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We present X-ray and optical observations of nova V407 Lup (Nova Lup 2016), previously well monitored in outburst, as it returned to quiescent accretion. The X-ray light curve in 2020 February revealed a clear flux modulation with a stable period of 564.64 |$\pm$| 0.64 s, corresponding to the period measured in outburst and attributed to the spin of a magnetized white dwarf in an intermediate polar (IP) system. This detection in quiescence is consistent with the IP classification proposed after the nova eruption. The XMM–Newton EPIC X-ray flux is |${\simeq} 1.3 \times 10^{-12}$|  erg cm |$^{-2}$|  s |$^{-1}$| at a distance, most likely, larger than 5 kpc, emitted in the whole 0.2–12 keV range without a significant cut-off energy. The X-ray spectra are complex; they can be fitted including a power-law component with a relatively flat slope (a power-law index |$\simeq$| 1), although, alternatively, a hard thermal component at kT |$\ge$| 19 keV also yields a good fit. The SALT optical spectra obtained in 2019 March and 2022 May are quite typical of IPs, with strong emission lines, including some due to a high ionization potential, like He  ii at 4685.7 Å. Nebular lines of O [ iii ] were prominent in 2019 March, but their intensity and equivalent width appeared to be decreasing during that month, and they were no longer detectable in 2022, indicating that the nova ejecta dispersed. Complex profiles of the He  ii lines of V407 Lup are also characteristic of IPs, giving further evidence for this classification. [ABSTRACT FROM AUTHOR]

We report on spectroscopic and photometric observations of the AM Canum Venaticorum (AM CVn) system ASASSN-21br, which was discovered in outburst by the All-Sky Automated Survey for Supernovae in 2021 February. The outburst lasted for around three weeks, and exhibited a pronounced brightness dip for |$\approx$| 4 d, during which the spectra showed a sudden transition from emission- to absorption-line dominated. Only |$\approx$| 60 AM CVn systems with derived orbital periods are found in the Galaxy, therefore increasing the sample of AM CVn systems with known orbital periods is of tremendous importance to (1) constrain the physical mechanisms of their outbursts and (2) establish a better understanding of the low-frequency background noise of future gravitational wave surveys. Time-resolved photometry taken during the outburst of ASASSN-21br showed modulation with a period of around 36.65 min, which is likely the superhump or orbital period of the system. Time-resolved spectroscopy taken with the Southern African Large Telescope did not show any sign of periodicity in the He  i absorption lines. This is possibly due to the origin of these lines in the outbursting accretion disc, which makes it challenging to retrieve periodicity from the spectral lines. Future follow-up spectral observations during quiescence might allow us better constrain the orbital period of ASASSN-21br. [ABSTRACT FROM AUTHOR]

The optical spectra of novae are characterized by emission lines from the hydrogen Balmer series and either Fe  ii or He/N, leading to their traditional classification into two spectral classes: 'Fe  ii ' and 'He/N'. For decades, the origins of these spectral features were discussed in the literature in the contexts of different bodies of gas or changes in the opacity of the ejecta, particularly associated with studies by R. E. Williams and S. N. Shore. Here, we revisit these major studies with dedicated, modern data sets, covering the evolution of several novae from early rise to peak all the way to the nebular phase. Our data confirm previous suggestions in the literature that the 'Fe  ii ' and 'He/N' spectral classes are phases in the spectroscopic evolution of novae driven primarily by changes in the opacity, ionization, and density of the ejecta, and most if not all novae go through at least three spectroscopic phases as their eruptions evolve: an early He/N (phase 1; observed during the early rise to visible peak and characterized by P Cygni lines of He  i and N  ii / iii), then an Fe  ii (phase 2; observed near visible peak and characterized by P Cygni lines of Fe  ii and O  i), and then a later He/N (phase 3; observed during the decline and characterized by emission lines of He  i / ii , N  ii / iii), before entering the nebular phase. This spectral evolution seems to be ubiquitous across novae, regardless of their speed class; however the duration of each of these phases differs based on the speed class of the nova. [ABSTRACT FROM AUTHOR]

We present early spectral observations of the very slow Galactic nova Gaia22alz, over its gradual rise to peak brightness that lasted 180 d. During the first 50 d, when the nova was only 3–4 mag above its normal brightness, the spectra showed narrow (FWHM ≈ 400 km s−1) emission lines of H Balmer, He  i , He  ii , and C  iv but no P Cygni absorption. A few weeks later, the high-excitation He  ii and C  iv lines disappeared, and P Cygni profiles of Balmer, He  i , and eventually Fe  ii lines emerged, yielding a spectrum typical of classical novae before peak. We propose that the early (first 50 d) spectra of Gaia22alz, particularly the emission lines with no P Cygni profiles, are produced in the white dwarf's optically thin envelope or accretion disc, reprocessing ultraviolet and potentially X-ray emission from the white dwarf after a dramatic increase in the rate of thermonuclear reactions, during a phase known as the 'early X-ray/UV flash'. If true, this would be one of the rare times that the optical signature of the early X-ray/UV flash has been detected. While this phase might last only a few hours in other novae and thus be easily missed, it was possible to detect in Gaia22alz due to its very slow and gradual rise and thanks to the efficiency of new all-sky surveys in detecting transients on their rise. We also consider alternative scenarios that could explain the early spectral features of Gaia22alz and its gradual rise. [ABSTRACT FROM AUTHOR]

Classical novae are shock-powered multiwavelength transients triggered by a thermonuclear runaway on an accreting white dwarf. V1674 Her is the fastest nova ever recorded (time to declined by two magnitudes is t 2 = 1.1 d) that challenges our understanding of shock formation in novae. We investigate the physical mechanisms behind nova emission from GeV γ-rays to cm-band radio using coordinated Fermi -LAT, NuSTAR ,  Swift , and VLA observations supported by optical photometry. Fermi -LAT detected short-lived (18 h) 0.1–100 GeV emission from V1674 Her that appeared 6 h after the eruption began; this was at a level of (1.6 ± 0.4) × 10−6 photons cm−2 s−1. Eleven days later, simultaneous NuSTAR and Swift X-ray observations revealed optically thin thermal plasma shock-heated to k T shock = 4 keV. The lack of a detectable 6.7 keV Fe Kα emission suggests super-solar CNO abundances. The radio emission from V1674 Her was consistent with thermal emission at early times and synchrotron at late times. The radio spectrum steeply rising with frequency may be a result of either free-free absorption of synchrotron and thermal emission by unshocked outer regions of the nova shell or the Razin–Tsytovich effect attenuating synchrotron emission in dense plasma. The development of the shock inside the ejecta is unaffected by the extraordinarily rapid evolution and the intermediate polar host of this nova. [ABSTRACT FROM AUTHOR]

V906 Carinae was one of the best observed novae of recent times. It was a prolific dust producer and harboured shocks in the early evolving ejecta outflow. Here, we take a close look at the consequences of these early interactions through study of high-resolution Ultraviolet and Visual Echelle spectrograph spectroscopy of the nebular stage and extrapolate backwards to investigate how the final structure may have formed. A study of ejecta geometry and shaping history of the structure of the shell is undertaken following a spectral line |$\rm {\small SHAPE}$| model fit. A search for spectral tracers of shocks in the nova ejecta is undertaken and an analysis of the ionized environment. Temperature, density, and abundance analyses of the evolving nova shell are presented. [ABSTRACT FROM AUTHOR]

The recurrent nova RS Oph underwent a new outburst on 2021 August 8, reaching a visible brightness of V  = 4.8 mag. Observations of the 2021 outburst made with the high-resolution UVES spectrograph at the Kueyen-UT2 telescope at ESO-VLT in Paranal enabled detection of the possible presence of 7Be newly made in the thermonuclear runaway reactions. The 7Be yields can be estimated at N (7Be)/ N (H)  = 5.7 × 10−6, which are close to the lowest yields measured in classical novae so far. 7Be is short-lived and decays only into 7Li. By means of a spectrum taken during the nebular phase we estimated an ejected mass of ≈ 1.1 × 10−5 M⊙, providing an amount of ≈ 4.4 × 10−10 M⊙ of 7Li created in the 2021 event. Recurrent novae of the RS Oph type may synthesize slightly lower amounts of 7Li per event than classical novae, but occur 103 times more frequently. The recurrent novae fraction is in the range of 10–30 per cent and they could have contributed to the making of the 7Li that we observe today. The detection of 7Be in RS Oph provides further support to the recent suggestion that novae are the most effective source of 7Li in the Galaxy. [ABSTRACT FROM AUTHOR]

We present a detailed study of the 2019 outburst of the cataclysmic variable V1047 Cen, which hosted a classical nova eruption in 2005. The peculiar outburst occurred 14 yr after the classical nova event and lasted for more than 400 days, reaching an amplitude of around 6 magnitudes in the optical. Early spectral follow-up revealed what could be a dwarf nova (accretion disk instability) outburst. However, the outburst duration, high-velocity (>2000 km sâˆ'1) features in the optical line profiles, luminous optical emission, and presence of prominent long-lasting radio emission together suggest a phenomenon more exotic and energetic than a dwarf nova outburst. The outburst amplitude, radiated energy, and spectral evolution are also not consistent with a classical nova eruption. There are similarities between V1047 Cen’s 2019 outburst and those of classical symbiotic stars, but pre-2005 images of the field of V1047 Cen indicate that the system likely hosts a dwarf companion, implying a typical cataclysmic variable system. Based on our multiwavelength observations, we suggest that the outburst may have started with a brightening of the disk due to enhanced mass transfer or disk instability, possibly leading to enhanced nuclear shell burning on the white dwarf, which was already experiencing some level of quasi-steady shell burning. This eventually led to the generation of a wind and/or bipolar, collimated outflows. The 2019 outburst of V1047 Cen appears to be unique, and nothing similar has been observed in a typical cataclysmic variable system before, hinting at a potentially new astrophysical phenomenon. [ABSTRACT FROM AUTHOR]

We characterize the extreme heartbeat star system MACHO 80.7443.1718 in the Large Magellanic Cloud using Transiting Exoplanet Survey Satellite (TESS) photometry and spectroscopic observations from the Magellan Inamori Kyocera Echelle (MIKE) and SOAR Goodman spectographs. MACHO 80.7443.1718 was first identified as a heartbeat star system in the All-Sky Automated Survey for SuperNovae (ASAS-SN) with |$P_{\rm orb}=32.836\pm 0.008\, {\rm d}$|⁠. MACHO 80.7443.1718 is a young (∼6 Myr), massive binary, composed of a B0 Iae supergiant with |$M_1 \simeq 35\, {\rm M}_\odot$| and an O9.5V secondary with |$M_2 \simeq 16\, {\rm M}_\odot$| on an eccentric (e = 0.51 ± 0.03) orbit. In addition to having the largest variability amplitude amongst all known heartbeats stars, MACHO 80.7443.1718 is also one of the most massive heartbeat stars yet discovered. The B[e] supergiant has Balmer emission lines and permitted/forbidden metallic emission lines associated with a circumstellar disc. The disc rapidly dissipates at periastron that could indicate mass transfer to the secondary, but re-emerges immediately following periastron passage. MACHO 80.7443.1718 also shows tidally excited oscillations at the N = 25 and N = 41 orbital harmonics and has a rotational period of 4.4 d. [ABSTRACT FROM AUTHOR]

The detection of GeV γ-ray emission from Galactic novae by the Fermi-Large Area Telescope has become routine since 2010, and is generally associated with shocks internal to the nova ejecta. These shocks are also expected to heat plasma to ∼107 K, resulting in detectable X-ray emission. In this paper, we investigate 13 γ-ray emitting novae observed with the Neil Gehrels Swift Observatory, searching for 1–10 keV X-ray emission concurrent with γ-ray detections. We also analyze γ-ray observations of novae V407 Lup (2016) and V357 Mus (2018). We find that most novae do eventually show X-ray evidence of hot shocked plasma, but not until the γ-rays have faded below detectability. We suggest that the delayed rise of the X-ray emission is due to large absorbing columns and/or X-ray suppression by corrugated shock fronts. The only nova in our sample with a concurrent X-ray/γ-ray detection is also the only embedded nova (V407 Cyg). This exception supports a scenario where novae with giant companions produce shocks with external circumbinary material and are characterized by lower density environments, in comparison with novae with dwarf companions where shocks occur internal to the dense ejecta. [ABSTRACT FROM AUTHOR]

The unprecedented sky coverage and observing cadence of the All-Sky Automated Survey for SuperNovae (ASAS-SN) has resulted in the discovery and continued monitoring of a large sample of Galactic transients. The vast majority of these are accretion-powered dwarf nova outbursts in cataclysmic variable systems, but a small subset are thermonuclear-powered classical novae. Despite improved monitoring of the Galaxy for novae from ASAS-SN and other surveys, the observed Galactic nova rate is still lower than predictions. One way classical novae could be missed is if they are confused with the much larger population of dwarf novae. Here, we examine the properties of 1617 dwarf nova outbursts detected by ASAS-SN and compare them to classical novae. We find that the mean classical nova brightens by ∼11 mag during outburst, while the mean dwarf nova brightens by only ∼5 mag, with the outburst amplitude distributions overlapping by roughly 15%. For the first time, we show that the amplitude of an outburst and the time it takes to decline by two magnitudes from maximum are positively correlated for dwarf nova outbursts. For classical novae, we find that these quantities are negatively correlated, but only weakly, compared to the strong anticorrelation of these quantities found in some previous work. We show that, even if located at large distances, only a small number of putative dwarf novae could be misclassified as classical novae, suggesting that there is minimal confusion between these populations. Future spectroscopic follow-up of these candidates can show whether any are indeed classical novae. [ABSTRACT FROM AUTHOR]

The physical mechanism driving mass ejection during a nova eruption is still poorly understood. Possibilities include ejection in a single ballistic event, a common-envelope interaction, a continuous wind, or some combination of these processes. Here, we present a study of 12 Galactic novae, for which we have premaximum high-resolution spectroscopy. All 12 novae show the same spectral evolution. Before optical peak, they show a slow P Cygni component. After peak, a fast component quickly arises, while the slow absorption remains superimposed on top of it, implying the presence of at least two physically distinct flows. For novae with high-cadence monitoring, a third, intermediate-velocity component is also observed. These observations are consistent with a scenario where the slow component is associated with the initial ejection of the accreted material and the fast component with a radiation-driven wind from the white dwarf. When these flows interact, the slow flow is swept up by the fast flow, producing the intermediate component. These colliding flows may produce the γ-ray emission observed in some novae. Our spectra also show that the transient heavy-element absorption lines seen in some novae have the same velocity structure and evolution as the other lines in the spectrum, implying an association with the nova ejecta rather than a preexisting circumbinary reservoir of gas or material ablated from the secondary. While this basic scenario appears to qualitatively reproduce multiwavelength observations of classical novae, substantial theoretical and observational work is still needed to untangle the rich diversity of nova properties. [ABSTRACT FROM AUTHOR]

We present a detailed study of the 2016 eruption of nova V407 Lupi (ASASSN-16kt), including optical, near-infrared, X-ray, and ultraviolet data from SALT, SMARTS, SOAR, Chandra, Swift, and XMM–Newton. Timing analysis of the multiwavelength light curves shows that, from 168 d post-eruption and for the duration of the X-ray supersoft source phase, two periods at 565 s and 3.57 h are detected. We suggest that these are the rotational period of the white dwarf and the orbital period of the binary, respectively, and that the system is likely to be an intermediate polar. The optical light-curve decline was very fast (t 2 ≤ 2.9 d), suggesting that the white dwarf is likely massive (≳ 1.25 M⊙). The optical spectra obtained during the X-ray supersoft source phase exhibit narrow, complex, and moving emission lines of He  ii, also characteristics of magnetic cataclysmic variables. The optical and X-ray data show evidence for accretion resumption while the X-ray supersoft source is still on, possibly extending its duration. [ABSTRACT FROM AUTHOR]

We report on multiwavelength observations of nova Small Magellanic Cloud Nova 2016-10a. The present observational set is one of the most comprehensive for any nova in the Small Magellanic Cloud, including low-, medium-, and high-resolution optical spectroscopy and spectropolarimetry from Southern African Large Telescope, Folded Low-Order Yte-Pupil Double-Dispersed Spectrograph, and Southern Astrophysical Research; long-term Optical Gravitational Lensing Experiment V- and I-bands photometry dating back to 6 yr before eruption; Small and Moderate Aperture Research Telescope System optical and near-IR photometry from ~11 d until over 280 d post-eruption; Swift satellite X-ray and ultraviolet observations from ~6 d until 319 d post-eruption. The progenitor system contains a bright disc and a main sequence or a sub-giant secondary. The nova is very fast with t2 ̃4.0 ± 1.0 d and t3 ̃ 7.8 ± 2.0 d in the V band. If the nova is in the SMC, at a distance of ~61 ± 10 kpc, we derive MV, max ̃-10.5 ± 0.5, making it the brightest nova ever discovered in the SMC and one of the brightest on record. At day 5 post-eruption the spectral lines show a He/N spectroscopic class and an Full Width at Half Maximum of ~3500 km s-1, indicating moderately high ejection velocities. The nova entered the nebular phase ~20 d post-eruption, predicting the imminent super-soft source turn-on in the X-rays, which started ~28 d post-eruption. The super-soft source properties indicate a white dwarf mass between 1.2 and 1.3M⨀ in good agreement with the optical conclusions. [ABSTRACT FROM AUTHOR]

We report spectroscopic and photometric follow-up of the peculiar nova V5852 Sgr (discovered as OGLE-2015-NOVA-01), which exhibits a combination of features from different classes nova. The photometry shows a flat-topped light curve with quasi-periodic oscillations, then a smooth decline followed by two fainter recoveries in brightness. Spectroscopy with the Southern African Large Telescope shows first a classical nova with an Fe II or Fe II b spectral type. In the latter spectrum, broad emissions from helium, nitrogen and oxygen are prominent, and the iron has faded which could be an indication to the start of the nebular phase. The line widths suggest ejection velocities around 1000 km s-1. The nova is in the direction of the Galactic bulge and is heavily reddened by an uncertain amount. The V magnitude 16 days after maximum enables a distance to be estimated and this suggests that the nova may be in the extreme trailing stream of the Sagittarius dwarf spheroidal galaxy. If so it is the first nova to be detected from that, or from any dwarf spheroidal galaxy. Given the uncertainty of the method and the unusual light curve we cannot rule out the possibility that it is in the bulge or even the Galactic disk behind the bulge. [ABSTRACT FROM AUTHOR]