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(circle dot), consistent with previous works. Results. The H alpha emission from the accretion disk is much weaker than in previous studies, possibly because of a decrease in disk activity. Doppler imaging of the H alpha line shows for the first time a narrow component coming from the secondary star, with an observed equivalent width of 1.4 +/- 0.3 angstrom, 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 angstrom. A bright hot spot is also detected at the position where the gas stream trajectory intercepts the accretion disk. Conclusions. The H alpha 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.