Parameters of X-ray binary system Scorpius X-1

TL;DR

This study models optical light curves of Sco X-1 using Kepler data, estimating key system parameters like mass ratio, inclination, and star temperature, highlighting the dominant role of X-ray heated accretion disc in optical emission.

Contribution

The paper provides the first detailed modeling of Sco X-1's optical light curves considering strong X-ray heating, estimating system parameters with quantified uncertainties.

Findings

Mass ratio q=3.6 (3.5-3.8)

Orbital inclination i=30° (25°-34°)

Optical star temperature 2500-3050 K

Abstract

We modelled optical light curves of Sco~X-1 obtained by the Kepler space telescope during K2 mission. Modelling was performed for the case of the strong heating of the optical star and accretion disc by X-rays. In the considered model the optical star fully filled its Roche lobe. We investigated the inverse problem in wide ranges of values of model parameters and estimated following parameters of Sco X-1: the mass ratio of components $q=M_x/M_v=3.6$ ($3.5-3.8$), where $M_x$ and $M_v$ were masses of the neutron and optical stars correspondingly, the orbital inclination was $i=30^{\circ}$ ($25^{\circ}-34^{\circ}$). In the brackets uncertainties of parameters $q$ and $i$ were shown, they originated due to uncertainties of characteristics of the physical model of Sco X-1. The temperature of non-heated optical star was $T_2 = 2500-3050$ K, its radius was $R_2=1.25R_{\odot}=8.7\times 10^{10}$ cm, and its bolometric luminosity was $L_{bol}=(2.1-4.6)\times 10^{32}$ erg s$^{-1}$. The mass of the star was $M_v\simeq 0.4M_{\odot}$. The contribution of the X-ray heated accretion disc dominated in the total optical emission of Sco~X-1. The transition between low and high states occurred due to the increase of X-ray luminosity by a factor $2-3$.