# Distributed Architectures and Constellations for \(\gamma\)-ray Burst Science
## 3 CBC Multi-Messenger Astrophysics Today  
It is often assumed that the multi-messenger revolution has a very precise starting date: 17 August 2017, with the detection of GW170817 by the LIGO/Virgo interferometers and short GRB170817 by the Fermi/GBM and INTEGRAL/SPI. Unfortunately, no other event was detected simultaneously by GWs and gamma-rays (or any electromagnetic radiation) during the third LIGO/VIRGO observing run (O3). Since one swallow does not make a summer, all questions presented in the previous section are still open. Certainly, GW/GRB1708017 has shown how powerful the multi-messenger approach could be. Its full impact, however, measured by the ability to open a long-lasting, brand-new field must still be determined, and it will greatly depend on the capability of collecting statistical samples of GW-electromagnetic events in the near future, that is, during the LIGO/VIRGO O4 and O5 observing runs planned for this decade. As of now, the LIGO/Virgo interferometers have detected two NS-NS coalescence events (GW170817 and GW190425) and one very likely BH-NS merger (GW190814). **The GW/GRB170817** event does not need to be commented on in detail here; many comprehensive reviews exist [6]. Briefly, Fermi/GBM and INTERGAL/SPI detected a short burst lasting for about 2 s, just 1.7 s after the NS-NS detection by LIGO/Virgo. The LIGO/Virgo error box was about 30 deg\({}^{2}\), a value that did not improve much after including the Inter Planetary Network (IPN) error box. The distance of the source 40\({}^{+8}_{-14}\)Mpc was determined directly from the GW detection and greatly limited the search for an optical counterpart to only about 50 galaxies. An optical counterpart was discovered in NGC4993 only after about 11 h from the GW/GRB event, which gave rise to an impressive follow-up by nearly one hundred ground-based and space-based telescopes, placing constraints on the optical/IR counterpart of the GW source/GRB and discovering the first confirmed kilonova in history. The GRB detection, which was nearly simultaneous with the GW signal, confirmed that, at least in this case, a merger of two NSs is the origin of short GRBs. The follow-up observations allowed the complex jet structure to be determined, and the jet was observed as off-axis for the first time [10; 11; 12]. It was also found that kilonovae were one of the prime sites of heavy element production through the r-processes [6; 13]. **GW190425** was detected with a high significance by one interferometer only, and thus the uncertainty region included a large fraction of the sky [14]. No GRB was detected at the same time, with an upper limit on the 50-300 keV fluence of \(\sim 10^{-6}\) ergs/cm\({}^{2}\) in 1 s [15], corresponding to a few ph/cm\({}^{2}\), by INTEGRAL/SPI. Swift/BAT and Fermi/GBM covered only a fraction of the region, and therefore could not contribute much to the INTEGRAL observation. The distance of the event inferred from the GW signal is \(159^{+69}_{-21}\) Mpc. Therefore, even for a gamma-ray luminosity comparable to that of GRB170817, its flux would need to be 16 times dimmer, \(<\)0.1 ph/cm\({}^{2}\); hence, it is relatively difficult to detect with the available detectors. Moreover, the observed gamma-ray flux is a strong function of the inclination at which the jet is observed (a power law with an exponent \(-5\div-6\)). Unfortunately, the inclination of this binary system was not well constrained by the interferometers, but assuming similar characteristics of GRB170817, we can infer a lower limit on the inclination between the jet and the line-of-sight of about 10 deg. **GW190814** was detected by all three interferometers, thus providing good constraints on the position of the source (error box of 18.5 deg\({}^{2}\)), distance (\(241^{+41}_{-45}\) Mpc) and inclination (\(46\div-11\) deg), in addition to the masses of the two coalescing objects and of the remnant (this was the GW event with the most unequal mass ratio, \(0.11+\setminus-\) 0.01 ever detected).