V1674 Her is the fastest (t2 similar to 1 day) classical nova in our Galaxy, and its absolute V peak of MV,max similar to-10.2 is 1 mag brighter than typical very fast novae. Such a nova is sometimes called a superbright nova. Using our fully self-consistent nova outburst model combined with the optically thick winds on a 1.35 M circle dot white dwarf (WD) with a mass accretion rate of 1 x 10-11 M circle dot yr-1, we have clarified that a strong reverse shock arises 0.3 day after the outburst, which is just after the maximum expansion of the WD photosphere. The shocked shell is optically thick and expanding with a velocity of similar to 3500 km s-1. Its V brightness reaches a maximum of MV,max=-10.2 when the shocked shell expands to Rshell similar to 300 R circle dot on day similar to 0.7. After that, the shocked shell turns optically thin and becomes fainter than the brightness of free-free emission from the nova wind. In chronological order, the optical brightness of free-free emission reaches a maximum of MV = -9 on day 0.3. However, it is overtaken on days 0.5-0.7 by the similar to 1 mag brighter luminosity of the optically thick shocked shell. The GeV gamma-ray flux reaches a maximum on day 0.4 because the gamma-rays are emitted by the shock that arises on day 0.3. Our model consistently explains both the superbrightness and chronological order that the gamma-ray peak precedes substantially before the optical V peak. We also present a similar light-curve model for another superbright nova, V1500 Cyg.

The rising phase toward the optical maximum of a classical nova is one of the last frontiers of nova study. Constructing free–free emission model light curves based on our fully self-consistent nova explosion models, we present several theoretical light curves of classical novae and compare them with the four novae having the observed rising phase toward the optical maximum. Our 1.25 M _⊙ white dwarf (WD) models show excellent agreements with the light curves of KT Eri, V339 Del, and V597 Pup, while our 1.35 M _⊙ WD models are consistent with the light curves of SMC NOVA 2016-10a. These agreements indicate that the light curves toward the optical maximum of these novae are dominated by free–free emission, rather than by photospheric emission. Our results justify the previously obtained WD masses and distance moduli for these novae and show that the postmaximum evolution can be well approximated with the evolution sequences of steady-state envelope solutions.

We have analyzed multiwavelength light curves of the very fast nova V1723 Sco, based on our fully self-consistent nova explosion models. The time-stretching method gives the distance modulus in the V band of (m-M)_V = 15.3± 0.2. Then the absolute V magnitude reaches M_V, max= m_V, max - (m-M)_V= 6.77 - 15.3± 0.2 = -8.5± 0.2. Using our fully self-consistent nova outburst model combined with the optically thick winds on a 1.25 M_⊙ white dwarf accreted by a mass accretion rate of Ṁ_ acc=1× 10^-9 M_⊙ yr^-1, we successfully reproduce the overall V light curve with a free-free emission model as well as the supersoft X-ray light curve with a blackbody approximation model. The epoch of the first GeV gamma-ray detection is almost coincident with the epoch of our model V peak. This supports the shock formation mechanism that a strong shock arises soon after the optical V maximum far outside the WD photosphere. We conclude that the shocked shell is optically thin.

The two very fast novae in our Galaxy, V1500 Cyg and V1674 Her, reached the absolute V magnitude of M_V, max≈ -10.4. These brightest novae are identified as superbright novae. Such superbrightnesses are realized if an optically thick shocked shell forms far outside the nova (white dwarf) photosphere and expands at the velocity of a few thousands km s^-1. We have analyzed V light curves of six candidates novae, CP Lac, CP Pup, V838 Her, V597 Pup, V5583 Sgr, and V5589 Sgr by comparing with our model nova light curves of 1.25 M_⊙ and 1.35 M_⊙ white dwarfs calculated with the free-free emission from a nova wind. Analyzing the data with our method, we obtain M_V, max=-10 for V838 Her, and we suggest that its large peak optical luminosity is due to the ejection of an optically thick shell like we suggested for the case of V1674 Her. The light curves of the other five novae, CP Lac, CP Pup, V597 Pup, V5583 Sgr, and V5589 Sgr are reproduced only with the free-free emission model V light curves without an optically thick shocked shell. Their M_V, max are fainter than -10 mag. We conclude that these five are not superbright novae.

We present the maximum ejecta mass (Mej)max and the maximum ratio of ejecta mass and accreted mass (Mej/Macc)max of a nova for various white dwarf (WD) masses (MWD = 0.6-1.38 M circle dot) and mass accretion rates ( Macc=1 & times;10-11 -3 & times; 10-7 M circle dot yr-1) based on the energy balance with nuclear burning. These maximum values serve as an upper bound of mass ejection for individual novae. Recently, B. E. Schaefer concluded that the WD masses in the recurrent novae U Sco and T CrB decreased at nova explosions, because the ejected mass is much larger than the accreted mass, i.e., Mej/Macc = 26 and 540, respectively. These values are derived from the orbital period change at the nova explosions. Recurrent novae have been considered to be a progenitor system of Type Ia supernovae (SNe Ia) because their WD masses are now close to, and will possibly grow up to, 1.38 M circle dot, at which WDs explode as SNe Ia. From the different viewpoint of energy generation at the thermonuclear runaway, we have obtained the much smaller value of the maximum ratio of Mej/Macc less than or similar to 2.6 for a 1.37 M circle dot WD. This conclusion simply means that the nuclear (hydrogen) burning cannot release energy enough to expel such a large ejecta mass as B. E. Schaefer's work claims. We also conclude that (Mej/Macc)max hardly increases, even if we include the effect of the frictional mass ejection process in the common envelope phase of a nova.

V1674 Her is one of the fastest novae, of which the very early phase is well observed, including optical rise to the peak over 10 mag. We present a full theoretical light-curve model of V1674 Her. Our 1.35 M _⊙ white dwarf (WD) model with the mass accretion rate of 1 × 10 ^−11 M _⊙ yr ^−1 explains overall properties including a very fast rise and decay of the optical V light curve. The WD photosphere expands up to 21 R _⊙ , thus a 0.26 M _⊙ companion star orbiting the WD every 3.67 hr is engulfed 2.7 hr after the onset of thermonuclear runaway and appears 5.3 days after that. The duration of X-ray flash is only 0.96 hr. The evolution of the expanding envelope and temporal change of the photospheric radius are very consistent with observed optical and X-ray modulations with the orbital and spin (501 s) periods. We confirm that the decay phase of the nova light curve is well approximated by a sequence of steady-state envelope solutions. Using the time-stretching method of nova light curves, we obtain the V -band distance modulus of ( m − M ) _V = 16.3 ± 0.2 and determine the distance to be d = 8.9 ± 1 kpc for the interstellar extinction of E ( B – V ) = 0.5 ± 0.05.

Millinovae are a new class of transient supersoft X-ray sources with no clear signature of mass ejection. They show similar triangle shapes of $V/I$ band light curves with thousand times fainter peaks than typical classical novae. Maccarone et al. regarded the prototype millinova, ASASSN-16oh, as a dwarf nova and interpreted the supersoft X-rays to originate from an accretion belt on a white dwarf (WD). Kato et al. proposed a nova model induced by a high-rate mass-accretion during a dwarf nova outburst; the X-rays originate from the photosphere of a hydrogen-burning hot WD whereas the $V/I$ band photons are from the irradiated accretion disk. Because each peak brightness differs largely from millinova to millinova, we suspect that not all the millinova candidates host a hydrogen burning WD. Based on the light curve analysis of the classical nova KT Eri that has a bright disk, we find that the disk is more than two magnitudes brighter when the disk is irradiated by the hydrogen burning WD than when not irradiated. We present the demarcation criterion for hydrogen burning to be $I_{\rm q} - I_{\rm max} > 2.2$, where $I_q$ and $I_{\rm max}$ are the $I$ magnitudes in quiescence and at maximum light, respectively. Among many candidates, this requirement is satisfied with the two millinovae in which soft X-rays were detected.

KT Eri is a classical nova that went into outburst in 2009. Recent photometric analysis in quiescence indicates a relatively longer orbital period of 2.6 days, so that KT Eri could host a very bright accretion disk during the outburst like in the recurrent nova U Sco, the orbital period of which is 1.23 days. We reproduced the optical V light curve as well as the supersoft X-ray light curve of KT Eri in outburst, assuming a large irradiated disk during a nova wind phase of the outburst while a normal size disk after the nova winds stop. This result is consistent with the temporal variation of wide-band V brightness that varies almost with the intermediate-band Str & ouml;mgren y brightness, because the V flux is dominated by continuum radiation, the origin of which is a photospheric emission from the very bright disk. We determined the white dwarf mass to be MWD = 1.3 +/- 0.02 M circle dot, the hydrogen-burning turnoff epoch to be similar to 240 days after the outburst, the distance modulus in the V band to be (m - M)V = 13.4 +/- 0.2, and the distance to KT Eri to be d = 4.2 +/- 0.4 kpc for the reddening of E(B - V) = 0.08. The peak absolute V brightness is about MV,max=-8.0 and the corresponding recurrence time is similar to 3000 yr from its ignition mass together with the mean mass-accretion rate of Macc similar to 1x10-9M circle dot yr-1 in quiescence. Thus, we suggest that KT Eri is not a recurrent nova.

V1674 Her is one of the fastest and brightest novae, characterized by dense optical photometry in the premaximum phase—a rise from g = 17 to 7 mag, over one-quarter of a day. We present a composite theoretical V light-curve model of its early rising phase, starting from a quiescent brightness of g = 19.2 mag. Our light-curve model consists of a hot and bright white dwarf (WD) and an irradiated accretion disk and companion star. We find that the earliest optical detection of an All Sky Automated Survey for Supernovae g -band brightness of g = 17.0 at t = 0.014 days from the onset of thermonuclear runaway can be explained with the irradiated accretion disk and companion star in the X-ray-flash phase of a 1.35 M _⊙ WD. This is the first detection in the optical of an X-ray-flash phase of a nova. Optically thick winds emerge from the WD photosphere at t = 0.04 days, and the optical flux is dominated by free–free emission from optically thin ejecta just outside the WD photosphere. Our free–free emission model V light curve reasonably reproduces the dense g light curve of Evryscope that spans from g = 14.8 (at 0.078 days) to g = 7.1 (at 0.279 days), including a sudden change of slope in the g light curve from a slow to a rapid rise at g = 14.3 on day 0.1. There is no indication of shocking power during the rising phase from g = 14.8 to 7.1.

We present time-dependent nova outburst models with optically thick winds for 1.2 and 1.35M(circle dot) white dwarfs (WDs) with a mass-accretion rate of 5x10(-9)M(circle dot) yr(-1) and for a 1.3M(circle dot) WD with 2x10(-9)M(circle dot) yr(-1). The X-ray flash occurs 11 d before the optical peak of the 1.2 circle dot WD and 2.5 d before the peak of the 1.3 circle dot WD. The wind mass-loss rate of the 1.2M(circle dot) WD (1.3M(circle dot) WD) reaches a peak of 6.4x10(-5)M(circle dot) yr(-1) (7.4x10(-5)M(circle dot) yr(-1)) at the epoch of the maximum photospheric expansion with the lowest photospheric temperature of log T-ph (K) = 4.33 (4.35). The nuclear energy generated during the outburst is lost in the form of radiation (61% for the 1.2M(circle dot) WD; 47% for the 1.3M(circle dot) WD), gravitational energy of ejecta (39%; 52%), and kinetic energy of the wind (0.28%; 0.29%). We found an empirical relation for fast novae between the time to optical maximum from the outburst t(peak) and the expansion timescale tau(exp). With this relation, we are able to predict the time to optical maximum t(peak) from the ignition model (at t = 0) without following a time-consuming nova wind evolution.

The classical nova V339 Del 2013 is characterized by a 1.5 mag dip of the V light curve owing to a dust shell formation, with which soft X-ray emissions coexist. We present a Strömgren y -band light curve, which represents continuum emission, not influenced by strong [O iii ] emission lines. The y light curve monotonically decreases in marked contrast to the V light curve that shows a 1.5 mag dip. We propose a multiwavelength light-curve model that reproduces the y and V light curves as well as the gamma-ray and X-ray light curves. In our model, a strong shock arises far outside the photosphere after optical maximum, because later ejected matter collides with earlier ejected gas. Our shocked shell model explains optical emission lines, H α , hard X-ray, and gamma-ray fluxes. A dust shell forms behind the shock that suppresses [O iii ]. This low flux of [O iii ] shapes a 1.5 mag drop in the V light curve. Then, the V flux recovers with an increasing contribution from [O iii ] lines, while the y flux does not. However, the optical depth of the dust shell is too small to absorb the photospheric (X-ray) emission of the white dwarf. This is the reason that a dust shell and soft X-ray radiation coexist. We determined the white dwarf mass to be M _WD = 1.25 ± 0.05 M _☉ and the distance modulus in the V band to be ( m − M ) _V = 12.2 ± 0.2; the distance is d = 2.1 ± 0.2 kpc for the reddening of E ( B − V ) = 0.18.

The classical nova V392 Per 2018 is characterized by a very fast optical decline, a long binary orbital period of 3.23 days, detection of GeV gamma rays, and almost identical decay trends of the B , V , and I _C light curves. The last feature is unique because most novae develop strong emission lines in the nebular phase and these lines contribute especially to the B and V bands and cause large differences between the BV and I _C light curves. This unique feature can be understood if the optical flux is dominated by continuum until the late phase of the nova outburst. Such continuum radiation is emitted by a bright accretion disk irradiated by a hydrogen-burning white dwarf (WD) and a viscous heating disk with a high mass accretion rate after the hydrogen burning ends. We present a comprehensive nova outburst model that reproduces all of these light curves. We determine the WD mass to be M _WD = 1.35–1.37 M _⊙ and the distance modulus in the V band to be ( m − M ) _V = 14.6 ± 0.2; the distance is d = 3.45 ± 0.5 kpc for the reddening of E ( B − V ) = 0.62.

A helium nova occurs on a white dwarf (WD) accreting hydrogen-deficient matter from a helium star companion. When the mass of a helium envelope on the WD reaches a critical value, unstable helium burning ignites to trigger a nova outburst. A bright soft X-ray phase appears in an early outbursting phase of a helium nova before it optically rises toward maximum. Such an X-ray bright phase is called the X-ray flash. We present theoretical light curves of X-ray flashes for 1.0, 1.2, and 1.35 M-circle dot helium novae with mass accretion rates of (1.6-7.5) x 10(-7) M-circle dot yr(-1). Long durations of the X-ray flashes (100 days-10 yr) and high X-ray luminosities (similar to 1038 erg s(-1)) indicate that X-ray flashes are detectable as a new type of X-ray transient or persistent X-ray sources. An X-ray flash is a precursor of optical brightening, so that the detection of X-ray flashes on helium novae enables us to plan arranged observation for optical premaximum phases that have been one of the frontiers of the study of novae. We found a candidate object of helium-burning X-ray flash from the literature on extragalactic X-ray surveys. This X-ray transient source is consistent with our X-ray flash model of a 1.35 M-circle dot WD.

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.

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.

CN Cha is a slow symbiotic nova characterized by a 3 yr 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 ∼0.6 M ☉ for a low metal abundance of Z = 0.004. These kinds 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 nova outbursts on a human timescale. Theoretically, there are two types of nova envelope solutions—static and optically thick wind—in low-mass WDs (≲0.7 M ☉ ). 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 the transition. CN Cha supports our explanation of the difference between long-lasting 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.

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 (≳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 ∼10 −4 M ☉ 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 kT sh ∼ 1 keV × ( ( v wind − v shock ) / 1000 km s − 1 ) 2 = 1 keV × ( ( v d − v p ) / 1000 km s − 1 ) 2 , where v d − v p is the velocity difference between the diffuse-enhanced ( v d ) and principal ( v p ) systems. We compare a 1.3 M ☉ 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.

The pre-maximum evolution of a nova is the last frontier in studying such objects. YZ Reticuli 2020 is the only nova whose X-ray flash was detected. The X-ray flash occurs immediately after the onset of thermonuclear runaway, so its physical properties impose severe constraints on the nova model. We discuss what we can learn from the early phase observations.

An X-ray flash, expected in a very early phase of a nova outburst, was at last detected with the 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 WD ∼ 1.3 M ☉ with mass-accretion rates of M ̇ acc ∼ 5 × 10 − 10 – 5 × 10 − 9 M ☉ yr −1 , including the case where the mass-accretion rate is changing between them, consistent with the 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 ∼0.1 R ☉ , 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 hydrogen-rich envelope by the core material seems to be preferred to explain the very short duration of the X-ray flash.

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., so-called optically thick winds. Evolution models are calculated by a Henyey code for a 1.0 M-circle dot, white dwarf with a mass-accretion rate of 5 x 10(-9) M-circle dot, yr(-1). The outermost part of the hydrogen-rich envelope is connected to a steadily moving envelope where optically thick winds occur. We confirm that no internal shock waves occur at thermonuclear runaway. The wind mass-loss rate reaches a peak of 1.4 x 10(-4) M-circle dot yr(-1) at the epoch of the maximum photospheric expansion, where the photospheric temperature decreases to log T-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 gravity, 35%), and the kinetic energy of the wind (0.23%). A classical nova should be very bright in a far-UV (100-300 angstrom) band for one day just after the onset of thermonuclear runaway (similar to 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.