The F-GAMMA programme: multi-frequency study of active galactic nuclei in the Fermi era

Programme description and the first 2.5 years of monitoring^⋆

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: lfuhrmann@mpifr-bonn.mpg.de
² IAPS-INAF, via Fosso del Cavaliere 100, 00133 Roma, Italy
³ Department of Physics/ Institute for Plasma Physics, University of Crete, and FORTH-IESL, 71003 Heraklion, Greece
⁴ California Institute of Technology, Pasadena, CA 91125, USA
⁵ Instituto de Radioastronomía Milimétrica, Avenida Divina Pastora 7, Local 20, 18012 Granada, Spain
⁶ KTH Royal Institute of Technology, Departement of Physics, AlbaNova, 10691 Stockholm, Sweden
⁷ Oskar Klein Centre, Department of Astronomy, AlbaNova, 10691 Stockholm, Sweden
⁸ Department of Physics, Purdue University, 525 Northwestern Ave, West Lafayette, IN 47907, USA
⁹ University of Science and Technology, 133 Gwahangno, Yuseong-gu, Daejeon, 305-333, Republic of Korea
¹⁰ Korea Astronomy and Space Science Institute, 776 Daedeok-Daero, Yuseong-gu, Daejeon, 305-348, Republic of Korea

Received: 23 December 2015
Accepted: 4 August 2016

Abstract

Context. To fully exploit the scientific potential of the Fermi mission for the physics of active galactic nuclei (AGN), we initiated the F-GAMMA programme. Between 2007 and 2015 the F-GAMMA was the prime provider of complementary multi-frequency monitoring in the radio regime.

Aims. We quantify the radio variability of γ-ray blazars. We investigate its dependence on source class and examine whether the radio variability is related to the γ-ray loudness. Finally, we assess the validity of a putative correlation between the two bands.

Methods. The F-GAMMA performed monthly monitoring of a sample of about 60 sources at up to twelve radio frequencies between 2.64 and 228.39 GHz. We perform a time series analysis on the first 2.5-yr data set to obtain variability parameters. A maximum likelihood analysis is used to assess the significance of a correlation between radio and γ-ray fluxes.

Results. We present light curves and spectra (coherent within ten days) obtained with the Effelsberg 100 m and IRAM 30 m telescopes. All sources are variable across all frequency bands with amplitudes increasing with frequency up to rest frame frequencies of around 60–80 GHz as expected by shock-in-jet models. Compared to flat-spectrum radio quasars (FSRQs), BL Lacertae objects (BL Lacs) show systematically lower variability amplitudes, brightness temperatures, and Doppler factors at lower frequencies, while the difference vanishes towards higher ones. The time scales appear similar for the two classes. The distribution of spectral indices appears flatter or more inverted at higher frequencies for BL Lacs. Evolving synchrotron self-absorbed components can naturally account for the observed spectral variability. We find that the Fermi-detected sources show larger variability amplitudes, brightness temperatures, and Doppler factors than non-detected ones. Flux densities at 86.2 and 142.3 GHz correlate with 1 GeV fluxes at a significance level better than 3σ, implying that γ rays are produced very close to the mm-band emission region.