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arXiv · 2112.09711

Constraining the properties of dense neutron star cores: The case of the low-mass X-ray binary HETE J1900.1-2455

N. Degenaar·D. Page·J. van den Eijnden·M. V. Beznogov·R. Wijnands·M. Reynolds
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Abstract

Measuring the time evolution of the effective surface temperature of neutron stars can provide invaluable information on the properties of their dense cores. Here, we report on a new Chandra observation of the transient neutron star low-mass X-ray binary HETE J1900.1-2455, which was obtained ~2.5 yr after the end of its ~10-yr long accretion outburst. The source is barely detected during the observation, collecting only six net photons, all below 2 keV. Assuming that the spectrum is shaped as a neutron star atmosphere model we perform a statistical analysis to determine a 1-sigma confidence upper range for the neutron star temperature of ~30-39 eV (for an observer at infinity), depending on its mass, radius and distance. Given the heat injected into the neutron star during the accretion outburst, estimated from data provided by all-sky monitors, the inferred very low temperature suggests that either the core has a very high heat capacity or undergoes very rapid neutrino cooling. While the present data do not allow us to disentangle these two possibilities, both suggest that a significant fraction of the dense core is not superfluid/superconductor. Our modeling of the thermal evolution of the neutron star predicts that it may still cool further, down to a temperature of ~15 eV. Measuring such a low temperature with a future observation may provide constraints on the fraction of baryons that is paired in the stellar core.

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N. Degenaar, D. Page, J. van den Eijnden, M. V. Beznogov, R. Wijnands, M. Reynolds. 2021-12-17. Constraining the properties of dense neutron star cores: The case of the low-mass X-ray binary HETE J1900.1-2455. https://doi.org/10.1093/mnras%2Fstab2202

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