arXiv subjects

Mariko Kato

Publications and source records attributed to Mariko Kato.

At least 37 records · Page 2Linked to original sources

arXiv2023

A helium nova in the Large Magellanic Cloud -- the faint supersoft X-ray source [HP99]159

We propose a helium nova model for the Large Magellanic Cloud (LMC) supersoft X-ray source (SSS) [HP99]159. This object has long been detected as a faint and persistent SSS for about 30 years, and recently been interpreted to be a source of steady helium-shell burning, because no hydrogen lines are observed. We find that the object can also be interpreted as in a decaying phase of a helium nova. The helium nova is slowly decaying toward the quiescent phase, during which the observed temperature, luminosity, and SSS lifetime ($\gtrsim 30$ years) are consistent with a massive white dwarf model of $\sim$ 1.2 $M_\odot$. If it is the case, this is the second discovery of a helium nova outburst after V445 Pup in our Galaxy and also the first identified helium nova in the LMC. We also discuss the nature of the companion helium star in relation to Type Ia supernova progenitors.

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arXiv2023

A multiwavelength light curve analysis of the classical nova YZ Ret: An extension of the universal decline law to the nebular phase

YZ Ret is the first X-ray flash detected classical nova, and is also well observed in optical, X-ray, and gamma-ray. We propose a comprehensive model that explains the observational properties. The white dwarf mass is determined to be $\sim 1.33 ~M_\odot$ that reproduces multiwavelength light curves of YZ Ret, from optical, X-ray, and to gamma-ray. We show that a shock is naturally generated far outside the photosphere because winds collide with themselves. The derived lifetime of shock explains some of the temporal variations of emission lines. The shocked shell significantly contributes to the optical flux in the nebular phase. The decline trend of shell emission in the nebular phase is close to $\propto t^{-1.75}$ and the same as the universal decline law of classical novae, where $t$ is the time from the outburst.

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arXiv2023

Theoretical light curve models of the symbiotic nova CN Cha -- Optical flat peak for three years

CN Cha is a slow symbiotic nova characterized by a three-years-long optical flat peak followed by a rapid decline. We present theoretical light curves for CN Cha, based on hydrostatic approximation, and estimate the white dwarf (WD) mass to be $\sim 0.6 ~M_\odot$ for a low metal abundance of Z = 0.004. This kind of flat peak novae are border objects between classical novae having a sharp optical peak and extremely slow novae, the evolutions of which are too slow to be recognized as a nova outburst in human timescale. Theoretically, there are two types of nova envelope solutions, static and optically-thick wind, in low mass WDs ($\lesssim 0.7 ~M_\odot$). Such a nova outburst begins first in a hydrostatic manner, and later it could change to an optically-thick wind evolution due to perturbation by the companion star in the nova envelope. Multiple peaks are a reflection of the relaxation process of transition. CN Cha supports our explanation on the difference between long-lasted flat peak novae like CN Cha and multiple peak novae like V723 Cas, because the companion star is located far outside, and does not perturb, the nova envelope in CN Cha.

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arXiv2022

A strong shock during a nova outburst: an origin of multiple velocity systems in optical spectra and of high-energy emissions

We propose a theoretical explanation of absorption/emission line systems in classical novae based on a fully self-consistent nova explosion model. We found that a reverse shock is formed far outside the photosphere ($\gtrsim 10^{13}$ cm) because later-ejected mass with a faster velocity collides with earlier-ejected matter. Optically thick winds blow continuously at a rate of $\sim 10^{-4} ~M_\odot$ yr$^{-1}$ near the optical maximum, but its velocity decreases toward the optical maximum and increases afterward, so that the shock arises only after the optical maximum. The nova ejecta is divided by the shock into three parts, the outermost expanding gas (earliest wind before maximum), shocked shell, and inner fast wind, which respectively contribute to pre-maximum, principal, and diffuse-enhanced absorption/emission line systems. A large part of nova ejecta is eventually confined to the shocked shell. The appearance of the principal system is consistent with the emergence of a shock. This shock is strong enough to explain thermal hard X-ray emissions. The shocked layer has a high temperature of $k T_{\rm sh} \sim 1$ keV $\times ((v_{\rm wind} - v_{\rm shock})/{\rm 1000 ~km~s}^{-1})^2 ={\rm 1 ~keV}\times ((v_{\rm d} - v_{\rm p})/{\rm 1000 ~km~s}^{-1})^2$, where $v_{\rm d} - v_{\rm p}$ is the velocity difference between the diffuse-enhanced ($v_{\rm d}$) and principal ($v_{\rm p}$) systems. We compare a 1.3 $M_\odot$ white dwarf model with the observational properties of the GeV gamma-ray detected classical nova V5856 Sgr (ASASSN-16ma) and discuss what kind of novae can produce GeV gamma-ray emissions.

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arXiv2022

A light-curve analysis of the X-ray flash first observed in classical novae

An X-ray flash, expected in a very early phase of a nova outburst, was at last detected with the {\it SRG}/eROSITA in the classical nova YZ Reticuli 2020. The observed flash timescale, luminosity, and blackbody temperature substantially constrain the nova model. We present light curve models of the X-ray flash for various white dwarf (WD) masses and mass accretion rates. We have found the WD mass in YZ Ret to be as massive as $M_{\rm WD}\sim 1.3 ~M_\odot$ with mass accretion rates of $\dot M_{\rm acc}\sim 5 \times 10^{-10}- 5\times 10^{-9} ~M_\odot$ yr$^{-1}$ including the case that the mass accretion rate is changing between them, to be consistent with the {\it SRG}/eROSITA observation. The X-ray observation confirms the luminosity to be close to the Eddington limit at the X-ray flash. The occurrence of optically thick winds, with the photospheric radius exceeding $\sim 0.1~R_\odot$, terminated the X-ray flash of YZ Ret by strong absorption. This sets a constrain on the starting time of wind mass loss. A slight contamination of the core material into the hydrogen rich envelope seems to be preferred to explain a very short duration of the X-ray flash.

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arXiv2022

Physics of nova outbursts: A theoretical model of classical nova outbursts with self-consistent wind mass loss

We present a model for one cycle of a classical nova outburst based on a self-consistent wind mass loss accelerated by the gradient of radiation pressure, i.e., the so-called optically thick winds. Evolution models are calculated by a Henyey code for a 1.0 $M_\odot$ white dwarf (WD) with a mass accretion rate of $5 \times 10^{-9}~M_\odot$ yr$^{-1}$. The outermost part of hydrogen-rich envelope is connected to a steadily moving envelope when optically thick winds occur. We confirm that no internal shock waves occur at the thermonuclear runaway. The wind mass loss rate reaches a peak of $1.4 \times 10^{-4}~M_\odot$ yr$^{-1}$ at the epoch of the maximum photospheric expansion, where the photospheric temperature decreases to $\log T_{\rm ph}$ (K)=3.90. Almost all of the accreted mass is lost in the wind. The nuclear energy generated in hydrogen burning is lost in a form of photon emission (64 %), gravitational energy (lifting-up the wind matter against the gravity, 35 %), and kinetic energy of the wind (0.23 %). A classical nova should be very bright in a far-UV (100 - 300 Å) band, during a day just after the onset of thermonuclear runaway ($\sim$25 d before the optical maximum). In the decay phase of the nova outburst, the envelope structure is very close to that of a steady state solution.

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arXiv2021

A light curve model of V2491 Cyg: Classical nova outburst on a cool and massive white dwarf

The classical nova V2491 Cyg was once suggested to be a recurrent nova. We have broadly reproduced the light curve of V2491 Cyg by a nova outburst model on a cold $1.36~M_\odot$ white dwarf (WD), which strongly suggests that V2491 Cyg is a classical nova outbursting on a cold very massive WD rather than a recurrent nova outbursting on a warmer WD like the recurrent nova RS Oph. In a long-term evolution of a cataclysmic binary, an accreting WD has been settled down to a thermal equilibrium state with the balance of gravitational energy release and neutrino loss. The central temperature of the WD is uniquely determined by the energy balance. The WD is hot (cold) for a high (low) mass-accretion rate. We present the central temperatures, ignition masses, ignition radii, and recurrence periods for various WD masses and mass-accretion rates. In a classical nova, which corresponds to a low mass-accretion rate, the WD is cool and strongly degenerated and the ignition mass is large, which result in a strong nova outburst. In a recurrent nova, the WD is relatively warmer because of a high mass accretion rate and the outburst is relatively weaker. The gravitational energy release substantially contributes to the luminosity during the recurrent nova outbursts. We compare physical properties between classical novae and recurrent novae and discuss the essential differences between them.

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arXiv2021

The $UBV$ Color Evolution of Classical Novae. IV. Time-Stretched $(U-B)_0$-$(M_B-2.5\log f_{\rm s})$ and $(V-I)_0$-$(M_I-2.5\log f_{\rm s})$ Color-Magnitude Diagrams of Novae in Outburst

Light curves and color evolutions of two classical novae can be largely overlapped if we properly squeeze or stretch the timescale of a target nova against that of a template nova by $t'=t/f_{\rm s}$. Then the brightness of the target nova is related to the brightness of the template nova by $(M[t])_{\rm template} = (M[t/f_{\rm s}] - 2.5 \log f_{\rm s})_{\rm target}$, where $M[t]$ is the absolute magnitude and a function of time $t$, and $f_{\rm s}$ is the ratio of timescales between the target and template novae. In the previous papers of this series, we show that many novae broadly overlap in the time-stretched $(B-V)_0$-$(M_V-2.5 \log f_{\rm s})$ color-magnitude diagram. In the present paper, we propose two other $(U-B)_0$-$(M_B-2.5\log f_{\rm s})$ and $(V-I)_0$-$(M_I-2.5\log f_{\rm s})$ diagrams, and show that their tracks overlap for 16 novae and for 52 novae, respectively. Here, $(U-B)_0$, $(B-V)_0$, and $(V-I)_0$ are the intrinsic $U-B$, $B-V$, and $V-I$ colors and not changed by the time-stretch, and $M_B$, $M_V$, and $M_I$ are the absolute $B$, $V$, and $I$ magnitudes. Using these properties, we considerably refine the previous estimates of their distance and reddening. The obtained distances are in reasonable agreement with those of {\it Gaia} Data Release 2 catalogue.

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arXiv2020

A Theory for the Maximum Magnitude versus Rate of Decline (MMRD) Relation of Classical Novae

We propose a theory for the MMRD relation of novae, using free-free emission model light curves built on the optically thick wind theory. We calculated $(t_3,M_{V,\rm max})$ for various sets of $(\dot M_{\rm acc}, M_{\rm WD})$, where $M_{V,\rm max}$ is the peak absolute $V$ magnitude, $t_3$ is the 3-mag decay time from the peak, and $\dot M_{\rm acc}$ is the mass accretion rate on to the white dwarf (WD) of mass $M_{\rm WD}$. The model light curves are uniquely characterized by $x\equiv M_{\rm env}/M_{\rm sc}$, where $M_{\rm env}$ is the hydrogen-rich envelope mass and $M_{\rm sc}$ is the scaling mass at which the wind has a certain wind mass-loss rate. For a given ignition mass $M_{\rm ig}$, we can specify the first point $x_0= M_{\rm ig}/M_{\rm sc}$ on the model light curve, and calculate the corresponding peak brightness and $t_3$ time from this first point. Our $(t_3, M_{V,\rm max})$ points cover well the distribution of existing novae. The lower the mass accretion rate, the brighter the peak. The maximum brightness is limited to $M_{V,\rm max} \gtrsim -10.4$ by the lowest mass-accretion rate of $\dot M_{\rm acc} \gtrsim1 \times 10^{-11}~M_\odot$ yr$^{-1}$. A significant part of the observational MMRD trend corresponds to the $\dot M_{\rm acc}\sim5\times10^{-9}~M_\odot$ yr$^{-1}$ line with different WD masses. A scatter from the trend line indicates a variation in their mass-accretion rates. Thus, the global trend of an MMRD relation does exist, but its scatter is too large for it to be a precision distance indicator of individual novae. We tabulate $(t_3, M_{V,\rm max})$ for many sets of $(\dot M_{\rm acc},M_{\rm WD})$.

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arXiv2020

Supersoft X-Ray Phases of Recurrent Novae as an Indicator of their White Dwarf Masses

We have examined the optical/X-ray light curves of seven well-observed recurrent novae, V745 Sco, M31N 2008-12a, LMC N 1968, U Sco, RS Oph, LMC N 2009a, T Pyx, and one recurrent nova candidate LMC N 2012a. Six novae out of the eight show a simple relation that the duration of supersoft X-ray source (SSS) phase is 0.70 times the total duration of the outburst ($=$ X-ray turnoff time), i.e., $t_{\rm SSS}=0.70 t_{\rm off}$, the total duration of which ranges from 10 days to 260 days. These six recurrent novae show a broad rectangular X-ray light curve shape, first half a period of which is highly variable in the X-ray count rate. The SSS phase corresponds also to an optical plateau phase that indicates a large accretion disk irradiated by a hydrogen-burning WD. The other two recurrent novae, T Pyx and V745 Sco, show a narrow triangular shape of X-ray light curve without an optical plateau phase. Their relations between $t_{\rm SSS}$ and $t_{\rm off}$ are rather different from the above six recurrent novae. We also present theoretical SSS durations for recurrent novae with various WD masses and stellar metallicities ($Z=$0.004, 0.01, 0.02, and 0.05) and compare with observed durations of these recurrent novae. We show that the SSS duration is a good indicator of the WD mass in the recurrent novae with a broad rectangular X-ray light curve shape.

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arXiv2020

ASASSN-16oh: A nova outburst with no mass ejection -- A new type of supersoft X-ray source in old populations

ASASSN-16oh is a peculiar transient supersoft X-ray source without a mass-ejection signature in the field of the Small Magellanic Cloud. Maccarone et al. (2019) concluded that ASASSN-16oh is the first dwarf nova with supersoft X-ray that originated from an equatorial accretion belt on a white dwarf (WD). Hillman et al. (2019) proposed a thermonuclear runaway model that both the X-rays and $V$/$I$ photons are emitted from the hot WD. We calculated the same parameter models as Hillman et al.'s and found that they manipulated on/off the mass-accretion, and their best fit $V$ light curves are 6 mag fainter, and decay about 10 times slower, than that of ASASSN-16oh. We propose a nova model induced by a high rate of mass accretion during a dwarf nova outburst, i.e., the X-rays originate from the surface of the hydrogen-burning WD whereas the $V/I$ photons are from the irradiated disk. Our model explains the main observational properties of ASASSN-16oh. We also obtained thermonuclear runaway models with no mass ejection for a wide range of parameters of the WD mass and mass accretion rates including both natural and forced novae in low-metal environments of $Z=0.001$ and $Z=0.0001$. They are a new type of periodic supersoft X-ray sources with no mass ejection, and also a bright transient in $V$/$I$ bands if they have a large disk. We suggest that such objects are candidates of Type Ia supernova progenitors because its mass is increasing at a very high efficiency $(\sim 100 \%)$.

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arXiv2019

A Light Curve Analysis of 32 Recent Galactic Novae --- Distances and White Dwarf Masses

We obtained the absolute magnitudes, distances, and white dwarf (WD) masses of 32 recent galactic novae based on the time-stretching method for nova light curves. A large part of the light/color curves of two classical novae often overlap each other if we properly squeeze/stretch their timescales. Then, a target nova brightness is related to the other template nova brightness by $(M_V[t])_{\rm template} = (M_V[t/f_{\rm s}] - 2.5 \log f_{\rm s})_{\rm target}$, where $t$ is the time, $M_V[t]$ is the absolute $V$ magnitude, and $f_{\rm s}$ is their timescaling ratio. Moreover, when these two time-stretched light curves, $(t/f_{\rm s})$-$(M_V-2.5 \log f_{\rm s})$, overlap each other, $(t/f_{\rm s})$-$(B-V)_0$ do too, where $(B-V)_0$ is the intrinsic $B-V$ color. Thus, the two nova tracks overlap each other in the $(B-V)_0$-$(M_V-2.5 \log f_{\rm s})$ diagram. Inversely using these properties, we obtain/confirm the distance and reddening by comparing each nova light/color curves with the well calibrated template novae. We classify the 32 novae into two types, LV Vul and V1500 Cyg types, in the time-stretched $(B-V)_0$-$(M_V-2.5 \log f_{\rm s})$ color-magnitude diagram. The WD mass is obtained by direct comparison of the model $V$ light curves with the observation. Thus, we obtain a uniform set of 32 galactic classical novae that provides the distances and WD masses from a single method. Many novae broadly follow the universal decline law and the present method can be applied to them, while some novae largely deviate from the universal decline law and so the method cannot be directly applied to them. We discuss such examples.

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arXiv2019

The $UBV$ Color Evolution of Classical Novae. III. Time-Stretched Color-Magnitude Diagram of Novae in Outburst

We propose a modified color-magnitude diagram for novae in outburst, i.e., $(B-V)_0$ versus $(M_V-2.5 \log f_{\rm s})$, where $f_{\rm s}$ is the timescaling factor of a (target) nova against a comparison (template) nova, $(B-V)_0$ is the intrinsic $B-V$ color, and $M_V$ is the absolute $V$ magnitude. We dub it the time-stretched color-magnitude diagram. We carefully reanalyzed 20 novae based on the time-stretching method and revised their extinctions $E(B-V)$, distance moduli in the $V$ band $(m-M)_V$, distances $d$, and timescaling factors $f_{\rm s}$ against the template nova LV Vul. We have found that these 20 nova outburst tracks broadly follow one of the two template tracks, LV Vul/V1668 Cyg or V1500 Cyg/V1974 Cyg group, in the time-stretched color-magnitude diagram. In addition, we estimate the white dwarf masses and $(m-M)_V$ of the novae by directly fitting the absolute $V$ model light curves ($M_V$) with observational apparent $V$ magnitudes ($m_V$). A good agreement in the two estimates of $(m-M)_V$ confirms the consistency of the time-stretched color-magnitude diagram. Our distance estimates are in good agreement with the results of Gaia Data Release 2.

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arXiv2018

Production of Silicon on Mass Increasing White Dwarfs -- Possible Origin of High-Velocity-Features in Type Ia Supernovae

Type Ia supernovae (SNe Ia) often show high-velocity absorption features (HVFs) in their early phase spectra; however the origin of the HVFs is unknown. We show that a near-Chandrasekhar-mass white dwarf (WD) develops a silicon-rich layer on a carbon-oxygen (CO) core before it explodes as an SN Ia. We calculated the nuclear yields in successive helium shell flashes for 1.0 $M_\odot$, 1.2 $M_\odot$, and 1.35 $M_\odot$ CO WDs accreting helium-rich matter with several mass-accretion rates ranging from $1 \times 10^{-7}~M_\odot$ yr$^{-1}$ to $7.5 \times 10^{-7}~M_\odot$ yr$^{-1}$. For the $1.35~M_\odot$ WD with the accretion rate of $1.6 \times 10^{-7}~M_\odot$ yr$^{-1}$, the surface layer developed as helium burning ash and consisted of 40% $^{24}$Mg, 33% $^{12}$C, 23% $^{28}$Si, and a few percent of $^{20}$Ne by weight. For a higher mass accretion rate of $7.5 \times 10^{-7}~M_\odot$ yr$^{-1}$, the surface layer consisted of 58% $^{12}$C, 31% $^{24}$Mg, and 0.43% $^{28}$Si. For the $1.2~M_\odot$ WDs, silicon is produced only for lower mass accretion rates (2% for $1.6 \times 10^{-7}~M_\odot$ yr$^{-1}$). No substantial silicon ($< 0.07%$) is produced on the $1.0~M_\odot$ WD independently of the mass-accretion rate. If the silicon-rich surface layer is the origin of Si II HVFs, its characteristics are consistent with that of mass increasing WDs. We also discuss possible Ca production on very massive WDs ($ \gtrsim 1.38~M_\odot$).

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arXiv2018

A Light Curve Analysis of Recurrent and Very Fast Novae in our Galaxy, Magellanic Clouds, and M31

We analyzed optical, UV, and X-ray light curves of 14 recurrent and very fast novae in our galaxy, Magellanic Clouds, and M31, and obtained their distances and white dwarf (WD) masses. Among the 14 novae, we found that eight novae host very massive ($\gtrsim 1.35 M_\odot$) WDs and are candidates of Type Ia supernova (SN Ia) progenitors. We confirmed that the same timescaling law and time-stretching method as in galactic novae can be applied to extra-galactic fast novae. We classify the four novae, V745 Sco, T CrB, V838 Her, and V1534 Sco, as the V745 Sco type (rapid-decline), the two novae, RS Oph and V407 Cyg, as the RS Oph type (circumstellar matter(CSM)-shock), and the two novae, U Sco and CI Aql, as the U Sco type (normal-decline). The $V$ light curves of these novae almost overlap with each other in the same group, if we properly stretch in the time direction (timescaling law). We apply our classification method to LMC, SMC, and M31 novae. YY Dor, LMC N 2009a, and SMC N 2016 belong to the normal-decline type, LMC N 2013 to the CSM-shock type, and LMC N 2012a and M31N 2008-12a to the rapid-decline type. We obtained the distance of SMC N 2016 to be $d=20\pm2$ kpc, suggesting that SMC N 2016 is a member of our galaxy. The rapid-decline type novae have very massive WDs of $M_{\rm WD}=1.37-1.385 M_\odot$ and are promising candidates of SN Ia progenitors. This type of novae are much fainter than the MMRD relations.

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arXiv2018

A Light Curve Analysis of Gamma-ray Nova V959 Mon -- Distance and White Dwarf Mass

V959 Mon is one of the gamma-ray detected novae. It was optically discovered about 50 days after the gamma-ray detection due to proximity to the Sun. The nova speed class is unknown because of lack of the earliest half of optical light curve and short supersoft X-ray phase due to eclipse by the disk rim. Using the universal decline law and time-stretching method, we analyzed the data of V959 Mon and obtained nova parameters. We estimated the distance modulus in the V band to be (m-M)_V=13.15\pm0.3 for the reddening of E(B-V)=0.38\pm0.01 by directly comparing with the similar type of novae, LV Vul, V1668 Cyg, IV Cep, and V1065 Cen. The distance to V959 Mon is 2.5\pm0.5 kpc. If we assume that the early phase light curve of V959 Mon is the same as that of time-stretched light curves of LV Vul, our model light curve fitting suggests that the white dwarf (WD) mass is 0.9-1.15 M_sun, being consistent with a neon nova identification. At the time of gamma-ray detection the photosphere of nova envelope extends to 5-8 R_sun (about two or three times the binary separation) and the wind mass-loss rate is (3-4)\times 10^{-5} M_sun yr^{-1}. The period of hard X-ray emission is consistent with the time of appearance of the companion star from the nova envelope. The short supersoft X-ray turnoff time is consistent with the epoch when the WD photosphere shrank to behind the elevating disk rim, that occurs 500 days before nuclear burning turned off.

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arXiv2017

Recurrent novae and long-term evolution of mass-accreting white dwarfs -- toward the accurate mass retention efficiency

The mass growth rate of mass-accreting white dwarfs (WDs) is a key factor in binary evolution scenarios toward Type Ia supernovae. Many authors have reported very different WD mass increasing rates. In this review, we clarify the reasons for such divergence, some of which come from a lack of numerical techniques, usage of old opacities, different assumptions for binary configurations, inadequate initial conditions, and unrealistic mass-loss mechanisms. We emphasize that these assumptions should be carefully chosen in calculating the long-term evolution of accreting WDs. Importantly, the mass-loss mechanism is the key process determining the mass retention efficiency: the best approach involves correctly incorporating the optically thick wind because it is supported by the multiwavelength light curves of novae.

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arXiv2017

A Millennium-Long Evolution of One-Year-Recurrence-Period Nova -- Search for Any Indication of the Forthcoming He Flash

We present 1500 cycles of hydrogen shell flashes on a 1.38 M_sun white dwarf (WD) for a mass accretion rate of 1.6 x 10^{-7} M_sun yr^{-1}, the mass ejection of which is calculated consistently with the optically thick winds. This model mimics the one-year-recurrence-period nova M31N 2008-12a. Through these hydrogen flashes a helium ash layer grows in mass and eventually triggers a helium nova outburst. Each hydrogen flash is almost identical and there is no precursor for the forthcoming He flash either in the outburst or in the quiescent until the next He flash suddenly occurs. Thus, M31N 2008-12a is a promising candidate of He novae, outbursting in any time within a millennium years. The prompt X-ray flash of He nova lasts as short as 15 min with the X-ray luminosity being about a half of the Eddington luminosity, making the observation difficult. In the very early phase of a He flash, the uppermost H-rich layer is convectively mixed into the deep interior and most of hydrogen is consumed by nuclear burning. In comparison with hydrogen shell flashes of M31N 2008-12a, we expect the forthcoming He nova with a very short prompt X-ray flash (15 min), a very bright optical/NIR peak (~3.5 mag brighter than M31N 2008-12a), a much longer nova duration (>2 years), and a longer supersoft X-ray source phase (40-50 days or more).

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