The optical light curves of XTE J2123--058: the mass of the binary components and the structure of the quiescent accretion disk

摘要

We present optical photometry of XTE J2123-058 during its quiescent statetaken in 1999 and 2000. The dominant feature of our R-band light curve is theellipsoidal modulation of the secondary star, however, in order to fit thissatisfactorily we require additional components which comprise an X-ray heatedRoche-lobe filling secondary star, and an accretion disk bulge, i.e. where thegas stream impacts the accretion disk. The observed dip near phase 0.8 isinterpreted as the eclipse of inner parts of the accretion disk by the bulge.This scenario is highly plausible given the high binary inclination. Our fitsallow us to constrain the size of the quiescent accretion disk to lie in therange 0.26-0.56R_L1 (68% confidence). Using the distance of 9.6 kpc and theX-ray flux inferred from the heated hemisphere of the companion, we obtain anunabsorbed X-ray luminosity of 1.2x10^33 erg/s for XTE J2123-058 in quiescence. From the observed quiescent optical/IR colors we find that the power-lawindex (-1.4) for the spectral distribution of the accretion disk compares wellwith other quiescent X-ray transients. We also re-analyse the optical lightcurves of the soft X-ray transient XTE J2123-058 taken during its outburst anddecay in 1998. We use a robust method to fit the data using a refined X-raybinary model. The model computes the light arising from a Roche-lobe fillingstar and flared accretion disk irradiated by X-rays, and calculates the effectsof shadowing and mutual star/disk eclipses. We obtain relatively accuratevalues for the binary inclination and mass ratio, which when combined withspectroscopic results obtained in paper II gives a neutron star mass in therange 1.04-1.56Mo (68% confidence).