  We present Chandra, XMM-Newton, and Swift observations of the quiescent
neutron star in the transient low-mass X-ray binary MAXI J0556-332.
Observations of the source made during outburst (with the Rossi X-ray Timing
Explorer) reveal tracks in its X-ray color--color and hardness--intensity
diagrams that closely resemble those of the neutron-star Z sources, suggesting
that MAXI J0556-332 had near- or super-Eddington luminosities for a large part
of its ~16 month outburst. A comparison of these diagrams with those of other Z
sources suggests a source distance of 46+/-15 kpc. Fits to the quiescent
spectra of MAXI J0556-332 with a neutron-star atmosphere model (with or without
a power-law component) result in distance estimates of 45+/-3 kpc, for a
neutron-star radius of 10 km and a mass of 1.4 Msun. The spectra show the
effective surface temperature of the neutron star decreasing monotonically over
the first ~500 days of quiescence, except for two observations that were likely
affected by enhanced low-level accretion. The temperatures we obtain for the
fits that include a power-law (kT_eff=184-308 eV) are much higher than those
seen for any other neutron star heated by accretion, while the inferred cooling
(e-folding) timescale (~200 days) is similar to other sources. Fits without a
power-law yield higher temperatures (kT_eff=190-336 eV) and a shorter e-folding
time (~160 days). Our results suggest that the heating of the neutron-star
crust in MAXI J0556-332 was considerably more efficient than for other systems,
possibly indicating additional or more efficient shallow heat sources in its
crust.
