Lithium production on a low-mass secondary in a black hole soft X-ray transient

TL;DR

This study models lithium production on low-mass secondaries in black hole soft X-ray transients via neutron spallation from hot accretion flows, matching observed lithium abundances and predicting a high isotopic ratio.

Contribution

It introduces a detailed model linking neutron spallation in ADAF flows to lithium enrichment on secondaries, aligning with observations and suggesting high Li6/Li7 ratios.

Findings

Lithium abundances match observed values in seven BHSXTs.

Predicted Li6/Li7 ratio is about 0.7-0.8, higher than meteorites.

Lithium abundance varies over timescales longer than a few months.

Abstract

We examine production of Li 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 ADAF 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. The resulting abundances are found to be in good agreement with the observed values in seven BHSXTs. We note that the abundances vary in a timescale longer than a few months in our model. Moreover, the isotopic ratio Li6/Li7 is calculated to be about 0.7--0.8 on the secondaries, which is much higher than the ratio measured in meteorites. Detection of such a high value is favorable to the production of Li via spallation and the existence of a hot accretion flow, rather than an accretion disk corona system in quiescent BHSXT.