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Title: 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 ∼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 M {sub ☉}. 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{sub eff}{sup ∞} = 184-308 eV) are much higher than those seen for any other neutron star heated by accretion, while the inferred coolingmore » (e-folding) timescale (∼200 days) is similar to other sources. Fits without a power law yield higher temperatures (kT{sub eff}{sup ∞} = 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.« less
Authors:
;  [1] ; ;  [2] ;  [3] ;  [4] ;  [5] ;  [6]
  1. MIT Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Avenue 37-582D, Cambridge, MA 02139 (United States)
  2. Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam (Netherlands)
  3. Department of Physics and Astronomy, Wayne State University, 666 W. Hancock St., Detroit, MI 48201 (United States)
  4. Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109 (United States)
  5. Instituto de Astrofísica de Canarias, c/ Vía Láctea s/n, E-38205 La Laguna, Tenerife (Spain)
  6. Space Science Center, University of New Hampshire, Durham, NH 03824 (United States)
Publication Date:
OSTI Identifier:
22370239
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 795; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCRETION DISKS; ATMOSPHERES; BINARY STARS; COLOR; COMPARATIVE EVALUATIONS; DISTANCE; HEAT SOURCES; HEATING; LUMINOSITY; MASS; NEUTRON STARS; SPECTRA; SURFACES; TRANSIENTS; X RADIATION