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arXiv · 1408.3276

A strongly heated neutron star in the transient Z source MAXI J0556-332

Jeroen Homan·Joel K. Fridriksson·Rudy Wijnands·Edward M. Cackett·Nathalie Degenaar·Manuel Linares·Dacheng Lin·Ronald A. Remillard
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Abstract

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.

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Jeroen Homan, Joel K. Fridriksson, Rudy Wijnands, Edward M. Cackett, Nathalie Degenaar, Manuel Linares, Dacheng Lin, Ronald A. Remillard. 2014-08-14. A strongly heated neutron star in the transient Z source MAXI J0556-332. https://doi.org/10.1088/0004-637x/795/2/131

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