A STRONGLY HEATED NEUTRON STAR IN THE TRANSIENT Z SOURCE MAXI J0556-332

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 similar to 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 1.4M(circle dot). The spectra show the effective surface temperature of the neutron star decreasing monotonically over the first similar to 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)(infinity) = 184-308 eV) are much higher than those seen for any other neutron star heated by accretion, while the inferred cooling (e-folding) timescale (similar to 200 days) is similar to other sources. Fits without a power law yield higher temperatures (kT 8(eff)(infinity) = 190-336 eV) and a shorter e-folding time (similar to 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.