2018MNRAS.478.3513Z

We performed quasi-simultaneous optical multi-band monitoring of the BL Lac object S5 0716+714 on seven nights from 2013 to 2016. Intra-day variability was found on all seven nights. The source was faintest on JD 245 6322 with 14.15 mag and brightest on JD 245 7437 with 12.51 mag in the R band. The maximum intra-day variation that we observed was 0.15 mag in the B band on JD 245 6322. Both bluer-when-brighter and achromatic spectral behaviours were observed on the intra-day timescale. On the longer-term scale, the object exhibited mild bluer-when-brighter behaviour between the B and R bands. We estimate the inter-band lags using two independent methods. The variation in the B band was observed to lag behind that in the I band by about 15 min on JD 245 7315. We compare this lag with one reported previously and discuss the origin of these lags.