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

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 (11 cm to 0.8 mm wavelength, F-GAMMA programme) and γ-ray (0.1–300 GeV) ∼3.5 yr 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 ± 9 d), systematically decreasing from cm to mm/sub-mm bands with a frequency dependence τ_(r, 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 3 mm core region (τ_(3mm, γ) = 12 ± 8 d), (iv) mean distances between the region of γ-ray peak emission and the radio ‘τ = 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/γ-ray correlations in nine individual sources at a significance level where one is expected by chance (probability: 4 × 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 γ-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 τ = 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 parsec distances from the SMBH.