Optical spectral variability of the very high energy gamma-ray blazar 1ES 1011+496

We present results of five years of optical (UBVRI) observations of the very high energy gamma-ray blazar 1ES 1011+496 at the MDM Observatory. We calibrated UBVRI magnitudes of five comparison stars in the field of the object. Most of our observations were done during moderately faint states of 1ES 1011+496 with R ≳ 15.0. The light curves exhibit moderate, closely correlated variability in all optical wavebands on timescales of a few days. A cross-correlation analysis between optical bands does not show significant evidence for time lags. We find a positive correlation (Pearson's r = 0.57; probability of non-correlation P(> r) ≈ 4 × 10^-8) between the R-band magnitude and the B-R color index, indicating a bluer-when-brighter trend. Snapshot optical spectral energy distributions exhibit a peak within the optical regime, typically between the V and B bands.We find a strong (r = 0.78; probability of non-correlation P(> r) ≈ 10^-15) positive correlation between the νF_ν peak flux and the peak frequency, best fit by a relation νF _ν ^pk α ν _pk^k with k = 2.05 ± 0.17. Such a correlation is consistent with the optical (synchrotron) variability of 1ES 1011+496 being primarily driven by changes in the magnetic field.