Investigating the origin of optical flares from the TeV blazar S4 0954+65

Aims. We aim to investigate the extreme variability properties of the TeV blazar S4 0954+65 using optical photometric and polarisation observations carried out between 2017 and 2023 using three ground-based telescopes. Methods. We examined an extensive dataset comprised of 138 intraday (observing duration shorter than a day) light curves (LCs) of S4 0954+65 for flux, spectral, and polarisation variations on diverse timescales. For the variable LCs, we computed the minimum variability timescales. We investigated flux-flux correlations and colour variations to look for spectral variations on long (several weeks to years) timescales. Additionally, we looked for connections between optical R-band flux and polarisation degree. Results. We found significant variations in 59 out of 138 intraday LCs. We detected a maximum change of 0.58$\pm$0.11 in V-band magnitude within $\sim$2.64 h and a corresponding minimum variability timescale of 18.21$\pm$4.87 mins on 2017 March 25. During the course of our observing campaign, the source brightness changed by $\sim$4 magnitudes in V and R bands; however, we did not find any strong spectral variations. The slope of the relative spectral energy distribution was 1.37$\pm$0.04. The degree of polarisation varied from $\sim$ 3% to 39% during our monitoring. We observed a change of $\sim$120 degrees in polarisation angle (PA) within $\sim$3 h on 2022 April 13. No clear correlation was found between optical flux and the degree of polarisation. Conclusions. The results of our optical flux, colour, and polarisation study provide hints that turbulence in the relativistic jet could be responsible for the intraday optical variations in the blazar S4 0954+65. However, the long-term flux variations may be caused by changes in the Doppler factor.