Soft X-ray Emission and Lithium Production in Cen X-4 during Quiescence

摘要

We investigate emission mechanism of soft X-ray radiation from a neutron star soft X-ray transient (NSSXT), Cen X-4 and unusually high abundances of Li on a low-mass secondary in Cen X-4 during a quiescent state. An accretion flow in NSSXTs in quiescence is though to be an advection-dominated one and truncated by magneto-centrifugal forces due to the magnetic field of a neutron star. The advection dominated accretion flow (ADAF) is very hot (> 1 MeV) enough to synthesize abundant neutrons through the spallation of ~4He via protons in the flow. Although almost all charged nuclei in the ADAF are outflowed by the magneto-centrifugal forces, the neutrons accrete onto the neutron star because of no influence of the magnetic field on the neutrons. Consequently, the accretion flow is composed of neutrons near the neutron star. The accretion energy of the neutrons liberated on the surface of the star is a possible energy budget for soft X-ray radiation in a NSSXT in quiescence. Moreover, Li is abundantly synthesized in the ADAF through α -α reactions and is transfered to the surface of the secondary by the magneto-centrifugal forces. We find that for reasonable values of the mass accretion rate and the magnetic field of the neutron star, the estimated soft X-ray luminosity and abundance of Li on the secondary are comparable to those observed in Cen X-4.