Testing the inverse-Compton catastrophe scenario in the intra-day variable blazar S5 0716+71

III. Rapid and correlated flux density variability from radio to sub-mm bands

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: lfuhrmann@mpifr-bonn.mpg.de
² Dipartimento di Fisica, Università di Perugia, via A. Pascoli, 06123 Perugia, Italy
³ INAF - Osservatorio Astronomico di Torino, via Osservatorio 20, 10025 Pino Torinese (TO), Italy
⁴ Instituto de Astrofísica de Andalucía, CSIC, Apartado 3004, 18080 Granada, Spain
⁵ Korea Astronomy & Space Science Institute, 61-1 Hwaam-dong, 305-348 Daejeon, Korea
⁶ Hungarian Academy of Sciences Research Group for Physical Geodesy and Geodynamics, Budapest, Hungary
⁷ FÖMI Satellite Geodetic Observatory, Budapest, Hungary
⁸ School of Physics & Astronomy, University of Southampton, Southampton, Hampshire SO17 1BJ, UK
⁹ Landessternwarte Heidelberg-Königstuhl, Königstuhl, 69117 Heidelberg, Germany
¹⁰ Dipartimento di Fisica Generale “Amedeo Avogadro”, Università degli Studi di Torino, via P. Giuria 1, 10125 Torino, Italy
¹¹ Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Torino, via P. Giuria 1, 10125 Torino, Italy
¹² Institut de Radio Astronomie Millimétrique, Avenida Divina Pastora 7, Local 20, 18012 Granada, Spain
¹³ Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, Domaine Universitaire de Grenoble, 38406 Saint-Martin d'Hères, France
¹⁴ Metsähovi Radio Observatory, Helsinki University of Technology, Metsähovintie 114, 02540 Kylmälä, Finland
¹⁵ Arizona Radio Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
¹⁶ University of Arizona, Steward Observatory, 933 N. Cherry Ave., Tucson, AZ 85721, USA
¹⁷ ASTRON, Postbus 2, 7990 AA Dwingeloo; and Astronomical Institute, University of Amsterdam, The Netherlands

Received: 19 October 2007
Accepted: 3 September 2008

Abstract

Aims. The BL Lac object S5 0716+71 was observed in a global multi-frequency campaign to search for rapid and correlated flux density variability and signatures of an inverse-Compton (IC) catastrophe during the states of extreme apparent brightness temperatures.

Methods. The observing campaign involved simultaneous ground-based monitoring at radio to IR/optical wavelengths and was centered around a 500-ks pointing with the INTEGRAL satellite (November 10-17, 2003). Here, we present the combined analysis and results of the radio observations, covering the cm- to sub-mm bands. This facilitates a detailed study of the variability characteristics of an inter- to intra-day variable IDV source from cm- to the short mm-bands. We further aim to constrain the variability brightness temperatures (T_B) and Doppler factors (δ) comparing the radio-bands with the hard X-ray emission, as seen by INTEGRAL at 3-200 keV.

Results. 0716+714 was in an exceptionally high state and different (slower) phase of short-term variability, when compared to the past, most likely due to a pronounced outburst shortly before the campaign. The flux density variability in the cm- to mm-bands is dominated by a ~4 day time scale amplitude increase of up to ~35%, systematically more pronounced towards shorter wavelengths. The cross-correlation analysis reveals systematic time-lags with the higher frequencies varying earlier, similar to canonical variability on longer time-scales. The increase of the variability amplitudes with frequency contradicts expectations from standard interstellar scintillation (ISS) and suggests a source-intrinsic origin for the observed inter-day variability. We find an inverted synchrotron spectrum peaking near 90 GHz, with the peak flux increasing during the first 4 days. The lower limits to T_B derived from the inter-day variations exceed the 10¹² K IC-limit by up to 3-4 orders of magnitude. Assuming relativistic boosting, our different estimates of δ yield robust and self-consistent lower limits of  – in good agreement with obtained from VLBI studies and the IC-Doppler factors  > 14-16 obtained from the INTEGRAL data.

Conclusions. The non-detection of S5 0716+714 with INTEGRAL in this campaign excludes an excessively high X-ray flux associated with a simultaneous IC catastrophe. Since a strong contribution from ISS can be excluded, we conclude that relativistic Doppler boosting naturally explains the apparent violation of the theoretical limits. All derived Doppler factors are internally consistent, agree with the results from different observations and can be explained within the framework of standard synchrotron-self-Compton (SSC) jet models of AGN.