2006A&A...456..117A

We report on a densely time sampled polarimetric flux density monitoring of the BL Lac object S5 0716+71 at 86GHz and 229GHz. The source was observed with the IRAM 30m telescope at Pico Veleta within a coordinated multi-frequency observing campaign, which was centred around a 500ks INTEGRAL observation during November 10 to 16, 2003. The aim of this campaign was to search for signatures of inverse-Compton catastrophes through the observation of the broad-band variability of the source. At 86GHz, S5 0716+71 showed no intra-day variability, but showed remarkable inter-day variability with a flux density increase of 34% during the first four observing days, which cannot be explained by source extrinsic causes. At this frequency, making use of a new calibration strategy, we reach a relative rms accuracy of the flux density measurements of 1.2%. Although the flux density variability at 229GHz was consistent with that at 86GHz, the larger measurement errors at 229GHz do not allow us to detect, with high confidence, inter-day variations at this frequency. At 86GHz, the linear polarization fraction of S5 0716+71 was unusually large (15.0±1.8)%. Inter-day variability in linear polarization at 86GHz, with significance level >95%; σ_P/<P≥15% and σ_χ=6°, was observed during the first four observing days. From the total flux density variations at the synchrotron turnover frequency (∼86GHz) we compute an apparent brightness temperature T_B^app>1.4x10¹⁴K at a redshift of 0.3, which exceeds by two orders of magnitude the inverse-Compton limit. A relativistic correction for T_B^app with a Doppler factor δ>7.8 brings the observed brightness temperature down to the inverse Compton limit. A more accurate lower limit of δ>14.0, consistent with previous estimates from VLBI observations, is obtained from the comparison of the 86GHz synchrotron flux density and the upper limits for the synchrotron self-Compton flux density obtained from the INTEGRAL observations. The relativistic beaming of the emission by this high Doppler factor explains the non-detection of ``catastrophic'' inverse-Compton avalanches by INTEGRAL.