GALEX J194419.33+491257.0: An unusually active SU UMa-type dwarf nova with a very short orbital period in the Kepler data

Publications of the Astronomical Society of Japan, Apr 2014

Kato, Taichi, Osaki, Yoji

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GALEX J194419.33+491257.0: An unusually active SU UMa-type dwarf nova with a very short orbital period in the Kepler data

Publ. Astron. Soc. Japan GALEX J194419.33+491257.0: An unusually active SU UMa-type dwarf nova with a very short orbital period in the Kepler data hi KATO 1 Yoji OSAKI 0 0 Department of Astronomy, School of Science, The University of Tokyo , 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 1 Department of Astronomy, Kyoto University , Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502 We studied a background dwarf nova of KIC 11412044 in the Kepler public data and identified it with GALEX J194419.33+491257.0. This object turned out to be a very active SU UMa-type dwarf nova that has a mean supercycle of ∼ 150 d and frequent normal outbursts with intervals of 4-10 d. The object showed a strong persistent signal of the orbital variation with a period of 0.0528164(4) d (76.06 min) and superhumps with a typical period of 0.0548 d during its superoutbursts. Most of the superoutbursts were accompanied by a precursor outburst. All these features are unusual for this very short orbital period. We succeeded in detecting an evolving stage of superhumps (stage A superhumps) and obtained a mass ratio of 0.141(2), which is unusually high for this orbital period. We suggest that the unusual outburst properties are a result of this high mass ratio. We suspect that this object is a member of the recently recognized class of cataclysmic variables (CVs) with a stripped core evolved secondary which are evolving toward AM CVn-type CVs. The present determination of the mass ratio by using stage A superhumps is the first case in such systems. accretion; accretion disks; novae; cataclysmic variables; stars; dwarf novae; stars; individual (GALEX J194419; 33+491257; 0) 1 Introduction The Kepler mission (Borucki et al. 2010; Koch et al. 2010) , which aimed to detect extrasolar planets, has provided unprecedented quality data on several cataclysmic variables (CVs). This satellite also recorded previously unknown CVs as by-products of the main target stars. The best documented example has been a background dwarf nova of KIC 4378554 (Barclay et al. 2012; Kato & Osaki 2013a) . In addition to this object, the Planet Hunters group (Fischer et al. 2012) detected several candidate background CVs.1 We studied one of these background dwarf novae, the one in the field of KIC 11412044 (hereafter J1944). This 1 http://talk.planethunters.org/objects/APH51255246/discussions/DPH101e5xe . 3 Characterization and identification of object 3.1 Outburst properties object was discovered by the Planet Hunters group to be a background SU UMa-type dwarf nova of KIC 11412044, of which superoutbursts and frequent normal outbursts were recognized.2 Since it was bright enough and was frequently included in the aperture mask of KIC 11412044, its outburst behavior can be immediately recognized in the Kepler SAP_FLUX light curve of KIC 11412044. The resultant light curve indicates that this object is an SU UMa-type dwarf nova with frequent outbursts (figure 1). There were eight observed superoutbursts, and from the regular pattern, another superoutburst most likely occurred between BJD 2455553 and 2455568 (a data gap in Q8) and 2 Data analysis we numbered the superoutburst and supercycle assuming We used Kepler public long cadence (LC) data (Q1–Q17) that there is a superoutburst in this gap. The interfor analysis. Since the outbursts were immediately recog- vals between successive superoutbursts (supercycles) were nizable in each light curve of the Kepler target pixel images, within a range of 120–160 d. We determined a mean superwe used a custom aperture consisting of 4–6 pixels showing cycle of 147(1) d. Most of the superoutbursts were associoutbursts as we did in the background dwarf nova of KIC ated with a precursor outburst which is variously separated 4378554 (Kato & Osaki 2013a). We used surrounding from the main superoutburst. The typical duration of the pixels to subtract the background from KIC 11412044. superoutburst is ∼ 8 d, inclusive of the precursor part. This We further corrected small long-term baseline variations duration is shorter than that of many other SU UMa-type by subtracting a locally weighted polynomial regression dwarf novae. (LOWESS: Cleveland 1979) and spline functions. Since the The number of normal outbursts in one supercycle quiescent magnitude is difficult of determination, we artifi- ranged from 11 to 21. The interval of normal outbursts cially set the level to be 22.0 mag. was 4–10 d, one of the shortest known to us except for ER UMa stars (Kato et al. 1999). The amplitude of normal outbursts increased as the supercycle phase progresses. Some of the normal outbursts “failed”; i.e., they decayed before 2 http://keplerlightcurves.blogspot.jp/2012/07/dwarf-novae-candidates-at-planet.html . reaching the full maximum. 3.2 Frequency analysis and source identification As shown in figure 2, a two-dimensional Fourier analysis (using the Hann window function) of the light curve of this object yielded two periods. There was a signal of a constant frequency (18.93 cycles d−1) with almost constant strength. Using the whole data segment, we determined the period; it measures 0.0528164(4) d (18.934 cycles d−1). We refer to this signal as the “0.0528 d” signal. Based on the high stability of the 0.0528 d signal during the entire Kepler observation of this object, we identified this period as the orbital period (Porb) of this object. During superoutbursts, there were transient signals of superhumps at frequencies around 18.1–18.5 cycles d−1 as expected. Let us now examine the source position of the background dwarf nova in figure 3. We checked the pixels which showed the dwarf nova-type variation. The peak signal of the dwarf nova-type outbursts was found to be two pixels away from the center of KIC 11412044 (star 1 on the DSS image). At this location, there is a GALEX (Martin et al. 2005) ultraviolet (UV) source, GALEX J194419.33+491257.0 [near-UV magnitude 21.3(3)], and we identified this source as the UV counterpart of this dwarf nova (figure 3, Q16), confirming the suggestion in the Planet Hunters’ page. The superhump component and 0.0528 d one were also confirmed at the location of this object (figure 3, Q14), and we consider that the 0.0528 d signal indeed comes from this dwarf nova. This has also been confirmed by the nondetection of the 0.0528 d signal in the SAP_FLUX of KIC 11412044 when this dwarf nova was outside the aperture of KIC 11412044. 3.3 Variation of superhump period Since the superhump period is less than three LC exposures, it is difficult to determine the times of superhump maxima by the conventional method. We employed the Markovchain Monte Carlo (MCMC) modeling used in Kato and Osaki (2013a). Although we show only the result of SO3 (figure 4), the pattern is similar to those of other superoutbursts. In the O − C diagram, stages A–C (for an explanation of these stages, see Kato et al. 2009) can be recognized. Long-period superhumps (stage A superhumps) with growing superhumps were recorded during the late part of the precursor outburst to the maximum of the superoutburst. The overall pattern is very similar to those of other SU UMa-type dwarf novae, including V1504 Cyg and V344 Lyr (Osaki & Kato 2013, 2014). The object is the fourth case (after V1504 Cyg, V344 Lyr, and V516 Lyr) in the Kepler field in which the growing superhumps lead smoothly from the precursor to the main superoutburst, and thus gives further support to the thermal tidal instability (TTI) model (Osaki 1989) as an explanation for the superoutburst. 3.4 System properties The inferred fractional superhump period excess ε ≡ (PSH/Porb) − 1, where PSH is the superhump period, of ∼ 3.8% is, however, unusually large for this Porb (cf. figure 15 in Kato et al. 2009) . Kato and Osaki (2013b) recently proposed that stage A superhumps can be used for determining the mass ratio (q = M2/M1) and the resultant mass ratios are as accurate as those obtained from eclipse modeling. We have succeeded in measuring the period of a stage A superhump during the three superoutbursts: 0.0555(2) d (SO3), 0.05546(5) d (SO6), and 0.05552(6) d (SO7). The corresponding fractional superhump excesses in the frequency unit ε∗ ≡ 1 − (Porb/PSH) are 4.8%, 4.77%, and 4.88%. These values correspond to the q values of 0.14, 0.139, and 0.143, respectively. We therefore adopted q = 0.141(2). This mass ratio implies a massive (approximately two times more massive) secondary for this very short orbital period as compared with most WZ Sge-type dwarf novae (figure 5). This result may alternatively suggest the possibility of an unusually low-mass white dwarf. If we assume that the secondary of J1944 has a normal mass for this orbital period, such as 0.066 M in WZ Sge (Kato & Osaki 2013b), the mass of the white dwarf must be ∼ 0.47 M . Zorotovic, Shreiber, and Ga¨ nsicke (2011) indicated that the fraction of CVs having white dwarf lighter than 0.5 M is only 7% ± 3%, even including suspicious measurements. Furthermore, there is evidence from modern eclipse observations that the mass of the white dwarf in short-Porb CVs is not diverse (Savoury et al. 2011). We therefore consider the interpretation of a massive secondary to be more likely. The presence of a precursor outburst and the high frequency of normal outbursts are usual features of longer-Porb systems such as V1504 Cyg and V344 Lyr. Systems with Porb such as J1944 are usually WZ Sge-type dwarf novae with very rare (super)outbursts (e.g., Kato et al. 2001) or ER UMa-type dwarf novae, a rare subgroup with very frequent outbursts and short supercycles (e.g., Kato & Kunjaya 1995; Robertson et al. 1995) . J1944 does not have any property of either group. This can be understood if the outburst properties are a reflection of the mass ratio rather than the orbital period since the q value of J1944 is closer to those of longer-Porb SU UMa-type dwarf novae. The presence of such a system would pose a problem in terms of the CV evolution since the secondary loses its mass during the CV evolution and the q value is expected to be as low as ∼ 0.08 around the orbital period of J1944. In recent years, some objects showing hydrogen lines in their spectra (this excludes the possibility of doubledegenerate AM CVn-type systems) have been discovered around this period or even at shorter one. These objects include EI Psc (Uemura et al. 2002; Thorstensen et al. 2002), V485 Cen (Augusteijn et al. 1996) , and GZ Cet (Imada et al. 2006) . These objects are considered to be CVs whose secondaries had an evolved core at the time of the contact, and to be progenitors of AM CVn-type double white dwarfs (Thorstensen et al. 2002; Uemura et al. 2002; Podsiadlowski et al. 2003; Nelemans et al. 2004). None of these objects have been reported for q determination directly from radial-velocity studies, and q values have only been inferred from the traditional ε, which has an unknown uncertainty (Kato & Osaki 2013b). The detection of stage A superhumps in J1944 allowed the first reliable determination of q in such stripped-core ultracompact binaries. There is, however, a marked difference in outburst frequency between J1944 and these known objects since the frequency of outbursts in such systems has been reported to be low (Thorstensen 2013). This suggests that J1944 has an anomalously high mass-transfer rate among these objects. The object may be in a phase analogous to ER UMa-type dwarf novae, whose high mass-transfer rates may be a result of a recent classical nova explosion (cf. Kato & Kunjaya 1995; Patterson et al. 2013) . Since the object can be within the reach of ground-based telescopes, the exact optical identification and the search for the feature of the secondary star encourage us to solve the mystery. Acknowledgement We thank the Kepler Mission team and the data calibration engineers for making Kepler data available to the public. We also thank the Planet Hunters group for making their information on the background dwarf novae public so as to enable us to study this interesting object. This work was supported by a Grant-in-Aid “Initiative for High-Dimensional Data-Driven Science through Deepening of Sparse Modeling” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. Kato, T., Nogami, D., Baba, H., Masuda, S., Matsumoto, K., & Kunjaya, C. 1999, in Disk Instabilities in Close Binary Systems, ed. S. Mineshige & J. C. 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Kato, Taichi, Osaki, Yoji. GALEX J194419.33+491257.0: An unusually active SU UMa-type dwarf nova with a very short orbital period in the Kepler data, Publications of the Astronomical Society of Japan, 2014, DOI: 10.1093/pasj/psu025