# Distributed Architectures and Constellations for \(\gamma\)-ray Burst Science
## 4 The Role of Distributed Architectures in Tomorrow's Multi-Messenger Astrophysics
### A Powerful Combination of Nano- and Micro-/Small Satellites  
Four years later, GW170817A remains the only gravitational wave source with a detection of an electromagnetic counterpart. While GW170817 was quickly followed by a short GRB seen at an angle of 19-42 degrees from the jet axis, it is likely that most kilonovae will not emit a GRB observable from the Earth. The prompt \(\gamma\)-ray emission is strongly beamed, and it is estimated that only about 1 in 100 kilonovae will be detectable at high energies [20]. However, GRB170817A is an unusually long and faint short-GRB, and detections of other GRB counterparts for gravitational wave events can be important discoveries. In the previous section, we discussed how a constellation of nano-satellites can be efficiently used to provide a powerful, high-energy, all-sky monitor at a relatively low cost and on short timescales. Here, we discuss how a synergic constellation of micro-satellites could be used for follow-up observations, greatly enhancing the scientific return. It will require coordination, but to a large extent, several nano- and micro-satellite constellations with different detectors could work together in conjunction, forming one network. The micro-satellites will perform rapid follow-up observations at near-UV/optical and near-IR wavelengths. To follow up kilonovae without GRB counterparts, detected only by GW observatories, these observations shall be triggered directly by their GW emission. This will require the near UV/optical/IR observatories to have large fields of view and fast repointing capabilities, enabling them to locate the electromagnetic counterparts of kilonovae after short mosaicing observations. The early time evolution of the near-UV to near-IR flux ratios will provide the key diagnostics to distinguish between various scenarios of kilonova explosions. No existing or proposed mission provides all-sky monitoring and localization together with rapid multi-wavelength follow-up capabilities. The rapid follow-up observations at near-UV/optical, near infrared, and X-ray wavelengths are expected to produce real breakthroughs in our understanding of kilonovae. The luminous optical counterpart of GW170817 was initially blue in colour with the emission peaking at near-UV wavelengths. Then, over the course of a few days the emission shifted to the near-IR wavelengths. This fast spectral evolution was unlike that of any previously observed event. However, the optical counterpart was discovered only about 11 h after the gravitational wave signal. A wide-field UV space telescope, able to rapidly slew on-source, could revolutionize our understanding of these exciting events (e.g., Ultrasat, [[http://space.gov.il/en/node/1129](http://space.gov.il/en/node/1129)]([http://space.gov.il/en/node/1129](http://space.gov.il/en/node/1129)); accessed on 1 December 2021). Theoretical modelling predicts that the first few hours might be dominated by near-UV emission from free neutrons, which do not have time to be captured by the nuclei. Observing this early emission is thus key for the understanding of the nucleosynthesis of kilonovae. Figure 1 of [PERSON] & [PERSON] 2016 [21] presents the phases of a binary neutron star merger as a function of time, showing the observational signatures, as well as the possible outcomes and the associated physical phenomena. The in-spiral and coalescence of neutron stars, which can be observed through gravitational waves, can result in a hyper-massive neutron star that quickly collapses into a black hole; into a stable, rapidly spinning, highly magnetised neutron star; or directly into a black hole. The merger gives rise to the ejection  
of 10\({}^{-4}\)-10\({}^{-2}\) solar masses of unbound matter, with velocities 0.1-0.3 c from the tidal tails in the equatorial region [20]. The ejected matter that remains bound to the resulting compact object falls back and forms an accretion disc that helps launch the ultra-relativistic jet, which produces the observed short GRB.