Binary accreting neutron star as a source of x-ray transients - Aql X-1, Cen X-4, 1608-522

Abstract

A thermonuclear model for x-ray transients has been considered to explain the observed characteristics of x-ray transient outbursts in Aql X-1, Cen X-4, 1608-522. The model is of a compact primary neutron star (NS) in a close binary system. When the secondary companion fills up the Roche-lobe, sometimes sudden rise of mass accretion is there onto the NS. When the accretion rate is quite high (~ 10-10M/sub theta/ yr-1 ), temperature and density increases up to ~ 10/sup8/ K and ~ 10/sup6/ g cm-3 respectively leading to thermonuclear flash. Using most recent proton capturing nuclear reaction rates and beta-decay rates, hydrogen burning cyclic reactions proceeding with seed nuclei C, O, Ne, Mg, have been studied and examined about energy generation during transient x-ray outburst time scale. Presently calculated values of energy production, luminosity etc. are found to tally well with the observed parameters. During x-ray outbursts, mass is ejected from the source. Abundances of the nuclei (Z = 6 ? 13) in the ejected mass have been calculated and compared with the recently observed cosmic ray source composition. Moreover, probable gamma-ray (g ) line spectra radiated by the excited product nuclei have been studied