arXiv subjects

Mariko Kimura

Publications and source records attributed to Mariko Kimura.

At least 19 recordsLinked to original sources

arXiv2026

Tracing the Orbital Motion of the Accreting White Dwarf in EX~Hydrae with XRISM/Resolve

Measuring the masses of accreting white dwarfs (WDs) is crucial for understanding their evolution and the physics of accretion. High-resolution X-ray spectroscopy can trace the WD motion through Doppler shifts of emission lines formed close to the WD. We report an 83~ks XRISM/Resolve observation of the intermediate polar EX~Hydrae and measure the orbital modulation of individual Fe K-shell line centroids. The Fe~{\sc xxv} K$α$ components show coherent orbital modulation, yielding $K_1 = 58.1 \pm 8.5\ \mathrm{km\ s^{-1}}$. This is the first detection of orbital modulation in individual Fe K-shell lines from an accreting WD, made possible by the high spectral resolution of Resolve and its frequent in-orbit gain calibration. The measured $K_1$ is consistent with optical/UV $K_1$ measurements, providing a cross-check that these distinct tracers follow the WD orbital motion. Combining this X-ray measurement with literature orbital parameters, we derive a WD mass of $M_1 = 0.79 \pm 0.04\ M_\odot$. These results demonstrate that high-resolution X-ray spectroscopy can use individual Fe K-shell line centroids to trace WD orbital motion in accreting WDs.

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arXiv2026

Plasma Dynamics of Radiative Cooling Accretion Flow in AM Herculis with XRISM

We present XRISM/Resolve high-resolution X-ray spectroscopy of the prototypical magnetic cataclysmic variable AM Herculis. All satellite lines of highly ionized Fe are fully resolved. Lighter element lines (Si, S, Ca) show 2 - 3 eV widths consistent with purely thermal broadening, while the broader 6 - 7 eV Fe lines require additional bulk Doppler broadening. Spin-phase-resolved modulations are clearly detected in the Fe XXV and Fe XXVI lines, with semi-amplitudes of $81.8\pm6$ km s$^{-1}$ and $132.5\pm9$ km s$^{-1}$, and mean velocities of $143.6\pm6$ km s$^{-1}$ and $225.6\pm8$ km s$^{-1}$, respectively. After removing these bulk Doppler shifts, we obtain intrinsic Doppler widths of $5.23_{-0.15}^{+0.16}$ eV for Fe XXV and $6.23_{-0.18}^{+0.19}$ eV for Fe XXVI, directly revealing gradients of bulk velocity and temperature in the cooling-flow plasma. We additionally examined the resonance anisotropy predicted by Terada et al. (1999, 2001): the equivalent widths of the Fe XXV and Fe XXVI resonance lines increase at the pole-on phase by factors of 1.30 - 1.35, in positive correlation with their oscillator strengths. Combining XRISM with simultaneous NuSTAR data and PSAC/MCVSPEC plasma models, we derive a self-consistent shock temperature of $24.0\pm0.1$ keV and shock velocity of $1,116\pm2$ km s$^{-1}$. Radiative transfer simulations of the resonance lines further constrain the shock density to about $(5 - 6)\times10^{15}$ cm$^{-3}$, providing a new density diagnostic for accretion columns. The resulting accretion column geometry has a height of 200 - 300 km and a radius of 200 - 400 km.

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arXiv2026

Disk instability model incorporating a variable inner disk radius in SS Cyg and U Gem

Previous theoretical studies indicate that the inner disk in dwarf novae evaporates into a high-temperature, optically thin, and geometrically thick accretion flow during quiescence, with the inner edge moving toward the white dwarf at the onset of an outburst. We incorporate this process into the numerical model developed by Kimura & Osaki (2023) and test the code on two representative dwarf novae, SS Cyg and U Gem. By modeling the inner accretion flow, we calculate the optical, ultraviolet (UV), and X-ray luminosities. Our results show that evaporation suppresses the inside-out outbursts without requiring a radially dependent viscosity parameter in the cold state. The observed time delay between the rise in UV luminosity and the onset of the optical outburst is more than one day, which is successfully reproduced when the inner disk is truncated at several $\times 10^{9}$ cm in the standard evaporation model. However, while the modeled accretion rate at the inner disk edge in U Gem accounts for its quiescent X-ray luminosity, the rate in SS Cyg remains insufficient. This discrepancy in SS Cyg suggests that SS Cyg may require either more efficient evaporation or an additional mass supply into the coronal cavity via gas-stream overflow. By accounting for disk evaporation, our simulations offer a refined version of the disk instability model for dwarf nova outbursts that naturally explains the observed multiwavelength light curves.

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arXiv2026

A probable inside-out dwarf nova outburst from the period bouncer candidate ASASSN-25dc

We report optical time-resolved photometric observations of a newly-discovered outbursting system, ASASSN-25dc. Its 8-mag amplitude, 40-day duration, 1-mag dip in the outburst plateau, and positive superhumps are characteristic of a dwarf nova superoutburst in a non-magnetic cataclysmic variable. We establish its stage-A and stage-B superhump periods as 0.059387(5) d and 0.058864(3) d, respectively. The negative superhump period derivative ($-$1.4(2)$\times10^{-5}$ cycle$^{-1}$) during the stage-B superhumps and the empirical relation indicate the mass ratio is 0.054(7), below the period bounce range. The long outburst decline timescale (35.2(1) d mag$^{-1}$) and small superhump amplitude ($\simeq$0.08 mag) observed in ASASSN-25dc are also seen in some period bouncer systems, but not seen in systems well before the period bounce. Despite its short superhump period and indicated small mass ratio, we find no evidence of the excitement of the 2:1 tidal resonance. Moreover, its outburst rise timescale (1.62(9) d mag$^{-1}$) is significantly longer than those measured at less than 0.4 d mag$^{-1}$ in other dwarf nova outbursts around the period minimum. Overall, an inside-out dwarf nova outburst from a massive disc in a system with a mass ratio around or even below the period minimum, but lacking the 2:1 tidal resonance, may explain all these observations. However, this challenges the existing models of dwarf nova superoutbursts, which do not predict these outburst properties in low-mass-ratio systems.

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arXiv2026

Spectral and photometric variability of SS 433 observed with XRISM and simultaneous optical and near-infrared telescopes

We present results from coordinated multiwavelength observations of the SS 433, obtained with XRISM, optical telescopes, and near-infrared camera during 2024 April and 2025 March. The XRISM exposures amounted to ~200 ks in 2024 and ~100 ks in 2025. With XRISM/Resolve's high spectral resolution and large effective area, we clearly resolved numerous emission lines even in short time segments, achieving improved accuracy in Doppler-shift measurements relative to earlier observations. The simultaneously obtained X-ray and optical Doppler shifts suggest a possible tendency for the optical emission to lag slightly behind the X-rays. In the Resolve data, the Doppler shifts of the two jet components exhibited apparent asymmetries, with jet speeds fluctuating around ~0.26$\pm$0.01$c$ in 2024 and ~0.30$\pm$0.01$c$ in 2025. The velocity variations indicated modulations on a timescale of ~6.3 d, with a phase offset of about -90$^{\circ}$ relative to the nutation cycle. The observed line widths and flux of the approaching and receding jets appear consistent with the expected geometrical effects, indicating systematically larger line widths in the inner regions of the jets, as proposed by Shidatsu et al. (2025). Optical light curves show flares of ~400 s in 2024 and ~1600 s in 2025, with amplitudes up to ~15% during out-of-eclipse intervals, while the XRISM/Xtend light curves show no significant variability within the overlapping intervals and given the statistical uncertainties. Near-infrared photometry in 2024, obtained during an out-of-eclipse interval at a different epoch from the optical observations, showed no flare-like variability, and the X-ray band also remained constant within uncertainties. These coordinated observations provide a foundation for future XRISM studies aimed at probing the dynamical properties of the relativistic jets in SS 433.

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arXiv2025

Evolution of the inner accretion flow and the white-dwarf spin pulse during the 2023 outburst in GK Persei

We present our X-ray and optical observations performed by NICER, NuSTAR, and Tomo-e Gozen during the 2023 outburst in the intermediate polar GK Persei. The X-ray spectrum consisted of three components: blackbody emission of several tens of eVs from the irradiated white-dwarf surface, a source possibly including several emission lines around 1 keV, and multi-temperature bremsstrahlung emission from the accretion column. The 351.3-s white-dwarf spin pulse was detected in X-rays, and the observable X-ray flux from the column drastically decreased at the off-pulse phase, which suggests that the absorption of the column by the accreting gas called the curtain was the major cause of the pulse. As the system became brighter in optical, the column became fainter, the pulse amplitude became higher, and the energy dependence of pulses became weaker at $<$8~keV. These phenomena could be explained by the column's more pronounced absorption by the denser curtain as mass accretion rates increased. The blackbody and line fluxes rapidly decreased at the optical decline, which suggests the expansion of the innermost disk edge with decreasing accretion rates. The electron scattering or the column geometry may be associated with almost no energy dependence of high-energy pulses. The irradiated vertically-thick structure at the disk may generate optical QPOs with a period of $\sim$5700 s.

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arXiv2025

ASASSN-24hd; a dwarf nova bridging WZ Sge-type and SU UMa-type superoutbursts

WZ Sge-type dwarf novae (DNe) form a subclass in cataclysmic variables, characterized by short-period variations called superhumps during an outburst. Here we present optical ground-based and TESS observations of ASASSN-24hd in its 2024-2025 outburst. ASASSN-24hd is the first reported WZ Sge-type DN outburst fully covered by TESS, providing a great opportunity to study the evolution of superhumps. Our observations establish its early and stage-A ordinary superhumps as 0.05711(4) and 0.05919(5) d, respectively, resulting in its mass ratio of 0.098(4). The TESS observations confirm that the evolution of its superhump period, amplitude, and profile after the appearance of ordinary superhumps is generally consistent with those of SU UMa-type DNe observed with Kepler and TESS. Furthermore, we find that ASASSN-24hd in outburst shares a great similarity to the 2010 superoutburst of an SU UMa-type DN V585 Lyr, observed by Kepler, particularly including the superhump evolution and the long waiting time ($\gtrsimeq$ 5 d) before the stage A--B transition of ordinary superhumps. The shorter superoutburst cycles and smaller outburst amplitude in V585 Lyr than those of ASASSN-24hd disfavor the interpretation that V585 Lyr is, in fact, a face-on WZ Sge-type DN where early superhumps are undetectable. Instead, one possibility of their critical differences is either low quiescence viscosity or inner disk truncation, which has been invoked to explain the extreme nature of WZ Sge-type DNe, but future observations in quiescence are vital to conclude. These findings emphasize the borderline between SU UMa-type and WZ Sge-type DNe.

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arXiv2024

Evolution of X-ray and optical rapid variability during the low/hard state in the 2018 outburst of MAXI J1820+070 = ASASSN-18ey

We performed shot analyses of X-ray and optical sub-second flares observed during the low/hard state of the 2018 outburst in MAXI J1820$+$070. Optical shots were less spread than X-ray shots. The amplitude of X-ray shots was the highest at the onset of the outburst, and they faded at the transition to the intermediate state. The timescale of shots was $\sim$0.2 s, and we detected the abrupt spectral hardening synchronized with this steep flaring event. The time evolution of optical shots was not similar to that of X-ray shots. These results suggest that accreting gas blobs triggered a series of magnetic reconnections at the hot inner accretion flow in the vicinity of the black hole, which enhanced X-ray emission and generated flaring events. The rapid X-ray spectral hardening would be caused by this kind of magnetic activity. Also, the synchrotron emission not only at the hot flow but also at the jet plasma would contribute to the optical rapid variability. We also found that the low/hard state exhibited six different phases in the hardness-intensity diagram and the correlation plot between the optical flux and the X-ray hardness. The amplitude and duration of X-ray shots varied in synchrony with these phases. This time variation may provide key information about the evolution of the hot flow, the low-temperature outer disk, and the jet-emitting plasma.

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arXiv2024

MASTER OT J030227.28+191754.5: an unprecedentedly energetic dwarf nova outburst

We present a detailed study of the MASTER OT J030227.28+191754.5 outburst in 2021-2022, reaching an amplitude of 10.2 mag and a duration of 60 d. The detections of (1) the double-peaked optical emission lines, and (2) the early and ordinary superhumps, established that MASTER OT J030227.28+191754.5 is an extremely energetic WZ Sge-type dwarf nova (DN). Based on the superhump observations, we obtained its orbital period and mass ratio as 0.05986(1) d and 0.063(1), respectively. These are within a typical range of low-mass-ratio DNe. According to the binary parameters derived based on the thermal-tidal instability model, our analyses showed that (1) the standard disk model requires an accretion rate $\simeq$ 10$^{20}$ g s$^{-1}$ to explain its peak optical luminosity and (2) large mass was stored in the disk at the outburst onset. These cannot be explained solely by the impact of its massive ($\gtrsim$ 1.15 M$_\odot$) primary white dwarf implied by Kimura et al. (2023). Instead, we propose that the probable origin of this enormously energetic DN outburst is the even lower quiescence viscosity than other WZ Sge-type DNe. This discussion is qualitatively valid for most possible binary parameter spaces unless the inclination is low ($\lesssim 40^\circ$) enough for the disk to be bright explaining the outburst amplitude. Such low inclinations, however, would not allow detectable amplitude of early superhumps in the current thermal-tidal instability model. The optical spectra at outburst maximum showed the strong emission lines of Balmer, He I, and He II series whose core is narrower than $\sim 800$ km s$^{-1}$. Considering its binary parameters, a Keplerian disk cannot explain this narrow component, but the presumable origin is disk winds.

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arXiv2023

MASTER OT J030227.28+191754.5: a dwarf nova at a massive oxygen-neon white-dwarf system ?

We present timing and spectral analysis results of the {\it NICER} and {\it NuSTAR} observations of the dwarf nova MASTER OT J030227.28$+$191754.5 during the 2021--2022 outburst. The soft X-ray component was found to be dominated by blackbody radiation with a temperature of $\sim$30 eV and also showed prominent oxygen and neon emission lines. The blackbody luminosity exceeded 10$^{34}$ ergs s$^{-1}$, which is consistent with theoretical predictions, and then decreased more than an order of magnitude in 3.5 days. The inferred abundances of oxygen and neon in the optically-thin coronal region surrounding the central white dwarf (WD) are several times higher than the respective solar values. Although inconclusive, the abundance enrichment may originate from the WD, indicating that it may be mainly composed of oxygen and neon. Assuming that the blackbody radiation comes from the belt-shaped boundary layer between the WD and the accretion disk, we estimated the WD radius to be $(2.9\pm1.1)\times10^{8}$ cm, which corresponds to the WD mass range of 1.15--1.34 $M_{\odot}$. If the accretion continues for another $\sim$Gyr, the WD may experience an accretion-induced collapse into a neutron star and form a so-called black-widow pulsar system.

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arXiv2022

The light curve simulations of the 2021 anomalous event in SS Cygni

The prototype dwarf nova SS Cyg unexpectedly exhibited an anomalous event in its light curve in the early few months of 2021 in which regular dwarf nova-type outbursts stopped, but small-amplitude fluctuations occurred only. Inspired by this event, we have performed numerical simulations of light curves of SS Cyg by varying mass transfer rates and varying viscosity parameters in the cool disk. We have also studied the effect of gas-stream overflows beyond the outer disk edge in the light curve simulations. We have confirmed that the enhanced mass transfer is unlikely responsible for the 2021 anomalous event and its forerunner. We have found that the enhancement of the viscosity in the disk may reproduce the forerunner of that event but may not be enough to explain the 2021 anomalous event, although the latter result might be particular to our thermal equilibrium curve used. Within our simulations, a model of the gas stream overflow with a slightly higher mass transfer rate than that of our standard model reproduces light curves similar to the 2021 anomalous event. We suggest that the gas-stream overflow is necessary to reproduce that event. The gas-stream overflow may also be responsible for the abnormally high X-ray flux during the normal quiescent state in SS Cyg.

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arXiv2021

Optical Variability Correlated with X-ray Spectral Transition in the Black-Hole Transient ASASSN-18ey = MAXI J1820+070

How a black hole accretes matter and how this process is regulated are fundamental but unsolved questions in astrophysics. In transient black-hole binaries, a lot of mass stored in an accretion disk is suddenly drained to the central black hole because of thermal-viscous instability. This phenomenon is called an outburst and is observable at various wavelengths (Frank et al., 2002). During the outburst, the accretion structure in the vicinity of a black hole shows dramatical transitions from a geometrically-thick hot accretion flow to a geometrically-thin disk, and the transition is observed at X-ray wavelengths (Remillard, McClintock, 2006; Done et al., 2007). However, how that X-ray transition occurs remains a major unsolved problem (Dunn et al., 2008). Here we report extensive optical photometry during the 2018 outburst of ASASSN-18ey (MAXI J1820$+$070), a black-hole binary at a distance of 3.06 kpc (Tucker et al., 2018; Torres et al., 2019) containing a black hole and a donor star of less than one solar mass. We found optical large-amplitude periodic variations similar to superhumps which are well observed in a subclass of white-dwarf binaries (Kato et al., 2009). In addition, the start of the stage transition of the optical variations was observed 5 days earlier than the X-ray transition. This is naturally explained on the basis of our knowledge regarding white dwarf binaries as follows: propagation of the eccentricity inward in the disk makes an increase of the accretion rate in the outer disk, resulting in huge mass accretion to the black hole. Moreover, we provide the dynamical estimate of the binary mass ratio by using the optical periodic variations for the first time in transient black-hole binaries. This paper opens a new window to measure black-hole masses accurately by systematic optical time-series observations which can be performed even by amateur observers.

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arXiv2021

On the nature of the anomalous event in 2021 in the dwarf nova SS Cygni and its multi-wavelength transition

SS Cyg has long been recognized as the prototype of a group of dwarf novae that show only outbursts. However, this object has entered a quite anomalous event in 2021, which at first appeared to be standstill, i.e., an almost constant luminosity state, observed in Z Cam-type dwarf novae. This unexpected event gives us a great opportunity to reconsider the nature of standstill in cataclysmic variables. We have observed this anomalous event and its forerunner, a gradual and simultaneous increase in the optical and X-ray flux during quiescence, through many optical telescopes and the X-ray telescopes NICER and NuSTAR. We have not found any amplification of the orbital hump during quiescence before the anomalous event, which suggests that the mass transfer rate did not significantly fluctuate on average. The estimated X-ray flux was not enough to explain the increment of the optical flux during quiescence via X-ray irradiation of the disk and the secondary star. It would be natural to consider that viscosity in the quiescent disk was enhanced before the anomalous event, which increased mass accretion rates in the disk and raised not only the optical flux but also the X-ray flux. We suggest that enhanced viscosity also triggered the standstill-like phenomenon in SS Cyg, which is considered to be a series of small outbursts. The inner part of the disk would always stay in the outburst state and only its outer part would be unstable against the thermal-viscous instability during this phenomenon, which is consistent with the observed optical color variations. This scenario is in line with our X-ray spectral analyses which imply that the X-ray emitting inner accretion flow became hotter than usual and vertically expanded and that it became denser and was cooled down after the onset of the standstill-like state.

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arXiv2021

KIC 9406652: A laboratory for tilted disks in cataclysmic variable stars. II. Modeling of the orbital light curves

KIC 9406652, one of the recently identified IW And-type dwarf novae, is the best target for studying the tilted disk in cataclysmic variable stars. In a previous paper by Kimura, Osaki, and Kato (2020), we analyzed its Kepler light curves and found that its orbital light curves during the brightening stage were dominated by the reflection effect of the secondary star and varied with the orientation of the tilted disk; the amplitude was maximized at the minimum of the super-orbital signal and the phase of the light maximum shifted to an earlier one with the advance of the super-orbital phase. We argued there that this was the direct evidence of the retrogradely precessing tilted disk as the secondary star acts like a reflecting object. In order to confirm this interpretation, we have performed numerical modeling of orbital light curves in this paper. We have succeeded in reproducing the main characteristics of the observed orbital light curves by a simple model in which the secondary star is irradiated by the tilted disk. We have also constrained the inclination angle, $i$, of the binary system and the tilt angle, $θ$, of the disk purely from photometric considerations. The best-fitting parameter set is found to be $i \sim$45~deg and $θ\sim$2.0~deg, respectively. The orbital inclination thus estimated is consistent with that obtained from the spectroscopic considerations within the uncertainty limit. On the other hand, the tilt angle of the disk could be underestimated by using only the semi-amplitude of super-orbital signals.

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arXiv2021

Spectroscopic and Photometric Observations of Dwarf Nova Superoutbursts by the 3.8 m Telescope Seimei and the Variable Star Network

We present spectroscopic and photometric observations of 17 dwarf-nova superoutbursts obtained by KOOLS-IFU mounted on the 3.8 m telescope Seimei at Okayama Observatory of Kyoto University and through VSNET collaboration. Our spectroscopic observations for six outbursts were performed within 1 d from their optical peak. 11 objects (TCP J00590972+3438357. ASASSN-19ado, TCP J06073081-0101501, ZTF20aavnpug, ASASSN-19ady, MASTER OT J061642.05+435617.9, TCP J20034647+1335125, ASASSN-20kv, ASASSN-20kw, MASTER OT J213908.79+161240.2, and ASASSN-20mf) were previously unknown systems, and our observations enabled quick classification of their transient type. These results illustrate that Seimei telescope has the capability to conduct quick follow-up observations of unknown transients. Our photometric observations yielded that 11 objects are WZ Sge-type dwarf novae and their candidates, and the other six objects are SU UMa-type dwarf novae and their candidates. The He II 4686Å~ emission line was clearly detected among ASASSN-19ado, TCP J06073081-0101501 and MASTER OT J213908.79+161240.2, whose association with a spiral arm structure in an accretion disk has been suggested in the previous studies. Our result suggests that a higher-inclination system shows a stronger emission line of He II 4686Å, as well as larger-amplitude early superhumps.

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arXiv2021

ASASSN-18aan: An Eclipsing SU UMa-type Cataclysmic Variable with a 3.6-hour Orbital Period and a Late G-type Secondary Star

We report photometric and spectroscopic observations of the eclipsing SU UMa-type dwarf nova ASASSN-18aan. We observed the 2018 superoutburst with 2.3 mag brightening and found the orbital period ($P_{\rm orb}$) to be 0.149454(3) d, or 3.59 hr. This is longward of the period gap, establishing ASASSN-18aan as one of a small number of long-$P_{\rm orb}$ SU UMa-type dwarf novae. The estimated mass ratio, ($q=M_2/M_1 = 0.278(1)$), is almost identical to the upper limit of tidal instability by the 3:1 resonance. From eclipses, we found that the accretion disk at the onset of the superoutburst may reach the 3:1 resonance radius, suggesting that the superoutburst of ASASSN-18aan results from the tidal instability. Considering the case of long-$P_{\rm orb}$ WZ Sge-type dwarf novae, we suggest that the tidal dissipation at the tidal truncation radius is enough to induce SU UMa-like behavior in relatively high-$q$ systems such as SU UMa-type dwarf novae, but that this is no longer effective in low-$q$ systems such as WZ Sge-type dwarf novae. The unusual nature of the system extends to the secondary star, for which we find a spectral type of G9, much earlier than typical for the orbital period, and a secondary mass $M_2$ of around 0.18 M$_{\odot}$, smaller than expected for the orbital period and the secondary's spectral type. We also see indications of enhanced sodium abundance in the secondary's spectrum. Anomalously hot secondaries are seen in a modest number of other CVs and related objects. These systems evidently underwent significant nuclear evolution before the onset of mass transfer. In the case of ASASSN-18aan, this apparently resulted in a mass ratio lower than typically found at the system's $P_{\rm orb}$, which may account for the occurrence of a superoutburst at this relatively long period.

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arXiv2020

Follow-up Observations for IceCube-170922A: Detection of Rapid Near-Infrared Variability and Intensive Monitoring of TXS 0506+056

We present our follow-up observations to search for an electromagnetic counterpart of the IceCube high-energy neutrino, IceCube-170922A. Monitoring observations of a likely counterpart, TXS 0506+056, are also described. First, we quickly took optical and near-infrared images of 7 flat-spectrum radio sources within the IceCube error region right after the neutrino detection and found a rapid flux decline of TXS 0506+056 in Kanata/HONIR J-band data. Motivated by this discovery, intensive follow-up observations of TXS 0506+056 are continuously done, including our monitoring imaging observations, spectroscopic observations, and polarimetric observations in optical and near-infrared wavelengths. TXS 0506+056 shows a large amplitude (~1.0 mag) variability in a time scale of several days or longer, although no significant variability is detected in a time scale of a day or shorter. TXS 0506+056 also shows a bluer-when-brighter trend in optical and near-infrared wavelengths. Structure functions of variabilities are examined and indicate that TXS 0506+056 is not a special blazar in terms of optical variability. Polarization measurement results of TXS 0506+056 are also discussed.

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arXiv2020

KIC 9406652: A laboratory of the tilted disk in cataclysmic variable stars

KIC 9406652 is a cataclysmic variable (CV), sub-classified as `IW And-type star', showing a repetition of standstills with oscillatory variations terminated by brightening. This system showed negative superhumps, semi-periodic variations having periods slightly shorter than the orbital period, and super-orbital signals, both of which are believed to originate from a precessing, tilted accretion disk. We have re-examined its Kepler light curve extending over 1500 d. In accordance with a cycle of the IW And-type light variation, the frequency of negative superhumps showed a reproducible variation: a rapid drop during the brightening and a gradual increase during the standstill. They are interpreted as the drastic change in the radial mass distribution and the expansion of the tilted disk, which is not expected from the existing models for IW And stars. The constancy in flux amplitudes of negative superhumps confirms that their light source is the bright spot sweeping across the surface of the tilted disk. The frequencies of negative superhumps and super-orbital signals varied in unison on long timescales, suggesting their common origin: the tilted disk. Orbital signals at the brightening were dominated by the irradiation of the secondary star and varied with the orientation of the tilted disk; the amplitude was maximized at the minimum of super-orbital signals and the light maximum shifted to early orbital phases as the super-orbital phase advances. This is the first direct evidence that the disk was tilted out of the binary orbital plane and retrogradely precessing. The tilt angle of the disk inferred from semi-amplitudes of super-orbital signals was lower than 3 degrees. The diversity in light curves of negative superhumps supports this and suggests that a part of the gas stream overflows the disk edge. This study thus offers rich information about the tilted disk in CVs.

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