MULTICOLOUR INVESTIGATION OF PNV J A NEW WZ SGE-TYPE DWARF NOVA IN DRACO

Transcription

1 ISSN MULTICOLOUR INVESTIGATION OF PNV J A NEW WZ SGE-TYPE DWARF NOVA IN DRACO N. KATYSHEVA 1, S. SHUGAROV 1,2, D. CHOCHOL 2, E. PAVLENKO 3, I. VOLKOV 1, M. ANDREEV 4,5, K. ANTONYUK 3, O. ANTONYUK 3, A. BAKLANOV 3, T. IRSMAMBETOVA 1, V. MALANUSHENKO 6, N. PIT 3 and R. ZVAGELSKY 3 1 Sternberg State Astronomical Institute, Lomonosov MSU 13, Universitetskij pr., Moscow, , Russia 2 Astronomical Institute of the Slovak Academy of Sciences, Tatranská Lomnica, Slovakia 3 Crimean Astrophysical observatory, Nauchny, 98409, Crimea, Ukraine 4 INASAN, Mt. Terskol, 3605, Kabardino-Balkaria, Russia 5 Inter. Center for Astronomical, Medical and Ecological Research of NASU, Ak. Zabolotnoho Str., Kiev, Ukraine 6 Apache Point Observatory, New Mexico State University, 2001 Apache Point Road, P.O. Box 59, Sunspot, New Mexico , USA Abstract. We presentourccd observations of anew WZSge-type dwarf novain Draco (PNV J ) during its superoutburst in At the plateau of the superoutburst of this unique object, superhumps with very small amplitude were detected. During a decline of brightness, the unusual long rebrightening followed by a short one were observed. The evolution of the multicolour light curves during all stages of the superoutburst is described. The superhumps periods were determined. We suggested the existence of the late superhumps at the final decline of the superoutburst, which lasted about 170 days. Key words: cataclysmic variables - superoutbursts - superhumps - photometry 1. Introduction The WZ Sge-type binaries are a subgroup of SU UMa-subtype of cataclysmic variables (CVs) with a long (several years or even decades) recurrence time of superoutbursts with a superhump modulation that appears shortly after maximum and maintains until the beginning of a quiescence. The humps reveal longer periods in comparison with orbital ones, and evolve during the brightness decline. The superhumps of WZ Sge-type stars evolve from Cent.Eur.Astrophys.Bull. 37 (2013) 1,

2 N. KATYSHEVA ET AL. "early superhumps" with a double-humped profile near the brightness maximum and the period extremely close to the orbital one, through "ordinary superhumps" with a single humped profile and the period of a few percent longer than the orbital one, to "late superhumps". The duration of superoutburst is about days. After the plateau of the superoutburst the fast decline occurs, sometimes with rebrightening(s) short-term increase(s) of the brightness. Most of the WZ Sge-type binaries have been discovered in the last few years. The four large papers of Kato et al. (2009, 2010, 2012a, 2012b) were devoted to the evolution of superoutburst, the O C-diagrams and variations of the superhump periods of SU UMa and WZ Sge dwarf novae (dne). "Early superhumps" are one of the most remarkable features of the WZ Sge-type objects. They appear near a maximum of superoutburst and last a few days. Their periods are very close to the orbital ones. "Early superhumps" exhibit a double-humped profile. Osaki and Meyer (2002)suggested that this profile is a manifestation of the tidal 2:1 resonance in accretion disks of binarysystems with extremely low mass ratios. Another explanation was given by Lin and Papaloizou (1979) and could be due to a two-armed spiral pattern of tidal dissipation generated by the 2:1 resonance. "Ordinarysuperhumps" with a single hump profile 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. "Late superhumps" are proposed to originate in the precessing eccentric disk near the tidal truncation. The eccentric disk slowly expands during the decline of the superoutburst and finally reaches the tidal truncation, where the period is stabilized (Kato et al., 2008). Central Bureau for Astronomical Telegrams (ed. Daniel W. E. Green) gave an information about a new cataclysmic variable in Draco (Nakano et al., 2011): PNV J (J ) was discovered by H. Nishimura on September 5, 2011 as a new transient with U = 11.8 mag. U. Munari reported that a spectrum, obtained on September 6 with the 1.22m telescope at the Asiago Astrophysical Observatory, is typical to a cataclysmic variable near maximum. It is dominated by an absorption 336 Cent.Eur.Astrophys.Bull. 37 (2013) 1,

3 INVESTIGATION OF PNV J A NEW WZ SGE-TYPE DWARF NOVA continuum similar to an A-type star, with H α in emission, filling in the corresponding absorption. This night A. Arai also performed low-resolution optical spectroscopic observations of PNV J at Koyama Astronomical Observatory. The spectrum showed Balmer lines on a blue continuum light. The H α line clearly showed a strong emission feature (E.W. 300Å) in its absorption component. The H β and the H γ lines were dominated by absorption components. These results suggested that the object would be a dwarf nova during its outburst 1 (Nakano et al., 2011, Kato et al., 2012b). Kato et al. (2012b) gave a brief description of a superoutburst. They found the "early superhumps" in the early days of the superoutburst. But later they either had very small amplitude (< 0.02 mag) or disappeared. VSNET-alerts reported the very unstable humps with a period of about 0.07 days. Up to September 24, 2011 narrow superhumps appeared and then vanished (Kato et al., 2012b). During the decline stage of the superoutburst therewere no clear "ordinarysuperhumps". This behaviouris very unusual for WZ Sge-type systems. Then the brightness of J increased and a long-term rebrightening began. Kato et al. (2012b) noted that this superoutburst could be considered as double outbursts consisting of an outburst with early superhumps and another with ordinary superhumps, separated by a temporary fading. Up to now, the sequence "early superhumps" "ordinary superhumps" "late superhumps" was observed for all WZ Sge-type dwarf novae except PNV J , investigated below. 2. Our Observations We observed a superoutburst of the J from the beginning of the superoutburst till very long decline of brightness (2011, September , May 26). Our observations were carried out by the CCD cameras mounted on telescopes in Slovakia: Stará Lesná: 50-cm, Zeiss-600; Ukraine: Mt. Koshka, Crimea: Zeiss-600, Zeiss-1000; SAI, Crimea: Zeiss-600; CrAO, Crimea: K-38 (38-cm), AZT-11 (125-cm), ZTSh (2.60-m); Russia: Mt. Terskol: 35-cm, Zeiss-600; Nyzhny Arkhyz: Zeiss-1000 and USA: Apache Point Observatory: 50-cm ARCSAT. We obtained more than CCD-frames 1 Cent.Eur.Astrophys.Bull. 37 (2013) 1,

4 N. KATYSHEVA ET AL. in UBV RcRjIcIj-bands. The observational data and detailed analysis will be publish in a paper of Katysheva et al. (2013). mag Rc+Rj V B U U, B, V, Rc+Rj, Ic+Ij JD Figure 1: Top: the UBV (R j+r c)-lc of J In the R j+r c plot R j are denoted by black asterisks and R c are marked by grey points. Bottom: The combined UBV R jr c- LC of J U - black dots, B +V - grey dots, R j +R c - black and I j +I j - grey asterisks Superoutburst plateau and two rebrightenings The light curves (LCs) of the object covering the superoutburst plateau and two rebrightenings in the UBV RI-bands are plotted in Figure 1. The first rebrightening was detected on October 6, 2011 (JD ) and its brightness maximum lasted for about 14 days (JD ). The long 338 Cent.Eur.Astrophys.Bull. 37 (2013) 1,

5 INVESTIGATION OF PNV J A NEW WZ SGE-TYPE DWARF NOVA mag U Ij+Ic Rj+Rc B V JD Figure 2: A view of rebrightening for the average UBV RI magnitudes. Grey and black crosses the values of I c + I j and R c + R j, accordingly. duration of the rebrightening is very unusual. The rebrightening usually lasts for about 2 6 days. UBV (R j + R c ) LCs exhibit the plateau-like stage of superoutburst with gradual decrease in brightness, then fast decline and two rebrightenings. In Figure 1 (bottom) all LCs were combined together without any shifts in magnitudes. Both rebrightenings are clearly seen. We detected the second rebrightening, not marked by VSNET (JD ). A view of the first rebrightening, for the average UBV RI magnitudes, is shown in Figure 2. The shape of the first rebrightening looks symmetrical but it could be due to a lack of data. The duration of rebrightening is very large compared to the duration of usual rebrightenings of SU UMa-and WZ Sge-type dne. The total duration of the 1 st rebrightening was about 20 days, and its plateau phase of 14 days. It was very uncommon for a superoutburst. The system was brighterin the U-band during the superoutburst and rebrightening. But after JD (bottom of the descending branch)the colours of the system havebecome more red. Kato et al. (2012b) suggested that this rebrightening was similar to the outburst and caused by the underdevelopment of the disk. In Figure 3 the nightly R j -LCs are presented: the LCs of the beginning of superoutburst (JD , 12, 13), during the superoutburst plateau (JD , 26, 30) and during the maximum of the first rebrightening JD (JD , 42, 43). The strong superhumps appeared only after JD In each panel the date in the form JD is indicated. Cent.Eur.Astrophys.Bull. 37 (2013) 1,

6 N. KATYSHEVA ET AL. R R Figure 3: The nightly R-LCs during the plateau of superoutburst and rebrightening. The x-axis is marked in parts of the day. The periodograms of the data, obtained during the plateau stage of superoutburst and maximum of the first rebrightening are presented in Figure 4. We found almost identical peaks corresponding to the periods of days and days. We believe that the superhumps were presented in both stages. The full amplitude 340 Cent.Eur.Astrophys.Bull. 37 (2013) 1,

7 INVESTIGATION OF PNV J A NEW WZ SGE-TYPE DWARF NOVA Figure 4: Periodograms for the plateau stage (left) and for the first rebrightening (right) P=0.0724d Figure 5: The average data with the standard error for the plateau stage LC (left), original observations (grey points) and average data (black points) for the maximum of the first rebrightening. Note the different scales. of superhumps in plateau superhumps reached 0.04 mag on the level of 4 σ (Figure 5, left). During the first rebrightening the amplitude of the superhumps was 10 times larger (Figure 5, right). The observations from the decline of the first rebrightening, obtained with the 2.6-m ZTSh at JD and phased with the period P = 0. d 073, are shown in Figure 6. The amplitude is even larger than during the maximum of the first rebrightening Thelast stageof thesuperoutburstandlatesuperhumps The decline from the second rebrightening and final decline of the superoutburst is presented in Figure 7. Due to the fact, that the brightness of the object in the final decline has been decreasing with the rate of Cent.Eur.Astrophys.Bull. 37 (2013) 1,

8 N. KATYSHEVA ET AL. mag P=0.073d Phase Figure 6: The folded R c LC, 2.6-m telescope, JD Rc+R j mag/day mag/day JD Figure 7: The R c + R j LC of J after the second rebrightening and during a final decline of the superoutburst. The data from AZT-11 and 50-cm ARCSAT telescopes are marked by open circles and filled circles, respectively. mag/day, it cannot be considered as a quiescence stage. Analysis of the 76 summed CCD-images obtained in 9 nights during JD showed the existence of the days period. This result is 342 Cent.Eur.Astrophys.Bull. 37 (2013) 1,

9 INVESTIGATION OF PNV J A NEW WZ SGE-TYPE DWARF NOVA Figure 8: Last R c observations of J (JD , 93): data, folded with a period of days, and the mean LC. confirmed by a more precise last set of observations in the R c -band, carried out on June 12 th and 14 th 2012 (JD , 93) at Mt. Terskol. These data are not presented in Figure 7. The original data, folded with the period of days and the mean phase LC are drawn in Figure 8. The mean amplitude is 0.07 mag. These variations could be a late superhumps. They could survive during 170 days of the last stage of the superoutburst Summary At present we know more than 40 WZ Sge-type dwarf novae. Observations and investigation of every object is very important for our understanding of the physical processes in close binaries. A new dwarf nova in Draco certainly belongs to the WZ Sge-type binaries. There is no information about any previous outbursts of this object. The period analysis of the data from the plateau of the superoutburst revealed low amplitude oscillations with a period P = days, which could be early superhumps. Clearly visible superhumps appeared only during the maximum of the first rebrightening. We used these observations to determine the mean period of "ordinary superhumps" as P = d. We detected the second rebrightening of the object. We also determined a period of d in the final decline of the superoutburst. It should also be noted that the superhumps period of the Cent.Eur.Astrophys.Bull. 37 (2013) 1,

10 N. KATYSHEVA ET AL. object is rather long. Most of the WZ Sge objects have superhump periods < 0.06 d. Acknowledgements We are grateful to Dr. V. Goranskij (SAO, SAI) for the observations of J at the Zeiss-1000 telescope of the Special Astrophysical Observatory (Russia) and Mrs. A. Zharova (SAI) for the observations at the Zeiss-600 telescope of SAI. Our thank to an anonymous referee for constructive comments. This work was supported by grants: NSh , RFBR , RFBR , VEGA-grant 2/0038/10. EP, NK and IV are thankful for support from SAIA (Slovakia), EP was partially supported by the program "Kosmomikrofizika" of Ukrainian Academy of Science. This article was supported by the realization of the Project ITMS No , based on the supporting operational Research and development program financed from the European Regional Development Fund. References Kato, T. et al.: 2008, Publ. Astron. Soc. Jpn. 60, L23. Kato, T., Imada, A., Uemura, M. et al.: 2009, Publ. Astron. Soc. Jpn. 61, S395. Kato, T., Maehara, H., Uemura, M., et al.: 2010, Publ. Astron. Soc. Jpn. 62, Kato, T., Maehara, H., Miller, I., et al.: 2012a, Publ. Astron. Soc. Jpn. 64, 21. Kato, T., Hambsch, F.-J., Maehara, H., et al.: 2012b, Publ. Astron. Soc. Jpn., astro-ph v1 (in press). Katysheva, N., Shugarov, S., Chochol, D. et al.: 2013, Contrib. Astron. Obs. Skalnaté Pleso (in preparation). Lin, D.N.C. and Papaloizou, J.: 1979, Mon. Not. R. Astron. Soc. 186, 799. Nakano, S., Nishimura, H., Noguchi, T., and Munari, U.: 2011, CBET Osaki, Y.: 1989, Publ. Astron. Soc. Jpn., 41, Osaki, Y. and Meyer, F.: 2002, Astron. Astrophys. 383, 574. Whitehurst, R.: 1988, Mon. Not. R. Astron. Soc. 232, Cent.Eur.Astrophys.Bull. 37 (2013) 1,