The unusual Nova Cygni 2006 (V2362 Cyg)

Context: Optical nova lightcurves often have structures, such as rapid declines and recoveries, due to nebular or dusty phases of the ejecta. Nova Cygni 2006 (V2362 Cyg) underwent an unusual brightening after an early rapid decline. The shape of the lightcurve can be compared to that of V1493 Aql, but the whole event in that case was not as bright and only lasted a couple of weeks. V2362 Cyg had a moderately fast decline of t_2 = 9.0 before rebrightening, which lasted 250 days after maximum. Aims: We present an analysis of our own spectroscopic investigations in combination with AAVSO photometric data covering the whole rebrightening phase until the return to the final decline. Methods: We used the medium resolution spectroscopy obtained in ten nights over a period of 79 nights to investigate the change of the velocity structure of the ejecta. The publicly available AAVSO photometry was used to analyze the overall properties and the energy of the brightening. Results: Although the behavior of the main outburst (velocity, outburst magnitude, and decline timescales) resembles a ``normal'' classical nova, the shell clearly underwent a second fast mass ejecting phase, causing the unusual properties. The integrated flux during this event contributes ~ 40 % to the total radiation energy of the outburst. The evolution of the H_alpha profile during the bump event is obtained by subtracting the emission of the detached shells of the main eruption by a simple optically thin model. A distance of D ~ 7.5 {+3.0}{-2.5} kpc and an interstellar extinction E(B-V) = 0.6 +/- 0.1 was also derived.Comment: 4 pages, 4 Postscript figures, accepted for A&A Letter