16317

The advent of GALEX and COS have revolutionized our view of HeII reionization comma the final major phase transition of the intergalactic medium. Efficient COS surveys have provided a statistical sample of strongly fluctuating HeII Lyman alpha absorption at 2.7<z<3 comma evoking a picture of overlapping HeIII zones around quasars at the end of HeII reionization. Unexpectedly comma the seven rare quasars probing z>3.5 have revealed a set of HeII transmission .spikes. and larger regions with nonzero transmission that suggest HeII reionization was well underway by z=4. This is in striking conflict with radiative transfer simulations of a HeII reionization driven by bright quasars. Explaining these measurements may require either faint quasars or more exotic sources of hard photons at z>4 comma with concomitant implications for HI reionization. A recent survey for high-redshift quasars in the southern hemisphere has uncovered the highest-redshift far-UV-bright quasar known (z=3.95 comma FUV=21.7). We propose here for COS G140L follow-up spectroscopy to (i) confirm its utility for HeII absorption studies by identifying possible interloping low-z HI absorbers comma (ii) provide accurate measurements of the HeII Lyman alpha effective optical depth that are limited by systematics for much fainter already observed quasars comma and (iii) constrain the lifetime of the background quasar from its ionized proximity zone. Moreover comma the proposed observations would provide only the third z>3.5 sightline that would allow for high-resolution G130M spectroscopy before the end of HST.s mission.