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

Abstract

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 ± 6km 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, < 2.49M⊙ 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.