2014MNRAS.441.1899F

The exact location of the γ-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 (11cm to 0.8mm wavelength, F-GAMMA programme) and γ-ray (0.1-300GeV) ∼ 3.5yr 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 multiband radio and γ-ray correlations (radio lagging γ rays) when averaging over the whole sample, (ii) average time delays (source frame: 76±23 to 7±9d), systematically decreasing from cm to mm/sub-mm bands with a frequency dependence τr, γ(ν) ∝ ν-1, in good agreement with jet opacity dominated by synchrotron self-absorption, (iii) a bulk γ-ray production region typically located within/upstream of the 3mm core region (τ3mm, γ = 12±8d), (iv) mean distances between the region of γ-ray peak emission and the radio `τ = 1 photosphere' decreasing from 9.8±3.0pc (11cm) to 0.9±1.1pc (2mm) and 1.4±0.8pc (0.8mm), (v) 3mm/γ-ray correlations in nine individual sources at a significance level where one is expected by chance (probability: 4x10<sup>–6</sup>), (vi) opacity and `time lag core shift' estimates for quasar 3C 454.3 providing a lower limit for the distance of the bulk γ-ray production region from the supermassive black hole (SMBH) of ∼ 0.8-1.6pc, i.e. at the outer edge of the broad-line region (BLR) or beyond. A 3mm τ = 1 surface at ∼ 2-3pc from the jet base (i.e. well outside the `canonical BLR') finally suggests that BLR material extends to several parsec distances from the SMBH.