Intra-day optical multi-band quasi-simultaneous observation of BL Lacertae object S5 0716+714 from 2013 to 2016

TL;DR

This study presents multi-band optical observations of BL Lac object S5 0716+714 over several years, revealing intra-day variability, spectral behaviors, and inter-band lags, contributing to understanding its emission mechanisms.

Contribution

It provides the first detailed intra-day multi-band variability analysis of S5 0716+714 over multiple years, including inter-band lag measurements and spectral behavior insights.

Findings

Intra-day variability detected on all observed nights.

Inter-band lag of approximately 15 minutes between B and I bands.

Both bluer-when-brighter and achromatic spectral behaviors observed.

Abstract

We perform quasi-simultaneous optical multi-band monitoring of BL Lac object S5 0716+714 on seven nights from 2013 to 2016. Intra-day variability (IDV) is found on all seven nights. The source was faintest on JD 2456322 with 14.15 mags and brightest on JD 2457437 with 12.51 mags in the $R$ band. The maximum intra-day variation we observed is 0.15 mags in the $B$ band on JD 2456322. Both bluer-when-brighter and achromatic spectral behaviours were observed on the intra-day timescale. On the longer-term scale, the object exhibited a 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 that in the $I$ band by about 15 minutes on JD 2457315. We compare this lag with one reported previously and discussed the origin of these lags.