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

¹ INAF-Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (Lc), Italy e-mail: davanzo@merate.mi.astro.it
² Università degli Studi dell'Insubria, Dipartimento di Fisica e Matematica, via Valleggio 11, 22100 Como, Italy
³ Instituto de Astrofìsica de Canarias, 38200 La Laguna, Tenerife, Spain
⁴ INAF-Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio Catone (Roma), Italy
⁵ School of Physics & Astronomy, University of Southampton, Southampton SO17 1BJ, UK
⁶ South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa

Received: 25 May 2005
Accepted: 31 July 2005

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  to the 5600–6900  flux and orbits the neutron star with a velocity semi-amplitude of  km s^-1. This, combined with a previous determination of the inclination angle and mass ratio, yields a neutron star and companion mass of and , 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 Hα 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 Hα emission from the companion can be explained by X-ray irradiation from the primary. No evidence is found for a hot spot in Hα, 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.