Production of light elements on a low-mass secondary in a soft X-ray transient

Abstract

We examine production of light elements (Li, Be, & B) on the surface of a low‐mass secondary in a black hole soft X‐ray transient (BHSXT) through the spallation of CNO nuclei by neutrons which are ejected from a hot (>10 MeV) advection‐dominated accretion flow (ADAF) around the black hole. Using updated binary parameters, cross sections of neutron‐induced spallation reactions, and mass accretion rates in ADAFs derived from the spectrum fitting of multi‐wavelength observations of quiescent BHSXTs, we obtain the equilibrium abundances of Li by equating the production rate of Li and the mass transfer rate through accretion to the black hole. We find that the resulting abundances are found to be in good agreement with the observed values in seven BHSXTs. Moreover, the isotopic ratio 6Li/7Li and the abundance ratios Be/Li and B/Li are calculated to be about 0.7–0.8, 1.0–1.3, and 1.8–2.1, respectively, on the secondaries.