Nova Outbursts Research Articles

High-resolution imaging of the evolving bipolar outflows in symbiotic novae: The case of the RS Ophiuchi 2021 nova outburst

High-resolution imaging of the evolving bipolar outflows in symbiotic novae: The case of the RS Ophiuchi 2021 nova outburst

Mass-transfer Outbursts Reborn: Modeling the Light Curve of the Dwarf Nova EX Draconis

EX Draconis is an eclipsing dwarf nova that shows outbursts with moderate amplitude (≃2 mag) and a recurrence timescale of ≃20–30 days. Dwarf novae outbursts are explained in terms of either a thermal-viscous instability in the disc or an instability in the mass-transfer rate of the donor star (MTIM). We developed simulations of the response of accretion discs to events of enhanced mass transfer, in the context of the MTIM, and applied them to model the light curve and variations in the radius of the EX Dra disc throughout the outburst. We obtain the first modeling of a dwarf nova outburst by using χ 2 to select, from a grid of simulations, the best-fit parameters to the observed EX Dra outbursts. The observed time evolution of the system brightness and the changes in the radius of the outer disc along the outburst cycle are satisfactorily reproduced by a model of the response of an accretion disc with a viscosity parameter α = 4.0 and a quiescent mass-transfer rate Ṁ2(quiescence)=4.0×1016 g s−1 to an event of width Δt = 6.0 × 105 s (∼7 days) where the mass-transfer rate increases to Ṁ2(outburst)=1.5×1018 g s−1.

The Impact of Nova Outbursts on the Chemical Abundance of the Interstellar Medium

Nova outbursts are the results of thermonuclear runaways, which occur when sufficient material accretes on the surfaces of white dwarfs (WDs). Using the MESA code, we construct a detailed grid for carbon-oxygen and oxygen-neon-magnesium novae. By employing population synthesis methods, we conduct a statistical analysis of the distribution of novae in the Milky Way. In our models, on average, a typical nova system may undergo about 8000 eruptions and the Galactic nova rate is ∼130 yr−1. The C, N, and O elements in nova ejecta are strongly affected by the mixing degree between WD core and accreted material. Our results show that the average value of 12C/13C in nova ejecta is about an order of magnitude lower than that on the surface of a red giant, that for 16O/17O is about 5 times lower, and that for 14N/15N is about 1.5 times lower. The annual yields of 13C , 15N, and 17O from nova ejection are larger than those from AGB stars. This indicates that compared to a red giant, nova eruptions are a more important source of the odd-numbered nuclear elements of 13C , 15N, and 17O in the Galactic interstellar medium.

X-Ray Flashes on Helium Novae

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 ☉ helium novae with mass accretion rates of (1.6–7.5) × 10−7 M ☉ yr−1. Long durations of the X-ray flashes (100 days–10 yr) and high X-ray luminosities (∼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 ⊙ WD.

Ancient Nova Shells of RX Pup Indicate Evolution of Mass Transfer Rate

RX Pup is a symbiotic binary that experienced a nova outburst in the 1970s. Here we report a discovery of a ∼1300 yr old nova shell around the system and a possible detection of a ∼7000 yr old nova shell. Together with the nova shell ejected in the 1970s, this makes RX Pup the first system with three nova shells observed. This triad of eruptions suggests a change in the nova recurrence time. The most likely explanation is an alteration in the mass transfer rate attributed to evolutionary changes of the mass donor in the system. Notably, comparative analyses with theoretical models indicate an increase in the average mass transfer rate by a factor of 3 over the past 10,000 yr. This makes RX Pup a unique system, which allows us to probe millennium-scale evolution of mass transfer rates in binary systems.

Investigation of WZ Sge-type Dwarf Nova ASASSN-19oc: Optical Spectroscopy and Multicolor Light Curve Analysis

In this study, we present an investigation of the newly discovered dwarf nova ASASSN-19oc during its superoutburst on 2019 June 2. We carried out detailed UBVR c I c -photometric observations and also obtained a spectrum on day 7 of the outburst, which shows the presence of hydrogen absorption lines commonly found in dwarf nova outbursts. Analysis of photometric data reveals the occurrence of early superhumps in the initial days of observations, followed by ordinary and late superhumps. We have accurately calculated the period of the ordinary superhumps as P ord = 0.05681(10) days and determined the periods at different stages, as well as the rate of change of the superhump period (P dot = Ṗ /P = 8.1 × 10−5). Additionally, we have derived the mass ratio of the components (q = 0.09), and estimated the color temperature during the outburst as ∼11,000 K, the distance to the system (d = 560 pc) and absolute magnitude of the system in outburst (M V = 5.3). We have shown that outbursts of this star are very rare: based on brightness measurements on 600 archival photographic plates, we found only one outburst that occurred in 1984. This fact, as well as the properties listed above, convincingly shows that the variable ASASSN-19oc is a dwarf nova of WZ Sge type.

A disc wind origin for the optical spectra of dwarf novae in outburst

ABSTRACT Many high-state cataclysmic variables (CVs) exhibit blue-shifted absorption features in their ultraviolet (UV) spectra – a smoking-gun signature of outflows. However, the impact of these outflows on optical spectra remains much more uncertain. During its recent outburst, the eclipsing dwarf nova V455 And displayed strong optical emission lines whose cores were narrower than expected from a Keplerian disc. Here, we explore whether disc + wind models developed for matching UV observations of CVs can also account for these optical spectra. Importantly, V455 And was extremely bright at outburst maximum: the accretion rate implied by fitting the optical continuum with a standard disc model is $\dot{M}_{\rm acc} \simeq 10^{-7}~{\rm M}_\odot ~{\rm yr^{-1}}$. Allowing for continuum reprocessing in the outflow helps to relax this constraint. A disc wind can also broadly reproduce the optical emission lines, but only if the wind is (i) highly mass-loaded, with a mass-loss rate reaching $\dot{M}_{\rm wind} \simeq 0.4 \dot{M}_{\rm acc}$, and/or (ii) clumpy, with a volume filling factor $f_V \simeq 0.1$. The same models can describe the spectral evolution across the outburst, simply by lowering $\dot{M}_{\rm acc}$ and $\dot{M}_{\rm wind}$. Extending these models to lower inclinations and into the UV produces spectra consistent with those observed in face-on high-state CVs. We also find, for the first time in simulations of this type, P-Cygni-like absorption features in the Balmer series, as have been observed in both CVs and X-ray binaries. Overall, dense disc winds provide a promising framework for explaining multiple observational signatures seen in high-state CVs, but theoretical challenges persist.

Current and secular accretion rates of EX Hydrae

We report an observed accretion rate of\ $ M_1\!=\! pm0.60) for the white dwarf in the short-period, intermediate polar This result is based upon the accretion-induced $4 energy flux from $2.45 to 100\,keV and the corresponding luminosity at the Gaia distance of 56.77\,pc. Our result is in perfect agreement with the theoretical mass transfer rate from the secondary star induced by gravitational radiation (GR) and the spin-up of the white dwarf, $- M_2\!=\! pm0.35) 24<!PCT!> of it is caused by the spin-up. The agreement indicates that mass transfer is conservative. The measured $ M_1$ obviates the need for angular momentum loss (AML) by any process other than GR. We complemented this result with an estimate of the mean secular mass transfer rate over $ by interpreting the non-equilibrium radius of the secondary star in based on published evolutionary calculations. This suggests a time-averaged mass transfer rate enhanced over GR by a factor GR Combined with the present-day lack of such an excess, we suggest that an enhanced secular AML is due to an intermittently active process, such as the proposed frictional motion of the binary in the remnants of nova outbursts. We argue that despite its weakly magnetic nature, has evolved in a very similar way to non-magnetic CVs. We speculate that the discontinuous nature of an enhanced secular AML may similarly apply to the latter.

Physics of nova outbursts: Theoretical models of classical nova outbursts with optically thick winds on 1.2 M⊙ and 1.3 M⊙ white dwarfs

Abstract We present time-dependent nova outburst models with optically thick winds for 1.2 and 1.35$\, M_{\odot }$ white dwarfs (WDs) with a mass-accretion rate of $5 \times 10^{-9}\, M_{\odot }$ yr−1 and for a 1.3$\, M_{\odot }$ WD with $2 \times 10^{-9}\, M_{\odot }$ yr−1. The X-ray flash occurs 11 d before the optical peak of the 1.2$\, M_{\odot }$ WD and 2.5 d before the peak of the 1.3$\, M_{\odot }$ WD. The wind mass-loss rate of the 1.2$\, M_{\odot }$ WD (1.3$\, M_{\odot }$ WD) reaches a peak of $6.4 \times 10^{-5}\, M_{\odot }$ yr−1 ($7.4 \times 10^{-5}\, M_{\odot }$ yr−1) at the epoch of the maximum photospheric expansion with the lowest photospheric temperature of log Tph (K) = 4.33 (4.35). The nuclear energy generated during the outburst is lost in the form of radiation (61% for the 1.2$\, M_{\odot }$ WD; 47% for the 1.3$\, M_{\odot }$ 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 tpeak and the expansion timescale τexp. With this relation, we are able to predict the time to optical maximum tpeak from the ignition model (at t = 0) without following a time-consuming nova wind evolution.

3D physical structure and angular expansion of the remnant of the recurrent nova T Pyx

ABSTRACT We present the analysis of archival Very Large Telescope (VLT) Multi-Unit Spectroscopic Explorer (MUSE) and multi-epoch Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) and Wide Field Camera 3/Ultraviolet-Visible Imaging Spectrograph (WFC3/UVIS) narrow-band observations of the remnant associated with the ejecta of the mid-nineteenth century outburst of the recurrent nova T Pyx. These data sets are used to investigate its true three-dimensional (3D) physical structure and the nebular expansion patterns along the line of sight and on the plane of the sky. The VLT MUSE emission line maps and 3D visualizations based on position–position–velocity diagrams reveal T Pyx as a bipolar nebula, with a knotty toroidal structure at its waist best seen in H $\beta$ and two open bowl-shaped bipolar lobes (a diabolo) best revealed by the [O iii] emission lines. The comparison of multi-epoch HST WFPC2 and WFC3/UVIS narrow-band images and VLT MUSE emission-line maps of T Pyx reveals the angular expansion of the remnant through the proper motion of individual knots and nebular features. The angular expansion is confirmed to be homologous in the period from 1994.2 to 2007.4 before the recent 2011 outburst, but there is suggestive evidence that the inner knots have experienced a higher expansion rate since then.

Burst-induced spin variations in the accreting magnetic white dwarf PBC J0801.2–4625

ABSTRACT PBC J0801.2–4625 is an intermediate polar with a primary spin frequency of 66.08 d−1 and an unknown orbital period. The long-term All Sky Automated Survey for Supernovae (ASAS-SN) light curve of this system reveals four bursts, all of which have similar peak amplitudes (∼2 mag) and durations (∼2 d). In this work, we primarily study the timing properties of this system’s 2019 February burst, which was simultaneously observed by both ASAS-SN and the Transiting Exoplanet Survey Satellite (TESS). Pre-burst, a frequency of 4.064 ± 0.002 d−1(5.906 ± 0.003 h period), likely attributed to the binary orbit, is identified in addition to previous measurements for the white dwarf’s spin. During the burst, however, we find a spin frequency of 68.35 ± 0.28 d−1. Post-burst, the spin returns to its pre-brust value but with a factor 1.82 ± 0.05 larger amplitude. The burst profile is double-peaked, and we estimate its energy to be 3.3 × 1039 erg. We conclude that the burst appears most consistent with thermonuclear runaway (i.e. a 'micronova'), and suggest that the spin variations may be an analogue to burst oscillations (i.e. 'micronova oscillations'). However, we also note that the above findings could be explained by a dwarf nova outburst. With the available data, we are unable to distinguish between these two scenarios.

The Long-term Activity of the Postnovae Q Cygni and BK Lyncis

We show the postnova activity of Q Cyg (Nova Cyg 1876) and BK Lyn (probable Nova Lyn 101). We use both CCD and photographic photometric observations. We show that both systems lie close to the upper limit of the luminosity in which dwarf nova (DN) outbursts occur. Q Cyg shows a novalike high-state activity. Random fluctuations (typically 0.6 mag) from a well-defined curve of the moving averages of brightness often occur on the timescale of weeks. The random fluctuations were suppressed during one fainter interval lasting several months but increased during another. In the author’s interpretation, clumps in the disk wind are likely to play a role in these fluctuations, especially when the luminosity of Q Cyg is near the upper limit of the range in which DN outbursts occur. BK Lyn was observed to spend about 100 yr in a very long state of a high luminosity on the upper limit of the region of DN outbursts before undergoing a time segment in which DN outbursts were present. We find that the individual DN outbursts in BK Lyn all show similar decay rates and fade more gradually than those of DNe that do not also show classical nova eruptions. We attribute it, along with the low amplitude of DN outbursts and the high quiescent luminosity, to the role of extra light. These outburst peaks, higher than the surrounding segments of the flat light curve, speak in favor of the ER UMa-type with superoutburst cycles and standstills rather than the high state in a novalike variable.

Swift monitoring of GK Persei during the 2018 dwarf nova outburst

ABSTRACT The old nova and intermediate polar (IP) GK Persei underwent one of its recurrent dwarf nova (DN) outbursts in 2018. We proposed monitoring it in UV and X-rays with the Neil Gehrels Swift Observatory, starting less than six days after the eruption, until 16 days after the eruption ended. For the first time, we could follow the decay to the minimum light UV and X-rays. We present the timing and spectral analysis, comparing the results with the previous outbursts and with the quiescent status. We confirm the spin modulation in X-rays with a period PWD = 351.325(9) s, only in the 2–10 keV range. The period was not detected in the 0.3–2 keV range and in the UV band, suggesting that the soft portion of the X-ray spectrum in GK Per does not originate near the poles, but in a wind or circumstellar material. The amplitude of the modulation was less prominent than in 2015, a fact that seems correlated with a lower average mass accretion rate. The spectral fits are consistent with a mass accretion rate increasing by a factor of 2 from rise to maximum and decreasing during the return to minimum, following the trend of the modulation amplitude. The maximum plasma temperature is higher than the Swift XRT energy range of 0.3–10 keV, thus it is not well constrained, but our spectral fits indicate that it may have varied irregularly during the outburst.

Cataclysmic variables and the disc instability model in the Gaia DR3 colour–magnitude diagram

Context. Cataclysmic variables (CVs) are semi-detached binaries composed of a white dwarf orbiting a lower-mass K or M star. Aims. We investigate whether CVs are responsible for a new intriguing feature (the “hook”) that appears in the Gaia DR3 colour–magnitude Hertzsprung-Russell diagram (HRD) when selecting sources with low extinction. We also aim to understand the location of CVs in the HRD based on the predictions of the disc instability model (DIM). The DIM forms the foundation of our basic understanding of stable (novae-like) and outbursting CVs (dwarf novae). Methods. We calculated the expected behaviour of CVs in the Gaia HRD while taking into account the variable light contributed by the accretion disc, the companion, the white dwarf, and the bright spot where the Roche lobe overflow stream from the companion intersects the disc. Results. We find that the hook feature is most likely composed of CVs. The hook corresponds to the limited region where stable (novae-like) CVs must be located in the HRD according to the DIM. Unstable systems giving rise to dwarf novae outbursts trace counterclockwise loops in the HRD. The overall behaviour is consistent with the location of the various CV subtypes in the HRD. Conclusions. These results can be used as a basis on which to pinpoint interesting outliers in the HRD, either due to their location or their tracks. These outliers could signal new subtypes, such as cold, stable CVs with truncated discs, or may challenge the disc instability model.

Classifying Optical (Out)bursts in Cataclysmic Variables: The Distinct Observational Characteristics of Dwarf Novae, Micronovae, Stellar Flares, and Magnetic Gating

Cataclysmic variables can experience short optical brightenings, which are commonly attributed to phenomena such as dwarf novae outbursts, micronovae, donor flares, or magnetic gating bursts. Since these events exhibit similar observational characteristics, their identification has often been ambiguous. In particular, magnetic gating bursts and micronovae have been suggested as alternative interpretations of the same phenomena. Here we show that the timescales and energies separate the optical brightenings into separate clusters consistent with their different classifications. This suggests that micronovae and magnetic gating bursts are in fact separate phenomena. Based on our findings, we develop diagnostic diagrams that can distinguish between these bursts/flares based on their properties. We demonstrate the effectiveness of this approach on observations of a newly identified intermediate polar, CTCV J0333-4451, which we classify as a magnetic gating system. CTCV J0333-4451 is the third highest spin-to-orbital period ratio intermediate polar with magnetic gating, suggesting that these bursts are common among these rare systems.

Nine New Cataclysmic Variable Stars with Negative Superhumps

Negative superhumps (NSHs) are signals a few percent shorter than the orbital period of a binary star and are considered to originate from the reverse precession of the tilted disk. Based on TESS photometry, we find nine new cataclysmic variable stars with NSHs. Three (ASAS J1420, TZ Per, and V392 Hya) of these stars similar to AH Her still have NSHs during dwarf nova outbursts, and the NSH amplitude varies with the outburst. The variation in the radius of the accretion disk partially explains this phenomenon. However, it does not explain the rebound of the NSH amplitude after the peak of the outburst and the fact that the NSH amplitude of the quiescence is sometimes not the largest, and it is necessary to include the disk instability model (DIM) and add other ingredients. Therefore, we suggest that the variation of NSH amplitude with outburst can be an important basis for studying the origin of NSHs and improving the DIM. The six (ASASSN-V J1137, ASASSN-V J0611, 2MASS J0715, LAMOST J0925, ASASSN-17qj, and ZTF 18acakuxo) remaining stars have been poorly studied, and for the first time we determine their orbital periods, NSHs, and superorbital signal (SOR) periods. The NSH periods and amplitudes of ASASSN-V J1137 and ASASSN-17qj vary with the SOR, and based on the comparison of the observations with the theory, we suggest that a single change in tilted disk angle does not explain the observations of the SOR and that other ingredients need to be considered as well.

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

ABSTRACT 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 yr, 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 (≳30 yr) are consistent with a massive white dwarf model of ∼1.2 M⊙. 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.

Theoretical Light-curve Models of the Symbiotic Nova CN Cha—Optical Flat Peak for 3 Yr

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.

Lepto-hadronic Interpretation of 2021 RS Ophiuchi Nova Outburst

Very-high-energy (VHE; 100 GeV < E ≤ 100 TeV) and high-energy (HE; 100 MeV < E ≤ 100 GeV) gamma rays were observed from the symbiotic recurrent nova RS Ophiuchi (RS Oph) during its outburst in 2021 August by various observatories, such as the High Energy Stereoscopic System (HESS), Major Atmospheric Gamma Imaging Cherenkov (MAGIC), and Fermi-Large Area Telescope (LAT). The models that have been explored so far tend to favor a hadronic scenario of particle acceleration over an alternative leptonic scenario. This paper explores a time-dependent lepto-hadronic scenario to explain the emission from the RS Oph source region. We have used simultaneous low-frequency radio data observed by various observatories along with the data provided by HESS, MAGIC, and Fermi-LAT to explain the multiwavelength spectral energy distributions corresponding to 4 days after the outburst. Our results show that a lepto-hadronic interpretation of the source not only explains the observed HE-VHE gamma-ray data but the corresponding model synchrotron component is also consistent with the first 4 days of low-radio-frequency data, indicating the presence of nonthermal radio emission at the initial stage of the nova outburst. We have also calculated the expected neutrino flux from the source region and discuss the possibility of detecting neutrinos.

Early Infrared and Visible Spectra from the 2022 Outburst of the Recurrent Novae U Scorpii

Spectroscopy from 0.46–2.5 μm is presented for the 2022 outburst of the recurrent nova U Scorpii from 2.54, 3.60, and 8.59 days after maximum light. During this period the spectra evolved from comparatively low excitation to the verge of displaying coronal lines. Between days 2.54 and 3.60, the nova showed a marked decrease in the low excitation lines of N i, O i, and Mg ii , an increase in He i, the emergence of N iii, and changes to the very broad line profiles. By day 8.59, the He i lines had been largely replaced by He ii emission and most broad component were weak or undetectable. No optical coronal lines were observed during or immediately following the detection of infrared coronal lines by Banerjee et al.