2012MNRAS.425.1357G

We report results from a one-week multiwavelength campaign to monitor the BLLacertae object (BL Lac) S5 0716+714 (on 2009 December 9–16). Nine ground-based telescopes at widely separated longitudes and one space-based telescope aboard the Swift satellite collected optical data. Radio data were obtained from the Effelsberg and Urumqi observatories and X-ray data from Swift. In the radio bands, the source shows rapid [ ∼ (0.5–1.5) d] intraday variability with peak amplitudes of up to ∼ 10 per cent. The variability at 2.8cm leads by about 1d the variability at 6 and 11cm. This time lag and more rapid variations suggest an intrinsic contribution to the source's intraday variability at 2.8cm, while at 6 and 11cm, interstellar scintillation (ISS) seems to predominate. Large and quasi-sinusoidal variations of ∼ 0.8mag were detected in the V, R and I bands. The X-ray data (0.2–10keV) do not reveal significant variability on a 4d time-scale, favouring reprocessed inverse Compton over synchrotron radiation in this band. The characteristic variability time-scales in radio and optical bands are similar. A quasi-periodic variation of 0.9–1.1d in the optical data may be present, but if so it is marginal and limited to 2.2 cycles. Cross-correlations between radio and optical bands are discussed. The lack of a strong radio–optical correlation indicates different physical causes of variability (ISS at long radio wavelengths, source intrinsic origin in the optical) and is consistent with a high jet opacity and a compact synchrotron component peaking at ≃ 100GHz in an ongoing very prominent flux-density outburst. For the campaign period, we construct a quasi-simultaneous spectral energy distribution, including γ-ray data from the Fermi satellite. We obtain lower limits for the relativistic Doppler boosting of δ ≥ 12–26, which for a BLLac-type object is remarkably high.