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Q0302-003 ($z=3.2860 \pm 0.0005$) was the first quasar discovered that showeda He II Gunn-Peterson trough, a sign of incomplete helium reionization at $z >2.9$. We present its HST/Cosmic Origins Spectrograph far-UV medium-resolutionspectrum, which resolves many spectral features for the first time, allowingstudy of the quasar itself, the intergalactic medium, and quasar proximityeffects. Q0302-003 has a harder intrinsic extreme-UV spectral index thanpreviously claimed, as determined from both a direct fit to the spectrum(yielding $\alpha_{\nu} = -0.8$) and the helium-to-hydrogen ion ratio in thequasar's line-of-sight proximity zone. Intergalactic absorption along thissightline shows that the helium Gunn-Peterson trough is largely black in therange $2.87 < z < 3.20$, apart from ionization due to local sources, indicatingthat helium reionization has not completed at these redshifts. However, wetentatively report a detection of nonzero flux in the high-redshift trough whenlooking at low-density regions, but zero flux in higher-density regions. Thisconstrains the He II fraction to be a few percent, suggesting heliumreionization has progressed substantially by $z \sim 3.1$. The Gunn-Petersontrough recovers to a He II Ly$\alpha$ forest at $z < 2.87$. We confirm atransmission feature due to the ionization zone around a $z = 3.05$ quasar justoff the sightline, and resolve the feature for the first time. We discover asimilar such feature possibly caused by a luminous $z = 3.23$ quasar furtherfrom the sightline, which suggests that this quasar has been luminous for >34Myr.

HST/COS Observations of the Quasar Q0302-003: Probing the He II Reionization Epoch and QSO Proximity Effects