The ongoing 2008–09 outburst of CI Cyg

• Published in: Monthly notices of the Royal Astronomical Society 2009-11, Vol.399 (4), p.2139-2145
• Main Authors: Siviero, A., Munari, U., Dallaporta, S., Valisa, P., Luppi, V., Moretti, S., Tomaselli, S., Bacci, S., Ballardini, F., Cherini, G., Graziani, M., Frigo, A., Vagnozzi, A.
• Format: Article
• Language: English
• Subjects: Astronomy; binaries: symbiotic; cataclysmic variables; Double stars; Earth, ocean, space; Exact sciences and technology; novae; novae, cataclysmic variables; Star & galaxy formation
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2009.15414.x

In this paper, we discuss the early phases of the ongoing outburst that CI Cyg, a prototype symbiotic star, is currently undergoing after 30-year quiescence. We have tightly monitored CI Cyg in B V RCIC bands, starting a whole year before the onset of the outburst, and in addition we obtained numerous Echelle high- and low-resolution absolutely flux-calibrated spectra. The outburst started while the accreting white dwarf (WD) was being eclipsed by the Roche lobe filling M giant companion, and it was discovered during the egress phase on the second half of 2008 August. The system reached peak V-band brightness in early 2008 October and has been characterized by amplitudes ΔB= 1.9, ΔV= 1.5, ΔRC= 0.9, ΔIC= 0.4 mag. At maximum V-band brightness, the outbursting WD had expanded to closely resemble an F3 II/Ib star, with MV=−3.5, Teff∼ 6900 K and R= 28 R⊙. The high-ionization emission lines ([Ne v], [Fe vii], He ii) disappeared and only lower ionization lines were visible. Balmer and He i emission lines declined in equivalent width but increased in absolute flux. The output radiated by the hot component during the outburst corresponds to nuclear burning proceeding at a 2 × 10−8 M⊙ yr−1 rate.