Detecting the signature of helium reionization through

Context. Helium reionization is the most recent phase change of the intergalactic medium, yet its timing and main drivers remain uncertain. Among the probes to trace to trace how it unfolds, the 3.46 cm hyperfine line of singly ionized helium has opened the study of helium reionization to upcoming radio surveys. Aims. We aim to evaluate the detectability of the 3.46 cm signal with radio surveys and the possible constraints it can place on helium reionization. In particular, we seek to determine whether it can distinguish between early and late helium reionization scenarios. Moreover, we performed a comprehensive study of the advantages of a single-dish versus an interferometric setup. Methods. Using hydrodynamic simulations post-processed with radiative transfer, we constructed mock data cubes for two models of helium reionization. We computed the power spectrum of the signal and forecasted the signal-to-noise ratio for SKA-1 MID, DSA-2000, and a PUMA-like survey using each observational setup. Results. The two scenarios produce distinct power spectra, but the faintness of the signal, largely caused by weak coupling between the spin temperature and the kinetic temperature in low-density regions of the IGM, combined with high instrumental noise, makes detection very difficult within realistic integration times for current surveys. A PUMA-like survey operating in single-dish mode could, however, detect the 3.46 cm signal with an integrated signal-to-noise ratio of a few in ≲1000 h in both scenarios. Conclusions. Distinguishing helium reionization scenarios with 3.46 cm line-intensity mapping therefore remains challenging for current facilities. Our results, however, indicate that next-generation high-sensitivity surveys with optimized observing strategies, especially when combined with complementary probes of the intergalactic medium, could begin to place meaningful constraints on the timing and morphology of helium reionization.