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Astron. Astrophys. 323, 797-800 (1997)

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Asymmetric [FORMULA] disk emission dominates the HST/FOS spectrum of Circinus X-1 *

R. Mignani 1, P.A. Caraveo 1 and G.F. Bignami 1, 2

1 Istituto di Fisica Cosmica del CNR, Via Bassini 15, I-20133 Milano, Italy
2 Dipartimento di Ingegneria Industriale, Universitá di Cassino, Via Zamosch 43, I-03043 Cassino, Italy

Received 11 July 1996 / Accepted 30 September 1996


CircinusX-1 is an highly variable X/radio binary source showing regular flux modulations with a periodicity of [FORMULA] days, 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 [FORMULA] in emission, itself accounting for almost 50 % of the optical flux between 5000 and 6800 [FORMULA], is visible. The [FORMULA] 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 [FORMULA] 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.

Key words: stars: Cir X-1 – X-rays: binaries – accretion

* Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555

Send offprint requests to: R. Mignani, mignani@ifctr.mi.cnr.it

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998