We report multi-color optical imaging and polarimetry observations of the afterglow of the first TeVdetected gamma-ray burst, GRB 190114C, using the RINGO3 polarimeter on the 2-m autonomous robotic Liverpool Telescope. Observations begin 201 s after the onset of the GRB and continue until ∼ 7000 s post-burst. High temporal resolution (∆t 2.3 − 4.6 s) and dense sampling of the RINGO3 light curves reveal a chromatic break at t ∼ 400 − 500 s -with initial temporal decay α ∼ 1.5 flattening to α ∼ 1 post-break -which we model as a combination of reverse and forward-shock components, with magnetization parameter R B ∼ 40. The observed polarization degree P ∼ 2 − 4% remains steady throughout the first ∼ 2000-s observation window, with a constant position angle. Broadband spectral energy distribution modeling of the afterglow confirms GRB 190114C is highly obscured (A v,HG = 1.49 ± 0.12 mag; N H,HG = (9.0 ± 0.03) × 10 22 cm −2 ). The measured polarization is therefore dominated by dust scattering and the intrinsic polarization is low -in contrast to P > 10% measured previously for other GRB reverse shocks. We test whether 1st and higher-order inverse Compton scattering in a magnetized reverse shock can explain the low optical polarization and the sub-TeV emission but conclude neither is explained in the reverse shock Inverse Compton model. Instead, the unexpectedly low intrinsic polarization degree in GRB 190114C can be explained if largescale jet magnetic fields are distorted on timescales prior to reverse shock emission.

Context. Fast Radio Bursts (FRBs) are millisecond-long bursts uniquely detected at radio frequencies. FRB 131104 is the only case for which a γ-ray transient positionally and temporally consistent was claimed. This high-energy transient had a duration of ∼ 400 s and a 15-150 keV fluence S γ ∼ 4 × 10 −6 erg cm −2 . However, the association with the FRB is still debated. Aims. We aim at testing the systematic presence of an associated transient high-energy counterpart throughout a sample of the FRB population.Methods. We used an approach like that used in machine learning methodologies to accurately model the highly-variable Fermi/GBM instrumental background on a time interval comparable to the duration of the proposed γ-ray counterpart of FRB 131104. A possible γ-ray signal is then constrained considering sample average lightcurves. Results. We constrain the fluence of the possible γ-ray signal in the 8-1000 keV band down to 6.4 × 10 −7 (7.1 × 10 −8 ) erg cm −2 for a 200-s (1-s) integration time. Furthermore, we found the radio-to-gamma fluence ratio to be η > 10 8 Jy ms erg −1 cm 2 . Conclusions. Our fluence limits exclude ∼ 94% of Fermi/GBM detected long gamma-ray bursts and ∼ 96% of Fermi/GBM detected short gamma-ray bursts. In addition, our limits on the radio-to-gamma fluence ratio point to a different emission mechanism from that of magnetar giant flares. Finally, we exclude a γ-ray counterpart as fluent as the one possibly associated with FRB 131104 to be a common feature of FRBs.

No transient electromagnetic emission has yet been found in association to fast radio bursts (FRBs), the only possible exception (3σ confidence) being the putative γ-ray signal detected in Swift/BAT data in the energy band 15-150 keV at the time and position of FRB 131104. Systematic searches for hard X/γ-ray counterparts to other FRBs ended up with just lower limits on the radio/γ-ray fluence ratios. In 2001, at the time of the earliest discovered FRBs, the BeppoSAX Gamma-Ray Burst Monitor (GRBM) was one of the most sensitive open sky γ-ray monitors in the 40-700 keV energy band. During its lifetime, one of the FRBs with the highest radio fluence ever recorded, FRB 010724 (800 ± 400 Jy ms), also known as the "Lorimer burst", was promptly visible to the GRBM. Upon an accurate modeling of the GRBM background, eased by its equatorial orbit, we searched for a possible γ-ray signal in the first 400 s following the FRB, similar to that claimed for FRB 131104 and found no significant emission down to a 5σ limit in the range (0.24-4.7) × 10 −6 erg cm −2 (corresponding to 1 and 400 s integration time, respectively), in the energy band 40-700 keV. This corresponds to η = F radio /F γ > 10 8−9 Jy ms erg −1 cm 2 , i.e. the deepest limit on the ratio between radio and γ-ray fluence, which rules out a γ-ray counterpart similar to that of FRB 131104. We discuss the implications on the possible mechanisms and progenitors that have been proposed in the literature, also taking into account its relatively low dispersion measure (375 ± 3 pc cm −3 ) and an inferred redshift limit of z < 0.4.