Astron. Astrophys. 353, 937-943 (2000)

Infrared, radio and optical variability of the BL Lacertae object 2007+777 ^*

B. Peng ^1,2, A. Kraus ¹, T.P. Krichbaum ¹, S.A.H. Müller ^3,4, S.J. Qian ^1,2, A. Quirrenbach ⁵, S.J. Wagner ⁶, A. Witzel ¹, J.A. Zensus ¹, C. Jin ^1,2 and H. Bock <sup>6

1</sup> Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Beijing Astronomical Observatory, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, P.R. China
³ Max-Planck-Institut für Astronomie, Königstuhl, 69117 Heidelberg, Germany
⁴ Ruhr-Universität Bochum, Universitätsstrasse 150, Postfach 102148, 44801 Bochum, Germany
⁵ University of California San Diego, Center for Astrophysics and Space Sciences, Department of Physics, Mail Code 0424, La Jolla, CA 92093-0424, USA
⁶ Landessternwarte Heidelberg, Königstuhl, 69117 Heidelberg, Germany

Received 10 September 1999 / Accepted 23 November 1999

Abstract

We present infrared observations at wavelengths of 60 and 100 µm and contemporaneous radio (five frequencies) and optical (R band) monitoring of the BL Lac object 2007+777. During a three-week campaign with the ISO ¹ spacecraft, the VLA ², the Effelsberg 100 m radio telescope, and optical telescopes in Heidelberg and on Calar Alto, Spain ³, we observed variability on time scales of a few days at 100 µm, at radio, and at optical frequencies. We discuss briefly several generic models which could account for our observations. The attempt at explaining the results by an intrinsic mechanism requires an extremely small source angular size of the order of a few µas ⁴, which can be reconciled with the K inverse Compton limit only when the Doppler factor of the bulk flow is about 40. The general behavior of the variability of the BL Lac object 2007+777 found in this IR-radio campaign follows the expectations of a shock-in-jet model, and we argue that this is the most likely explanation of our observations.

Key words: galaxies: active galaxies: BL Lacertae objects: individual: 2007+777 radio continuum: galaxies

* Based on observations made with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with participation of ISAS and NASA.

Send offprint requests to: B. Peng (bpeng@mpifr-bonn.mpg.de)

© European Southern Observatory (ESO) 2000

Online publication: January 18, 2000

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