Probing the jet base of the blazar PKS 1830−211 from the chromatic variability of its lensed images

Serendipitous ALMA observations of a strong gamma-ray flare^⋆

¹ Department of Earth and Space SciencesChalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
e-mail: mivan@chalmers.se
² Observatoire de Paris, LERMA, CNRS, 61 Av. de l’Observatoire, 75014 Paris, France
³ Institut d’Astrophysique Spatiale, Bât. 121, Université Paris-Sud, 91405 Orsay Cedex, France
⁴ Center for Astrophysics and Space Astronomy, Department of Astrophysical and Planetary Sciences, Box 389, University of Colorado, Boulder, CO 80309-0389, USA
⁵ Institut de Radioastronomie Millimétrique, 300 rue de la piscine, 38406 St Martin d’Hères, France
⁶ École Normale Supérieure/LERMA, 24 rue Lhomond, 75005 Paris, France
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁸ Astron. Dept., King Abdulaziz University, PO Box 80203, Jeddah, Saudi Arabia
⁹ Departamento de Astronomía y Astrofísica, C/ Doctor Moliner 50, 46100 Burjassot, Valencia, Spain
¹⁰ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
¹¹ Institute of Physics, Vietnam Academy of Science and Technology, 10 DaoTan, ThuLe, BaDinh, Hanoi, Vietnam
¹² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany

Received: 24 June 2013
Accepted: 3 September 2013

Abstract

The launching mechanism of the jets of active galactic nuclei is poorly constrained observationally, owing to the large distances to these objects and the very small scales (sub-parsec) involved. To better constrain theoretical models, it is especially important to get information from the region close to the physical base of the jet, where the plasma acceleration takes place. In this paper, we report multi-epoch and multifrequency continuum observations of the z = 2.5 blazar PKS 1830−211 with ALMA, serendipitously coincident with a strong γ-ray flare reported by Fermi-LAT. The blazar is lensed by a foreground z = 0.89 galaxy, with two bright images of the compact core separated by 1′′. Our ALMA observations individually resolve these two images (although not any of their substructures), and we study the change in their relative flux ratio with time (four epochs spread over nearly three times the time delay between the two lensed images) and frequency (between 350 and 1050 GHz, rest frame of the blazar), during the γ-ray flare. In particular, we detect a remarkable frequency-dependent behavior of the flux ratio, which implies the presence of a chromatic structure in the blazar (i.e., a core-shift effect). We rule out the possibility of micro- and milli-lensing effects and propose instead a simple model of plasmon ejection in the blazar’s jet to explain the time and frequency variability of the flux ratio. We suggest that PKS 1830−211 is likely to be one of the best sources to probe the activity at the base of a blazar’s jet at submillimeter wavelengths, thanks to the peculiar geometry of the system. The implications of the core shift in absorption studies of the foreground z = 0.89 galaxy (e.g., constraints on the cosmological variations of fundamental constants) are discussed.