Asymmetric Hα disk emission dominates the HST/FOS spectrum of Circinus X-1.

Abstract

CircinusX-1 is an highly variable X/radio binary source showing regular flux modulations with a periodicity of ~16.6days, probably coincident with the orbital period of the binary system. Here we present HST/FOS spectra of the optical counterpart taken around the predicted X-ray maximum. Strong Hα in emission, itself accounting for almost 50% of the optical flux between 5000 and 6800Å, is visible. The Hα profile, entirely ascribable to the accretion disk, is characterized by a remarkable asymmetry, pointing to the presence of at least two different components, i.e. a broad one at rest plus a narrower redshifted (v~400km/s) one. No other emission/absorption line is clearly visible superimposed on the source continuum, thus making troublesome any tentative spectral classification of the companion star. Furthermore, the noisy background together with a substantial, although uncertain, interstellar reddening, makes it difficult to fit reliably any continuum spectrum at the source.