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The predicted Galactic nova rate based on 1000 Monte Carlo trials, from ASAS-SN (blue histogram), Gaia (red histogram), and a combination of both surveys (purple histogram). We give the median values of the distributions as the most likely Galactic nova rate and include the 16% and 84% confidence regions. The results are all consistent at the 1σ level, with the most likely rate from both surveys predicting a Galactic nova rate of R = 26 ± 5 yr −1 .
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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 fro...
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... In particular, they showed that previous optical searches were systematically missing a substantial population of dust-obscured novae, resulting in inconsistencies between observed and estimated nova rates in optical searches. More recently, Kawash et al. (2021b) and Kawash et al. (2022) used novae discovered in the ASASSN and Gaia surveys to carry out a systematic analysis of the nova rate, providing a rate of 26 ± 5 yr −1 . With the estimated optical nova rate nominally inconsistent with the higher NIR rate, further progress requires additional investigation of independent surveys and observing wavebands to assess these discrepancies. ...
... However, the WISE sample of novae, being discovered in the midinfrared bands over a broad-sky area including the Galactic plane, is tightly clustered around the central Galactic plane in regions of heavy extinction. This trend corroborates previous suggestions (De et al. 2021;Kawash et al. 2022) that infrared searches are substantially more sensitive to the (most frequent) nova eruptions near the Galactic plane, even though these events have been historically overlooked in optical surveys. ...
... As this work focuses on an archival search for events, spectroscopic confirmation of the events remains highly unfeasible. Therefore, although our search identifies more events than other recent systematic searches (De et al. 2021;Kawash et al. 2022), there is likely contamination from other fast evolving outbursts (e.g. X-ray binaries and young stars) near the Galactic plane (noting that the sample of Mróz et al. 2015 as well as Kawash et al. 2022 also consist of several unconfirmed events). ...
The Galactic nova rate is intimately linked to our understanding of its chemical enrichment and progenitor channels of Type Ia supernovae. Yet past estimates have varied by more than an order of magnitude ($\approx10-300$ yr$^{-1}$) owing to limitations in both discovery methods as well as assumptions regarding the Galactic dust distribution and extragalactic stellar populations. Recent estimates utilizing synoptic near-infrared surveys have begun to provide a glimpse of a consensus ($\approx25-50$ yr$^{-1}$); however, a consistent estimate remains lacking. Here, we present the first all-sky search for Galactic novae using 8 years of data from the NEOWISE mid-infrared (MIR) survey. Operating at $3.4$ and $4.6$ $\mu$m where interstellar extinction is negligible, the 6-month cadence NEOWISE dataset offers unique sensitivity to discover slowly evolving novae across the entire Galaxy. Using a novel image subtraction pipeline together with systematic selection criteria, we identify a sample of 49 rapidly evolving MIR outbursts as candidate Galactic novae. While 27 of these sources are known novae, the remaining are previously missed nova candidates discovered in this work. The unknown events are spatially clustered along the densest and most heavily obscured regions of the Galaxy where previous novae are severely underrepresented. We use simulations of the NEOWISE survey strategy, the pipeline detection efficiency and our criteria to derive a Galactic nova rate of $47.9^{+3.1}_{-8.3}$ yr$^{-1}$. The discovery of these exceptionally bright (yet overlooked) nova candidates confirm emerging suggestions that optical surveys have been highly incomplete in searches for Galactic novae, highlighting the potential for MIR searches in revealing the demographics of Galactic stellar outbursts.
... We can estimate the number of potential novae contaminants using the estimated novae rate of 26 ± 5 in the Milky Way (Kawash et al. 2022) and 363 +33 −45 in M87 (Shafter et al. 2017). The year-to decadelong evolution timescale in Figure 1 for novae translates to ∼ 10−100 years as contaminants. ...
Many double white dwarf (WD) mergers likely do not lead to a prompt thermonuclear explosion. We investigate the prospects for observationally detecting the surviving remnants of such mergers, focusing on the case of mergers of Carbon-Oxygen WDs. For $\sim 10^4$ yr, the merger remnant is observationally similar to an extreme AGB star evolving to become a massive WD. Identifying merger remnants is thus easiest in galaxies with high stellar masses (high WD merger rate) and low star formation rates (low birth rate of $\sim 6-10 \,{\rm M_{\odot}}$ stars). Photometrically identifying merger remnants is challenging even in these cases because the merger remnants appear similar to He stars and post-outburst classical novae. We propose that the most promising technique for discovering WD merger remnants is through their unusual surrounding photoionized nebulae. We use CLOUDY photoionization calculations to investigate their unique spectral features. Merger remnants should produce weak hydrogen lines and strong carbon and oxygen recombination and fine-structure lines in the UV, optical and IR. With integral field spectrographs, we predict that hundreds of candidates are detectable in M87 and other nearby massive galaxies. Dust somewhat reduces the optical line emission for nebula radii $\lesssim 10^{17}$ cm, but the sources spend most of their time with larger radii when dust is less important. Our models roughly reproduce the WISE nebula surrounding the Galactic WD merger candidate IRAS 00500+6713; we predict detectable [Ne VI] and [Mg VII] lines with JWST but that the mid-IR WISE emission is dominated by dust not fine-structure lines.
... While the optical continuum light of a nova fades on a time-scale of days to months, the warm ejected envelope remains the source of optical line and radio continuum emission for months and years after the eruption (Strope et al. 2010;Chomiuk et al. 2021b). About 30 such events occur in the Galaxy each year, with only 10 events per year typically observed while others remain hidden by dust extinction (Shafter 2017;De et al. 2021;Kawash et al. 2022;Rector et al. 2022). ...
Classical novae are shock-powered multi-wavelength 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 gamma-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)x10^-6 photons cm^-2 s^-1. Eleven days later, simultaneous NuSTAR and Swift X-ray observations revealed optically thin thermal plasma shock-heated to kT_shock = 4 keV. The lack of a detectable 6.7 keV Fe K_alpha 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.
... The current nova rate in the Galaxy is also largely uncertain. Recent studies report rates of 43:7 þ19:5 À8:7 yr À1 , 26 AE 5 yr À1 (Kawash et al. 2022), and 28 þ5 À4 yr À1 (Rector et al. 2022). The past Galactic nova rate is basically unknown, though some population synthesis studies suggest the existence of an anticorrelation between metallicity and the number of novae produced: for instance, according to Kemp et al. (2022), the number of novae at Z ¼ 0:03 is roughly half that at Z ¼ 10 À4 . ...
After hydrogen and helium, oxygen, carbon, and nitrogen—hereinafter, the CNO elements—are the most abundant species in the universe. They are observed in all kinds of astrophysical environments, from the smallest to the largest scales, and are at the basis of all known forms of life, hence, the constituents of any biomarker. As such, their study proves crucial in several areas of contemporary astrophysics, extending to astrobiology. In this review, I will summarize current knowledge about CNO element evolution in galaxies, starting from our home, the Milky Way. After a brief recap of CNO synthesis in stars, I will present the comparison between chemical evolution model predictions and observations of CNO isotopic abundances and abundance ratios in stars and in the gaseous matter. Such a comparison permits to constrain the modes and time scales of the assembly of galaxies and their stellar populations, as well as stellar evolution and nucleosynthesis theories. I will stress that chemical evolution models must be carefully calibrated against the wealth of abundance data available for the Milky Way before they can be applied to the interpretation of observational datasets for other systems. In this vein, I will also discuss the usefulness of some key CNO isotopic ratios as probes of the prevailing, galaxy-wide stellar initial mass function in galaxies where more direct estimates from the starlight are unfeasible.
TÜBİTAK-TUG RTT150'de bulutsu evredeki novaları gözlemek için bir kampanya başlattık. Şimdiye kadar 11 nova gözlendi. Bunların içerisinde bulunan N Sct 2019, 29-10-2019 tarihinde keşfedildi, bundan 245 gün sonra 30-06-2020 tarihinde gözlemlerini yaptık. Klasik novaların bolluk analizi, içteki beyaz cüce, patlamanın mekanizması ve patlamadan önceki sistemin fiziksel koşulları hakkında önemli bilgiler verebilir. Bu, bir fotoiyonizasyon kodu kullanarak novanın kabuğunu bulutsu aşamasında modelleyerek ve çıktılarını gözlemlerle karşılaştırarak yapılabilir. Bulut fotoiyonizasyon kodunu kullanarak patlamanın bulutsu aşamasında nova kabuklarının bolluk analizini yapıyoruz. Bulut kodu, kabuğunun termal ve istatistiksel denge denklemlerini çözerek elektron sıcaklığını ve sayı yoğunluğunu, emisyon tayflarını ve iyonlaşmayı hesaplar. Model tarafından tahmin edilen parametreleri gözlem sonuçlarıyla karşılaştırıyoruz. Burada N Sct 2019 için şu ön sonuçları sunuyoruz: Model atmosfer parametreleri olarak etkin sıcaklık 100000 K ve log g= 5.5. Elementlerin yüzde olarak kütle kesirleri H (74.08), He (25.04), C (0.2376), N (0.0696) ve O (0.576). Nova'yı modellemek için Cloudy kodu tarafından kullanılan parametrelerden beyaz cücenin toplam ışıma gücü 36.500 ergs−1, zarfın iç bölgesinin büyüklüğü (rmin 15.43 log cm ve rmax 16.43 log cm), hidrojen yoğunluğu (6.2 log cm−3 ), doldurma faktörü (0.6), kaplama faktörü (0.9) ve uzaklık (2.2 kpc).
The population of optical transients evolving within a time-scale of a few hours or a day (so-called fast optical transients, FOTs) has recently been debated extensively. In particular, our understanding of extragalactic FOTs and their rates is limited. We present a search for extragalactic FOTs with the Tomo-e Gozen high-cadence survey. Using the data taken from 2019 August to 2022 June, we obtain 113 FOT candidates. Through light curve analysis and cross-matching with other survey data, we find that most of these candidates are in fact supernovae, variable quasars, and Galactic dwarf novae that were partially observed around their peak brightness. We find no promising candidate of extragalactic FOTs. From this non-detection, we obtain upper limits on the event rate of extragalactic FOTs as a function of their time-scale. For a very luminous event (absolute magnitude M < −26 mag), we obtain the upper limits of 4.4 × 10−9 Mpc−3 yr−1 for a time-scale of 4 h, and 7.4 × 10−10 Mpc−3 yr−1 for a time-scale of 1 d. Thanks to our wide (although shallow) surveying strategy, our data are less affected by the cosmological effects, and thus, give one of the more stringent limits to the event rate of intrinsically luminous transients with a time-scale of <1 d.
Many double white dwarf (WD) mergers likely do not lead to a prompt thermonuclear explosion. We investigate the prospects for observationally detecting the surviving remnants of such mergers, focusing on the case of mergers of double Carbon–Oxygen WDs. For ∼104 yr, the merger remnant is observationally similar to an extreme AGB star evolving to become a massive WD. Identifying merger remnants is thus easiest in galaxies with high-stellar masses (high WD merger rate) and low star formation rates (low birth rate of ∼6–10 M⊙ stars). Photometrically identifying merger remnants is challenging even in these cases because the merger remnants appear similar to He stars and post-outburst classical novae. We propose that the most promising technique for discovering WD merger remnants is through their unusual surrounding photoionized nebulae. We use CLOUDY photoionization calculations to investigate their unique spectral features. Merger remnants should produce weak hydrogen lines, strong carbon and oxygen recombination, and fine-structure lines in the UV, optical and IR. With narrow-band imaging or integral field spectrographs, we predict that multiple candidates are detectable in the bulge of M31, the outskirts of M87 and other nearby massive galaxies, and the Milky Way. Our models roughly reproduce the WISE nebula surrounding the Galactic WD merger candidate IRAS 00500+6713; we predict detectable [Ne vi] and [Mg vii] lines with JWST but that the mid-IR WISE emission is dominated by dust not fine-structure lines.
The Galactic nova rate is intimately linked to our understanding of its chemical enrichment and progenitor channels of Type Ia supernovae. Yet past estimates have varied by more than an order of magnitude, (≈10 − 300 yr−1) owing to limitations in both discovery methods as well as assumptions regarding the Galactic dust distribution and extragalactic stellar populations. Recent estimates utilizing synoptic near-infrared surveys have begun to provide a glimpse of a consensus (≈25 − 50 yr−1); however, a consistent estimate remains lacking. Here, we present the first all-sky search for Galactic novae using 8 years of data from the NEOWISE mid-infrared (MIR) survey. Operating at 3.4 and 4.6 μm where interstellar extinction is negligible, the 6-month cadence NEOWISE dataset offers unique sensitivity to discover slowly evolving novae across the entire Galaxy. Using a novel image subtraction pipeline together with systematic selection criteria, we identify a sample of 49 rapidly evolving MIR outbursts as candidate Galactic novae. While 27 of these sources are known novae, the remaining are previously missed nova candidates discovered in this work. The unknown events are spatially clustered along the densest and most heavily obscured regions of the Galaxy where previous novae are severely underrepresented. We use simulations of the NEOWISE survey strategy, the pipeline detection efficiency, and our criteria to derive a Galactic nova rate of $47.9^{+3.1}_{-8.3}$ yr−1. The discovery of these exceptionally bright (yet overlooked) nova candidates confirm emerging suggestions that optical surveys have been highly incomplete in searches for Galactic novae, highlighting the potential for MIR searches in revealing the demographics of Galactic stellar outbursts.
The All-Sky Automated Survey for Supernovae (ASAS-SN) began observing in late-2011 and has been imaging the entire sky with nightly cadence since late 2017. A core goal of ASAS-SN is to release as much useful data as possible to the community. Working towards this goal, in 2017 the first ASAS-SN Sky Patrol was established as a tool for the community to obtain light curves from our data with no preselection of targets. Then, in 2020 we released static V-band photometry from 2013--2018 for 61 million sources. Here we describe the next generation ASAS-SN Sky Patrol, Version 2.0, which represents a major progression of this effort. Sky Patrol 2.0 provides continuously updated light curves for 111 million targets derived from numerous external catalogs of stars, galaxies, and solar system objects. We are generally able to serve photometry data within an hour of observation. Moreover, with a novel database architecture, the catalogs and light curves can be queried at unparalleled speed, returning thousands of light curves within seconds. Light curves can be accessed through a web interface (http://asas-sn.ifa.hawaii.edu/skypatrol/) or a Python client (https://asas-sn.ifa.hawaii.edu/documentation). The Python client can be used to retrieve up to 1 million light curves, generally limited only by bandwidth. This paper gives an updated overview of our survey, introduces the new Sky Patrol, and describes its system architecture. These results provide significant new capabilities to the community for pursuing multi-messenger and time-domain astronomy.
