Discovery of superhumps during a normal outburst of SU Ursae Majoris
Akira Imada, Hideyuki Izumiura, Daisuke Kuroda, Kenshi Yanagisawa, Nobuyuki Kawai, Toshihiro Omodaka, Ryo Miyanoshita
aa r X i v : . [ a s t r o - ph . S R ] O c t PASJ:
Publ. Astron. Soc. Japan , 1– ?? , c (cid:13) Discovery of superhumps during a normal outburst of SU Ursae Majoris
Akira
Imada , Hideyuki Izumiura , Daisuke Kuroda , Kenshi Yanagisawa ,Nobuyuki Kawai , Toshihiro Omodaka , and Ryo Miyanoshita Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, Asakuchi,Okayama 719-0232 Department of Physics, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro-ku, Tokyo 152-8551 Faculty of Science, Kagoshima University, 1-21-30 Korimoto, Kagoshima, Kagoshima 890-0065 (Received ; accepted )
Abstract
We report on time-resolved photometry during a 2012 January normal outburst of SU UMa. Thelight curve shows hump-like modulations with a period of 0.07903(11) d, which coincides with the knownsuperhump period of SU UMa during superoutbursts. We interpret this as superhump, based on theobserved periodicity, profiles of the averaged light curve, and the g ′ − I c variation during the normaloutburst. This is the first case that superhumps are detected during an isolated normal outburst of SUUMa-type dwarf novae. The present result strongly suggests that the radius of the accretion disk alreadyreaches the 3:1 resonance even in the midst of the supercycle. Key words: accretion, accretion disks — stars: dwarf novae — stars: individual (SU Ursae Majoris)— stars: novae, cataclysmic variables — stars: oscillations
1. Introduction
SU UMa-type dwarf novae, a subclass of dwarf novae,exhibit two types of outbursts: normal outburst and su-peroutburst (for a review, see Warner 1995; Osaki 1996).During superoutburst, hump-like modulations called su-perhumps are visible. Basically, the light source of super-humps is understood as phase-dependent tidal dissipationin an eccentric accretion disk (Whitehurst 1988; Hirose,Osaki 1990). The general consensus is that an eccentricityof the accretion disk is excited by a 3:1 orbital resonance(Osaki 1989). According to the original thermal-tidal in-stability model (TTI model, Osaki 1989), the radius of theaccretion disk monotonically increases with each normaloutburst. When the disk finally reaches the 3:1 resonanceradius, the accretion disk is tidally deformed and triggersa superoutburst. The TTI model reproduces well basicbehavior of SU UMa-type dwarf novae. However, a re-form of the TTI model may be required, particularly insystems with unusual recurrence times of superoutbursts(Hellier 2001; Patterson et al. 2002; Osaki, Meyer 2003).Over the past few years, research activity concerning SUUMa-type dwarf novae has been significantly improved.One of the most important research is that unprecedentedphotometric surveys during superoutbursts have been car-ried out by T. Kato and his colleagues (Kato et al. 2009;Kato et al. 2010; Kato et al. 2012). They collected allof available data and analyzed the light curves during su-peroutbursts, from which they have established the basicpicture of the superhump period changes (see figure 3 ofKato et al. (2009)). Another important research to benoted is that the
Kepler satellite has provided us withunprecedentedly precise light curves at the one-minitecadence (Borucki et al. 2010; Haas et al. 2010). This allows us to investigate detailed light curves that can-not be achieved under ground-based observations (Stillet al. 2010; Cannizzo et al. 2010; Wood et al. 2011; Katoet al. 2012; Cannizzo et al. 2012). Although these surveysimprove our understanding of SU UMa-type dwarf novae,the diversity of the observations claims further modifica-tion of the TTI model.In order to decipher and understand the observed diver-sity of SU UMa-type dwarf novae, we started a new ap-proach: simultaneous multicolor photometry of dwarf no-vae not only during outburst but also during quiescence.As a first step, we performed multicolor photometry ofSU UMa itself from 2011 December to 2012 February. SUUMa is a prototype of SU UMa-type dwarf novae rang-ing V =11.3-15.7 (Ritter, Kolb 2008) and its orbital pe-riod is determined to be P orb =0.07635 d (Thorstensenet al. 1986). However, anomalous behavior with shortand long time scales were reported in the previous studies(Rosenzweig et al. 2000; Kato 2002). In this letter, wereport on detection of superhumps during a 2012 Januarynormal outburst. This is the first recorded superhumpsthat emerged in the middle of a supercycle. Results ofthe whole observations will be discussed in a forthcomingpaper.
2. Observation and Result
Time-resolved CCD photometry were performed from2011 December 1 to 2012 February 20 at OkayamaAstrophysical Observatory using the 50-cm MITSuMEtelescope, which is able to obtain g ′ , R c , and I c bandssimultaneously (Kotani et al. 2005). In this letter, weextracted data from 2012 January 4 to 7, during whichSU UMa experienced a normal outburst. The exposure A.Imada et al. [Vol. ,
12 13 14 15 16 0 10 20 30 40 50 60 R c m agn i t ude HJD-2455900
Fig. 1. R c band light curve of SU UMa. The abscissa and ordinate denote HJD − R c magnitude, respectively. Thenormal outburst in which superhumps are detected is marked with an arrow. Note that this normal outburst is held between twonormal outbursts. T he t a Period(day)
Fig. 2.
PDM analysis during HJD 2455932-34, correspond-ing to the declining stage of the normal outburst. A weak sig-nal can be seen at P =0.07903(11) d, almost identical to themean superhump period during the 1989 April superoutburstof SU UMa. Also seen is the periodicity at P =0.07616(11) d,slightly shorter than the orbital period of SU UMa. time was 30 s with a read-out time as short as 1 s.The data were processed under the standard manner us-ing IRAF/daophot . After removing bad images, we ac-quired available 1577 images for g ′ band, 1588 imagesfor R c band, and 1587 images for I c band, respectively.Differential photometry were carried out using Tycho-2 4126-00036-1 (RA: 08:12:45.104, Dec: +62:26:17.57),whose constancy was checked by nearby stars in the sameimage. Heliocentric correction was made before the fol-lowing analyses.Figure 1 shows R c band light curve of SU UMa between2011 December 3 and 2012 February 2. On 2012 January 4 IRAF (Image Reduction and Analysis Facility) is distributedby the National Optical Astronomy Observatories, which isoperated by the Association of Universities for Research inAstronomy, Inc., under cooperative agreement with NationalScience Foundation. (HJD 2455931), the magnitude monotonically brightenedat a rate of -1.41(4) mag/d, indicating the initiation ofthe outburst. The magnitude reached R c ∼ ∼ bright quiescence lasted until the next normal outburst. More details arebeyond the scope of this letter and will be discussed in aforthcoming paper. Taking this observation into consider-ation, we infer that the normal outburst ended until 2012January 8 (HJD 2455935).We performed the phase dispersion minimizationmethod (PDM, Stellingwerf 1978) for estimation of pe-riods during the normal outburst. After removing the de-clining trends, we combined light curves on 2012 January5, 6, and 7. The resultant theta diagram on R c bandis displayed in figure 2. As can be seen in this fig-ure, two strong signals coincide with P =0.07616(11) dand P =0.07903(11) d, respectively. The former period isvery close to the orbital period of the system but slightlyshorter. The latter period, on the other hand, is in ex-cellent agreement with the mean superhump period dur-ing the 1989 April superoutburst of SU UMa reported byUdalski (1990).In order to clarify the nature of this periodicity, weobtained phase-averaged R c band light curve and g ′ − I c color folded with 0.07903 d, which are given in figure 4.Although the data contain a secondary peak around phase0.4, a rapid rise and slow decline, characteristic of super-humps are visible. Furthermore, this profile bears signif-icant resemblance to that obtained in the previous study(Kato 2002). As for g ′ − I c color, the bluest peak is priorto the maximum timing of the R c light curve by φ ∼ We also folded the R c light curve See also Uemura et al. (2008) in which phase discordance be- o. ] 3 -0.15-0.1-0.05 0 0.05 0.1 0.15 0.2-0.5 0 0.5 1 1.5 D i ff. m agn i t ude D i ff. g ’ - I c Phase
Fig. 3.
Phase averaged R c (filled circle) and g ′ − Ic color(filled square) after folding with P =0.07903 d. Although R c light curve shows the secondary peak, a rapid rise and slow de-cline, characteristic of superhumps are visible. Note that thebluest peak in g ′ − I c is prior to R c by phase ∼ -0.15-0.1-0.05 0 0.05 0.1 0.15 0.2-0.5 0 0.5 1 1.5 D i ff. m agn i t ude D i ff. g ’ - I c Phase
Fig. 4.
Same as figure 3 but folding with P =0.07916 d.Double-peaked modulations, remiscent of orbital humps, arevisible. Note that the bluest peak in g ′ − I c is in accordancewith the magnitude peak in R c . Datapoints are verticallyshifted for display purpose. and g ′ − I c with 0.07616 d, which are given in figure 4. Inthis figure, a significant difference compared with figure 3is that the peak R c magnitudes coincide with the bluestpeaks in g ′ − I c .
3. Discussion
In the previous studies, many authors have reportedthe presence of (late) superhumps after the end of the su-peroutburst (Patterson et al. 2002; Patterson et al. 1998;Uemura et al. 2002; Kato et al. 2004; Kato et al. 2008;Kato et al. 2009). This phenomenon can be understoodif the accretion disk persist on eccentricity after the end tween V and J was reported during the 2006 superoutburst ofSDSS J102146.44+234926.3. of the superoutburst. Recently, Kato et al. (2012) de-tected superhumps during a normal outburst of V1504Cyg. After this normal outburst, V1504 Cyg returned toquiescence and the subsequent outburst was erupted assuperoutburst. In this case, full development of super-humps may have been prevented despite the radius of theaccretion exceeding the 3:1 resonance. In any case, super-humps are observed in the vicinity of a superoutburst.According to the AAVSO light curve generator, SUUMa experienced superoutbursts on 2011 July and 2012May. This means that the 2012 January normal outburstis independent of the superoutbursts. As for magnitudesand color behavior, Matsui et al. (2009) suggest that su-perhump phase discordance between them is associatedwith the heating or cooling process in the accretion disk.In combination with the observed periodicity, profile ofthe light curve, and behavior of color index in g ′ − I c , wecan conclude that this is the first example that super-humps are observed during an isolated normal outburstof SU UMa-type dwarf novae. Our present result furtherindicate that the radius of the accretion disk exceeds the3:1 resonance radius even in the middle of the supercycle.Recently, Cannizzo et al. (2012) studied Kepler data ofV344 Lyr, in which the radius at which the thermal in-stability sets in ( r trig ) may be the largest roughly in themidst of a supercycle. Although r trig may not be nec-essarily linked with the radius of the accretion disk, thisresult provides the potential possibility that the accretiondisk exceeds the 3:1 resonance radius even in the midstof quiescence. In order to test our hypothesis, quiescentspectroscopy should be performed, which enables us tomeasure the radius of the accretion disk.Finally, we briefly discuss figure 4. As already describedabove, the most important point is that the peak magni-tude coincides with color index in g ′ − I c . This impliesthat the main light source is influenced by a geometriceffect of the accretion disk rather than the disk processitself. If this is the case, then negative superhumps in ERUMa stars, possibly originated from a tilted disk, showphase accordance between magnitude and color (Ohshimaet al. 2012). This should be clarified in future observa-tions.We express our gratitude to Daisaku Nogami for hisconstructive comments on the manuscript of the letter.We acknowledge with thanks the variable star obser-vations from the AAVSO International Database con-tributed by observers worldwide and used in this research.A.I. and H.I. are supported by Grant-In-Aid for ScientificResearch (A) 23244038 from Japan Society for Promotionof the Science (JSPS). This work is partly supportedby Optical & Near-infrared Astronomy Inter-UniversityCooperation Program, supported by the MEXT of Japan. References
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