The spotty donor star in the X-ray transient Cen X-4

Abstract

We accurately determine the fundamental system parameters of the neutron-star X-ray transient Cen X-4 solely using phase-resolved high-resolution UVES spectroscopy. We first determine the radial-velocity curve of the secondary star and then model the shape of the phase-resolved absorption line profiles using an X-ray binary model. The model computes the exact rotationally broadened phase-resolved spectrum and does not depend on assumptions about the rotation profile, limb-darkening coefficients and the effects of contamination from an accretion disk. We determine the secondary star-to-neutron star binary mass ratio to be 0.1755+/-0.0025, which is an order of magnitude more accurate than previous estimates. We also constrain the inclination angle to be 32 (+8; -2) degrees, Combining these values with the results of the radial velocity study gives a neutron star mass of 1.94 (+0.37; -0.85) Msun consistent with previous estimates. Finally, we perform the first Roche tomography reconstruction of the secondary star in an X-ray binary. The tomogram reveals surface inhomogeneities that are due to the presence of cool starspots. A large cool polar spot, similar to that seen in Doppler images of rapidly-rotating isolated stars is present on the Northern hemisphere of the K7 secondary star and we estimate that about 4 per cent of the total surface area of the donor star is covered with spots. This evidence for starspots supports the idea that magnetic braking plays an important role in the evolution of low-mass X-ray binaries.

10 pages, 8 figures accepted for publication in MNRAS

References in corpus (6)

• Roche tomography of cataclysmic variables - IV. Starspots and slingshot prominences on BV Cen
• Roche tomography of cataclysmic variables -- III. Starspots on AE Aqr
• An X-ray-UV correlation in Cen X-4 during quiescence
• The isotopic 6Li/7Li ratio in Cen X-4 and the origin of Li in X-ray binaries
• The rotational broadening of V395 Car - implications on compact object's mass
• A search for evidence of irradiation in Centaurus X-4 during quiescence

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• HST spectrum and timing of the ultra-compact X-ray binary candidate 47 Tuc X9
• On the Optical -- X-ray correlation from outburst to quiescence in Low Mass X-ray Binaries: the representative cases of V404 Cyg and Cen X-4
• Roche tomography of cataclysmic variables - VIII: The irradiated and spotted dwarf nova, SS Cygni
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• The intermediate polar cataclysmic variable GK Persei 120 years after the nova explosion: a first dynamical mass study
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• Roche tomography of cataclysmic variables - VII. The long-term magnetic activity of AE Aqr
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