Testing the inverse-Compton catastrophe scenario in the intra-day variable blazar S5 0716+71, III. Rapid and correlated flux density variability from radio to sub-mm bands

TL;DR

This study investigates rapid, correlated flux variability in blazar S5 0716+71 across radio to sub-mm bands, testing the inverse-Compton catastrophe scenario and confirming relativistic boosting as a key factor.

Contribution

It provides multi-frequency observational evidence that supports relativistic Doppler boosting as an explanation for extreme brightness temperatures in blazar jets.

Findings

Flux variability is intrinsic, not due to interstellar scintillation.

Brightness temperatures exceed the inverse-Compton limit by 3-4 orders of magnitude.

Relativistic Doppler factors are consistent with VLBI and X-ray data.

Abstract

The BL Lac object S5 0716+71 was observed in a global multi-frequency campaign to search for rapid and correlated flux density variability and signatures of an inverse-Compton (IC) catastrophe during the states of extreme apparent brightness temperatures. The observing campaign involved simultaneous monitoring at radio to IR/optical wavelengths centered around a 500-ks INTEGRAL pointing (November 10-17, 2003). We present the combined analysis and results of the cm- to sub-mm observations including a detailed study of the inter- to intra-day variability and spectral characteristics of 0716+714. We further constrain the variability brightness temperatures (T_B) and Doppler factors (delta) comparing the radio-bands with the hard X-ray emission (3-200 keV). 0716+714 was in an exceptionally high state (outburst) and different (slower) phase of short-term variability. The flux density variability in the cm- to mm-bands is dominated by a correlated, ~4 day time scale amplitude increase of up to ~35% systematically more pronounced towards shorter wavelengths. This contradicts expectations from standard interstellar scintillation (ISS) and suggests a source-intrinsic origin of the variability. The derived lower limits to T_B exceed the 10^12 K IC-limit by up to 3-4 orders of magnitude. Assuming relativistic boosting, we obtain robust and self-consistent lower limits of delta >= 5-33, in good agreement with delta_VLBI obtained from VLBI studies and the IC-Doppler factors delta_IC > 14-16 obtained from the INTEGRAL data. Since a strong contribution from ISS can be excluded and a simultaneous IC catastrophe was not observed, we conclude that relativistic Doppler boosting naturally explains the apparent violation of the theoretical limits within standard synchrotron-self-Compton (SSC) jet models of AGN.