The "K-Correction" for Irradiated Emission Lines in LMXBs: Evidence for a Massive Neutron Star in X1822-371 (V691 CrA)

Abstract

We study the K-correction for the case of emission lines formed in the X-ray illuminated atmosphere of a Roche lobe filling star. We compute the K-correction as function of the mass ratio 'q' and the disc flaring angle 'alpha' using a compact binary code where the companion's Roche lobe is divided into 10^5 resolution elements. We also study the effect of the inclination angle in the results. We apply our model to the case of the neutron star low-mass X-ray binary X1822-371 (V691 CrA), where a K-emission velocity K_em=300 +-8 km/s has been measured by Casares et al. (2003). Our numerical results, combined with previous determination of system parameters, yields 1.61Msun < M_NS < 2.32Msun and 0.44Msun < M_2 < 0.56Msun for the two binary components(i. e. 0.24 < q < 0.27), which provide a compelling evidence for a massive neutron star in this system. We also discuss the implications of these masses into the evolutionary history of the binary.

6 pages, 5 figures. Accepted for publication in ApJ