arXiv subjects

Taichi Kato

Publications and source records attributed to Taichi Kato.

At least 37 records · Page 2Linked to original sources

arXiv2023

2021 superoutburst of WZ Sge-type dwarf nova V627 Pegasi lacks an early superhump phase

Superoutbursts in WZ Sge-type dwarf novae (DNe) are characterized by both early superhumps and ordinary superhumps originating from the 2:1 and 3:1 resonances, respectively. However, some WZ Sge-type DNe show a superoutburst lacking early superhumps; it is not well established how these differ from superoutbursts with an early superhump phase. We report time-resolved photometric observations of the WZ Sge-type DN V627 Peg during its 2021 superoutburst. The detection of ordinary superhumps before the superoutburst peak highlights that this 2021 superoutburst of V627 Peg, like that {in} 2014, did not feature an early superhump phase. The duration of stage B superhumps was slightly longer in the 2010 superoutburst accompanying early superhumps than that in the 2014 and 2021 superoutbursts which lacked early superhumps. This result suggests that an accretion disk experiencing the 2:1 resonance may have a larger mass at the inner part of the disk and hence take more time for the inner disk to become eccentric. The presence of a precursor outburst in the 2021 superoutburst suggests that the maximum disk radius should be smaller than that of the 2014 superoutburst, even though the duration of quiescence was longer than that before the 2021 superoutburst. This could be accomplished if the 2021 superoutburst was triggered as an inside-out outburst or if the mass transfer rate in quiescence changes by a factor of two, suggesting that the outburst mechanism and quiescence state of WZ Sge-type DNe may have more variety than ever thought.

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arXiv2023

MGAB-V240: 23-min AM CVn star showing both 12-d supercycle and standstills

Using Zwicky Transient Facility (ZTF) data, I noticed that MGAB-V240 = PS1-3PI J185529.82+323017.8 showed two different states: regularly outbursting state with a cycle length of 12 d and standstills. I found that the regularly outbursting state was in fact a sequence of superoutburst and intervening normal outbursts comprising a 12 d supercycle. During one of the superoutbursts, superhumps with a period of 0.015824(9) d (=22.79 min) were detected in the ZTF time-resolved data. This period and behavior have confirmed that MGAB-V240 is an AM CVn-type object with the shortest known supercycle and the second known AM CVn star showing genuine standstills. The standstills in this system were interrupted by short drops and the system often brightened after these drops. This phenomenon can be explained by the accumulation of the transferred matter in the outer part of the disk during the drops. This phenomenon favors a constant mass-transfer from the secondary combined with the difficulty in maintaining the hot state in a helium disk rather than a temporary decrease of the mass-transfer rate as the cause of these drops. MGAB-V240 should be close to the border of the thermal instability of a helium disk, and the observed superhump period agrees very well with the activity sequence expected by the disk instability theory and the evolutionary sequence of AM CVn stars.

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arXiv2023

ASASSN-15cm: an SU UMa star with an orbital period of 5.0 hours

ASASSN-15cm had been identified as a dwarf nova with an orbital period of 5.0 hours and a hot, luminous secondary of of a spectral type around K2.5 in the previous study. Using the Zwicky Transient Facility (ZTF) data, the Asteroid Terrestrial-impact Last Alert System (ATLAS) forced photometry and the All-Sky Automated Survey for Supernovae (ASAS-SN) Sky Patrol data, I found that this object underwent a superoutburst in 2019. I obtained a refined orbital period of 0.2084652(3) d and a superhump period of 0.2196(1) d, which gave a mass ratio q=0.22. Modeling of quiescent ellipsoidal variations yielded an inclination of i=55 deg, consistent with the lack of eclipses during outbursts. The object adds another example of SU UMa stars above or in the period gap containing a secondary with an evolved core, and has the longest orbital period among the established ones. ASASSN-15cm showed relatively regular superoutbursts with a supercycle of 849(18) d between 2015 and 2022, and the next superoutburst is expected to occur in the early half of 2024. Coordinated detailed observations during the next superoutburst are expected to better clarify the nature of this object.

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arXiv2023

On the orbital period of the dwarf nova CW Mon

CW Mon is a relatively bright and nearby SS Cyg-type dwarf nova frequently used in detailed analysis of cataclysmic variables and statistical studies. Using Transiting Exoplanet Survey Satellite (TESS) observations, we found that the orbital period is different from what has been adopted. Using the combined data (TESS, the Zwicky Transient Facility data and VSNET campaigns), we updated the period to be 0.19346802(4) d. The previously adopted period of 0.1766 d turned out to be its 2-day alias, probably introduced by a confusion between the two maxima/minima of the ellipsoidal variations. We confirmed that the object showed grazing eclipses during the 2016 and 2002 outbursts, and also in quiescence before and after the 2016 outburst. These eclipses were not necessarily always present and were not remarkable during some past outbursts and in the TESS data. The presence/absence of eclipses may be related to the disk radius or the brightness of the outer part of the disk. A 37-min quasi-period oscillation (QPO) signal was reported during the 2002 outburst. Combined with a recent report of the detection of QPOs around the peaks of long outbursts of a dwarf nova, we suspect that such QPOs during long outbursts may have been excited when the accretion disk reaches the maximum radius, the tidal truncation radius as being a possibility.

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arXiv2023

Genuine standstill in the AM CVn star CR Boo

CR Boo is one of the brightest and most famous AM CVn stars showing dwarf nova-type outbursts. Previous studies showed different modes of outbursts in this object ranging from the one equivalent to a hydrogen-rich ER UMa star or WZ Sge star to a low-amplitude oscillating state. We for the first time identified a bona fide standstill in this object in 2022 and we consider that CR Boo is a helium analog of Z Cam stars in addition to its SU UMa/ER UMa-type classification. The standstill lasted for ~60 d with variations typically less than 0.2 mag and ended with fading. This standstill was not preceded by a superoutburst and was different from a post-superoutburst phenomenon. The brightness after the standstill was similar to those after superoutbursts and the standstill appears to have acted like a superoutburst in effectively accreting the disk mass. The existence of a standstill in an AM CVn star be a challenge to theories of a helium disk or a degenerate secondary to explain how such a state could be maintained.

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arXiv2023

SU UMa-type supercycle in the IW And-type dwarf nova BO Cet above the period gap

BO Cet is renowned cataclysmic variable having an orbital period above the period gap (0.139835 d) and showing both features of a Z Cam/IW And star and an SU UMa star. Using the Asteroid Terrestrial-impact Last Alert System (ATLAS) forced photometry and the All-Sky Automated Survey for Supernovae (ASAS-SN) Sky Patrol data, I found that BO Cet underwent a superoutburst in 2022 October-November after a series of short, normal outbursts with increasing amplitudes. This sequence of outbursts (supercycle) is what is seen in many SU UMa stars and this observation strengthened the suggestion that the accumulating mass and angular momentum in the disk during repeated normal outbursts caused a superoutburst even in the unusual system BO Cet. The outburst just preceding the superoutburst bore characteristics of an IW And-type standstill. This phenomenon reinforces the suggestion that the terminal outburst in IW And stars occurs when the disk radius reaches a certain limit. I consider that this outburst was a failed superoutburst, during which the disk reached the radius of the 3:1 resonance but the outburst faded before superhumps developed. In BO Cet with a mass ratio on the borderline of the stability of the 3:1 resonance, there may have been a competition between the effects of tidal truncation and the 3:1 resonance as the disk radius grew and the latter won in the current case. This finding in BO Cet might suggest that IW And-type and SU UMa-type phenomena are more strongly physically related than have been thought.

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arXiv2023

LS And: WZ Sge-type outburst first time since the 1971 discovery

LS And was a transient discovered in 1971 in the M31 region and it has been argued whether it could be an intergalactic nova or a dwarf nova. Using the Zwicky Transient Facility (ZTF) data, I found that the object underwent the second known outburst in 2022 April. The behavior was that of a WZ Sge-type dwarf nova with a long fading tail and the light curves of the 1971 and 2022 outbursts matched very well. The light curves suggest that LS And is a typical WZ Sge-type dwarf nova near (but before reaching) the period minimum of cataclysmic variables. The true observed peak of the 1971 outburst was likely 12.2 mag. The outburst parameters were similar to those of other WZ Sge-type dwarf novae. The fading tail lasts more than a year and the object is still currently on this tail. There was a hint of 0.5-mag temporary brightening on the fading tail and the object appears still active after the outburst.

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arXiv2023

Very low state in PY Per in 2022

Using VSNET, VSOLJ, ASAS-SN and ATLAS observations, I found that the Z Cam star PY Per spent a long, faint low state reaching 19.1 mag at least between 2022 June and November. No dwarf nova outburst was recorded during this interval. TESS data during this low state showed two maxima in one orbital cycle and can be interpreted as an ellipsoidal modulation arising from the secondary. These observations suggest that the mass-transfer almost stopped during this low state and strengthen the identification of PY Per as a VY Scl star. PY Per had shown an unusual outburst resembling an SU UMa-type superoutburst less than half a year before (Kato 2022, arXiv:2204.12056) and these phenomena may have been physically related.

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arXiv2022

PNV J00444033+4113068: early superhumps with 0.7 mag amplitude and non-red color

In the first days of WZ Sge-type dwarf nova (DN) outbursts, the 2:1 resonance induces a spiral arm structure in the accretion disk, which is observed as early superhumps in optical light curves. This paper reports our optical observations of an eclipsing WZ Sge-type DN PNV J00444033+4113068 during its 2021 superoutburst with the 3.8m Seimei telescope and through VSNET collaboration. The eclipse analysis gave its orbital period as 0.055425534(1) d. Our observations confirmed early superhumps with an amplitude of 0.7 mag, the largest amplitude among known WZ Sge-type DNe. More interestingly, its early superhumps became the reddest around their secondary minimum, whereas other WZ Sge-type DNe show the reddest color around the early superhump maximum. The spectrum around the peak of the outburst showed the double-peaked emission lines of He II 4686Å~ and H$α$ with a peak separation of $\ge 700$ km/s, supporting a very high-inclination system. With the early superhump mapping, the unique profile and color of the early superhump of PNV J00444033+4113068 are successfully reproduced by the accretion disk with vertically extended double arm structure. Therefore, the large amplitude and unique color behavior of the early superhumps in PNV J00444033+4113068 can be explained by the 2:1 resonance model along with other WZ Sge-type DNe.

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arXiv2022

Analysis of TESS observations of V844 Her during the 2020 superoutburst

I analyzed Transiting Exoplanet Survey Satellite (TESS) observations of the 2020 superoutburst of the SU~UMa-type dwarf nova V844 Her. This object showed "textbook" superhump stages A, B and C confirmed by modern satellite observations. The resultant figure can be used for an illustration of the concept of superhump stages under the Creative Commons (CC-BY-NC) licence. During the growing phase of superhumps, the period was initially close to, but slightly longer than the orbital period. Observers should pay attention to the presence of such a phenomenon not to confuse the phenomenon with early superhumps seen in WZ Sge stars. After the superoutburst, superhumps were detected for additional 18 d with the same period and no orbital signal was detected. Small wiggles with a period of ~0.5 d were recorded in the post-superoutburst phase and they may be the same phenomenon recorded in the Kepler data of the SU UMa star V585 Lyr.

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arXiv2022

Negative superhumps in the eclipsing Z Cam + VY Scl star ES Dra

I analyzed All-Sky Automated Survey for Supernovae (ASAS-SN) Sky Patrol data of ES Dra and classified it to be a Z Cam star with VY Scl-type fading episodes. An analysis of Transiting Exoplanet Survey Satellite (TESS) observations showed that this object shows shallow eclipses and that the orbital period is 0.17749895(17) d. Negative superhumps with a period of 0.167830(2) d and the beat phenomenon between the period of negative superhumps and the orbital period were detected in the TESS data between 2020 January and March. The orbital profile systematically varied depending on the beat phase and eclipses were missing in some phases. The eclipses in ES Dra were grazing and the disk was probably not eclipsed in some phases depending on the orientation of the tilted disk. These observations added a support to the interpretation of the precessing, tilted disk as the origin of negative superhumps. Negative superhumps disappeared 4 d before the VY Scl-type fading started. It was likely that the mass-transfer rate quickly dropped when negative superhumps disappeared and the decline of the total luminosity of the disk took 4 d. This provides a measurement of the time-scale of the response of the disk against a sudden decrease of the mass transfer in a VY Scl star. Although one of standstills in ES Dra was terminated by brightening, the identity of ES Dra as an IW And star would require further events.

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arXiv2022

Romanov V48: unusual intermediate polar in the period gap

We studied Romanov V48, which had been considered as a possible polar below the period minimum (orbital period of 0.0420991 d). We analyzed the publicly available Zwicky Transient Facility data and found that this object is an intermediate polar with an orbital period of 0.102312 d (in the period gap) and a spin period of 0.0420991 d. The amplitude of the spin variation was very large (0.6 mag) and the profile has been confirmed to vary with the beat period of 0.071534 d. The ratio between the spin and orbital periods was large and the system resembles the intermediate polar DW Cnc below the period gap. Infrared emission from Romanov V48, however, could not be explained by radiation from the secondary. The infrared excess and the shape of the spectral energy distribution resembled those of polars, suggesting that the emission mechanism in Romanov V48 is similar to those of polars. Romanov V48 may be an intermediate object between intermediate polars and polars.

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arXiv2022

Z Cam star PY Per in SU UMa state?

PY Per has been known as a Z Cam star. Using VSOLJ, VSNET, AAVSO, ASAS-SN and ZTF observations, I found that this object experienced faint states and classified it to be a Z Cam + VY Scl star. Furthermore, the object changed the outburst behavior since 2020 and it has been showing regularly recurring long, bright outbursts with cycles of 110-160 d and short, faint outbursts between them. The 2021 December--2022 January outburst particularly resembled a superoutburst with a gradually fading plateau phase having a duration more than 25 d and two rebrightenings in the fading part. This object has a long orbital period of 0.15468(5) d based on the TESS data and the previous radial-velocity study. The existence of a superoutburst in a system with a long orbital period would be unusual, but not unprecedented. Since the phenomenon may still be ongoing, I draw attention to this object when it emerges from in the morning sky after the solar conjunction.

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arXiv2022

Gaia19bxc: possible polar below the period minimum

Gaia19bxc is a transient detected on 2019 May 9 by the Gaia Photometric Science Alerts Team. I analyzed the past public Zwicky Transient Facility (ZTF) data and found that Gaia19bxc has a period of 0.04473647(3) d and two different maxima in one cycle. This object also showed high and low states in the ZTF data. Based on the high amplitude (2.0 mag) of the short-term variations, short period, almost zero color indices between the different ZTF bands and the absence of a longer period, I classified it to be a likely polar. There has been no established polar below the period minimum of cataclysmic variables (CVs) and Gaia19bxc could be the first such an object. CVs below the period minimum usually have a secondary star with a stripped evolved core and Gaia19bxc is expected to have a similar secondary. If this is indeed the case, Gaia19bxc could become a highly magnetized exotic ultracompact binary during its secular evolution.

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arXiv2022

Gaia22ayj: outburst from a deeply eclipsing 9.36-min binary?

Gaia22ayj (=ZTF19aagmvuk) was detected as a transient on 2022 March 3 both by the Gaia satellite and the Zwicky Transient Facility (ZTF). I analyzed the past public ZTF data and found that Gaia22ayj showed coherent large-amplitude double-wave variations with a period of 0.00649910257(13) d = 9.36 min. The period can be either from the spin period of the white dwarf or the orbital period of the binary. I consider the latter possibility more likely based on the light curve resembling an eclipsing binary and on the stability of the profile. The presence of an outburst lasting at least ~1 d suggests that this system has an accretion disk. If Gaia22ayj is indeed an eclipsing binary with a period of 9.36 min, this is the shortest orbital period ever measured in eclipsing accreting binaries and is the shortest record of a system with a dwarf nova-type outburst. The presence of an outburst is unusual for an AM CVn system with this orbital period and this object might be in the turn-on phase of the mass-transfer. If the ultrashort orbital period of Gaia22ayj is confirmed, this object would an ideal target to detect period variations within several years to confirm its evolutionary state and to identify the future evolutionary consequence.

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arXiv2022

WFI J161953.3+031909: eclipsing ER UMa-type and Z Cam-type star in the period gap

WFI J161953.3+031909 was considered to be an eclipsing novalike object in the period gap. Using the Zwicky Transient Facility (ZTF) public database, I found that this object is the first eclipsing Z Cam star in the period gap, and is also most likely an ER UMa star with supercycles of 60-80 d. The longest outburst (most likely a superoutburst) comprised 35-46% of the supercycle in the extreme case. If superhumps are confirmed, this becomes the second object showing both ER UMa and Z Cam states after NY Ser. These objects have anomalously high mass-transfer rates despite that they are in the period gap. I refined the orbital period to be 0.099419808(8) d. We can expect to learn from WFI J161953.3+031909 using eclipses what is actually happening in the ER UMa-type and Z Cam-type disk. I also provide a revised classification of an eclipsing IW And star for BMAM-V383 = IPHAS J200822.55+300341.6 by detecting an IW And-type standstill.

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arXiv2022

V476 Cyg (Nova Cyg 1920) is currently a dwarf nova -- first such an object in the period gap?

V476 Cyg (Nova Cyg 1920) is a bright, fast nova reaching a photographic magnitude of 2.0. Using the Zwicky Transient Facility (ZTF) public database, I found that this nova is currently a dwarf nova with a cycle length of ~24 d. Compared to other classical novae currently in dwarf nova-type states, outbursts of V476 Cyg are rapidly rising and short with durations of a few days. Based on the AAVSO observations, this nova was probably already in the dwarf nova-type phase in 2016, 96 years after the nova eruption. I found a possible orbital period of 0.1018002(6) d using the ZTF data, which would place the object in the period gap. This supposed short orbital period appears to explain the features and faint absolute magnitudes of the observed dwarf nova outbursts. If this period is confirmed, V476 Cyg is a classical nova with the shortest orbital period with distinct dwarf nova outbursts and in which a nova eruption was recorded in the modern era. I also compared with the outburst properties with V446 Her (Nova Her 1960), which currently shows SS Cyg-type outbursts. The transition to the dwarf nova-phase in V476 Cyg occurred much earlier (~100 yr) than what has been supposed (~1000 yr) for classical novae below the period gap. V476 Cyg would not only provide an ideal laboratory of the behavior of an irradiated accretion disk in which tidal instability is expected to work, but also an ideal laboratory of the effect of a massive white dwarf on dwarf nova outbursts.

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arXiv2022

Long-lasting standstill and fading episode in the IW And star V507 Cyg

We studied the IW And star V507 Cyg using ASAS-SN data, ZTF data and our snapshot photometry. The star has been found to be in a long standstill in 2020 May-2022 March (and it still continues now). Such long-lasting standstills have been found in a few other IW And stars in the past literature. We found that this system was systematically fainter by 0.3 mag between 2018 March and 2019 January when it showed dwarf nova-type variations, including IW And-type states. This is one of the clearest pieces of evidence that the dwarf nova-type state in Z Cam stars (in a broader sense) is associated with a decrease in the mass-transfer rate (~25% for a duration of 300 d in this case). The object also showed IW And-type phenomena in 2019, when the system was as bright as in the current long standstill. This finding suggests that the condition whether the IW And-type phenomenon occurs or not is subtle under the same mass-transfer rate.

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