Here we report on the results of the WEBT photo-polarimetric campaign targeting the blazar S5 0716+71, organized in March 2014 to monitor the source simultaneously in BVRI and near IR filters. The campaign resulted in an unprecedented dataset spanning ∼ 110 h of nearly continuous, multi-band observations, including two sets of densely sampled polarimetric data mainly in R filter. During the campaign, the source displayed pronounced variability with peak-to-peak variations of about 30% and "bluer-when-brighter" spectral evolution, consisting of a day-timescale modulation with superimposed hourlong microflares characterized by ∼ 0.1 mag flux changes. We performed an in-depth search for quasi-periodicities in the source light curve; hints for the presence of oscillations on timescales of ∼ 3 h and ∼ 5 h do not represent highly significant departures from a pure rednoise power spectrum. We observed that, at a certain configuration of the optical polarization angle relative to the positional angle of the innermost radio jet in the source, changes in the polarization degree led the total flux variability by about 2 h; meanwhile, when the relative configuration of the polarization and jet angles altered, no such lag could be noted. The microflaring events, when analyzed as separate pulse emission components, were found to be characterized by a very high polarization degree (> 30%) and polarization angles which differed substantially from the polarization angle of the underlying background component, or from the radio jet positional angle. We discuss the results in the general context of blazar emission and energy dissipation models.

The occurrence of low-amplitude flux variations in blazars on hourly timescales, commonly known as microvariability, is still a widely debated subject in high-energy astrophysics. Several competing scenarios have been proposed to explain such occurrences, including various jet plasma instabilities leading to the formation of shocks, magnetic reconnection sites, and turbulence. In this letter we present the results of our detailed investigation of a prominent, five-hour-long optical microflare detected during recent WEBT campaign in 2014, March 2-6 targeting the blazar 0716+714. After separating the flaring component from the underlying base emission continuum of the blazar, we find that the microflare is highly polarized, with the polarization degree ∼ (40−60)% ±(2−10)%, and the electric vector position angle ∼ (10−20) deg ±(1−8) deg slightly misaligned with respect to the position angle of the radio jet. The microflare evolution in the (Q, U ) Stokes parameter space exhibits a looping behavior with a counter-clockwise rotation, meaning polarization degree decreasing with the flux (but higher in the flux decaying phase), and approximately stable polarization angle. The overall very high polarization degree of the flare, its symmetric flux rise and decay profiles, and also its structured evolution in the Q − U plane, all imply that the observed flux variation corresponds to a single emission region characterized by a highly ordered magnetic field. As discussed in the paper, a small-scale but strong shock propagating within the outflow, and compressing a disordered magnetic field component, provides a natural, though not unique, interpretation of our findings.

We present the results of a long-term study designed to investigate the nature of micro-variability in blazars carried out primarily at the Southeastern Association for Research in Astronomy (SARA) observatories. We analyzed micro-variability data of fifteen OVV quasars and BL Lac sources collected from 1995 to 2021. The data set consists of single-band light curves interspersed with multi-color and micro-variability observations. This paper reports over 900 nights of CCD observations. We also incorporated observations from other observers as well as observations gleaned from the literature into our analysis. We employed differential photometry to measure magnitudes and then construct the long-term and micro-variability light curves. Our results indicate that there is no correlation between the presence of micro-variations and the brightness of the source. We present a viable theory to explain the intermittent micro-variability as pulses of radiation emitted by individual turbulent cells in the relativistic jet, which are stimulated by a passing shock wave. We present model fits and test results for various data sets, including WEBT light curves, Kepler light curves and a TESS light curve. Although the consensus in the community is that blazar jets must be turbulent, the identification of micro-variations as manifestations of actual turbulent cells is important for modeling these turbulent jets. We can obtain estimates of cell sizes (assuming a shock speed), and the distribution of cell sizes derived from observations is consistent with numerical simulation predictions.

Abstract:A critical observation for testing the KRM jet model, in development at FIU, is to observe high time resolution, high accuracy photometry and polarimetry over a wide range of frequencies in the optically thin portion of the synchrotron spectrum. The detection of micro-variability during these observations would be ideal so the background and flaring components could be separated. Target of opportunity H-band photometry and polarimetry observations on the Gran Telescopio Canarias (GTC) 10.4-m with the Canarias InfraRed Camera Experiment (CIRCE) instrument were made in conjunction with the Southeastern Association for Research in Astronomy (SARA) JKT observing in the optical VRI bands in order to test the model. Here we present simultaneous micro-variability observations of Blazar S5 0716+71 made on 14 April 2017, with the CIRCE instrument on the GTC 10.4-m telescope and optical observations made with the 1.0-m SARA JKT in La Palma. The CIRCE observations consisted of high time resolution polarimetric observations in the H band over a period of 2.4 h on source, measuring both the H-band flux and the polarization degree and angle. Simultaneous observations with the SARA JKT 1.0-m yielded VRI light curves with about three minute time resolution over ∼4 h. 0716+71 showed only small amounts of variability during the observation. We present here the resulting data and a comparison to previous observations.