a linear model (Fig. \[spec\_flux\]) with the test statistics of $\chi ^2/\text{d.o.f.}= 2.97/5$, corresponding to Pearson correlation coefficient of 0.76. Moreover, we tried to fit a log-parabola model to the data obtained on MJD 57228. It reveals that the power law model with an index $\Gamma_X = 2.58 \pm 0.05$ ($\chi ^2/\text{d.o.f.}= 48.2/50$) can describe the spectrum better. The X-ray flux ($F_{0.3-10\,\text{keV}}$) on the flare night was $>6$ times higher than the flux from 2006 observations.

Long-term behaviour
-------------------

Recently two studies of optical and radio behaviour of TeV blazars have been published. studied the long-term optical and radio behaviour of 32 VHE gamma-ray blazars using data from the OVRO and Tuorla blazar monitoring programs. They found correlated flares in half of the sources, and correlated long-term trends in 13 sources. performed a first statistical study of the optical polarization variability of TeV blazars, and found that they are not different from the control sample of non-TeV blazars. S2 0109+22 was not part of those studies. In order to compare its optical and radio behaviour with the sample of VHE gamma-ray blazars, we have performed the same analysis of the long-term optical and radio data and optical-polarization data as done 