Doppler tomography of the black hole binary A0620-00 and the origin of chromospheric emission in quiescent X-ray binaries

Context. Doppler tomography of emission line profiles in low-mass X-ray binaries allows us to disentangle the different emission sites and study the structure and variability of accretion disks. Aims. We present UVES high-resolution spectroscopic observations of the black hole binary A0620-00 at quiescence. Methods. These spectroscopic data constrain the orbital parameters P orb = 0.32301405(1) d and K 2 = 437.1 ± 2.0 km s -1 . These values, together with the mass ratio q = M 2 /M 1 = 0.062 ± 0.010, imply a minimum mass for the compact object of M 1 sin 3 i = 3.15 ± 0.10 M ⊙ , consistent with previous works. Results. The Hα emission from the accretion disk is much weaker than in previous studies, possibly because of a decrease in disk activity. Doppler imaging of the Hα line shows for the first time a narrow component coming from the secondary star, with an observed equivalent width of 1.4 ± 0.3 A, perhaps associated to chromospheric activity. Subtracting a K-type template star and correcting for the veiling of the accretion disk yields an equivalent width of 2.8 ± 0.3 A. A bright hot spot is also detected at the position where the gas stream trajectory intercepts the accretion disk. Conclusions. The Hα flux associated to the secondary star is too large to be powered by X-ray irradiation. It is comparable to those observed in RS CVn binaries with similar orbital periods and is probably triggered by the rapid stellar rotation.