The presentation of the light curves of the novae on the logarithmic scale of the radius of the shell ejected during the outburst and in the scale of the amplitude of the outburst simplified the review of the light curves of all the known, about 500, classical novae of our Galaxy and the Large and Small Magellanic Clouds. As a result, the modified light curves about of 250 classical novae were grouped into 7 typical groups with subgroups defined by the light curves of prototypes. The largest group is the DQ Her group with V1280 Sco as the main prototype and the key to grouping. Novae of this group have three or four variants of a dust condensation. Novae of the small GQ Mus group may be a bright X-ray source during an outburst. CP Pup, CP Lac, V1974 Cyg, and V1493 Aql are prototypes for 4 groups with close tilts of the initial brightness decline phase of the light curves. The group with RR Pic as prototype, including the HR Del subgroup, has a prolonged state of maximal brightness with the presence of several brightness variations before the final decline phase. The relationship of groups of novae, or form of the light curve, with processes in the circumstellar and circum-binary system environment is shortly discussed. The material ejected during an outburst forms expanding shells of a generally ellipsoidal shape. In the Lac, Pup, and Cyg groups, the shells do not show a pronounced regular structure, except for their ellipsoidal shape. Shells of novae with a dust condensation (the DQ Her group) have an ellipsoidal shape with an equatorial belt. The unique shell of CK Vul, Nova Vul 1670, gives an idea about the possible shape of the shells of other members in the V1493 Sql group. Each of groups has own position along the well-known "absolute maximal magnitude, rate of brightness decline" relation.
We reviewed the light curves of 10 known recurrent novae (RNs) in the modified scales of the "logarithmic of time, amplitude of outburst". A unified rule was established to normalize the time scale. Confirmation of the known result: in a series of recurrent outbursts, the light curve of nova retains its shape, was realized by distinguishing three groups amongst these 10 novae. The CI Aql (+V2487 Oph) and T Pyx (+ IM Nor) groups contain by one additional member. The V745 Sco group includes V394 CrA and V3890 Sgr. Three RNs: T CrB, RS Oph and U Sco, have its own uniqueness. After comparing the l…
Unique Nova Centauri 2005, V1047 Cen, by a form of the modified light curve was included into the CP Pup group. It means that the re-brightening in 2019 occurred still during the finishal stage of the 2005 outburst and can be connected with the current activity of the post-nova. The light curve of V1047 Cen in 2019 as a separate event is some details similar to the HR Del, Nova Del 1967, and also it is distinguish considerable from dwarf nova outbursts in post-nova systems (GK Per, etc.). There is proposed possible correct identification of Nova Sgr 1970, V3645 Sgr, which was included into the…
In recent years, recurrent nova eruptions are often observed very intensely in wide range of wavelengths from radio to optical to X-rays. Here I present selected highlights from recent multi-wavelength observations. The enigma of T Pyx is at the heart of this paper. While our current understanding of CV and symbiotic star evolution can explain why certain subset of recurrent novae have high accretion rate, that of T Pyx must be greatly elevated compared to the evolutionary mean. At the same time, we have extensive data to be able to estimate how the nova envelope was ejected in T Pyx, and it t…
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…
I review the basics of the disc instability model (DIM) for dwarf novae and soft-X-ray transients and its most recent developments, as well as the current limitations of the model, focusing on the dwarf nova case. Although the DIM uses the Shakura-Sunyaev prescription for angular momentum transport, which we know now to be at best inaccurate, it is surprisingly efficient in reproducing the outbursts of dwarf novae and soft X-ray transients, provided that some ingredients, such as irradiation of the accretion disc and of the donor star, mass transfer variations, truncation of the inner disc, et…
The spectroscopic development of classical novae is described as a narrative of the various stages of the outburst. The review highlights the multiwavelength aspects of the phenomenology and the recent developments related to structure, inhomogeneity, and dynamics of the ejecta. Special emphasis is placed on the distinct behavior of the symbiotic-like recurrent novae.
In this review, I present our current state of knowledge regarding both Classical Nova and Recurrent Nova systems. Two particular objects (V1974 Cyg and RS Oph) are chosen to illustrate the range of phenomena that may be associated with their outbursts. The wider importance of nova research is emphasised and some of the most pressing unsolved problems are summarised.
We review the near-infrared properties of classical novae in the J, H and K bands at wavelengths between 1.08 to 2.4 micron. A classification system exists for the early post-outburst optical spectra of novae on the basis of the strength of group of non-hydrogen emission lines. A similar scheme for the near-infrared regime, which is not available at present, is presented here. In the optical system there are two principal classes, namely, "Fe II" and "He/N" for novae with either prominent Fe II lines or prominent "He/N" lines. There is also a small subset of the hybrid Fe IIb type. From spectr…
Recurrent novae are binaries harboring a very massive white dwarf (WD), as massive as the Chandrasekhar mass, because of their short recurrence periods of nova outbursts of 10-100 years. Thus, recurrent novae are considered as candidates of progenitors of Type Ia supernovae (SNe Ia). In fact, the SN Ia PTF11kx showed evidence that its progenitor is a symbiotic recurrent nova. The binary parameters of recurrent novae have been well determined, especially for the ones with frequent outbursts, U Sco and RS Oph, which provide useful information on the elementary processes in binary evolution towar…
Detection of X-rays from classical novae, both in outburst and post-outburst, provides unique and crucial information about the explosion mechanism. Soft X-rays reveal the hot white dwarf photosphere, whenever hydrogen (H) nuclear burning is still on and expanding envelope is transparent enough, whereas harder X-rays give information about the ejecta and/or the accretion flow in the reborn cataclysmic variable. The duration of the supersoft X-ray emission phase is related to the turn-off of the classical nova, i.e., of the H-burning on top of the white dwarf core. A review of X-ray observation…
In this review I summarize the observational attempts done so far to unveil the nature of the progenitor system/s of Type Ia Supernovae. In particular, I focus on the most recent developments that followed the alleged detection of circumstellar material around a few events, and on the link this possibly establishes with recurrent novae. In this framework, I then discuss the case of RS Oph, what we know of its circumstellar environment, and what this is telling us about its supposed connection to Type Ia supernova explosions.
There is a wide consensus in the astrophysics community that the mechanism underlying the observed Classical Nova eruptions is a surface thermonuclear runaway. We start this short review with the main observational facts that lead to the theoretical model of a thermonuclear runaway that takes place in an accreted hydrogen rich envelope placed on top of a cool degenerate core of a white dwarf. According to the theory, the accreted envelope becomes unstable to convection days to weeks prior to the runaway. During the extreme stages of the runaway itself, when the burning is most efficient, the e…
We survey our understanding of classical novae: non-terminal, thermonuclear eruptions on the surfaces of white dwarfs in binary systems. The recent and unexpected discovery of GeV gamma-rays from Galactic novae has highlighted the complexity of novae and their value as laboratories for studying shocks and particle acceleration. We review half a century of nova literature through this new lens, and conclude: --The basics of the thermonuclear runaway theory of novae are confirmed by observations. The white dwarf sustains surface nuclear burning for some time after runaway, and until recently, it…
Ten weeks of daily imaging of the giant elliptical galaxy M87 with the Hubble Space Telescope (HST) has yielded 41 nova light curves of unprecedented quality for extragalactic cataclysmic variables. We have recently used these light curves to demonstrate that the observational scatter in the so-called Maximum-Magnitude Rate of Decline (MMRD) relation for classical novae is so large as to render the nova-MMRD useless as a standard candle. Here we demonstrate that a modified Buscombe - de Vaucouleurs hypothesis, namely that novae with decline times t2 > 10 days converge to nearly the same absolu…
Classical novae produce radioactive nuclei which are emitters of gamma-rays in the MeV range. Some examples are the lines at 478 and 1275 keV (from 7Be and 22Na) and the positron-electron annihilation emission (511 keV line and a continuum below this energy, with a cut-off at 20-30 keV). The analysis of gamma-ray spectra and light curves is a potential unique and powerful tool both to trace the corresponding isotopes and to give insights on the properties of the expanding envelope determining its transparency. Another possible origin of gamma-rays is the acceleration of particles up to very hi…
Novae have been reported as transients for more than two thousand years. Their bright optical outbursts are the result of explosive nuclear burning of gas accreted from a binary companion onto a white dwarf. Novae containing a white dwarf close to the Chandrasekhar mass limit and accreting at a high rate are potentially the unknown progenitors of the type Ia supernovae used to measure the acceleration of the Universe. Swift X-ray observations have radically transformed our view of novae by providing dense monitoring throughout the outburst, revealing new phenomena in the super-soft X-rays from…
Recurrent novae are binary stars in which a white dwarf accretes matter from a less evolved companion, either a red giant or a main-sequence star. They have dramatic optical brightenings of around 5-6 mag in V in less than a day, several times a century. These occur at variable and unpredictable intervals, and are followed by an optical decline over several weeks, and activity from the X-ray to the radio. The unpredictability of recurrent novae and related stellar types can hamper systematic study of their outbursts. Here we analyse the long-term lightcurve of RS Ophiuchi, a recurrent nova wit…
Over the last 40 years, multi-frequency observations, coupled with advances in theoretical modelling, have led to a much fuller understanding of the nova phenomenon. Here I give a brief review of the current state of knowledge of Classical and Recurrent Novae including their central systems; the causes and consequences of their outbursts; sub-types, and possible relationships to Type Ia Supernovae. Particular attention is paid to the Recurrent Nova RS Ophiuchi as it shows a wealth of phenomena associated with its 2006 outburst. Finally, some open questions and avenues for future work are summa…
