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The launching mechanism of the jets of active galactic nuclei isobservationally poorly constrained, due to the large distances to these objectsand the very small scales (sub-parsec) involved. In order to better constraintheoretical models, it is especially important to get information from theregion close to the physical base of the jet, where the plasma accelerationtakes place. In this paper, we report multi-epoch and multi-frequency continuumobservations of the z=2.5 blazar PKS1830-211 with ALMA, serendipitouslycoincident with a strong $\gamma$-ray flare reported by Fermi-LAT. The blazaris lensed by a foreground z=0.89 galaxy, with two bright images of the compactcore separated by 1". Our ALMA observations individually resolve these twoimages (although not any of their substructures), and we study the change oftheir relative flux ratio with time (four epochs spread over nearly three timesthe time delay between the two lensed images) and frequency (between 350 and1050 GHz, rest-frame of the blazar), during the $\gamma$-ray flare. Inparticular, we detect a remarkable frequency-dependent behaviour of the fluxratio, which implies the presence of a chromatic structure in the blazar (i.e.,a core-shift effect). We rule out the possiblity of micro- and milli-lensingeffects and propose instead a simple model of plasmon ejection in the blazar'sjet to explain the time and frequency variability of the flux ratio. We suggestthat PKS1830-211 is likely one of the best sources to probe the activity at thebase of a blazar's jet at submillimeter wavelengths, thanks to the peculiargeometry of the system. The implications of the core-shift in absorptionstudies of the foreground z=0.89 galaxy (e.g., constraints on the cosmologicalvariations of fundamental constants) are discussed.

astronomy and astrophysics

Probing the jet base of the blazar PKS1830-211 from the chromatic  variability of its lensed images. Serendipitous ALMA observations of a strong  gamma-ray flare