EDP Sciences
Free access
Volume 417, Number 1, April I 2004
Page(s) 169 - 179
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20034191

A&A 417, 169-179 (2004)
DOI: 10.1051/0004-6361:20034191

Thermal states of coldest and hottest neutron stars in soft X-ray transients

D. G. Yakovlev1, K. P. Levenfish1, 2, A. Y. Potekhin1, 2, O. Y. Gnedin3 and G. Chabrier4

1  Ioffe Physico-Technical Institute, Politekhnicheskaya 26, 194021 St. Petersburg, Russia
2  Isaac Newton Institute of Chile, St. Petersburg Branch, Russia
3  Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
4  Ecole Normale Supérieure de Lyon (C.R.A.L., UMR CNRS No. 5574), 46 allée d'Italie, 69364 Lyon Cedex 07, France

(Received 13 August 2003 / Accepted 8 October 2003 )

We calculate the thermal structure and quiescent thermal luminosity of accreting neutron stars (warmed by deep crustal heating in accreted matter) in soft X-ray transients (SXTs). We consider neutron stars with nucleon and hyperon cores and with accreted envelopes. It is assumed that an envelope has an outer helium layer (of variable depth) and deeper layers of heavier elements, either with iron or with much heavier nuclei (of atomic weight $A\gtrsim100$) on the top [CITE]. The relation between the internal and surface stellar temperatures is obtained and fitted by simple expressions. The quiescent luminosity of the hottest (low-mass) and coldest (high-mass) neutron stars is calculated, together with the ranges of its possible variations due to variable thickness of the helium layer. The results are compared with observations of SXTs, particularly, containing the coldest ( SAX J1808.4-3658 ) and the hottest ( Aql X-1 ) neutron stars. The observations of SAX J1808.4-3658 in a quiescent state on March 24, 2001 [CITE] can be explained only if this SXT contains a massive neutron star with a nucleon/hyperon core; a hyperon core with a not too low fraction of electrons is preferable. Future observations may discriminate between the various models of hyperon/nucleon dense matter. The thermal emission of SAX J1808.4-3658 is also sensitive to the models of plasma ionization in the outermost surface layers and can serve for testing such models.

Key words: stars: neutron -- dense matter -- equation of state -- stars: individual: Aql X-1 , SAX J1808.4-3658 -- X-rays: binaries

Offprint request: A. Y. Potekhin, palex@astro.ioffe.ru

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