Lithium Production and Soft X-ray Emission in Neutron Star Soft X-Ray Transients Cen X-4 and Aql X-1

概要

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We investigate production of Li in soft X-ray transients (NSSXTs) during quiescence. Lithium is abundantly synthesized in a hot accretion flow and is transfered to the surface of a secondary star. Almost all charged nuclei in the accretion flow are outflowed by the magnetocentrifugal forces, so that the accretion flow is mainly composed of neutrons. Accretion energies of neutrons liberated on the surface of the neutron star naturally explain the origin of blackbody-like soft X-rays from a quiescent NSSXT. It is found that for reasonable values of the mass accretion rate M=2-4x10^-11 M_sun yr^-1 from the secondary and the magnetic field B_NS=2-3x10^8 G of the neutron star, the abundance of Li on the secondary is LI/H≈10^-9 and the soft X-ray luminosity is Lx=2-3x10^32 erg s^-1, which are comparable to the observed values in Cen X-4 during quiescence. Moreover, the isotopic ratio ^6Li/^7Li is found to be comparable to the observed value. The scenario for the Li production and the emission mechanism of the soft X-ray in terms of neutron accretion explains the non-detection of Li in Aql X-1 due to strong magnetic fields. These facts strongly suggest the existence of a hot accetion flow in an NSSXT during quiescence.
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