HST/COS OBSERVATIONS OF THE QUASAR HE 2347-4342: PROBING THE EPOCH OF He II PATCHY REIONIZATION AT REDSHIFTS z=2.4-2.9

We report ultraviolet spectra of the high-redshift (z(em) approximate to 2.9) quasar, HE 2347-4342, taken by the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope. Spectra in the G130M (medium resolution, 1135-1440 angstrom) and G140L (low resolution, 1030-2000 angstrom) gratings exhibit patchy Gunn-Peterson absorption in the 303.78 angstrom Ly alpha line of He II between z = 2.39-2.87 (G140L) and z = 2.74-2.90 (G130M). With COS, we obtain better spectral resolution, higher signal-to-noise ratio (S/N), and better determined backgrounds than previous studies, with sensitivity to abundance fractions xHe II approximate to 0.01 in filaments of the cosmic web. The He II optical depths from COS are higher than those with the Far Ultraviolet Spectroscopic Explorer and range from tau He II <= 0.02 to tau He II >= 5, with a slow recovery in mean optical depth to <= 2 at z < 2.7. The He II/H I optical-depth ratio varies (eta approximate to 10-100 for 2.4 < z < 2.73 and eta = 5-500 for 2.75 < z < 2.89) on scales Delta z less than or similar to 0.01 (10.8 Mpc in comoving radial distance at z = 2.8), with numerous flux-transmission windows between 1135 and 1186 angstrom. The He II absorption extends to 1186.26 angstrom (z = 2.905), including associated absorbers with z(abs) approximate to z(QSO) and minimal proximity effect of flux transmission at the He II edge. We propose a QSO systemic redshift z(QSO) = 2.904 +/- 0.002, some Delta z = 0.019 higher than that derived from O I lambda 1302 emission. Three long troughs (4-10 angstrom or 25-60 Mpc comoving distance) of strong He II absorption between z = 2.75 and 2.90 are uncharacteristic of the intergalactic medium if He II reionized at z(r) approximate to 3. Contrary to recent indirect estimates (z(r) = 3.2 +/- 0.2) from H I optical depths, the epoch of He II reionization may extend to z less than or similar to 2.7.