Rotational broadening and Doppler tomography of the quiescent X-ray nova Centaurus X-4

We present high- and intermediate-resolution spectroscopy of the X-ray nova Centaurus X-4 during its quiescent phase. Our analysis of the absorption features supports a K3–K5 V spectral classification for the companion star, which contributes ≃75 per cent of the total flux at Hα. Using the high-resolution spectra we have measured the rotational broadening of the secondary star to be v sin i=43±6 km s-1 and determined a binary mass ratio of q=0.17±0.06. Combining our results for K2 and q with the published limits for the binary inclination, we constrain the mass of the compact object and the secondary star to the ranges 0.49<M1<2.49 M⊙ and 0.04<M2<0.58 M⊙. A Doppler image of the Hα line shows emission coming from the secondary star, but no hotspot is present. We discuss the possible origins of this emission.