Doppler tomography of the transient X-ray binary Centaurus X-4 in quiescence

Abstract

We present ESO-NTT low resolution spectroscopy of the neutron star X-ray transient Cen X-4 in quiescence over a complete orbital cycle. Our data reveal the presence of a K3-7 V companion which contributes 63% to the 5600-6900A flux and orbits the neutron star with a velocity semi-amplitude of K_2=145.8 +/- 1.0 km s^{-1}. This, combined with a previous determination of the inclination angle and mass ratio, yields a neutron star and companion mass of M_1=1.5 +/- 1.0 M_Sun and M_2=0.31 +/- 0.27 M_Sun, respectively. The mass donor is thus undermassive for the inferred spectral type indicating it is probably evolved, in agreement with previous studies. Doppler tomography of the H_alpha line shows prominent emission located on the companion and a slightly asymmetric accretion disc distribution similar to that seen in systems with precessing eccentric discs. Strong H_alpha emission from the companion can be explained by X-ray irradiation from the primary. No evidence is found for a hot spot in H_alpha, whereas one is revealed via Doppler tomography of the HeI lines. This can be interpreted as the hot spot and outer regions of the disc being at a higher temperature than in other systems.