Rapid Variability in S5 0716+714 Across the Electromagnetic Spectrum

Abstract

We have investigated the rapid variations of the BL Lac object S5 0716+714 in the radio, optical, ultraviolet, and x-ray regimes. Several simultaneous multifrequency campaigns have been carried out. Here we describe the complete analysis of a four-week monitoring campaign at three radio and three optical frequency bands, simultaneous optical-UV and optical-x-ray observations, and the correlations between rapid variations and changes on longer time scales. We also investigate the polarization properties in the radio regime. The major observational results are the high duty cycle of variability in all frequency bands and correlations between the rapid variations in different frequency regimes. The source is almost always variable. Most power resides on time scales of a few days. These rapid variations are closely related over a wide range in frequencies. A close correlation is observed through the optical-radio regime. The simultaneous optical/x-ray investigations are consistent with a close correlation, but the sampling is too poor to determine a transfer function. While no spectral changes have been found in the optical regime, the radio spectral index varies in a similar way as the flux densities do in the radio regime. We find a close correlation with zero lag between the spectral index between 5 and 8.4 GHz, and the optical flux density, indicating that the source gets optically thick in the low-frequency part of the synchrotron branch during states of high optical flux. We interpret the close correlations between the variations in the optical and the radio regime as an indication that the rapid radio variability is not caused by interstellar scintillation. Various arguments rule out gravitational microlensing. Intrinsic intraday radio variability requires enormous brightness temperatures and calls for extremely high boosting factors to avoid the Compton catastrophe, if the radiation is due to incoherent synchrotron emission. We discuss the implications of our observations for various models which aim at explaining rapid variation of nonthermal radiation by either disk or jet models.