Intra-day variability observations of S5 0716+714 over 4.5 years at 4.8 GHz

¹ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 150 Science 1-Street, 830011 Urumqi, PR China
e-mail: liux@xao.ac.cn
² Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 210008 Nanjing, PR China
³ Graduate University of the Chinese Academy of Sciences, 100049 Beijing, PR China
⁴ Max-Plank-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Dipartimento di Astronomia, Università di Padova, Vicolo dellsservatorio 3, 35122 Padova, Italy
⁶ College of Physical Science and Technology, Guangxi University, 530004 Nanning, Guangxi, PR China

Received: 8 April 2012
Accepted: 15 May 2012

Abstract

Aims. We aim to search for evidence of annual modulation in the time scales of the BL Lac object S5 0716+714.

Methods. The intra-day variability (IDV) observations were carried out monthly from 2005 to 2009, with the Urumqi 25 m radio telescope at 4.8 GHz.

Results. The source has shown prominent IDV as well as long-term flux variations. The IDV time scale does show evidence in favor of an annual modulation, suggesting that the IDV of 0716+714 is dominated by interstellar scintillation. The source underwent a strong outburst phase between mid-2008 and mid-2009; a second intense flare was observed in late 2009, but no correlation between the total flux density and the IDV time scale is found, implying that the flaring state of the source does not have serious implications for the general characteristics of its intra-day variability. However, we find that the inner-jet position angle is changing throughout the years, which could result in an annual modulation noise in the anisotropic ISS model fit. There is also an indication that the lowest IDV amplitudes (rms in flux density) correspond to the slowest time scales of IDV, which would be consistent with an ISS origin of the IDV of 0716+714.