Spectroscopic observations of the candidate sgB[e]/X-ray binary CI Camelopardalis

¹ Department of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
² Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
³ Special Astrophysical Observatory, Nizhnij Arkhyz, 369167, Russia
⁴ University College, Department of Physics, Cork, Ireland
⁵ School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, UK
⁶ Harvard Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁷ Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
⁸ University of Toledo, Dept. of Physics and Astronomy, Toledo, OH 43606, USA
⁹ Pulkovo Observatory, 196140 Saint-Petersburg, Russia
¹⁰ Observatoire de Strasbourg, 11 rue de l'Université, 67000 Strasbourg, France
¹¹ Physics Department, University of Crete, PO Box 2208, 71003 Heraklion, Greece
¹² Foundation for Research and Technology–Hellas, 71110 Heraklion, Greece
¹³ Dept. of Astronomy, San Diego State University, San Diego, CA 92182, USA

Corresponding author: R. I. Hynes, rih@astro.as.utexas.edu

Received: 7 March 2002
Accepted: 28 June 2002

Abstract

We present a compilation of spectroscopic observations of the sgB[e] star CI Cam, the optical counterpart of XTE J0421+560. This includes data from before, during, and after its 1998 outburst, with quantitative results spanning 37 years. The object shows a rich emission line spectrum originating from circumstellar material, rendering it difficult to determine the nature of either star involved or the cause of the outburst. We collate all available pre-outburst data to determine the state of the system before this occurred and provide a baseline for comparison with outburst and post-outburst data. During the outburst all lines become stronger, and hydrogen and helium lines become significantly broader and asymmetric. After the outburst, spectral changes persist for at least three years, with Fe ii and [N ii] lines still a factor of ~2 above the pre-outburst level and He i, He ii, and N ii lines suppressed by a factor of 2–10. We find that the spectral properties of CI Cam are similar to other sgB[e] stars and therefore suggest that the geometry of the circumstellar material is similar to that proposed for the other objects: a two component outflow, with a fast, hot, rarefied polar wind indistinguishable from that of a normal supergiant and a dense, cooler equatorial outflow with a much lower velocity. Based on a comparison of the properties of CI Cam with the other sgB[e] stars we suggest that CI Cam is among the hotter members of the class and is viewed nearly pole-on. The nature of the compact object and the mechanism for the outburst remain uncertain, although it is likely that the compact object is a black hole or neutron star, and that the outburst was precipitated by its passage through the equatorial material. We suggest that this prompted a burst of supercritical accretion resulting in ejection of much of the material, which was later seen as an expanding radio remnant. The enhanced outburst emission most likely originated either directly from this supercritical accretion, or from the interaction of the expanding remnant with the equatorial material, or from a combination of both mechanisms.