astronomy. The corresponding trigger threshold is $\gtrsim 50$GeV [@2016APh....72...76A]. S2 0109+22 is visible from the MAGIC site at zenith angle below 40$^{\circ}$ between mid-July and February.

Triggered by increased activity in HE gamma rays, MAGIC observed S2 0109+22 for 9.63h in 2015 July within a multi-wavelength blazar monitoring program. The observations were performed during 7 consecutive nights from July 22 to July 28 (MJD 57225–57231) with zenith angle range between 11$^\circ$ and 39$^\circ$. The data have been analysed using the MAGIC Standard Analysis Software [MARS, @2009arXiv0907.0943M; @2016APh....72...61A; @2017MNRAS.468.1534A]. Part of the data were affected by clouds, therefore we applied atmospheric transmission correction based on the information obtained with the MAGIC elastic LIDAR [@2015EPJWC..8902003F].

High-energy gamma rays (*Fermi*-LAT)
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The Large Area Telescope (LAT) is the primary instrument on-board the *Fermi Gamma-ray Space Telescope*. Based on the pair-conversion technique, it is designed to investigate the gamma-ray sky in the energy band from 30MeV to &gt;300GeV [@2009ApJ...697.1071A]. In its standard operation mode it surveys the sky, covering it fully every 3h.

The data analysed in this paper were selected from a region of interest around S2 0109+22 with a radius of $15^{\circ}$, in a period lasting around three weeks (MJD 57220–57240) roughly 