# Intraday Optical Variability of BL Lacertae

## TL;DR

This study monitored BL Lacertae across multiple optical bands over 13 nights, revealing significant intraday variability, chromatic behavior, and potential inter-band time delays, interpreted through synchrotron pulse models.

## Contribution

First simultaneous multi-band optical monitoring of BL Lacertae over multiple nights revealing detailed variability and spectral behavior.

## Key findings

- Significant intraday variability detected in all bands.
- Bluer-when-brighter chromatism observed on intra-night timescales.
- Possible 10-minute time delay between V and R band variations.

## Abstract

We monitored BL Lacertae simultaneously in the optical B, V, R and I bands for 13 nights during the period 2012-2016. The variations were well correlated in all bands and the source showed significant intraday variability (IDV). We also studied its optical flux and colour behaviour, and searched for inter-band time lags. A strong bluer-when-brighter chromatism was found on the intra-night time-scale. The spectral changes are not sensitive to the host galaxy contribution. Cross-correlation analysis revealed possible time delay of about 10 min between variations in the V and R bands. We interpreted the observed flares in terms of the model consisting of individual synchrotron pulses.