Fast radio bursts ( FRBs ) are bright flashes observed typically at GHz frequencies with millisecond duration , whose origin is likely extragalactic .
Their nature remains mysterious , motivating searches for counterparts at other wavelengths .
FRB 121102 is so far the only source known to repeatedly emit FRBs and is associated with a host galaxy at redshift z \simeq 0.193 .
We conducted simultaneous observations of FRB 121102 with the Arecibo and MAGIC telescopes during several epochs in 2016–2017 .
This allowed searches for millisecond-timescale burst emission in very-high-energy ( VHE ) gamma rays as well as the optical band .
While a total of five FRBs were detected during these observations , no VHE emission was detected , neither of a persistent nature nor burst-like associated with the FRBs .
The average integral flux upper limits above 100 GeV at 95 % confidence level are 6.6 \times 10 ^ { -12 } ~ { } \mathrm { photons cm ^ { -2 } s ^ { -1 } } ( corresponding to luminosity L _ { VHE } \lesssim 10 ^ { 45 } ~ { } \mathrm { erg s ^ { -1 } } ) over the entire observation period , and 1.2 \times 10 ^ { -7 } ~ { } \mathrm { photons cm ^ { -2 } s ^ { -1 } } ( L _ { VHE } \lesssim 10 ^ { 49 } ~ { } \mathrm { erg s ^ { -1 } } ) over the total duration of the five FRBs .
We constrain the optical U-band flux to be below 8.6 mJy at 5- \sigma level for 1-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 the arrival of one FRB .
However , the probability of spuriously detecting such a signal within the sampled time space is 1.5 % ( 2.2 \upsigma , post-trial ) , i.e .
consistent with the expected background .
We discuss the implications of the obtained upper limits for constraining FRB models .