Nova Research Articles

Relation between Long-term Activity and Luminosity of the Pre- and Post-novae CT Ser and V446 Her

We show a relation of the pre- and post-nova activity of CT Ser (nova 1948) and V446 Her (nova 1960), known to have remarkably similar orbital periods P orb. We use both photographic and CCD photometric observations. Post-nova CT Ser shows a nova-like high-state activity with only a slight decrease of absolute magnitude M opt. In the interpretation, its M opt is above the region of the thermal-viscous instability (TVI) regime, so the disk is ionized out to its outer rim, similarly to before its classical nova (CN) outburst. On the contrary, V446 Her started its CN outburst from the TVI regime. Extensive changes in activity type in years after CN outburst were observed in V446 Her. The features consistent with the dwarf-nova outbursts appeared at most three decades after the end of the CN outburst. It suggests that its disk transitioned into the TVI region in that it was before its CN outburst. The pre-nova and post-nova states of activity did not change significantly for a given system, except for several decades after the CN outburst. Even pre-novae and post-novae with mutually similar P orb, hence secondary star (donor) spectral types and dimensions and disk radii, can show largely discrepant M opt and disk states in the similar epochs (several decades) surrounding the CN outburst.

Classical novae in the ASKAP pilot surveys

Abstract We present a systematic search for radio counterparts of novae using the Australian Square Kilometer Array Pathfinder (ASKAP). Our search used the Rapid ASKAP Continuum Survey, which covered the entire sky south of declination $+41^{\circ}$ ( $\sim$ $34000$ square degrees) at a central frequency of 887.5 MHz, the Variables and Slow Transients Pilot Survey, which covered $\sim$ $5000$ square degrees per epoch (887.5 MHz), and other ASKAP pilot surveys, which covered $\sim$ 200–2000 square degrees with 2–12 h integration times. We crossmatched radio sources found in these surveys over a two–year period, from 2019 April to 2021 August, with 440 previously identified optical novae, and found radio counterparts for four novae: V5668 Sgr, V1369 Cen, YZ Ret, and RR Tel. Follow-up observations with the Australian Telescope Compact Array confirm the ejecta thinning across all observed bands with spectral analysis indicative of synchrotron emission in V1369 Cen and YZ Ret. Our light-curve fit with the Hubble Flow model yields a value of $1.65\pm 0.17 \times 10^{-4} \rm \:M_\odot$ for the mass ejected in V1369 Cen. We also derive a peak surface brightness temperature of $250\pm80$ K for YZ Ret. Using Hubble Flow model simulated radio lightcurves for novae, we demonstrate that with a 5 $\sigma$ sensitivity limit of 1.5 mJy in 15-min survey observations, we can detect radio emission up to a distance of 4 kpc if ejecta mass is in the range $10^{-3}\rm \:M_\odot$ , and upto 1 kpc if ejecta mass is in the range $10^{-5}$ – $10^{-3}\rm \:M_\odot$ . Our study highlights ASKAP’s ability to contribute to future radio observations for novae within a distance of 1 kpc hosted on white dwarfs with masses $0.4$ – $1.25\:\rm M_\odot$ , and within a distance of 4 kpc hosted on white dwarfs with masses $0.4$ – $1.0\:\rm M_\odot$ .

Understanding the cosmic abundance of 22Na: Lifetime measurements in 23Mg

Simulations of explosive nucleosynthesis in novae predict the production of 22Na, a key astronomical observable to constrain nova models. Its gamma-ray line at 1.275 MeV has not yet been observed by the gamma-ray space telescopes. The 20Ne/22Ne ratio in presolar grains, a possible tool to identify nova grains, also depends on 22Na produced. Uncertainties on its yield in classical novae currently originate from the rate of the 22Na(p, γ)23Mg reaction. At peak novae temperatures, this reaction is dominated by a resonance at ER=0.204 MeV, corresponding to the Ex=7.785 MeV excited state in 23Mg. The resonance strengths measured so far disagree by one order of magnitude. An experiment has been performed at GANIL to measure the lifetime and the proton branching ratio of this key state, with a femtosecond resolution for the former. The reactions populating states in 23Mg have been studied with a high resolution detection set-up, i.e. the particle VAMOS, SPIDER and gamma tracking AGATA spectrometers, allowing the measurements of lifetimes and proton branchings. We present here a comparison between experimental results and shell-model calculations, that allowed us to assign the spin and parity of the key state. Rather small values obtained for reduced M1 matrix elements, M(M1) ≲ 0.5 µN, and proton spectroscopic factors, C2Sp<10−2, seem to be beyond the accuracy of the shell model. With the reevaluated 22Na(p, γ)23Mg rate, the 22Na detectability limit and its observation frequency from novae are found promising for the future space telescopes.

Observations of Recent Novae Visible on the Northern Hemisphere

We are experiencing an increased rate of nova explosion observed from the northern hemisphere in the recent 2 years. Starting from July 2020 with the strongly reddened nova V1391 Cas, we are continuously observing some novae in outburst. Here, we report the observations of 5 classical novae: Nova Cas 2020 = V1391 Cas, Nova Per 2020 = V1112 Per, Nova Cas 2021 = V1405 Cas, Nova Her 2021 = V1674 Her and Nova Vul 2021 = V606 Vul. The best at the end - the recurrent symbiotic nova RS Oph. In this paper, we report our photometric and spectroscopic observations. Thorough analysis is to be done in the near future. At this moment we want to highlight two important facts. In the case of V1405 Cas, we know the orbital period of the progenitor. The progenitor of V1674 Her is an intermediate polar with known white dwarf spin period. In both cases, it will be very interesting to compare the values after the nova explosion. In the case of V1674 Her, we have already measured both the orbital and the spin period.

Ultraviolet and X-ray Light-Curves of Novae Observed by the Neil Gehrels Swift Observatory

With rapid response capabilities, and a daily planning of its observing schedule, the Neil Gehrels Swift Observatory is ideal for monitoring transient and variable sources. Here we present a sample of the 12 novae with the most detailed ultraviolet (UV) follow-up by Swift—the first uniform analysis of such UV light-curves. The fading of these specific light-curves can be modelled as power-law decays (plotting magnitude against log time), showing that the same physical processes dominate the UV emission for extended time intervals in individual objects. After the end of the nuclear burning interval, the X-ray emission drops significantly, fading by a factor of around 10–100. The UV changes, however, are of a lower amplitude, declining by 1–2 mag over the same time period. The UV light-curves typically show a break from flatter to steeper around the time at which the X-ray light-curve starts a steady decline from maximum, ∼0.7–1.3 TSSSend. Considering populations of both classical and recurrent novae, and those with main sequence or giant companions, we do not find any strong differences in the UV light-curves or their evolution, although the long-period recurrent novae are more luminous than the majority of the classical novae.

γ-Ray Emission from Classical Nova V392 Per: Measurements from Fermi and HAWC

This paper reports on the γ-ray properties of the 2018 Galactic nova V392 Per, spanning photon energies ∼0.1 GeV–100 TeV by combining observations from the Fermi Gamma-ray Space Telescope and the HAWC Observatory. As one of the most rapidly evolving γ-ray signals yet observed for a nova, GeV γ-rays with a power-law spectrum with an index Γ = 2.0 ± 0.1 were detected over 8 days following V392 Per’s optical maximum. HAWC observations constrain the TeV γ-ray signal during this time and also before and after. We observe no statistically significant evidence of TeV γ-ray emission from V392 Per, but present flux limits. Tests disfavor the extension of the Fermi Large Area Telescope spectrum to energies above 5 TeV by 2 standard deviations (95%) or more. We fit V392 Per’s GeV γ-rays with hadronic acceleration models, incorporating optical observations, and compare the calculations with HAWC limits.

Interpretation of the light curve of gamma-ray emission from the 2021 outburst of the recurrent nova RS Ophiuchi

Nova outbursts take place in binary star systems comprising a white dwarf (WD) and either a low-mass Sun-like star (classical novae) or, a red giant. GeV gamma-ray emission has been detected from a dozen of classical novae and from one novae in symbiotic system (V407 Cyg) by Fermi-LAT. For classical novae, gamma-ray emission is generally thought to be related to internal shocks formed as fast outflow collides with the slow outflow. However, for V407 Cyg, the origin of the gamma-ray emission has been debated, as both internal shock and external shock, resulted from the collision between the nova ejecta and the ambient wind of the giant companion, were suggested to explain the gamma-ray data. Recently, bright GeV and TeV gamma-ray emission has been detected from a nova in symbiotic system, RS Ophiuchi, during its 2021 outburst, which shows a remarkably smooth power-law decay in time up to about one month after the outburst. We show that this temporal decay behavior can be interpreted as arising from an adiabatic external shock expanding in the red giant wind. In this interpretation, the gamma-rays are produced by shock-accelerated protons interacting with the dense wind through the hadronic process. We also derive the scaling relations for the decay slopes for both adiabatic and radiative nova shocks in the self-similar deceleration phase.

The 2019 Outburst of the 2005 Classical Nova V1047 Cen: A Record Breaking Dwarf Nova Outburst or a New Phenomenon?

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 s−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.

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 (≳1013 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 , 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.

Classical novae with CUBES

Classical novae with CUBES

Vector leptoquark U_3 and CP violation at T2K, NOvA experiments

In the current epoch of neutrino physics, many experiments are aiming for precision measurements of oscillation parameters. Thus, various new physics scenarios which alter the neutrino oscillation probabilities in matter deserve careful investigation. In this context, we study the effect of a vector leptoquark which induces non-standard neutrino interactions (NSI) that modify the oscillation probabilities of neutrinos in matter. We show that such interactions provide a relatively large value of NSI parameter varepsilon _{e mu }. Considering this NSI parameter, we successfully explain the recent discrepancy between the observed delta _{CP} results of T2K and NOvA.

Binary Interaction Dominates Mass Ejection in Classical Novae

Recent observations suggest our understanding of mass loss in classical novae is incomplete, motivating a new theoretical examination of the physical processes responsible for nova mass ejection. In this paper, we perform hydrodynamical simulations of classical nova outflows using the stellar evolution code MESA. We find that, when the binary companion is neglected, white dwarfs with masses ≳0.8 M ⊙ successfully launch radiation-pressure-driven optically thick winds that carry away most of the envelope. However, for most of the mass-loss phase, these winds are accelerated at radii beyond the white dwarf’s Roche radius assuming a typical cataclysmic variable donor. This means that, before a standard optically thick wind can be formed, mass loss will instead be initiated and shaped by binary interaction. An isotropic, optically thick wind is only successfully launched when the acceleration region recedes within the white dwarf’s Roche radius, which occurs after most of the envelope has already been ejected. The interaction between these two modes of outflow—a first phase of slow, binary-driven, equatorially focused mass loss encompassing most of the mass ejection and a second phase consisting of a fast, isotropic, optically thick wind—is consistent with observations of aspherical ejecta and signatures of multiple outflow components. We also find that isolated lower-mass white dwarfs ≲0.8 M ⊙ do not develop unbound optically thick winds at any stage, making it even more crucial to consider the effects of the binary companion on the resulting outburst.

Investigation of a likely orbital periodicity of Nova Hercules 2021 in X-rays and γ-rays

ABSTRACT We report a detection of a ∼0.153 d period in the classical nova V1674 Her using the NICER observations taken within a month since the outburst (i.e. ∼MJD 59405). The X-ray period is consistent with the orbital period previously found in the optical band, strongly suggesting the NICER signal as the X-ray orbital periodicity of the system. A seemingly double-humped profile was obtained by folding the detrended X-ray light curve with the period after removing the rotational X-ray pulsations of the nova. The profile may be caused by occultation by the companion or the accretion disc, possibly indicating a high inclination of the system. The γ-ray emission of V1674 Her with a significance level ≳ 5 σ was detected by Fermi-LAT close to its optical peak and the emission faded away within 1 d, which is the shortest duration known for a γ-ray nova. Folded on 0.153 d, a marginal γ-ray variability can be also seen in the LAT light curve, but without the double-hump feature observed in X-rays. If the γ-ray modulation is real, its origin is probably different from that observed in the X-ray and optical bands.

The Galactic Nova Rate: Estimates from the ASAS-SN and Gaia Surveys

We present the first estimate of the Galactic nova rate based on optical transient surveys covering the entire sky. Using data from the All-Sky Automated Survey for Supernovae (ASAS-SN) and Gaia—the only two all-sky surveys to report classical nova candidates—we find 39 confirmed Galactic novae and 7 additional unconfirmed candidates discovered from 2019 to 2021, yielding a nova discovery rate of ≈14 yr−1. Using accurate Galactic stellar mass models and three-dimensional dust maps and incorporating realistic nova light curves, we have built a sophisticated Galactic nova model to estimate the fraction of Galactic novae discovered by these surveys over this time period. The observing capabilities of each survey are distinct: the high cadence of ASAS-SN makes it sensitive to fast novae, while the broad observing filter and high spatial resolution of Gaia make it more sensitive to highly reddened novae across the entire Galactic plane and bulge. Despite these differences, we find that ASAS-SN and Gaia give consistent Galactic nova rates, with a final joint nova rate of 26 ± 5 yr−1. This inferred nova rate is substantially lower than found by many other recent studies. Critically assessing the systematic uncertainties in the Galactic nova rate, we argue that the role of faint, fast-fading novae has likely been overestimated, but that subtle details in the operation of transient alert pipelines can have large, sometimes unappreciated effects on transient recovery efficiency. Our predicted nova rate can be directly tested with forthcoming red/near-infrared transient surveys in the southern hemisphere.

7Be detection in the 2021 outburst of RS Oph

ABSTRACT 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.

Gap Transients Interacting with Circumstellar Medium

In the last 20 years, modern wide-field surveys discovered a new class of peculiar transients, which lie in the luminosity gap between standard supernovae and classical novae. These transients are often called “intermediate luminosity optical transients” or “gap transients”. They are usually distinguished in subgroups based on their phenomenology, such as supernova impostors, intermediate luminosity red transients, and luminous red novae. In this review, we present a brief overview of their observational features and possible physical scenarios to date, in the attempt to understand their nature.

Very light sterile neutrinos at NOvA and T2K

Over the last several years, our understanding of neutrino oscillations has developed significantly due to the long-baseline measurements of muon-neutrino disappearance and muon-to-electron-neutrino appearance at the T2K and NOvA experiments. However, when interpreted under the standard-three-massive-neutrinos paradigm, a tension has emerged between the two experiments' data. Here, we examine whether this tension can be alleviated when a fourth, very light neutrino is added to the picture. Specifically, we focus on the scenario in which this new neutrino has a mass similar to, or even lighter than, the three mostly-active neutrinos that have been identified to date. We find that, for some regions of parameter space, the four-neutrino framework is favored over the three-neutrino one with moderate (a little under two sigma) significance. Interpreting these results, we provide future outlook for near-term and long-term experiments if this four-neutrino framework is indeed true.

Searching the SETI Ellipsoid with Gaia

The search for extraterrestrial intelligence (SETI) Ellipsoid is a geometric method for prioritizing technosignature observations based on the strategy of receiving signals synchronized to conspicuous astronomical events. Precise distances to nearby stars from Gaia makes constraining Ellipsoid crossing times possible. Here we explore the utility of using the Gaia Catalog of Nearby Stars to select targets on the SN 1987A SETI Ellipsoid, as well as the Ellipsoids defined by 278 classical novae. Less than 8% of stars within the 100 pc sample are inside the SN 1987A SETI Ellipsoid, meaning the vast majority of nearby stars are still viable targets for monitoring over time. We find an average of 734 stars per year within the 100 pc volume will intersect the Ellipsoid from SN 1987A, with ∼10% of those having distance uncertainties from Gaia better than 0.1 lyr.

Improved measurement of neutrino oscillation parameters by the NOvA experiment

We present new $\nu_\mu\rightarrow\nu_e$, $\nu_\mu\rightarrow\nu_\mu$, $\overline{\nu}_\mu\rightarrow\overline{\nu}_e$, and $\overline{\nu}_\mu\rightarrow\overline{\nu}_\mu$ oscillation measurements by the NOvA experiment, with a 50% increase in neutrino-mode beam exposure over the previously reported results. The additional data, combined with previously published neutrino and antineutrino data, are all analyzed using improved techniques and simulations. A joint fit to the $\nu_e$, $\nu_\mu$, $\overline{\nu}_e$, and $\overline{\nu}_\mu$ candidate samples within the 3-flavor neutrino oscillation framework continues to yield a best-fit point in the normal mass ordering and the upper octant of the $\theta_{23}$ mixing angle, with $\Delta m^{2}_{32} = (2.41\pm0.07)\times 10^{-3}$ eV$^2$ and $\sin^2\theta_{23} = 0.57^{+0.03}_{-0.04}$. The data disfavor combinations of oscillation parameters that give rise to a large asymmetry in the rates of $\nu_e$ and $\overline{\nu}_e$ appearance. This includes values of the CP-violating phase in the vicinity of $\delta_\text{CP} = \pi/2$ which are excluded by $>3\sigma$ for the inverted mass ordering, and values around $\delta_\text{CP} = 3\pi/2$ in the normal ordering which are disfavored at 2$\sigma$ confidence.

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 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 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.