2011ApJ...733L..24W

We report on the detection of strongly varying intergalactic He II absorption in HST/COS spectra of two z_em≃ 3 quasars. From our homogeneous analysis of the He II absorption in these and three archival sightlines, we find a marked increase in the mean He II effective optical depth from <τ_{eff,He ii}>≃1 at z ≃ 2.3 to <τ_{eff,He ii} >\gtrsim 5 at z ≃ 3.2, but with a large scatter of 2\lesssim τ_{eff,He ii}\lesssim 5 at 2.7 < z < 3 on scales of ∼10 proper Mpc. This scatter is primarily due to fluctuations in the He II fraction and the He II-ionizing background, rather than density variations that are probed by the coeval H I forest. Semianalytic models of He II absorption require a strong decrease in the He II-ionizing background to explain the strong increase of the absorption at z ≳ 2.7, probably indicating He II reionization was incomplete at z_reion ≳ 2.7. Likewise, recent three-dimensional numerical simulations of He II reionization qualitatively agree with the observed trend only if He II reionization completes at z_reion≃ 2.7 or even below, as suggested by a large τ_{eff,He  ii}\gtrsim 3 in two of our five sightlines at z < 2.8. By doubling the sample size at 2.7 ≲ z ≲ 3, our newly discovered He II sightlines for the first time probe the diversity of the second epoch of reionization when helium became fully ionized.