1. Detection of the photometric period of the classical nova V2468 Cygni. D. Chochol 1, S.Y. Shugarov 1,2, I.M. Volkov 2, V.P. Goranskij 3 1) Astronomical Institute of the Slovak Academy of Sciences, 059 60 Tatranská Lomnica, Slovakia 2) Sternberg State Astronomical Institute, Universitetskij Prospect 13, 119992, Moscow, Russia 3) Special Astrophysical Observatory of the Russian Academy of Sciences, 369167, Nizhniy Arkhyz, Karachai-Cherkesia, Russia Stella Novae: Past and Future Decades, February 4-8, 2013, Cape Town, South Africa Introduction Classical novae are semidetached binaries with orbital periods of a few hours, consisting of a mass losing red dwarf and an accreting white dwarf. Classification to the fast and slow novae is based on a time interval, during which nova declines by 2 or 3 magnitudes from its maximum brightness, The fast novae (t2 13 days, t3 > 30 days) have structured light curves and many of them have standstills at maximum and signatures of a dust formation at later stages (Downes & Duerbeck, 2000). The fast novae have higher ejection velocities of the envelope than the slow novae. Nova V2468 Cygni – discovery, classification, basic characteristics and outburst evolution Classical nova V2468 Cyg (Nova Cygni 2008) was discovered by H. Kaneda on March 7.801 UT (JD 2454534.301) at mag 8.2 in the position α 2000 =19 h 58 m 33.39 s, δ 2000 = +29 ° 52´06.5´´, measured by K.Kadota on the CCD image, taken on March 8.716 UT (Nakano, 2008). It was classified as a Williams Fe II class nova (Beaky, 2008). Henden and Munari (2008) found a faint star USNO-B1 1198-0459968 (R = 18 mag) at this position, visible only on POSS-II red plates, not the blue ones. The amplitude of the outburst was larger than 12 mag in the B band. Chochol et al. (2011) used the V and B photometric light curves to find the rates of decline: t 2;V = 9 d, t 2;B = 10 d, t 3;V = 20 d and t 3;B = 22 d of this fast nova. They determined its basic parameters: M V;max = -8.70± 0.07, interstellar extinction E(B-V ) = 0.79±0.01 and distance d = 5.4±0.6 kpc. Nova entered to a nebular stage 122 days after the outburst (Iijima & Naito, 2011). The SSS stage of the nova started in June 2009 (~460 days after outburst) when the object was detected by Swift with the XRT and UVOT instruments. The X-ray light curve was variable with a large amplitude on timescale of 500 seconds, UV observations showed large amplitude variations of the order 0.3 mag (Schwarz et al., 2009). In July 2012 (1602 days after outburst) the X-ray flux of the nova strongly increased (Page et al., 2012). At that time, the duration of the SSS stage already exceeded 3 years. References: Beaky, M.M. 2008, IAUC, No. 8928 Downes, R. A., & Duerbeck, H. W. 2000, AJ, 120, 2007 Chochol, D., Shugarov, S.Yu., Pribulla, T. & Volkov, I. 2012, Mem. Soc. Astron. It., 83, 767 Henden, A., & Munari, U. 2008, IBVS, No. 5822 Iijima, T., & Naito, H. 2011, A&A, 526, A73 Kahabka, P., & van den Heuvel, E.P.G. 1997, Annual Review of Astron. Astrophys., 35, 60 Nakano, S. 2008, IAUC, No. 8927 Page, K.L. et al. 2012, The Astronomer's Telegram, No. 4286 Schwarz, G.J. et al. 2009, The Astronomer's Telegram, No. 2157 Acknowledgments: DC and SYS acknowledge support from the VEGA grant 2/0002/13. Fig. 1. The V light curve of V2468 Cygni in 2011-12 during the decline from the outburst. The full circles are mean night values of the individual CCD observations presented by grey points. The number of days, which elapsed from the outburst on March 7, 2008 are marked in the upper axis. Fig. 6. The phase V light curve corresponding to the ephemeris (1). Fig. 4. The longest night runs obtained on Sept. 23, and Nov. 18, 2012. Fig. 3. Histogram of QPOs periods in 2011 V data. Our CCD observations of V2468 Cyg In 2011 and 2012 we monitored V2468 Cyg in the V passband, with the time resolution from one to two minutes, using the 0.5m and 0.6m telescopes of the Stará Lesná observatory (Slovakia), 0.6m and 1.25m telescopes at the Crimean Laboratory of the Sternberg Institute in Nauchnyj (Ukraine), 0.6m and 1m Zeiss telescopes at Simeiz Observatory at Mt. Koshka (Ukraine), and 1m Zeiss SAO RAS telescope of the Special Astrophysical Observatory in Nizhniy Arkhyz (Karachai-Cherkesia, Russia). Quasi-periodic oscillations The light curve, presented in Fig. 1, is highly variable, due to the presence of quasi-periodic oscillations (QPOs) with amplitudes up to 0.5 mag in 2011 and 0.9 mag in May 2012. They are clearly seen in night runs taken on June 12, 2011 and May 30, 2012 (Fig. 2). The histogram of QPO periods of the nova, found from the subsequent brightness maxima in night runs obtained during the year 2011, is shown in Fig. 3. The most of the detected periods of QPOs are in the range 20 - 50 minutes.(2 The stable photometric period. The QPOs either disappeared or their amplitudes were very small after the increase of X-ray brightness in July 2012. The night runs taken on September 23 and November 18, 2012 are presented in Fig. 4. Fourier period analysis of our observations obtained in nights JD 2456006-007 and JD 2456156-250 revealed the best period 0.14525 day (3.486 hour). The periodogram is shown in Fig. 5. We determined the following ephemeris for the minima of brightness: Min = JD2456006.355 + 0.145250(5)xE (1) The phase light curve corresponding to this ephemeris is presented in Fig. 6. The amplitude of the light variations is 0.08 mag.. 1 Fig. 5. Fourier period analysis revealed the best period 0.14525 day (3.486 hour). Is the detected period orbital? The nature of SSS sources in classical novae is explained with the steady thermonuclear burning on the surface of a white dwarf with high accretion rate (Kahabka and van den Heuvel, 1997). The light variations with the period 3.486 hour are most probably caused by the orbital motion in a binary system and irradiation of the secondary star by the hot white dwarf. The rotation of the magnetic white dwarf with the magnetic axis inclined to the rotational axis can explain the periodic variations, too. In such a case a thermonuclear burning runs nearby the magnetic pole of the white dwarf. Fig. 2. V light curve of V2468 Cyg obtained on June 12, 2011 and May 30, 2012.