# Gamma-ray Bursts: 50 Years and Counting!
## 8 The Birth of the Multi-Messenger Era (2017-Ongoing)  
GRBs have been believed to be multi-messenger emitters since 1989. In 1995, three independent groups proposed that GRBs could be a dominant source of Ultra-High-Energy Cosmic Rays (UHECRs) through different mechanisms. Two groups proposed that the origin of UHECRs is in the internal shocks [145] or external shocks [146] of a GRB fireball. The latter group suggested a GRB-UHECR association after noticing that each of the error boxes of the two highest-energy cosmic-ray-shower events known at the time overlapped very well with that of a strong GRB [147]. GRB-accelerated cosmic rays can interact with background photons or other baryons through hadronic processes (\(p\gamma\)) to produce high-energy neutrinos, and as early as 1997, it was suggested that these neutrinos could reach PeV energies [148]. Predictions of the neutrino fluxes from GRBs are still a matter of debate, as they depend on many unknown parameters and are highly model-dependent [149; 150]. Even though the theoretical evidence is particularly convincing, to date, no detection of neutrinos in conjunction with GRB signals has been observed, and progressively more stringent constraints on the neutrino flux from GRBs have been reportedfrom both the IceCube neutrino observatory in the South Pole [151] and the ANTARES neutrino observatory in the Mediterranean Sea [152; 153]. Ever since it was suggested that the merger of compact objects, such as two neutron stars, could be the progenitors of some GRBs [13], it was clear that this would make them ideal candidates for emitting gravitational waves together with gamma rays. In fact, these systems were already known sources of gravitational waves (GWs) [154]. Over the years, the possible association between short GRBs and GWs was revisited several times and consolidated. On 17 August 2017, GW170817 was detected by the gravitational wave observatories Advanced LIGO in the USA and Advanced Virgo in Italy, identifying it as an NS-NS merger event and obtaining an accurate localization [155]. Then, 1.7 s later, the low-luminosity short GRB GRB170817A was observed by _[PERSON]_/GBM and INTEGRAL, with a localization compatible with that obtained by LIGO-Virgo [156; 157]. The joint detection thus confirmed the association between short GRBs and mergers of compact objects (NS-NS) [158]. At the same time, this detection triggered a massive observational follow-up campaign, which led to the observation of the signal in the optical, radio, and X-ray bands and allowed the localization of the signal in a nearby galaxy, NGC4993, at \(\sim\)40 Mpc [159; 160; 161; 162; 163; 164; 165]. Particularly remarkable is the observation of the simultaneous emission of a \"kilonova\", a short-lived (days) IR-UV signal powered by the radioactive decay of heavy elements synthesized in the ejected outflow, predicted by many models [166; 88; 167; 168]. Other than these, various electromagnetic counterparts originating from gravitational wave (GW) sources have been linked to compact binary mergers, such as a faint radio afterglow (\"radio flare\"), arising from the interaction between the ejected material and the ambient medium [168], and an X-ray counterpart due to magnetic dissipation if the neutron-star-neutron-star (NS-NS) merger product takes the form of a millisecond magnetar or a black hole. It is speculated that collapsars might exhibit strong GW burst emissions, presenting the intriguing prospect of long GRBs and core-collapse hypernovae as potential multi-messenger targets [169]. The joint detection of GRB-GW170817 had enormous importance for fundamental physics, as it allowed a constraint to be imposed on the difference between the speed of gravity and the speed of light:  
\[-3\times 10^{-15}\leq\frac{\Delta\
u}{\
u_{EM}}\leq 7\times 10^{-16} \tag{6}\]  
This allowed new bounds to placed on Lorentz Invariance Violations and allowed a new test of the equivalence principle by constraining the Shapiro delay between gravitational and electromagnetic radiation [157].