Detection of significant cm to sub-mm band radio and gamma-ray correlated variability in Fermi bright blazars

The exact location of the gamma-ray emitting region in blazars is still controversial. In order to attack this problem we present first results of a cross-correlation analysis between radio (11 cm to 0.8 mm wavelength, F-GAMMA program) and gamma-ray (0.1-300 GeV) ~ 3.5 year light curves of 54 Fermi-bright blazars. We perform a source stacking analysis and estimate significances and chance correlations using mixed source correlations. Our results reveal: (i) the first highly significant multi-band radio and gamma-ray correlations (radio lagging gamma rays) when averaging over the whole sample, (ii) average time delays (source frame: 76+/-23 to 7+/-9 days), systematically decreasing from cm to mm/sub-mm bands with a frequency dependence tau_r,gamma (nu) ~ nu^-1, in good agreement with jet opacity dominated by synchrotron self-absorption, (iii) a bulk gamma-ray production region typically located within/upstream of the 3 mm core region (tau_3mm,gamma=12+/-8 days), (iv) mean distances between the region of gamma-ray peak emission and the radio tau=1 photosphere decreasing from 9.8+/-3.0 pc (11 cm) to 0.9+/-1.1 pc (2 mm) and 1.4+/-0.8 pc (0.8 mm), (v) 3 mm/gamma-ray correlations in 9 individual sources at a significance level where one is expected by chance (probability: 4 times 10^-6), (vi) opacity and time lag core shift estimates for quasar 3C 454.3 providing a lower limit for the distance of the bulk gamma-ray production region from the supermassive black hole (SMBH) of ~ 0.8-1.6 pc, i.e. at the outer edge of the Broad Line Region (BLR) or beyond. A 3 mm tau=1 surface at ~ 2-3 pc from the jet-base (i.e. well outside the canonical BLR) finally suggests that BLR material extends to several pc distances from the SMBH.

Testing the inverse-Compton catastrophe scenario in the intra-day variable blazar S5 0716+71, III. Rapid and correlated flux density variability from radio to sub-mm bands

The BL Lac object S5 0716+71 was observed in a global multi-frequency campaign to search for rapid and correlated flux density variability and signatures of an inverse-Compton (IC) catastrophe during the states of extreme apparent brightness temperatures. The observing campaign involved simultaneous monitoring at radio to IR/optical wavelengths centered around a 500-ks INTEGRAL pointing (November 10-17, 2003). We present the combined analysis and results of the cm- to sub-mm observations including a detailed study of the inter- to intra-day variability and spectral characteristics of 0716+714. We further constrain the variability brightness temperatures (T_B) and Doppler factors (delta) comparing the radio-bands with the hard X-ray emission (3-200 keV). 0716+714 was in an exceptionally high state (outburst) and different (slower) phase of short-term variability. The flux density variability in the cm- to mm-bands is dominated by a correlated, ~4 day time scale amplitude increase of up to ~35% systematically more pronounced towards shorter wavelengths. This contradicts expectations from standard interstellar scintillation (ISS) and suggests a source-intrinsic origin of the variability. The derived lower limits to T_B exceed the 10^12 K IC-limit by up to 3-4 orders of magnitude. Assuming relativistic boosting, we obtain robust and self-consistent lower limits of delta >= 5-33, in good agreement with delta_VLBI obtained from VLBI studies and the IC-Doppler factors delta_IC > 14-16 obtained from the INTEGRAL data. Since a strong contribution from ISS can be excluded and a simultaneous IC catastrophe was not observed, we conclude that relativistic Doppler boosting naturally explains the apparent violation of the theoretical limits within standard synchrotron-self-Compton (SSC) jet models of AGN.