S5 1803+78 revisited

TL;DR

This study presents a long-term optical monitoring of the BL Lac object S5 1803+78, revealing no clear periodicity but suggesting a possible 1300-day cycle, and discusses its multi-wavelength behavior and modeling challenges.

Contribution

The paper provides the first extensive optical monitoring data of S5 1803+78 over 15 years and analyzes its variability, color behavior, and gamma-ray correlations.

Findings

No clear periodicity detected in optical light curve.

Possible ~1300-day timescale between major flares.

One gamma-ray flare coincided with optical brightening.

Abstract

We report on our optical monitoring of the BL Lac object S5 1803+78 from 1996 to 2011. The source showed no clear periodicity, but a time scale of about 1300 days between major flares is possibly present. No systematic trend of the color index with flux variations is evident, at variance with other BL Lacs. In one flare, however, the source was bluer in the rising phase and redder in the falling one. Two Gamma-ray flares were detected by Fermi-GST during our monitoring: on the occasion of only one of them we found simultaneous optical brightening. A one-zone Synchrotron Self Compton (SSC) model appears too simple to explain the source behavior.