Gamma ray astronomy with Antares

It has been suggested that underwater neutrino telescopes could detect muons from gamma ray showers. Antares’ ability to detect high energy muons produced by TeV photons is discussed in the light of a full Monte Carlo study. It is shown that currently known sources would be hardly detectable

💡 Research Summary

The paper investigates whether the ANTARES underwater neutrino telescope, originally designed to detect muons produced by high‑energy astrophysical neutrinos, could also be used to observe muons generated in extensive air showers initiated by TeV gamma‑rays. The authors perform a full Monte‑Carlo chain: primary gamma‑rays with energies from 1 TeV to 100 TeV are injected into the atmosphere using CORSIKA, where they interact with nuclei and produce secondary particles, including muons. The secondary muons are then propagated through the sea water with the MUSIC code, accounting for continuous energy loss, stochastic processes, and multiple scattering. Finally, a GEANT‑4 based detector simulation models the Cherenkov light production, propagation, and the response of the ANTARES optical modules (photomultiplier tubes, electronics, and trigger logic).

Key performance metrics derived from the simulation are the effective area for gamma‑induced muons, the angular resolution, and the energy threshold required for a muon to generate a detectable signal. The study finds that a 1 TeV gamma‑ray typically produces only ~10⁻⁶ muons in the atmosphere, and fewer than 10⁻⁴ of those retain enough energy (>300 GeV) after traversing the water to be seen by ANTARES. Consequently, the probability that a single TeV photon results in a detectable muon is of order 10⁻¹⁰. Even for the brightest known TeV sources (e.g., Crab, Markarian 421, Markarian 501), whose photon fluxes are ∼10⁻¹¹ cm⁻² s⁻¹ TeV⁻¹, the expected number of gamma‑induced muon events per year is well below 0.01.

In contrast, the background from atmospheric muons amounts to several thousand events per year, and atmospheric neutrinos contribute a few hundred events. The resulting signal‑to‑background ratio is therefore ≤10⁻⁶, far too low for a statistically significant detection within realistic observation times. Moreover, the directional information of gamma‑induced muons is degraded by multiple scattering, making them indistinguishable from the overwhelming atmospheric muon flux with the current reconstruction algorithms.

The authors discuss possible avenues to improve sensitivity: lowering the muon energy threshold, increasing the density of optical modules, employing higher‑quantum‑efficiency photomultipliers, or implementing a “burst mode” that temporarily relaxes trigger conditions during transient gamma‑ray flares. Each of these strategies, however, faces practical limitations. Reducing the threshold would dramatically increase noise and background rates; adding more modules or upgrading hardware is constrained by the existing infrastructure and cost; and burst‑mode operation would require precise external alerts and still yields only modest gains.

The conclusion is that, with its present design and operating parameters, ANTARES is not capable of detecting muons from known TeV gamma‑ray sources. Gamma‑induced muon detection remains a challenging secondary science case that may become feasible only with next‑generation, larger‑scale water‑Cherenkov detectors such as KM3NeT or IceCube‑Gen2, which will feature larger instrumented volumes, denser optical module layouts, and more sophisticated background‑rejection techniques. The paper recommends further work on refined gamma‑ray flux models, advanced background suppression algorithms, and coordinated multi‑messenger observations to explore this niche capability in future experiments.