4.3.3. GALI

A new concept for identifying the direction of GRBs was suggested recently by Rahin et al. [34]. The concept was named GALI (GAmma-ray burst Localizing Instrument). Its basic idea is to use numerous small scintillators (e.g., 1 <sup>×</sup> <sup>1</sup> <sup>×</sup> 1 cm<sup>3</sup> cubes) in a 3D array utilizing their mutual shielding. Consequently, the relative γ-photon count of each scintillator varies strongly with the direction of the burst. In a sense, GALI can be thought of as a coded-mask detector, but where the mask itself has detecting elements. Moreover, the detector (and mask) have no preferred direction, and thus provide full-sky coverage, as opposed to coded-mask instruments. A configuration such as CAMELOT, benefits from the SiPMs, which occupy little volume; hence, they enable the compact packing of the scintillators. As with GRBAlpha/CAMELOT, the SiPMs are radiation-sensitive, and need to be protected. A GALI laboratory prototype was successfully tested, and a flight model to be launched to the International Space Station (ISS) is being built. The laboratory prototype is shown in Figure 4. *Galaxies* **2021**, *9*, x FOR PEER REVIEW 11 of 13

**Figure 4.** Laboratory model of the GALI detector system with 90 scintillators crystals. The individual scintillators in their reflective wrappings can be seen, arranged in a random order. A larger version is currently being built for the ISS. **Figure 4.** Laboratory model of the GALI detector system with 90 scintillators crystals. The individual scintillators in their reflective wrappings can be seen, arranged in a random order. A larger version is currently being built for the ISS.

The GALI concept can be scaled to any size, and thus can fit many platforms. Clearly, larger versions will be more sensitive, and, more importantly, they will provide a better angular resolution. An advantage of the GALI configuration is the reduced sky background on the inner scintllators, which will light up only for GRBs in specific directions. This allows for a high signal-to-background ratio on these scintillators, and the exploitation of soft γ-rays below 50 keV for directionality. The aforementioned flight model will consist of 350 scintillators, occupying a total volume of merely ~1 L. Simulations show that even such a small detector can identify the direction of a burst down to approximately ±2**°** for 1 s GRBs with a 10 keV–1 MeV flux of 10 ph cm−2 s −1, ±5**°**for 5 ph cm−2 s −1, and ±10**°**for 2.5 ph cm−2 s −1 [34]. Although GALI can operate onboard a single satellite, it can also be incorporated into a distributed satellite architecture to enhance the sky coverage and directional capabilities of the entire constellation. The GALI concept can be scaled to any size, and thus can fit many platforms. Clearly, larger versions will be more sensitive, and, more importantly, they will provide a better angular resolution. An advantage of the GALI configuration is the reduced sky background on the inner scintllators, which will light up only for GRBs in specific directions. This allows for a high signal-to-background ratio on these scintillators, and the exploitation of soft γ-rays below 50 keV for directionality. The aforementioned flight model will consist of 350 scintillators, occupying a total volume of merely ~1 L. Simulations show that even such a small detector can identify the direction of a burst down to approximately ±2 ◦ for 1 s GRBs with a 10 keV–1 MeV flux of 10 ph cm−<sup>2</sup> s −1 , ±5 ◦ for 5 ph cm−<sup>2</sup> s −1 , and ±10◦ for 2.5 ph cm−<sup>2</sup> s −1 [34]. Although GALI can operate onboard a single satellite, it can also be incorporated into a distributed satellite architecture to enhance the sky coverage and directional capabilities of the entire constellation.

#### 4.3.4. Other Projects 4.3.4. Other Projects

by the second unit [38].

grant from the Israel Space Agency.

BurstCube is a 6U CubeSat developed by NASA, which will detect GRBs using four CsI scintillators, each with an effective area ~90 cm<sup>2</sup> [35]. BurstCube is expected to be launched in 2022. BurstCube is a 6U CubeSat developed by NASA, which will detect GRBs using four CsI scintillators, each with an effective area ~90 cm<sup>2</sup> [35]. BurstCube is expected to be launched in 2022.

The Educational Irish Research Satellite 1 (EIRSAT-1), supported by ESA's Fly Your Satellite program, will carry a gamma-ray module (GMOD) to detect gamma-ray bursts

Nanosatellite constellations include the Chinese Gamma-Ray Integrated Detectors (GRID), which will consist of GRB detectors (as secondary payloads) on 10–24 CubeSats [37]. Two GRID units have been launched so far and one GRB has been recently detected

**Author Contributions:** conceptualization, F.F., N.W., E.B., writing, F.F., N.W., E.B. All authors have

**Funding:** This research was funded by European Union Horizon 2020 Research and Innovation Framework Programme under grant agreements HERMES-SP n. 821896 and AHEAD2020 n. 871158, by ASI INAF Accordo Attuativo n. 2018-10-HH.1.2020 HERMES—Technologic Pathfinder Attivita' scientifiche, by a Center of Excellence of the Israel Science Foundation, grant No. 2752/19, and by a

read and agreed to the published version of the manuscript.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

The Educational Irish Research Satellite 1 (EIRSAT-1), supported by ESA's Fly Your Satellite program, will carry a gamma-ray module (GMOD) to detect gamma-ray bursts [36]. GMOD uses SensL B-series SiPM detectors and a CeBr scintillator. EIRSAT-1 will be launched from the ISS in 2022.

Nanosatellite constellations include the Chinese Gamma-Ray Integrated Detectors (GRID), which will consist of GRB detectors (as secondary payloads) on 10–24 CubeSats [37]. Two GRID units have been launched so far and one GRB has been recently detected by the second unit [38].

**Author Contributions:** Conceptualization, F.F., N.W. and E.B., writing, F.F., N.W. and E.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by European Union Horizon 2020 Research and Innovation Framework Programme under grant agreements HERMES-SP n. 821896 and AHEAD2020 n. 871158, by ASI INAF Accordo Attuativo n. 2018-10-HH.1.2020 HERMES—Technologic Pathfinder Attivita' scientifiche, by a Center of Excellence of the Israel Science Foundation, grant No. 2752/19, and by a grant from the Israel Space Agency.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This article benefited from the invaluable input of the HERMES pathfinder, CAMELOT and GALI teams. In particular we would like to thank J. Ripa, A. Sanna, L. Burderi, M. Lavagna, P. Bellutti, Y. Evangelista, M. Trenti for inspiring discussions.

**Conflicts of Interest:** The authors declare no conflict of interest.
