  We report on the detection of Ne VIII in the HST/Cosmic Origins Spectrograph
spectrum of the intervening absorption system at z = 0.495096 towards PKS
0405-123. The high S/N COS spectrum also covers absorption from H I, C III, O
III, O IV and O VI associated with this multiphase system. The Ne VIII is
detected with high significance in both lines of the doublet, with integrated
column densities of log Na(Ne VIII 770) = 13.96 +/- 0.06 dex and log Na(Ne VIII
780) = 14.08 +/- 0.07 dex. We find the origin of Ne VIII consistent with
collisionally ionized gas at T ~ 5 x 10^5 K with a large baryonic column
density of N(H) ~ 10^{19} - 10^{20} cm^-2. The metallicity in the Ne VIII gas
phase is estimated to be [Ne/H] ~ -0.6 +/- 0.3 dex. The intermediate ions such
as C III, O III, O IV and H I are consistent with photoionization in lower
ionization gas at T ~ 10,000 K. The O V and O VI in this absorber can have
contributions from both the photoionized and collisionally ionized gas phases.
The absorber is at 180 km/s systematic velocity and 110 kpc projected
separation from a M_R = -19.6 galaxy of extended morphology. The collisionally
ionized gas at T ~ 5 x 10^5 K points to an origin in multiphase gas embedded in
the hot halo of the galaxy, or in a nearby WHIM structure. The high sensitivity
UV spectroscopy afforded by COS has opened up new opportunities for discovering
large reservoirs of "missing baryons" in the low-z universe through the
detection of Ne VIII systems.
