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

I. Simultaneous broadband observations during November 2003

¹ Landessternwarte Heidelberg-Königstuhl, Königstuhl, 69117 Heidelberg, Germany e-mail: L.Ostorero@lsw.uni-heidelberg.de
² Tuorla Observatory, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
³ Section of Astrophysics, Astronomy & Mechanics, Department of Physics, University of Athens, Panepistimiopolis, 157 84 Zografos, Athens, Greece
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Istituto Nazionale di Astrofisica (INAF), Osservatorio Astronomico di Torino, via Osservatorio 20, 10025 Pino Torinese (TO), Italy
⁶ Department of Physics and Astronomy, Western Kentucky University, 1 Big Red Way, Bowling Green, KY 42104, USA
⁷ Institute of Astronomy, National Central University, 300 Jungda Road, Jungli City 320-54, Taoyuan, Taiwan, ROC
⁸ Department of Physics, National Central University, 300 Jungda Road, Jungli City 320-54, Taoyuan, Taiwan, ROC
⁹ Osservatorio Astronomico, Università di Perugia, via B. Bonfigli, 06126 Perugia, Italy
¹⁰ Crimean Laboratory of the Sternberg Astronomical Institute, University of Moscow, Russia; P/O Nauchny, 98409 Crimea, Ukraine
¹¹ Astronomical Institute, St. Petersburg State University, Universitetsky pr. 28, Petrodvoretz, 198504 St. Petersburg, Russia
¹² Cork Institute of Technology, Dept. of Applied Physics & Instrumentation, Rossa Avenue, Bishoptown, Cork, Ireland
¹³ Ulugh Beg Astronomical Institute, Center for Space Research, Uzbek Academy of Sciences, Astronomicheskaya 33, 700052 Tashkent, Uzbekistan
¹⁴ Isaac Newton Institute, Uzbekistan Branch, Astronomicheskaya 33, 700052 Tashkent, Uzbekistan
¹⁵ Metsähovi Radio Observatory, Helsinki University of Technology, Metsähovintie 114, 02540 Kylmälä, Finland
¹⁶ Institute of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotlárská 2, 611 37 Brno, Czech Republic
¹⁷ Abastumani Astrophysical Observatory, 383762 Abastumani, Georgia
¹⁸ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
¹⁹ Michael Adrian Observatory, Astronomie Stiftung Trebur, Fichtenstrasse 7, 65468 Trebur, Germany
²⁰ Coyote Hill Observatory, PO Box 930, Wilton, CA 95693, USA
²¹ Department of Physics and Astronomy, University of Alaska Anchorage, 3211 Providence Dr., Anchorage, AK 99508, USA
²² Department of Physics and Astronomy, University of Victoria, BC, Canada
²³ Faculty of Information Sciences, Hiroshima-City University, 3-4-1, Ozuka-Higashi, Asa-minami-ku, Hiroshima 731-3194, Japan
²⁴ Department of Physics and Astronomy, University of Missouri-St. Louis, 8001 Natural Bridge Road, St. Louis, MO 63121, USA
²⁵ Department of Astronomy, Dennison Building, University of Michigan, Ann Arbor, MI 48109, USA
²⁶ ASTRON, Postbus 2, 7990 AA Dwingeloo, The Netherlands
²⁷ Astronomical Institute, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
²⁸ IRAM, Avd. Div. Pastora 7NC, 18012 Granada, Spain
²⁹ University of Arizona, Steward Observatory, 933 N. Cherry Ave., Tucson, AZ 85721, USA

Received: 19 August 2005
Accepted: 14 January 2006

Abstract

Context.Some intra-day variable, compact extra-galactic radio sources show brightness temperatures severely exceeding 10¹² K, the limit set by catastrophic inverse-Compton (IC) cooling in sources of incoherent synchrotron radiation. The violation of the IC limit, actually possible under non-stationary conditions, would lead to IC avalanches in the soft-γ-ray energy band during transient periods.

Aims.For the first time, broadband signatures of possible IC catastrophes were searched for in a prototypical source, S5 0716+71.

Methods.A multifrequency observing campaign targetting S5 0716+71 was carried out during November 06-20, 2003. The observations, organized under the framework of the European Network for the Investigation of Galactic nuclei through Multifrequency Analysis (ENIGMA) together with a campaign by the Whole Earth Blazar Telescope (WEBT), involved a pointing by the soft-γ-ray satellite INTEGRAL, optical, near-infrared, sub-millimeter, millimeter, radio, as well as Very Long Baseline Array (VLBA) monitoring.

Results.S5 0716+71 was very bright at radio frequencies and in a rather faint optical state () during the INTEGRAL pointing; significant inter-day and low intra-day variability was recorded in the radio regime, while typical fast variability features were observed in the optical band. No obvious correlation was found between the radio and optical emission. The source was not detected by INTEGRAL, neither by the X-ray monitor JEM-X nor by the γ-ray imager ISGRI, but upper limits to the source emission in the 3-200 keV energy band were estimated. A brightness temperature K (violating the IC limit) was inferred from the variability observed in the radio regime, but no corresponding signatures of IC avalanches were recorded at higher energies.

Conclusions.In the most plausible scenario of negligible contribution of the interstellar scintillation to the observed radio variability, the absence of the signatures of IC catastrophes provides either a lower limit to the Doppler factor affecting the radio emission or strong constraints for modelling of the Compton-catastrophe scenario in S5 0716+71.