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E.F. Brown (Enrico Fermi Inst./Univ. of Chicago), L. Bildsten (ITP/UC-Santa Barbara), P. Chang (Dept. Physics/UC-Santa Barbara)
We examine how the varying atmospheric composition of a accreting transient neutron star affects its quiescent thermal luminosity. During an outburst, the neutron star in a soft X-ray transient accretes hydrogen (H) and helium (He), which then burn via the rp-process to heavier species (up to 104Ru) during a type I X-ray burst. The composition of the atmosphere during quiescence thus depends on how much H and He were accreted following the last type I X-ray burst before quiescence, and on how much H was stably consumed by the HCNO cycle as the accretion rate decreased. While the core temperature is unchanging over a timescale > 104 yr, the effective temperature depends on the thermal gradient through the H, He and ash layers, and in particular depends strongly on the depth of the layer of residual H/He. We calculate the possible variation in effective temperature and find that it can vary by a factor of 2 from one quiescent epoch to another. This may contribute to the variability in the quiescent luminosity observed from transients such as Aql X-1 and Cen X-4. This work is partially supported by the Department of Energy under grant B341495 to the Center for Astrophysical Thermonuclear Flashes at the University of Chicago, by NASA via grant NAG 5-8658 and by the National Science Foundation under Grants PHY99-07949
The author(s) of this abstract have provided an email address for comments about the abstract: brown@flash.uchicago.edu