A seasonal cycle and an abrupt change in the variability characteristics of the intraday variable source S4 0954+65

N. Marchili^1,2, T. P. Krichbaum¹, X. Liu³, H.-G. Song³, K. É. Gabányi^4,5, L. Fuhrmann¹, A. Witzel¹ and J. A. Zensus¹

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: marchili@mpifr-bonn.mpg.de
² Dipartimento di Astronomia, Università di Padova, Vicolo dellOsservatorio 3, 35122 Padova, Italy
³ Xinjiang Astronomical Observatory, the Chinese Academy of Sciences, 150 Science 1-Street, 830011 Urumqi, PR China
⁴ FÖMI, Satellite Geodetic Observatory, Budapest, Hungary
⁵ Konkoly Thege Miklós Astronomical Institute of the Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, Hungary

Received: 7 February 2012
Accepted: 13 April 2012

Abstract

The BLLac object S4 0954+65 is one of the main targets of the Urumqi monitoring program targeting intraday variable (IDV) sources. Between August 2005 and December 2009, the source was included in 41 observing sessions, carried out at a frequency of 4.8 GHz. The time analysis of the collected light curves, performed by applying both a structure function analysis and a specifically developed wavelet-based algorithm, discovered an annual cycle in the variability timescales, suggesting that there is a fundamental contribution by interstellar scintillation to the IDV pattern of the source. The combined use of the two analysis methods also revealed that there was a dramatic change in the variability characteristics of the source between February and March 2008, at the starting time of a strong outburst phase. The analysis’ results suggest that the flaring state of the source coincides with the appearance of multiple timescales in its light curves, indicating that changes in the structure of the relativistically moving emitting region may strongly influence the variability observed on IDV timescales.