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G. Ushomirsky, R. E. Rutledge (California Institute of Technology)
Nuclear reactions deep in a neutron star crust, occuring during accretion events as the material is hydrostatically compressed, can maintain the core of a transiently accreting neutron star at a temperature ~107K. Thermal emission from such a hot core can account for the observed quiescent luminosity. Because of the core's long thermal time, this emission does not vary between outbursts. On the other hand, the crust can cool substantially between outbursts. We calculate the response of the crust to heating during an accretion outburst and the resulting post-outburst thermal emission from the neutron star. Nuclear energy releases heat the crust, and the deposited heat diffuses slowly towards the surface. This leads to slow variability of the quiescent thermal emission, with shorter timescale variations at early times after an outburst, and longer timescale variability at late times. The relative magnitude of this variability depends on the time-averaged accretion rate and the outburst recurrence time. Quiescent X-ray luminosities of the observed neutron star transients vary over timescales of days to years. This variability, if disentangled from the possible continued accretion in quiescence, can be used to map out the thermal time as a function of depth in the crust, and to constrain the radii of neutron stars.
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