  The mass of the Galactic dark matter halo is under vivid discussion. A recent
study by Xue et al. (2008, ApJ, 684, 1143) revised the Galactic halo mass
downward by a factor of ~2 relative to previous work, based on the
line-of-sight velocity distribution of ~2400 blue horizontal-branch (BHB) halo
stars. The observations were interpreted in a statistical approach using
cosmological galaxy formation simulations, as only four of the 6D phase-space
coordinates were determined. Here we concentrate on a close investigation of
the stars with highest negative radial velocity from that sample. For one star,
SDSSJ153935.67+023909.8 (J1539+0239 for short), we succeed in measuring a
significant proper motion, i.e. full phase-space information is obtained. We
confirm the star to be a Population II BHB star from an independent
quantitative analysis of the SDSS spectrum - providing the first NLTE study of
any halo BHB star - and reconstruct its 3D trajectory in the Galactic
potential. J1539+0239 turns out as the fastest halo star known to date, with a
Galactic rest-frame velocity of 694$^{+300}_{-221}$ km/s (full uncertainty
range from Monte Carlo error propagation) at its current position. The extreme
kinematics of the star allows a significant lower limit to be put on the halo
mass in order to keep it bound, of M_halo$\ge1.7_{-1.1}^{+2.3}\times10^{12}$
Msun. We conclude that the Xue et al. results tend to underestimate the true
halo mass as their most likely mass value is consistent with our analysis only
at a level of 4%. However, our result confirms other studies that make use of
the full phase-space information.
