jet structure, which we interpret as resulting either from the differential Doppler boosting in a mildly rotating jet, or from a helical magnetic field.
Speaker: Roberto Angioni
Title: Gamma-ray emission in radio galaxies under the VLBI scope
We report on the first systematic VLBI and gamma-ray monitoring study of a representative sample of radio galaxies with strong compact radio emission, with the aim of exploring the intrinsic relationship between high-energy emission and pc-scale jet properties in active galactic nuclei (AGN). While a number of studies have firmly established a close relationship between the gamma-ray and radio properties of AGN in general, the samples considered are dominated by blazars, i.e. AGN featuring well-aligned, Doppler-boosting-dominated jets. This poses a challenge in disentangling the orientation-dependent effects from the intrinsic emission produced in AGN jets. Radio galaxies, on the other hand, have misaligned jets whose emission is much less affected by Doppler boosting. We find that the high-energy emission in the compact jets of radio galaxies is not strongly driven by orientation-dependent Doppler boosting effects, much unlike the situation in their blazar counterparts. However, a significant correlation between gamma-ray flux and radio flux still holds, suggesting a direct physical link between the intrinsic emission properties of AGN jets in the two wavebands. We base our study on the decade(s)-long VLBI monitoring provided by the TANAMI and MOJAVE programs, in combination with gamma-ray data from Fermi-LAT, and also report on the interplay between pc-scale jet kinematics and gamma-ray emission in key individual sources.