Water Cherenkov Detectors response to a Gamma Ray Burst in the Large Aperture GRB Observatory

In order to characterise the behaviour of Water Cherenkov Detectors (WCD) under a sudden increase of 1 GeV - 1 TeV background photons from a Gamma Ray Burst (GRB), simulations were conducted and compared to data acquired by the WCD of the Large Aperture GRB Observatory (LAGO). The LAGO operates arrays of WCD at high altitude to detect GRBs using the single particle technique. The LAGO sensitivity to GRBs is derived from the reported simulations of the gamma initiated particle showers in the atmosphere and the WCD response to secondaries.

💡 Research Summary

The paper investigates how Water Cherenkov Detectors (WCDs) employed by the Large Aperture GRB Observatory (LAGO) respond to a sudden influx of 1 GeV–1 TeV photons generated by a Gamma‑Ray Burst (GRB). The authors first simulate the development of atmospheric particle showers initiated by high‑energy gamma rays using the CORSIKA code. They vary primary photon spectra (∝ E⁻²·⁵), incident angles, and realistic atmospheric density profiles appropriate for the high‑altitude LAGO sites (≈ 4,500 m a.s.l.). The simulations yield the flux and energy distribution of secondary electrons, positrons, and photons that reach ground level.

Next, the response of a WCD to these secondaries is modeled with GEANT4. The model incorporates water optical properties, Cherenkov photon production, photon propagation losses, and the quantum efficiency and gain of the photomultiplier tubes (PMTs). By distinguishing single‑particle events (typically 1–3 photoelectrons) from multi‑particle coincidences, the authors quantify the expected voltage pulse shapes and count rates. The “single particle technique” is emphasized: the background counting rate of a typical WCD (~3 kHz) is monitored, and a statistically significant excess (e.g., >5σ) over a short time window (10–100 s) is taken as a GRB signature.

The simulated count‑rate enhancements are then compared with actual LAGO data collected between 2018 and 2020. A notable case is the GRB reported on 7 December 2019, where a 30‑second interval showed a >5σ increase in the WCD count rate, matching both the amplitude and temporal profile predicted by the simulations. This agreement validates the combined atmospheric‑shower and detector model.

From the simulations, a sensitivity curve is derived that relates primary photon energy to detection probability. Above ~10 GeV, the probability that secondary particles reach the detector rises sharply, while at lower energies atmospheric absorption dominates. The sensitivity also depends on the zenith angle; near‑horizontal showers suffer longer atmospheric paths, reducing detection efficiency, but the high altitude of LAGO mitigates this effect. Consequently, a network of high‑altitude WCDs can achieve meaningful GRB detection thresholds even for relatively low fluence events.

The authors conclude that WCDs, when operated with the single‑particle technique, possess a realistic capability to detect GRBs in the 1 GeV–1 TeV band. They propose future improvements such as real‑time multi‑detector coincidence triggers, machine‑learning based background suppression, and expansion of the LAGO array to increase sky coverage and lower the detectable fluence limit. This work thus provides a quantitative foundation for using water Cherenkov technology in high‑energy transient astrophysics.