Nuclear composition and heating in accreting neutron-star crusts.pdf

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Nuclear composition and heating in accreting neutron-star crusts.pdf

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Nuclear composition and heating in accreting neutron-star crusts

a r X i v : a s t r o - p h / 0 3 0 5 2 2 0 v 1 1 3 M a y 2 0 0 3 Astronomy Astrophysics manuscript no. Fc111 February 2, 2008 (DOI: will be inserted by hand later) Nuclear composition and heating in accreting neutron-star crusts P. Haensel and J.L. Zdunik N. Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, PL-00-716 Warszawa, Poland haensel@camk.edu.pl, jlz@camk.edu.pl Received / Accepted Abstract. Nuclear reactions in accreting neutron-star crusts and the heat release accompanying them are studied, under different assumptions concerning the composition of the outermost layer formed of the ashes of X-ray bursts. Particular examples of ashes containing nuclides with A ? 90?110 are considered and compared with a standard A = 56 case. In all cases, evolution of a crust shell is followed from 108 g cm?3 to a few times 1013 g cm?3. The total crustal heating produced in the non-equilibrium processes in the accreting crust is 1.1 ? 1.5 MeV per one accreted nucleon. The composition of the accreted crust at densities exceeding the threshold for the pycnonuclear fusion (ρ 1012 g cm?3) is essentially independent of the assumed initial composition of the X-ray burst ashes. Key words. dense matter – equation of state – stars: neutron – stars :general – X-rays: bursts – X-ray: binaries – nuclear reactions 1. Introduction Accretion of matter onto neutron stars in close binary sys- tems plays crucial role in many objects studied by the X- ray astronomy, such as X-ray bursters, X-ray pulsars, and transient X-ray sources. Particular attention was focused recently on soft X-ray transients (SXRTs) in quiescence, when the accretion from a disk formed of matter flow- ing from the low-mass companion star is switched off or strongly suppressed. The nature of the X-ray radiation during quiescence is still a matter of debate. Typically, the quiescent emission is much higher than the expected one for an old cooling neutron star. It has been suggested that this