The Thermal Proximity Effect: A New Probe of the He II Reionization History and Quasar Lifetime

Despite decades of effort, the timing and duration of He II reionization and the properties of the quasars believed to drive it are still not well constrained. We present a new method to study both via the thermal proximity effect-the heating of the intergalactic medium (IGM) around quasars when their radiation doubly ionizes helium. We post-process hydrodynamical simulations with 1D radiative transfer and study how the thermal proximity effect depends on the He II fraction, chi(He) (II,0), which prevailed in the IGM before the quasar turned on, and the quasar lifetime t(Q). We find that the amplitude of the temperature boost in the quasar environment depends on chi(He) (II,0), with a characteristic value of Delta T similar or equal to 10(4) K for chi(He) (II,0) = 1.0, whereas the size of the thermal proximity zone is sensitive to t(Q), with typical sizes of similar or equal to 100 cMpc for t(Q) = 10(8) yr. This temperature boost increases the thermal broadening of H I absorption lines near the quasar. We introduce a new Bayesian statistical method based on measuring the Ly alpha forest power spectrum as a function of distance from the quasar, and demonstrate that the thermal proximity effect should be easily detectable. For a mock data set of 50 quasars at z similar or equal to 4, we predict that one can measure chi(He) (II,0) to an (absolute) precision approximate to 0.04 and t(Q) to a precision of approximate to 0.1 dex. By applying our formalism to existing highresolution Lya forest spectra, one should be able to reconstruct the He II reionization history, providing a global census of hard photons in the high-z universe.