A geometrical interpretation for the properties of multiband optical variability of the blazar S5 0716+714

Abstract

We present the results of multiband observations of the blazar S5 0716+714 intra-night variability performed during 23 nights in the period from 04.2014 through 04.2015. The bluer-when-brighter trend is detected in both intra- and inter-night data. We assume that the jet component crossing the region where the medium becomes transparent to the optical radiation forms almost all optical emission of S5 0716+714. Deviations of some parts of the component from the general trajectory of the component can cause the Doppler factor of these parts to increase. Various maximum Doppler factors achieved by these parts of the component and different volumes occupied by them with the concave synchrotron self-absorption spectrum result in both the observed various color index behavior in variability and explain the absence of dependence of the bluer-when-brighter behavior on the object magnitude. We estimated spectral maximum frequency νm≈(0.6−1.7)·1014 GHz from intra- and inter-night data. Assuming that the size of emitting region is comparable with the gravitational radius of a black hole with a mass of 5·108 solar masses, the magnetic field obtained from synchrotron self-absorption is B∼102−104 G, which corresponds to the values of other independent estimations. The obtained values of νm and B confirm our assumption about the nature of the blazar S5 0716+714 region radiating in the optical range.