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Fast radio bursts (FRBs) are bright flashes observed typically at GHzfrequencies with millisecond duration, whose origin is likely extragalactic.Their nature remains mysterious, motivating searches for counterparts at otherwavelengths. FRB 121102 is so far the only source known to repeatedly emit FRBsand is associated with a host galaxy at redshift $z \simeq 0.193$. We conductedsimultaneous observations of FRB 121102 with the Arecibo and MAGIC telescopesduring several epochs in 2016--2017. This allowed searches formillisecond-timescale burst emission in very-high-energy (VHE) gamma rays aswell as the optical band. While a total of five FRBs were detected during theseobservations, no VHE emission was detected, neither of a persistent nature norburst-like associated with the FRBs. The average integral flux upper limitsabove 100 GeV at 95% confidence level are $6.6 \times 10^{-12}~\mathrm{photons\cm^{-2}\ s^{-1}}$ (corresponding to luminosity $L_{\rm VHE} \lesssim10^{45}~\mathrm{erg\ s^{-1}}$) over the entire observation period, and $1.2\times 10^{-7}~ \mathrm{photons\ cm^{-2}\ s^{-1}}$ ($L_{\rm VHE} \lesssim10^{49}~\mathrm{erg\ s^{-1}}$) over the total duration of the five FRBs. Weconstrain the optical U-band flux to be below 8.6 mJy at 5-$\sigma$ level for1-ms intervals around the FRB arrival times. A bright burst with U-band flux$29~\mathrm{mJy}$ and duration $\sim 12$ ms was detected 4.3 s before thearrival of one FRB. However, the probability of spuriously detecting such asignal within the sampled time space is 1.5% (2.2 $\sigma$, post-trial), i.e.consistent with the expected background. We discuss the implications of theobtained upper limits for constraining FRB models.

Constraining very-high-energy and optical emission from FRB 121102 with the MAGIC telescopes