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V2491 Cyg - a possible recurrent nova?

ATel #1485; T. Tomov, M. Mikolajewski, T. Brozek, E. Ragan, E. Swierczynski, P. Wychudzki, C. Galan (Uniwersytet Mikolaja Kopernika, Torun, Poland)
on 20 Apr 2008; 18:55 UT
Credential Certification: Andrzej Niedzielski (aniedzi@astri.uni.torun.pl)

Subjects: Optical, Binary, Cataclysmic Variable, Nova, Star

Referred to by ATel #: 1504, 1514

We report on further optical observations of nova V2491 Cyg obtained in the Piwnice Observatory (Torun, Poland). Spectral observations were secured with the Canadian Copernicus Spectrograph attached to the 60/90 cm Schmidt-Cassegrain telescope. The UBVRI photometry was carried out with the 60 cm Cassegrain telescope.

Our brightness estimations obtained on Apr. 14.99 UT were 9.23, 9.53, 9.33, 8.28, 8.01 for UBVRI, respectively; on Apr. 17.06 UT were 9.84, 9.76, 8.70 for BVR, respectively; on Apr. 18.99 UT were 9.72, 9.98, 9.98, 9.05, 8.88 for UBVRI, respectively. The accuracy and the comparison star are the same as reported in ATel #1475. The VRI decline rate was relatively high and changed from ~0.3 to ~0.15 mag per day between Apr. 13 and Apr. 18. Assuming Apr. 11.36 and V=7.54 mag for the maximum (IAUC # 8934) the t2 time in V band is about 4.6 days.

Spectra with a resolution ~4 A were obtained on Apr. 15.08 UT (3600-9300 AA) and Apr. 17.08 UT (5400-9300 AA). In the better quality spectrum on Apr. 15.08 UT the Balmer lines and the Balmer jump are well visible in emission. The strongest non-hydrogen emissions are OI (8446 A and 7773 A) and different nitrogen ions (for instance NI 8212 A, NII 5679 A, NIII 4640 A, etc.). Relatively weak HeI lines (e.g. 7065 A, 5876 A) present as well. It seems that our previous identification (ATel #1475) for FeII lines of multiplets 37 and 38 is not correct. Nevertheless, faint emissions of FeII (multiplets 42, 48, 49) probably present as well. For now, we did not identify any forbidden emission lines in the spectrum of V2491 Cyg. The FWHM of H-alpha (~4000 km/s) and H-beta (~3900 km/s) show a small decrease while their equivalent widths (900 A and 190 A, respectively) increased on Apr 15.08 in comparison to the values reported closer to the maximum (CBET #1334, ATel #1475). The flux ratio of OI(8446)/H-beta is about 1.5 on Apr. 15.08 UT. All this suggests that V2491 Cyg can be classified (Williams et #al., 1991, ApJ 376, 721) as a 'He/N' nova with post-maximum spectrum of subclass Pn and OI 8446 A dominating in the near-IR.

A complex shape P Cyg absorption presents in all Balmer lines from H-beta to H9. The measured radial velocity of the strongest absorption component is -3550 km/s and -3470 km/s for H-beta and H-gamma, respectively. Slightly higher radial velocities we measured for two relatively strong absorptions which we identified as P Cyg absorption components of HeI 5876 A (-3980 km/s) and NII 5679 A (-4030 km/s). The extended blue wing of the P Cyg absorption suggest the presence of higher velocity components. The only one, we were able to measure in H-gamma, shows a radial velocity about -5350 km/s.

The profile shapes of the strongest emissions in the spectrum of V2491 Cyg, the Balmer lines and the OI lines, are quite different. The Balmer lines present a Gaussian-like profile while the OI line profiles are rectangle-like with a multi-peak top. The two extremely blue and red shifted emission peaks in OI profiles are placed symmetrically at radial velocities -2020+/-40 km/s and +2180+/-20 km/s, and are clearly seen in the Balmer emissions as well.

Most of the described spectral characteristics of V2491 Cyg seem to be similar to these observed, at the same outburst phase, in at least two recurrent novae, U Sco and V394 CrA (Munari et al., 1999, A&A 347, L39; Williams, 1992, AJ 104, 725). Moreover, V2491 Cyg is the second very fast nova which has been observed in X-rays before the outburst. The first one was V2487 Oph (ATel #1478). Hachisu et al. (2002, ASP Conf. Ser. 261, 629) analysed the observed light curve of V2487 Oph after the eruption and concluded that it can be a recurrent nova. A search in the archival observations in the fields of both, V 2487 Oph and V2491 Cyg, to check their historical brightness could be very useful.