Simultaneous optical monitoring of BL Lacertae object S5 0716+714 with high temporal resolution

TL;DR

This study conducted high-temporal-resolution optical monitoring of BL Lacertae S5 0716+714, revealing intra-day variability, spectral hysteresis loops, and inter-band time delays, providing insights into jet electron processes.

Contribution

First clear observation of intra-day spectral hysteresis loops in optical band and measurement of inter-band time delays in S5 0716+714.

Findings

Intra-day variability up to 0.3 mag observed.

Bluer-when-brighter color behavior detected.

Inter-band lag of about 1.5 minutes between R/B and I bands.

Abstract

We have monitored the BL Lacertae object S5 0716+714 simultaneously in the B, R and I bands on three nights in November 2014. The average time resolution is quite high (73s, 34s, 58s for the filters B, R and I), which can help us trace the profile of the variation and search for the short inter-band time delay. Intra-day variability was about 0.1 mag on the first two nights and more than 0.3 mag on the third. A bluer-when-brighter color behavior was found. An clear loop path can be seen on the color-magnitude diagram of the third night, revealing possible time delays between variations at high and low energies. It is the first time that the intra-day spectral hysteresis loop has been found so obviously in the optical band. We used the interpolated cross-correlation function method to further confirm the time delay and calculated the values of lag between light curves at different wavelengths on each night. On the third night, variations in the R and B bands is approximately 1.5 minutes lagging behind the I band. Such optical time delay is probably due to the interplay of different processes of electrons in the jet of the blazar.