We have run a new suite of simulations that solve hydrodynamics and radiative transfer simultaneously to study helium ii reionization .
Our suite of simulations employs various models for populating quasars inside of dark matter halos , which affect the He ii reionization history .
In particular , we are able to explore the impact that differences in the timing and duration of reionization have on observables .
We examine the thermal signature that reionization leaves on the IGM , and measure the temperature-density relation .
As previous studies have shown , we confirm that the photoheating feedback from helium ii reionization raises the temperature of the intergalactic medium ( IGM ) by several thousand kelvin .
To compare against observations , we generate synthetic Ly \alpha forest sightlines on-the-fly and match the observed effective optical depth \tau _ { \mathrm { eff } } ( z ) of hydrogen to recent observations .
We show that when the simulations have been normalized to have the same values of \tau _ { \mathrm { eff } } , the effect that helium ii reionization has on observations of the hydrogen Ly \alpha forest is minimal .
Specifically , the flux PDF and the one-dimensional power spectrum are sensitive to the thermal state of the IGM , but do not show direct evidence for the ionization state of helium .
We show that the peak temperature of the IGM typically corresponds to the time of 90-95 % helium ionization by volume , and is a relatively robust indicator of the timing of reionization .
Future observations of helium reionization from the hydrogen Ly \alpha forest should thus focus on measuring the temperature of the IGM , especially at mean density .
Detecting the peak in the IGM temperature would provide valuable information about the timing of the end of helium ii reionization .