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Title: Late-time Cooling of Neutron Star Transients and the Physics of the Inner Crust

Abstract

An accretion outburst onto a neutron star transient heats the neutron star's crust out of thermal equilibrium with the core. After the outburst, the crust thermally relaxes toward equilibrium with the neutron star core, and the surface thermal emission powers the quiescent X-ray light curve. Crust cooling models predict that thermal equilibrium of the crust will be established $$\approx 1000\,\mathrm{days}$$ into quiescence. Recent observations of the cooling neutron star transient MXB 1659-29, however, suggest that the crust did not reach thermal equilibrium with the core on the predicted timescale and continued to cool after $$\approx 2500\,\mathrm{days}$$ into quiescence. Because the quiescent light curve reveals successively deeper layers of the crust, the observed late-time cooling of MXB 1659-29 depends on the thermal transport in the inner crust. In particular, the observed late-time cooling is consistent with a low thermal conductivity layer near the depth predicted for nuclear pasta that maintains a temperature gradient between the neutron star's inner crust and core for thousands of days into quiescence. As a result, the temperature near the crust–core boundary remains above the critical temperature for neutron superfluidity, and a layer of normal neutrons forms in the inner crust. We find that the late-time cooling of MXB 1659-29 is consistent with heat release from a normal neutron layer near the crust–core boundary with a long thermal time. We also investigate the effect of inner crust physics on the predicted cooling curves of the accreting transient KS 1731-260 and the magnetar SGR 1627-41.

Authors:
ORCiD logo [1]; ORCiD logo [2]; ORCiD logo [1];  [3]
  1. Michigan State Univ., East Lansing, MI (United States)
  2. Michigan State Univ., East Lansing, MI (United States); McGill Univ., Montreal, QC (Canada)
  3. Univ. of Washington, Seattle, WA (United States); Michigan State Univ., East Lansing, MI (United States)
Publication Date:
Research Org.:
Univ. of Washington, Seattle, WA (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1537184
Grant/Contract Number:  
FG02-00ER41132
Resource Type:
Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal (Online); Journal Volume: 839; Journal Issue: 2; Journal ID: ISSN 1538-4357
Publisher:
Institute of Physics (IOP)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; Astronomy & Astrophysics

Citation Formats

Deibel, Alex, Cumming, Andrew, Brown, Edward F., and Reddy, Sanjay. Late-time Cooling of Neutron Star Transients and the Physics of the Inner Crust. United States: N. p., 2017. Web. doi:10.3847/1538-4357/aa6a19.
Deibel, Alex, Cumming, Andrew, Brown, Edward F., & Reddy, Sanjay. Late-time Cooling of Neutron Star Transients and the Physics of the Inner Crust. United States. doi:10.3847/1538-4357/aa6a19.
Deibel, Alex, Cumming, Andrew, Brown, Edward F., and Reddy, Sanjay. Thu . "Late-time Cooling of Neutron Star Transients and the Physics of the Inner Crust". United States. doi:10.3847/1538-4357/aa6a19. https://www.osti.gov/servlets/purl/1537184.
@article{osti_1537184,
title = {Late-time Cooling of Neutron Star Transients and the Physics of the Inner Crust},
author = {Deibel, Alex and Cumming, Andrew and Brown, Edward F. and Reddy, Sanjay},
abstractNote = {An accretion outburst onto a neutron star transient heats the neutron star's crust out of thermal equilibrium with the core. After the outburst, the crust thermally relaxes toward equilibrium with the neutron star core, and the surface thermal emission powers the quiescent X-ray light curve. Crust cooling models predict that thermal equilibrium of the crust will be established $\approx 1000\,\mathrm{days}$ into quiescence. Recent observations of the cooling neutron star transient MXB 1659-29, however, suggest that the crust did not reach thermal equilibrium with the core on the predicted timescale and continued to cool after $\approx 2500\,\mathrm{days}$ into quiescence. Because the quiescent light curve reveals successively deeper layers of the crust, the observed late-time cooling of MXB 1659-29 depends on the thermal transport in the inner crust. In particular, the observed late-time cooling is consistent with a low thermal conductivity layer near the depth predicted for nuclear pasta that maintains a temperature gradient between the neutron star's inner crust and core for thousands of days into quiescence. As a result, the temperature near the crust–core boundary remains above the critical temperature for neutron superfluidity, and a layer of normal neutrons forms in the inner crust. We find that the late-time cooling of MXB 1659-29 is consistent with heat release from a normal neutron layer near the crust–core boundary with a long thermal time. We also investigate the effect of inner crust physics on the predicted cooling curves of the accreting transient KS 1731-260 and the magnetar SGR 1627-41.},
doi = {10.3847/1538-4357/aa6a19},
journal = {The Astrophysical Journal (Online)},
number = 2,
volume = 839,
place = {United States},
year = {2017},
month = {4}
}

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