**4. Monitoring GRBs with SWGO**

Both theoretical and observational arguments suggest that VHE emission is an important property of GRBs. This is a well verified feature in the afterglow of some bright events. Evidence for the existence of energetic photons in the prompt phase is much harder to obtain, although there are some *Fermi*-LAT detected bursts that hint in this direction. Solving questions such as the occurrence rate of energetic spectral components in GRB afterglows, the existence of fast VHE pulses associated with the prompt stage, and assess the delay in the onset of VHE components is a problem that requires an extensive monitoring campaign, possibly covering a large sample of GRBs with an instrument characterized by a large collecting area. At present, several experiments are available or are being constructed to provide VHE spectral coverage with different sensitivities and resolution.

The SWGO collaboration is currently investigating the design of a new WCD arraybased EAS observatory, to be constructed in the Southern Hemisphere [47]. The target performance domain is summarized in Figure 4, together with the sensitivities achieved by other instruments and with a comparison of the expected limiting fluxes with the ones emitted by GRBs that were detected in the HE and VHE domains. The role of such a new instrument will be to provide constant scanning of a wide FoV in the Southern sky, thus complementing the FoV covered by Northern facilities such as HAWC and LHAASO and providing a triggering and alert system for CTA. Adopting a compact array concept with a collecting area of 80,000 m<sup>2</sup> , located in a high-altitude site (>4400 m a.s.l.), this type of instrument has the possibility to probe the flux range that we expect to be characteristic of GRB emission, with a transient localization accuracy *α*<sup>68</sup> 6 1 <sup>o</sup> below 1 TeV, although the possibility to detect different types of events depends critically on the overall instrument performance and on its ability to probe the lower part of the spectral range, where the effects of EBL opacity are less severe and a larger volume of the Universe is potentially accessible.

**Figure 4.** (**Left panel**) Differential sensitivity to the flux of a point-like source located at zenith distance *ϑ* = 20<sup>o</sup> for SWGO, HAWC and LHAASO (computed for 1 year of data taking) and for MAGIC, H.E.S.S. and CTA (computed for 50 h of exposure time). Different fractions of the Crab Nebula flux are also shown for comparison. (**Right panel**) Expected detection times for a GRB with the temporal and spectral characteristics of GRB 130427A, located at redshift *z* = 0.34, for different fractions of the optimal SWGO performance, taken integrating the spectrum above thresholds of *E* = 125 GeV, 250 GeV and 500 GeV. For each case, the blue crosses mark the time required to accumulate an integrated flux above the corresponding detection threshold. The black points with error bars denote the temporal evolution of the VHE flux detected by MAGIC for *E* > 300 GeV from GRB 190114C [1].

To test the potential role of SWGO as a monitoring and alert system, we took the sample of GRBs with simulated redshifts, discussed in §3, and we calculated the expected VHE fluxes, integrating Equation (4), with the inclusion of Equation (5), in time and in energy, using the spectral and the temporal characteristics extracted from 2FLGC and applying the *γγ* absorption effects predicted by an EBL opacity model [39]. The results of these calculations are summarized in Figure 5 for different possible performances of the experiment. The plots shown in Figure 5 represent the number of simulated redshift distributions that result in the GRB detections reported on the *x*-axes out of a total of 1000 simulations. Although we observe that all the explored configurations have some degree of detection chances, we can easily verify that an instrument performing at the optimal sensitivity, down to a low energy threshold of *Elow* = 125 GeV, has a predicted ability to detect significantly more than 10 GRBs in 10 years in approximately 75% of the simulated scenarios, provided that they occur within a zenith distance of *ϑ* 6 20<sup>o</sup> . Adopting lower performance solutions, or using a spectral window with a higher limiting threshold, such as 250 GeV or 500 GeV, leads to lower detection chances, due to the loss of many relatively low-flux events and to a more severe effect of EBL opacity, which limits the visible horizon to a smaller volume of the Universe. Incidentally, these calculations approximately correspond to what we would expect at larger zenith distances and, therefore, within increasing fractions of the available FoV, from an optimally performing solution. As a result, even if the FoV covered by a ground-based instrument below 1 TeV is smaller than the one that was available in the case of the *Fermi*-LAT observations, by taking into account the estimated distribution of expected GRB fluxes, we can conclude that the possibility to detect more than 1 event per year is a reasonable expectation. Due to the characteristic shapes of the tested light-curves, which are generally dominated by the peak flux, a fraction of approximately (78 ± 16)% of these detections is estimated to occur within the first 10 s of the event, resulting in promisingly good chances to investigate the elusive properties of the prompt emission.

**Figure 5.** From upper left to lower right: histograms of the number of expected detections of GRBs extracted from 2FLGC and associated with 1000 random redshift values, computed for flux integrations above a low energy threshold of *Elow* = 125 GeV (red histograms), 250 GeV (blue histograms) and 500 GeV (green histograms), with an overall performance reaching up to 100%, 75%, 50% and 25% of the optimal SWGO sensitivity and zenith distance up to *ϑ* = 20<sup>o</sup> . The optimal SWGO design is expected to be able to detect more than 10 GRBs in an observing period of 10 years in more than 750 simulations out of 1000. Reductions of performance or higher energy thresholds reflect in gradually lower expected detection chances.

#### **5. Conclusions**

Investigating the VHE properties of GRB will have fundamental implications in our understanding of these extremely powerful events. The existence of VHE radiation components, particularly if associated with the prompt stage, represents a fundamental piece of information to model the physics of the radiating environment, thanks to the strong implications that VHE spectral and temporal properties have on the radiating species. The degree of correlation, or the occurrence of delays of the energetic components with respect to lower energy emission, depend on the particle energy distribution and on the interactions within the emitting regions, or between these regions and the environment. Testing the distribution of these properties among long and short GRBs will reduce the ambiguity implied by the partial overlap of these classes and further characterize the jets produced in the two cases. In addition, the possibility to verify whether the early VHE emission is dominated by smooth temporal evolution or irregular variability will improve our understanding of the transition between the prompt stage and the afterglow emission, providing invaluable information on the close GRB environment.

In our investigation, we used the HE data from GRBs detected during the first 10 years of *Fermi*-LAT observations, to infer the expected properties of GRBs in the VHE domain. Using the spectral and timing information of the LAT detected events, to estimate the expected fluxes, with the aid of ancillary simulations to derive fiducial redshift distributions

for most of the sources with unknown *z*, we extrapolated the predicted spectra up to the TeV scale, taking into account the effects of EBL opacity. We then calculated the detection prospects of the resulting GRB distributions for different performances of the experiment, based on estimates of the SWGO potential. We found that a new monitoring facility, with the characteristics investigated by the SWGO collaboration, could effectively monitor VHE emission from GRBs, providing localization information at the level of a few square degrees and reaching timescales of less than 10 s for the brightest events. This is a time domain that is very hard to explore with IACT facilities, despite being critical to distinguish whether the high-energy component is a spectral extension, characteristic of powerful bursts, or the result of an additional process, dominated by some external contribution. With the ability to trigger on VHE transient signals, the localization regions of Multi-Messenger source candidates will further improve and the efficiency in the execution of follow-up campaigns will subsequently increase.

**Author Contributions:** Conceptualization, U.B.d.A. and F.L.; methodology, G.L.M.; software, G.L.M., R.C. and B.T.; validation, F.L.; investigation, M.P. and A.d.A.; writing—original draft preparation, G.L.M.; funding acquisition, M.P. and U.B.d.A.; modeling, D.M. and G.L.M.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Fundação para a Ciência e Tecnologia, under project PTDC/FIS-PAR/4300/2020 and grant DL57/2016/cP1330/cT0002, by CNPq Productivity Research, Grant no. 311997/2019-8, by Serrapilheira Institute Grant number Serra—1812-26906, and by FAPERJ Young Scientist Fellowship no. E-26/202.818/2019.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** There are no new data associated with this article. The data used to estimate the GRB HE properties are provided by the *Fermi*-LAT collaboration and publicly available online at the HEASARC website https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermilgrb.html (accessed on 20 September 2021).

**Acknowledgments:** The authors gratefully thank Edna Ruiz-Velasco and Brian Reville for discussion and comments leading to the improvement of the manuscript. The SWGO Collaboration acknowledges the support from the agencies and organizations listed here: https://www.swgo.org/ SWGOWiki/doku.php?id=acknowledgements (accessed on 20 September 2021). Part of this work is based on public data provided by the *Fermi*-LAT Collaboration. The *Fermi*-LAT Collaboration acknowledges generous ongoing support from several agencies and institutes that have supported both the development and the operation of the LAT as well as scientific data analysis. These include the National Aeronautics and Space Administration and the Department of Energy in the United States, the Commissariat à l'Energie Atomique and the Centre National de la Recherche Scientifique/Institut National de Physique Nucléaire et de Physique des Particules in France, the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica Nucleare in Italy, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), High-Energy Accelerator Research Organization (KEK) and Japan Aerospace Exploration Agency (JAXA) in Japan, and the K. A. Wallenberg Foundation, the Swedish Research Council and the Swedish National Space Agency in Sweden. Additional support for science analysis during the operations phase is gratefully acknowledged from the Istituto Nazionale di Astrofisica in Italy and the Centre National d'Études Spatiales in France. This work performed in part under DOE Contract DEAC02-76SF00515.

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