Time-variable emission from transiently accreting neutron stars in quiescence due to deep crustal heating |
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Authors: | Greg Ushomirsky Robert E Rutledge |
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Institution: | 1Theoretical Astrophysics, California Institute of Technology, MS 130-33, Pasadena, CA 91125;2Space Radiation Laboratory, California Institute of Technology, MS 220-47, Pasadena, CA 91125 |
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Abstract: | Transiently accreting neutron stars in quiescence ( L X ?1034 erg s?1) have been observed to vary in intensity by factors of few, over time-scales of days to years. If the quiescent luminosity is powered by a hot neutron star core, the core cooling time-scale is much longer than the recurrence time, and cannot explain the observed, more rapid variability. However, the non-equilibrium reactions which occur in the crust during outbursts deposit energy in isodensity shells, from which the thermal diffusion time-scale to the photosphere is days to years. The predicted magnitude of variability is too low to explain the observed variability unless – as is widely believed – the neutrons beyond the neutron-drip density are superfluid. Even then, the variability due to this mechanism in models with standard core neutrino cooling processes is less than 50 per cent – still too low to explain the reported variability. However, models with rapid core neutrino cooling can produce a variability by a factor as great as 20, on time-scales of days to years following an outburst. Thus, the factors of ~ few intensity variability observed from transiently accreting neutron stars can be accounted for by this mechanism only if rapid core cooling processes are active. |
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Keywords: | nuclear reactions nucleosynthesis abundances stars: neutron X-rays: binaries |
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