We report on the detection of strongly varying intergalactic He ii absorption in HST /COS spectra of two z _ { \mathrm { em } } \simeq 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 \left < \tau _ { \mathrm { eff,HeII } } \right > \simeq 1 at z \simeq 2.3 to \left < \tau _ { \mathrm { eff,HeII } } \right > \gtrsim 5 at z \simeq 3.2 , but with a large scatter of 2 \lesssim \tau _ { \mathrm { eff,HeII } } \lesssim 5 at 2.7 < z < 3 on scales of \sim 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 co-eval 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 \gtrsim 2.7 , probably indicating He ii reionization was incomplete at z _ { \mathrm { reion } } \gtrsim 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 _ { \mathrm { reion } } \simeq 2.7 or even below , as suggested by a large \tau _ { \mathrm { eff,HeII } } \gtrsim 3 in two of our five sightlines at z < 2.8 .
By doubling the sample size at 2.7 \lesssim z \lesssim 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 .