Evidence for Shocks as the Origin of Gamma-Ray Flares in Blazars

We present centimeter-band total flux density and linear polarization light curves illustrating the signature of shocks during radio band outbursts associated in time with -ray flares detected by the Fermi LAT. The general characteristics of the spectral evolution during these events is well-explained by new radiative transfer simulations incorporating propagating oblique shocks and assuming an initially turbulent magnetic field. This finding supports the idea that oblique shocks in the jet are a viable explanation for activity from the radio to the -ray band in at least some -ray flares. 1. Overview Since the mid-1980s, the leading paradigm for the production of flares in AGN in the radio-to-optical bands has been shocks which propagate down the relativistic jets of these sources [1, 2]; hydrodynamical simulations have demonstrated that these structures develop naturally within the jets [3], making this scenario a plausible explanation. In contrast, the location and nature of the emission site giving rise to the GeV -ray flares, detected first by EGRET and now by the Fermi LAT, have remained contentious issues. While both a site near to the central engine and a location in the parsec scale jet have been discussed in the literature, mounting evidence based on correlated broadband activity (including high resolution VLBA imaging of the inner jet) supports a location within the jet at a site near to the millimeter-band radio core (e.g. [4] and references therein). This result supports a direct relation between the flaring in the radio and in the -ray spectral bands and the production of the flaring by the same disturbance. The emitting region itself could take the form of a shock (standing or propagating) or a ‘blob’ with a chaotic magnetic field where turbulence accelerates the high energy electrons. The propagating shock scenario, used successfully for the radio band data has now been proposed as a possible explanation for the -ray flares in some recent studies, e.g. [5]. However, this hypothesis has not been rigorously tested. Radiative transfer modeling carried out in the mid-to-late 1980s and early 1990s incorporating transverse shocks successfully reproduced the spectral evolution of the total flux density and linear polarization in centimeter-band monitoring data during a few carefully selected radio band events. However, the transverse shock models failed to match the variability in later events in the same sources; there the swings in electric vector position