2026ApJ...998..179E

The reionization of helium is thought to occur at 2.5 ≲ z ≲ 4, marking the last phase transition and final global heating event of the intergalactic medium (IGM). Since it is driven by rare quasars, helium reionization should give rise to strong temperature fluctuations in the IGM between neutral and recently ionized regions of order $\sigma (\mathrm{ln}T)\sim {\rm{\Delta }}T/T=20 \% -50 \% $. We introduce a novel method to search for reionization-induced temperature fluctuations in the IGM, by using the effective optical depths of the Lyα forest toward a large number of background quasars. Higher IGM temperatures give rise to lower effective optical depths in the Lyα forest, implying that temperature fluctuations will broaden the observed optical depth distribution. We measure the distributions of the effective Lyα forest optical depths across 71 X-Shooter spectra from the XQ-100 survey in four redshift bins, from z = 3.76 to z = 4.19, and compare them to a large-volume cosmological hydrodynamical simulation. A good agreement is found between the observations and the simulation, which does not include temperature fluctuations; therefore, we do not detect a signature of helium reionization. We then postprocess the simulations to include an increasing amount of temperature fluctuations, until the model becomes inconsistent with the observations. We obtain tight constraints on $\sigma (\mathrm{ln}T)\lt 0.29\,(\lt 0.40)$ at 2σ (3σ) at z = 3.76 when averaging over a scale of 100 comoving Mpc, with weaker constraints for higher redshifts and smaller scales. Our constraints are the tightest to date, implying that either the IGM temperature contrast caused by helium reionization is less than ∼30% or the process has not yet significantly started at z = 3.76.