*4.1. A Constellation of Nano-Satelliters for High-Energy Transient Detection and Localization*

Today, X-ray and gamma-ray monitors dedicated to the search and localization of high-energy transients are mostly monolithic instruments (e.g., NASA Swift/BAT, ESA INTEGRAL/IBIS, INTEGRAL/SPI, ASI AGILE/SuperAgile, AGILE/Microcalorimeter) or multiple detectors hosted by the same large spacecraft (NASA FERMI/GBM). One good example of distributed architecture used for GRB science since the beginning of this enterprise is the Inter-Planetary network (IPN). The accurate timing from X-ray and γ-ray detectors hosted on different spacecrafts for both Low-Earth Orbit (LEO) and High-Earth orbit (HEO) or even inter-planetary routes, is joined together to determine the transient position in the sky. This is achieved by using the delay time of arrival of the transient signal on different detectors, a strategy used since the earliest GRB detections by the VELA satellites at the end of the 1960s.

While the VELA satellites were fully dedicated to the detection of gamma-ray flashes (in this case the main target was the detection of gamma-rays from nuclear tests above

the Earth's atmosphere), today, the IPN includes instruments on satellites fully dedicated to high-energy astrophysics, and instruments hosted by spacecrafts with quite different primary purpose. The diverse gamma-ray instrumentation used by the IPN, the poor knowledge of the spacecraft position when outside the Earth's GNSS infrastructure, as well as the difficulty of defining an absolute time with a good precision for all spacecrafts and the delay in communication with remote solar system spacecrafts, significantly limit the ability of the IPN to routinely provide accurate (better than a few degrees) and timely localizations. In other words, systematic errors associated with IPN transient localization are usually much greater than the statistical errors.

All of these difficulties could be overcome by a constellation of satellites in LEO hosting similar, if not identical, X-ray and gamma-ray detectors. The GPS and Galileo infrastructures offers the opportunity to constrain LEO satellite positions to within a few tens of meters, and the absolute time within a few tens of nanoseconds, orders of magnitude better than what is possible to achieve outside of the GNSS infrastructure. Adopting identical detectors ensures similar responses to cosmic events, reducing systematic uncertainties.

Since GRBs, and high-energy transients in general, are relatively bright (the flux from GRB170817 was about 2 ph/cm2/s in the 50–300 keV band (and this was a faint GRB) seen from a relatively large off-axis jet angle), even a small instrument can be efficiently used for their detection. In fact, the collecting area of the Fermi/GBM modules is about 120 cm<sup>2</sup> . Today, this class of instruments can be hosted by compact nano-satellites. Nanosatellites were developed at the end of the 20th century for didactical purposes, but today they are used for the most diverse applications: Earth observation, aircraft and ship tracking, telecommunication, and science. Nano-satellites have several major advantages with respect to traditional satellites. First and foremost, they can be developed on a relatively short timescale (a few years and, in extreme cases, a few months) compared with one to several decades usually required for standard space missions. Second, their cost is a few orders of magnitude smaller than standard large satellites. Both of these advantages imply that modularity can be exploited to its maximum. Modularity can allow us to (a) avoid single (or even multiple) point failures (if one or several units are lost the constellation and the experiment can still be operative); (b) fully test the hardware in orbit with the first launches and then improve it, if needed, with the following launches (iterative development, used in the context of gamma-ray flashes and GRBs early on by the VELA satellites. The second generation included gamma-ray detectors with much better timing capabilities, strongly improving the localization capabilities of the constellation); (c) to build the final mission step by step, gradually increasing its performance while diluting costs and risks.

An all-sky monitor, capable of monitoring the whole sky, or a large fraction of the sky, at all times, requires either a distributed infrastructure on LEO or a dedicated spacecraft far from the Earth. Given the advantages of miniaturized instrumentation hosted by CubeSats, it is relatively natural to propose building a sensitive all-sky monitor based on a constellation of nano-satellites.

In the previous section, in the context of multi-messenger astrophysics, we discussed the scientific relevance of even simple high-energy transient detections at the time of GWEs. Of course, their accurate localization would multiply the scientific return, prompting multi-wavelength (and even multi-messenger) follow-ups.

In the context of multi-wavelength astrophysics, it must be considered that, in a few years, the Vera Rubin Observatory (VRO) and the Cerenkov Array Telescope (CTA) will come online. VRO will have the ability to cover ~1/4 of the sky every night, finding millions of transients per night down to a magnitude r < 24.5 using real-time data analysis. On the one hand, optical counterparts of GRBs and GWEs will likely be serendipitously found in VRO images (covering each about 10 deg<sup>2</sup> ), providing arcsec positions and immediately prompting multiwavelength follow-up. On the other hand, the detection of X-ray and gamma-ray emission will promptly characterize the VRO transients (magnetars, soft gammaray repeaters, tidal distruption events, thermonuclear bursts from accreting NS, novae, AGN jets, etc., in addition to GRBs), thus better focusing the multiwavelength follow-up.

CTA will boost the study of the 20 GeV–300 TeV energy range. Only three GRBs have been detected at TeV energies so far: one by MAGIC [18] and two by HESS [19]. The CTA's fast re-positioning capabilities (20 s) and the improved sensitivity, due to the larger collecting area and lower energy threshold (~20 GeV) compared to MAGIC and HESS, will aid the study of GRB high-energy radiation routine, opening the possibility to accurately derive the jet Lorentz factor, assessing the role of synchrotron and inverse Compton radiation, and constraining the magnetic field strength and configuration. Given the limited field of view (FoV) of CTA at GeV energies (4.3◦ ), an instrument operating during the 20 s and providing the localization of GRBs with errors smaller than the CTA FoV is of paramount importance for triggering CTA follow-up observations.

#### *4.2. 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 γ-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 microsatellites 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 onsource, could revolutionize our understanding of these exciting events (e.g., Ultrasat, 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 Fernandez & Mezger 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−<sup>2</sup> 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. The equatorial ejecta are expected to be rich in heavier elements, known as lantanides, and produce long-lasting infrared emission. UV and blue emissions are produced early in the kilonova and last for only about a day. This may arise from free neutrons or from lantanide-poor polar ejecta with a higher electron fraction. While the properties of the tidal ejecta are sensitive to the mass ratio of the neutron stars, the properties of the polar and wind ejecta are sensitive to the neutron star radii and to the nature of the merger product. The different ratios between the observed kilonova fluxes obtained by near-UV, optical and near-infrared observations, will allow us to identify and constrain the properties of the different ejecta [13]. In particular, UV observations, performed in the first few hours of the kilonova (unavailable for GW170817), will probe the mass, composition, and thermal content of the fastest ejecta and allow us to constrain its geometry, quantity, and kinematics, as well as the nature of the merger product. Micro-satellites carrying relatively small (with a collecting area around 200 cm<sup>2</sup> ) near-IR and near-UV space telescopes will probe the emission from kilonovae out to the distance beyond 200 Mpc.

Observations in the near-IR should also allow us to detect afterglows at redshifts z > 5, corresponding to the first billion years of the Universe. Long GRBs are mostly observed at cosmological distances, with the most distant GRB detected at the redshift of z = 9.4, which corresponds to a look-back time of 13.3 billion years, only 500 million years after the Big Bang. Long GRBs are thus excellent probes for examining the early Universe, when the first massive stars and their host galaxies were being formed, the first heavier elements were produced, and the diffuse interstellar and intergalactic matter was re-ionised. To truly exploit long GRBs as probes of the early Universe, we need to identify more GRBs from the first billion years after the Big Bang. This can only be achieved by rapid near-infrared, follow-up observations from space, capable of imaging the afterglows of GRBs at the edge of the observable Universe. This was the main goal of the Theseus mission [22], which unfortunately was not selected by ESA for realization. CubSats again can provide a contribution here; see, for example, the SkyHopper proposal [23] https://skyhopper.research.unimelb.edu.au; accessed on 1 December 2021.

Next to the near-IR and near-UV telescopes, the constellation would also benefit from a microsatellite carrying a wide-field 10 cm2–20 cm<sup>2</sup> X-ray telescope observing in the 0.5–8 keV band. Since GRBs are bright in X-rays, a rapidly slewing X-ray telescope can aid the quick arcmin scale identification of the GRB position in the sky. The time and spectral evolution of the early X-ray emission can also provide valuable information about the possible two-step collapse model (through a short-lived massive neutron star) and the jet geometry of the source.
