We report on a densely time sampled polarimetric flux density monitoring of the BL Lac object S5 0716+71 at 86 GHz and 229 GHz .
The source was observed with the IRAM 30 m telescope at Pico Veleta within a coordinated multi-frequency observing campaign , which was centred around a 500 ks 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 86 GHz , 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 can not 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 229 GHz was consistent with that at 86 GHz , the larger measurement errors at 229 GHz do not allow us to detect , with high confidence , inter-day variations at this frequency .
At 86 GHz , the linear polarization fraction of S5 0716+71 was unusually large ( 15.0 \pm 1.8 ) \% .
Inter-day variability in linear polarization at 86 GHz , with significance level { \raise - 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } } 95 \% ; \sigma _ { P } / < P > = 15 % and \sigma _ { \chi } = 6 ^ { \circ } , was observed during the first four observing days .
From the total flux density variations at the synchrotron turnover frequency ( \sim 86 GHz ) we compute an apparent brightness temperature T _ { B } ^ { app } > 1.4 \times { 10 } ^ { 14 } K 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 \delta > 7.8 brings the observed brightness temperature down to the inverse Compton limit .
A more accurate lower limit of \delta > 14.0 , consistent with previous estimates from VLBI observations , is obtained from the comparison of the 86 GHz 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 .