Monitoring of the WZ Sge-type system V455 And before, during and after superoutburst.

1. Monitoring of the WZ Sge-type system V455 And before, during and after superoutburst. N.A. Katysheva2, S.Y. Shugarov1,2 1) Astronomical Institute of the Slovak Academy of Sciences, 059 60 Tatranská Lomnica, The Slovak Republic 2) Sternberg State Astronomical Institute, Universitetskij Prosp. 13, 119992, Moscow, Russia We summed the light curves of V455 And with the orbital period 0.05630921 d. This ephemeris : Min = 24551812.67765+ 0.05630921*E was determined in paper by Araujo-Betancor et al. (2005) and works till now, so during the 60000 cycles (till the end of 2009) the orbital period does not change. On the Fig.4 the phase light curve of October 2008 is plotted. Two-hump LC is similar the light curves of WZ Sge-type stars in quiescence. This LC was constructed from reduced B,V & R after removal of the declining trends and averages. WZ Sge-type dwarf novae The WZ Sge-type objects are the most extreme subgroup of SU UMa-type DNe with a long (several years) superoutbursts recurrence time. Their orbital periods are the shortest ones observed for SU UMa-type stars. Some of them exhibit a complex post-superoutburst rise of brightness called rebrightening(s), rarely seen in other SU UMa-type DNe. One of the most remarkable signatures of WZ Sge-type objects is a presence of ”early superhumps” during the earliest stages of superoutbursts. This feature is also referred to as orbital superhumps or outburst orbital humps. Early superhumps have a period extremely close to the binary period and commonly show a double–humped profile, in contrast to the ordinary superhumps of SU UMa-type dwarf novae. Early superhumps are the most discriminative feature of the WZ Sge-type objects, and have not been detected in other DNe. V455 And (= HS2331+3905) was discovered by Gaensicke et al. from the Hamburg Quasar SUrvey. Araujo-Betancor et al. (2005) analyzed their data described the main characteristics of this. They defined HS2331 as a cataclysmic variable with very wide spectra of the periods – orbital (81.08 min) and superhump ones (83.38 min), non-radial pulsations of white dwarf (5 – 6 min) and a coherent signal at 1.12 min. The spectroscopic period was about of 3.5 hour. Araujo-Betancor et al. (2005) proposed a possible superoutburst of HS2331+3905 as WZ Sge-type binary. And superoutburst happened in September, 2007. The paper by Matsui et al. (2009), Maegara et al. (2009)described the photometrical observations of superoutburst. Nogami et al. (2009) carried out the spectroscopic observations V455 And during the superoutburst. Fig. 4. Two-hump phase light curve of the 16-17 October 2008 (below, left). (delta BVR and mean, folded with the orbital period). Fig. 1. The del V light curves summed with the orbital period on the zero-day of the outburst (05 September 2007). In the left-top corner – gaps on the LC are because of clouds, in the right-bottom - a part of V-light curve from  ~0.4 till 0.8 (P=0.05630921 d). Our CCD observations of V455 And Most bulk of our BVR-CCD observations were taken with the camera Apogee-47a mounted in the 0.6m Zeiss-reflector at the Crimean laboratory of SAI, further CCD observations were taken with the Pictor-416 cameras (0.5m Maksutov (f/4.0) telescope at the Crimean Laboratory ofSAI. We carried out the monitoring of V455 And from 2004 to 2009 yr. Katysheva and Shugarov (2009) observed this system before superoutburst (in 2004  2006) and studied the photoplates from Moscow archive. The limit of these Moscow plates is about of 12 –13 magnitudes. On the 45th plates from 1907 till 1957 there were not any sign of outbursts on them. The nine month before superoutburst of 2007 yr there were no significant signs of any activities except mentioned by Araujo-Betancor et al. (2005) (Katysheva & Shugarov, 2009). Zero day of superoutburst  the 5th of September 2007. We carried out the observations on the 5, 8, 9, 10 and 11 and then 17, 18, 19 and 20 September 2007 yr. So it was the beginning of superoutburst. The further set of observations was in November of 2007. In Fig.1 we present the nightly LCs in V-band during the zero-day of superoutburst. The numerous humps are seen clearly between phase 04 - 0.8. In Fig.2 we plotted the overall V-LC and nightly LCs summed with the orbital period. The complex structure of LC (with many humps) especially at the beginning of superoutburst seems to connect with the turbulent processes in accrecion disc. The interesting detail is double-peak maxima at 5th (10 Sept.) and 12th days (17 Sept.) of superoutburst and (for example) in quiescence (the 13th and 15th of November 2007). On the LC of Nov. 18, 2007 – there is very clear orbital minimum. Three days in November 13, 15 and 18 – and there are so different LCs in spite of the transition to quiescence. It says about unsteady disc processes. Fig. 4. Two-hump phase light curve of the 16-17 October 2008 (below, left). (delta BVR and mean, folded with the orbital period). Fig. 5. Phased light curve of V455 And and the mean light curve (November 2009) –left. At the right – average light curve folded with the superhump period 0.05735 d. Early, ordinary and late superhumps The early superhumps are a remarkable feature of WZ Sge-type stars. They appear near the maximum magnitude of superoutburst and have periods almost identical to the orbital one. Osaki and Meyer (2002) suggested that a double peaked profile of early superhumps is manifestation of the tidal 2:1 resonance in accretion disks of binary systems with extremely low mass ratios. Early superhumps can be explained by a two-armed spiral pattern of tidal dissipation generated by the 2:1 resonance. According to Kato (2002), the expansion beyound the 3:1 resonance radius can be responsible for the appearance of “early superhumps”. The common, or ordinary superhumps, with a single peak profile, appear during the plateau of superoutburst of WZ Sge-type stars. Their periods are a few percent longer than the orbital period. The ordinary superhumps can be explained by the thermal tidal instability model of an accretion disk (Osaki, 1989; Whitehurst, 1988). The presence of the tidal 3:1 resonance in the disk (with the radius smaller than the 2:1 resonance radius) results in the formation of an eccentric outer ring undergoing apsidal precession with a period appreciably longer than the orbital one. The beating of the orbital and precessional periods cause periodic variations, identified as superhumps. This model is supported by numerical simulations (Bisikalo et al., 2005). The late superhumps appear several days after the rapid decline from the plateau of a superoutburst and may continue for several hundred cycles after the end of the superoutburst. According to Kato et al. (2008), the late superhumps originate in the precessing eccentric disk near the tidal truncation. The eccentric disk slowly expands during the superoutburst decline and finally reaches the tidal truncation, where the period is stabilized. Fig. 2. The overall light curve of the beginning of superoutburst (top) and the nightly light curves of V455 And (days 4-6 and 12-15 after outburst (AO)) – from top to bottom (HJD 2400000+). Fig. 3. The nightly and mean orbital phase light curves (R-band). The ephemeris: JDmin = 2451812.67765 + 0.05630921*E. The 13th, 15th and 18th of November 2007. (Top) – Nov. 13, (bottom, right -Nov. 15. Bottom – Nov. 18) References S. Araujo-Betancor, B.T. Gaensicke, J.-H. Hagen et al., 2005, Astron. Astroph., 430, 629. D. VBisikalo,., A.A. Boyarchuk, P.V. Kaygorodov, O.A. Kuznetsov, T. Matsuda, T. 2005, AIP Conf. Proc. 797, 295 T. Kato, 2002, Publ. Astron. Soc. Japan, 54, L11. T. Kato, A. Imada, M. Uemura, Makoto, 2009, Publ. Astron. Soc. Japan, 61S, 395 T. Kato, H. Maehara, B. Monard, 2008, Publ. Astron. Soc.Japan 60, L23 N. Katysheva, S. Shugarov, 2009, Journal of Physics, Conf. Ser. S.172a2044K (http://iopscience.iop.org/1742-6596/172/1/012044) H. Maehara, A. Imada, K.Kubota, et al., in: “The Eighth Pacific Rim Conference on Stellar Astrophysics: A Tribute to Kam-Ching Leung”, ASP Conf. Ser., Vol. 404, San Francisco, p. 57 R. Matsui, M. Uemura, A. Arai et al., 2009, Publ. Astron. Soc. Japan, 61, 1081. D. Nogami, K. Hiroi, Y. Suzuki et al., in: “The Eighth Pacific Rim Conference on Stellar Astrophysics: A Tribute to Kam-Ching Leung”, ASP Conf. Ser., Vol. 404, San Francisco, p. 52 • Y. Osaki, 1989, Publ. Astron. Soc. Japan, 41, 1005 Y. Osaki, F..Meyer, 2002, Astron. Astrophys. 383, 574 • R. Whitehurst, 1988, Mon. Not. R. Astron. Soc. 232, 35 Physics of Accreting Compact Binaries, July 26(Mon) - 30(Fri), 2010, Kyoto University, Kyoto, Japan