[show abstract][hide abstract] ABSTRACT: We report the observation of a new dwarf nova, OT J055717+683226, during its first-ever recorded superoutburst in December 2006. Our observation shows that this object is an SU UMa-type dwarf nova having a very short superhump period of 76.67+/- 0.03 min (0.05324+/-0.00002 d). The next superoutburst was observed in March 2008. The recurrence time of superoutbursts (supercycle) is, hence, estimated to be ~480 d. The supercycle is much shorter than those of WZ Sge-type dwarf novae having supercycles of >~ 10 yr, which are a major population of dwarf novae in the shortest orbital period regime (<~85 min). Using a hierarchical cluster analysis, we identified seven groups of dwarf novae in the shortest orbital period regime. We identified a small group of objects that have short supercycles, small outburst amplitudes, and large superhump period excesses, compared with those of WZ Sge stars. OT J055717+683226 probably belongs to this group. Comment: 14 pages, 11 figures, accepted for publication in PASJ
[show abstract][hide abstract] ABSTRACT: We report on optical and infrared photometric observations of a WZ Sge-type dwarf nova, V455 And during a superoutburst in 2007. These observations were performed with the KANATA (V, J, and K_s bands) and MITSuME (g', Rc, and Ic bands) telescopes. Our 6-band simultaneous observations allowed us to investigate the temporal variation of the temperature and the size of the emitting region associated with the superoutburst and short-term modulations, such as early and ordinary superhumps. A hot (>11000 K) accretion disk suddenly disappeared when the superoutburst finished, while blackbody emission, probably from the disk, still remained dominant in the optical region with a moderately high temperature (~8000 K). This indicates that a substantial amount of gas was stored in the disk even after the outburst. This remnant matter may be a sign of an expected mass-reservoir which can trigger echo outbursts observed in several WZ Sge stars. The color variation associated with superhumps indicates that viscous heating in a superhump source stopped on the way to the superhump maximum, and a subsequent expansion of a low-temperature region made the maximum. The color variation of early superhumps was totally different from that of superhumps: the object was bluest at the early superhump minimum. The temperature of the early superhump light source was lower than that of an underlying component, indicating that the early superhump light source was a vertically expanded low-temperature region at the outermost part of the disk. Comment: 14 pages, 12 figures, PASJ accepted
Publications- Astronomical Society of Japan 08/2009; · 2.44 Impact Factor
[show abstract][hide abstract] ABSTRACT: WZ Sge-type dwarf novae are characterized by very long recurrence times (>= 10 yr). They also exhibit various types of rebrightenings after main outbursts, while the mechanism of the rebrightening phenomenon is poorly understood. Here, we report simultaneous g, V, Rc, Ic, J, and Ks-band observations throughout WZ Sge-type superoutbursts. We found unprecedented infrared activities, for example, prominent infrared superhumps and a temporary quite red state just after superoutbursts. They provide evidence for the presence of a mass reservoir at the outermost part of accretion disks, which is responsible for the rebrightening phenomenon.
[show abstract][hide abstract] ABSTRACT: We report detailed, long term near-infrared (NIR) light curves of GRS 1915+105 in 2007-2008, covering its long "soft state" for the first time. From our NIR monitoring and the X-ray data of the All Sky Monitor (ASM) onboard Rossi X-ray Timing Explorer (RXTE), we discovered that the NIR flux dropped by > 1 mag during short X-ray flares with a time-scale of days. With the termination of the soft state, the H-Ks color reddened and the anti-correlation pattern was broken. The observed H-Ks color variation suggests that the dominant NIR source was an accretion disk during the soft state. The short X-ray flares during the soft state were associated with spectral hardening in X-rays and increasing radio emission indicating jet ejection. The temporal NIR fading during the X-ray flares, hence, implies a sudden decrease of the contribution of the accretion disk when the jet is ejected.
Publications- Astronomical Society of Japan 05/2009; · 2.44 Impact Factor
[show abstract][hide abstract] ABSTRACT: We discovered the first outburst of V455 And (=HS2331+3905) on Sep. 4 2007 at V=14.59, which has been considered a good candidate for the WZ Sge-type dwarf nova. A time-resolved photometry campaign was soon coordinated and covered the whole of the superoutburst from the rising phase to the slow fading tail. At the time of our outburst detection, the object was in the rising phase at a rate of 10 mag/day. We found that there were no orbital humps nor early superhumps during the rapid rising phase of superoutburst. Early superhumps with a period of 0.05626(1) days appeared around the maximum. Ordinary superhumps appeared 9 days after the maximum. The superhump period was 0.057202(1) days which was 1.6 decline from the plateau 19 days after the maximum. After the rapid fading, the object showed a slow fading trend at a rate of 0.03 mag/day. During the early phase of the slow fading tail, superhumps with a period of 0.05728(3) days were observed. The object showed no rebrightenings. We will present the results from our observation campaign on this object.