1\arcsec$).

Using aperture (radius of $1.5\arcsec$) photometry, the sky-subtracted target counts were measured for ordinary and extraordinary beams. By using the intensity ratios of two beams and standard formulae in @2007ASPC..364..495L, we calculated normalized Stokes parameters, polarization fraction, and position angle for each observation. Systematic uncertainties are included in our error estimation.

Radio (OVRO and Metsähovi)
--------------------------

S2 0109+22 was observed at 15GHz as part of a high-cadence gamma-ray blazar monitoring program using the Owens Valley Radio Observatory (OVRO) 40 m telescope [@2011ApJS..194...29R]. The observations are calibrated by using a temperature-stable diode noise source to remove receiver gain drifts, and the flux density scale is derived from observations of 3C 286 assuming the value of 3.44Jy at 15.0GHz . The systematic uncertainty of about 5% in the flux density scale is not included in the error bars. Complete details of the reduction and calibration procedure are found in @2011ApJS..194...29R.

The Mets[ä]{}hovi radio telescope, operating at 37GHz, has been observing the source for two decades. We selected radio data obtained after mid-2005 for the long-term study of the source. The instrument and data reduction procedures are described by .

Results {#sec3}
=======

Very high energy gamma rays {#sec3-1}
---------------------------

The VHE 