Search

We present an extensive optical photometric and spectroscopic investigation into the calcium-rich supernova (SN) – SN 2023xwi. Observations from a variety of ground-based telescopes follow the SN from 8 d pre-peak brightness to 87 d post-peak, covering both early-time (photospheric) and late-time (nebular) phases of the SN. Objects of this class are characterized by nebular spectra that are dominated by [Ca  ii ] |$\lambda \lambda$| 7291, 7324 emission. SN 2023xwi displays a unique peculiarity in that its forbidden [Ca  ii ] feature is visible in its peak photospheric spectrum – far earlier than expected in current models. This is one of the strongest and earliest detections of this feature in Ca-rich SNe in photospheric-phase spectra. We investigate the velocity evolution of this spectral feature and show that it cannot be easily explained by conventional progenitor systems. From our observations, we propose an SN progenitor embedded in an environment polluted by a recurrent He-nova AM CVn system. [ABSTRACT FROM AUTHOR]

SDSS-V is carrying out a dedicated survey for white dwarfs, single and in binaries, and we report the analysis of the spectroscopy of 504 cataclysmic variables (CVs) and CV candidates obtained during the first 34 months of observations of SDSS-V. We developed a convolutional neural network (CNN) to aid with the identification of CV candidates among the over 2 million SDSS-V spectra obtained with the BOSS spectrograph. The CNN reduced the number of spectra that required visual inspection to |$\simeq 2$|  per cent of the total. We identified 776 CV spectra among the CNN-selected candidates, plus an additional 27 CV spectra that the CNN misclassified, but that were found serendipitously by human inspection of the data. Analysing the SDSS-V spectroscopy and ancillary data of the 504 CVs in our sample, we report 61 new CVs, spectroscopically confirm 248 and refute 13 published CV candidates, and we report 82 new or improved orbital periods. We discuss the completeness and possible selection biases of the machine learning methodology, as well as the effectiveness of targeting CV candidates within SDSS-V. Finally, we re-assess the space density of CVs, and find |$1.2\times 10^{-5}\, \mathrm{pc^{-3}}$|⁠. [ABSTRACT FROM AUTHOR]

V745 Sco is a Galactic symbiotic recurrent nova with nova eruptions in 1937, 1989, and 2014. We study the behaviour of V745 Sco at radio wavelengths (0.6–37 GHz), covering both its 1989 and 2014 eruptions and informed by optical, X-ray, and |$\gamma$| -ray data. The radio light curves are synchrotron-dominated. Surprisingly, compared to expectations for synchrotron emission from explosive transients such as radio supernovae, the light curves spanning 0.6–37 GHz all peak around the same time (⁠|$\sim$| 18–26 d after eruption) and with similar flux densities (5–9 mJy). We model the synchrotron light curves as interaction of the nova ejecta with the red giant wind, but find that simple spherically symmetric models with wind-like circumstellar material (CSM) cannot explain the radio light curve. Instead, we conclude that the shock suddenly breaks out of a dense CSM absorbing screen around 20 d after eruption, and then expands into a relatively low-density wind (⁠|$\dot{M}_{out} \approx 10^{-9}\!-\!10^{-8}$|  M |$_{\odot }$|  yr |$^{-1}$| for |$v_w = 10$|  km s |$^{-1}$|⁠) out to |$\sim$| 1 yr post-eruption. The dense, close-in CSM may be an equatorial density enhancement or a more spherical red giant wind with |$\dot{M}_{in} \approx [5\!-\!10] \times 10^{-7}$| M |$_{\odot }$| yr |$^{-1}$|⁠ , truncated beyond several |$\times 10^{14}$|  cm. The outer lower-density CSM would not be visible in typical radio observations of Type Ia supernovae: V745 Sco cannot be ruled out as a Type Ia progenitor based on CSM constraints alone. Complementary constraints from the free–free radio optical depth and the synchrotron luminosity imply the shock is efficient at accelerating relativistic electrons and amplifying magnetic fields, with |$\epsilon _e$| and |$\epsilon _B \approx 0.01\!-\!0.1$|⁠. [ABSTRACT FROM AUTHOR]

We present a dynamical study of the intermediate polar cataclysmic variable YY Dra based on time-series observations in the K band, where the donor star is known to be the major flux contributor. We covered the 3.97-h orbital cycle with 44 spectra taken between 2020 and 2022 and two epochs of photometry observed in 2021 March and May. One of the light curves was simultaneously obtained with spectroscopy to better account for the effects of irradiation of the donor star and the presence of accretion light. From the spectroscopy, we derived the radial velocity curve of the donor star metallic absorption lines, constrained its spectral type to M0.5–M3.5 with no measurable changes in the effective temperature between the irradiated and non-irradiated hemispheres of the star, and measured its projected rotational velocity vrot sin i = 103 ± 2 km s−1. Through simultaneous modelling of the radial velocity and light curves, we derived values for the radial velocity semi-amplitude of the donor star, K2 = 188−2+1 km s−1, the donor to white dwarf mass ratio, q = M2/M1 = 0.62 ± 0.02, and the orbital inclination, i = 42°−1°+2°. These binary parameters yield dynamical masses of M1 = 0.99−0.09+0.10 M⊙ and M2 = 0.62−0.06+0.07 M⊙ (68 per cent confidence level). As found for the intermediate polars GK Per and XY Ari, the white dwarf dynamical mass in YY Dra significantly differs from several estimates obtained by modelling the X-ray spectral continuum. [ABSTRACT FROM AUTHOR]

Context. Supersoft X-ray sources (SSSs) have been proposed as one of the progenitors for Type Ia supernovae. However, the exact origin of the quasi-periodic variability in the optical light curve remains a mystery. Aims. In this work, our goal is to investigate the effect of the feedback of an evolved main-sequence companion star on X-ray irradiation and find whether periodic X-ray irradiation of the companion star could reproduce periodic mass transfer. Methods. Using the Modules for Experiments in Stellar Astrophysics (MESA) code, we modeled the evolutionary track of the companion star under the influence of supersoft X-ray irradiation, and we calculated the resulting mass transfer rate. Results. We find that the supersoft X-ray heating of the companion star can result in the expansion of the companion, causing it to greatly overflow its Roche lobe and thereby increasing the mass transfer rate. The periodic X-ray irradiation on the companion stars leads to periodic changes in the mass transfer rate. For a given companion star, higher irradiation efficiencies result in a higher mass transfer rate. Additionally, the mass transfer rate increases as the mass of the companion star decreases for a given irradiation efficiency. Conclusions. The companion star undergoing thermal timescale mass transfer is periodically irradiated by the X-rays from the WD, which can lead to periodic enhancement of the mass transfer rate. The mechanism could be the origin of the quasi-periodic optical light curve in supersoft X-ray sources. [ABSTRACT FROM AUTHOR]

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  T ph (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 t peak and the expansion timescale τexp. With this relation, we are able to predict the time to optical maximum t peak from the ignition model (at t = 0) without following a time-consuming nova wind evolution. [ABSTRACT FROM AUTHOR]

Dynamical interactions in globular clusters (GCs) significantly impact the formation and evolution of binary sources, including cataclysmic variables (CVs). This study investigates the connection between dynamical states of GCs and X-ray luminosity (Lx) distributions of CV populations through both simulations and actual observations. Utilizing a Monte Carlo simulation tool, mocca , we categorize the simulated GCs into three different evolutionary stages which are referred to as Classes I/II/III. Significant differences are found in the Lx distributions of the CVs among these three Classes. In observational aspects, we have analysed 179 CV candidates in 18 GCs observed by the Chandra X-ray Observatory. By dividing these GCs into three Families of different dynamical ages, namely Families I/II/III, the Lx distributions of the CV candidates also show significant differences among these three Families. Both simulations and observational results suggest that CVs in more dynamically evolved clusters (Class/Family III) exhibit brighter X-ray emission. This highlights the influence of the dynamical status of a GC on the properties of its hosted compact binaries. Similar to blue stragglers, CV populations can serve as tracers of a GC's dynamical history. Our findings provide insights for understanding the interplay between intracluster dynamics and the evolution of compact binaries in GCs. [ABSTRACT FROM AUTHOR]

We report an observed accretion rate of Ṁ1 = (3.86 ± 0.60) × 10−11M⊙ yr−1 for the white dwarf in the short-period, intermediate polar EX Hya. This result is based upon the accretion-induced 4π-averaged energy flux from 2.45 μm 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, − Ṁ2 = (3.90 ± 0.35) × 10−11M⊙ yr−1; 24% of it is caused by the spin-up. The agreement indicates that mass transfer is conservative. The measured Ṁ1 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 ∼107 yr by interpreting the non-equilibrium radius of the secondary star in EX Hya based on published evolutionary calculations. This suggests a time-averaged mass transfer rate enhanced over GR by a factor fGR ≳ 2. 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 EX Hya, 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. [ABSTRACT FROM AUTHOR]

We report on spin variations in the intermediate polar and cataclysmic variable CC Scl, as seen by the Transiting Exoplanet Survey Satellite (TESS). By studying both the spin period and its harmonic, we find that the spin has varied since it was first observed in 2011. We find the latest spin value for the source to be 389.473(6) s, equivalent to 0.00450779(7) d, 0.02 s shorter than the first value measured. A linear fit to these and intermediate data give a rate of change of spin (Ṗ) ∼ −4.26(2.66)× 10−11 and a characteristic time-scale τ ∼ 2.90 × 105 yr, in line with other known intermediate polars with varying spin. The spin profile of this source also matches theoretical spin profiles of high-inclination intermediate polars, and furthermore, appears to have changed in shape over a period of three years. Such 'spin-up' in an intermediate polar is considered to be from mass accretion on to the white dwarf (the primary), and we note the presence of dwarf nova eruptions in this source as being a possible catalyst of the variations. [ABSTRACT FROM AUTHOR]

IGR J18434−0508 is a Galactic Intermediate Polar (IP) type Cataclysmic Variable (CV) previously classified through optical spectroscopy. The source is already known to have a hard Chandra spectrum. In this paper, we have used follow-up XMM–Newton and NuSTAR observations to measure the white dwarf (WD) mass and spin period. We measure a spin period of P  = 304.4 ± 0.3 s based on the combined MOS1, MOS2, and pn light curve. Although this is twice the optical period found previously, we interpret this value to be the true spin period of the WD. The source has an |$8\pm 2~{{\ \rm per\ cent}}$| pulsed fraction in the 0.5–10 keV XMM–Newton data and shows strong dips in the soft energy band (0.5–2 keV). The XMM–Newton and NuSTAR joint spectrum is consistent with a thermal bremsstrahlung continuum model with an additional partial covering factor, reflection, and Fe-line Gaussian components. Furthermore, we fit the joint spectrum with the post-shock region 'ipolar' model which indicates a high WD mass ≳1.36 M⊙, approaching the Chandrasekhar limit. [ABSTRACT FROM AUTHOR]

Cataclysmic variables (CVs) that have evolved past the period minimum during their lifetimes are predicted to be systems with a brown dwarf donor. While population synthesis models predict that around 40–70 per cent of the Galactic CVs are post-period minimum systems referred to as 'period bouncers', only a few dozen confirmed systems are known. We report the study and characterization of a new eclipsing CV, SRGeJ041130.3+685350 (SRGeJ0411), discovered from a joint SRG/eROSITA and ZTF programme. The optical spectrum of SRGeJ0411 shows prominent hydrogen and helium emission lines, typical for CVs. We obtained optical high-speed photometry to confirm the eclipse of SRGeJ0411 and determine the orbital period to be P orb ≈ 97.530 min. The spectral energy distribution suggests that the donor has an effective temperature of ≲ 1800 K. We constrain the donor mass with the period–density relationship for Roche lobe-filling stars and find that M donor ≲ 0.04  M ⊙. The binary parameters are consistent with evolutionary models for post-period minimum CVs, suggesting that SRGeJ0411 is a new period bouncer. The optical emission lines of SRGeJ0411 are single-peaked despite the system being eclipsing, which is typically only seen due to stream-fed accretion in polars. X-ray spectroscopy hints that the white dwarf in SRGeJ0411 could be magnetic, but verifying the magnetic nature of SRGeJ0411 requires further investigation. The lack of optical outbursts has made SRGeJ0411 elusive in previous surveys, and joint X-ray and optical surveys highlight the potential for discovering similar systems in the near future. [ABSTRACT FROM AUTHOR]

AM CVn-type systems are ultracompact, helium-accreting binary systems that are evolutionarily linked to the progenitors of thermonuclear supernovae and are expected to be strong Galactic sources of gravitational waves detectable to upcoming space-based interferometers. AM CVn binaries with orbital periods ≲20–23 min exist in a constant high state with a permanently ionized accretion disc. We present the discovery of TIC 378898110, a bright (G  = 14.3 mag), nearby (309.3 ± 1.8 pc), high-state AM CVn binary discovered in TESS two-minute-cadence photometry. At optical wavelengths, this is the third-brightest AM CVn binary known. The photometry of the system shows a 23.07172(6) min periodicity, which is likely to be the 'superhump' period and implies an orbital period in the range 22–23 min. There is no detectable spectroscopic variability. The system underwent an unusual, year-long brightening event during which the dominant photometric period changed to a shorter period (constrained to 20.5 ± 2.0 min), which we suggest may be evidence for the onset of disc-edge eclipses. The estimated mass transfer rate, |$\log (\dot{M} / \mathrm{M_\odot } \, \mathrm{yr}^{-1}) = -6.8 \pm 1.0$|⁠ , is unusually high and may suggest a high-mass or thermally inflated donor. The binary is detected as an X-ray source, with a flux of |$9.2 ^{+4.2}_{-1.8} \times 10^{-13}$|  erg cm−2 s−1 in the 0.3–10 keV range. TIC 378898110 is the shortest-period binary system discovered with TESS , and its large predicted gravitational-wave amplitude makes it a compelling verification binary for future space-based gravitational wave detectors. [ABSTRACT FROM AUTHOR]

We present a catalogue of 507 cataclysmic variables (CVs) observed in SDSS I to IV including 70 new classifications collated from multiple archival data sets. This represents the largest sample of CVs with high-quality and homogeneous optical spectroscopy. We have used this sample to derive unbiased space densities and period distributions for the major sub-types of CVs. We also report on some peculiar CVs, period bouncers and also CVs exhibiting large changes in accretion rates. We report 70 new CVs, 59 new periods, 178 unpublished spectra, and 262 new or updated classifications. From the SDSS spectroscopy, we also identified 18 systems incorrectly identified as CVs in the literature. We discuss the observed properties of 13 peculiar CVS, and we identify a small set of eight CVs that defy the standard classification scheme. We use this sample to investigate the distribution of different CV sub-types, and we estimate their individual space densities, as well as that of the entire CV population. The SDSS I to IV sample includes 14 period bounce CVs or candidates. We discuss the variability of CVs across the Hertzsprung–Russell diagram, highlighting selection biases of variability-based CV detection. Finally, we searched for, and found eight tertiary companions to the SDSS CVs. We anticipate that this catalogue and the extensive material included in the Supplementary Data will be useful for a range of observational population studies of CVs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present follow-up spectroscopy of 21 cataclysmic variables (CVs) with evolved secondaries and ongoing or recently terminated mass transfer. Evolutionary models predict that the secondaries should have anomalous surface abundances owing to nuclear burning in their cores during their main-sequence evolution and subsequent envelope stripping by their companion white dwarfs. To test these models, we measure sodium (Na) abundances of the donors from the Fraunhofer 'D' doublet. Accounting for interstellar absorption, we find that all objects in our sample have enhanced Na abundances. We measure 0.3 dex ≲ [Na/H] ≲ 1.5 dex across the sample, with a median [Na/H]  = 0.956 dex, i.e. about an order of magnitude enhancement over solar values. To interpret these values, we run Modules for Experiments in Stellar Astrophysics binary evolution models of CVs in which mass transfer begins just as the donor leaves the main sequence. These generically predict Na enhancement in donors with initial donor masses |$\gtrsim 1\, {\rm M}_{\odot }$|⁠ , consistent with our observations. In the models, Na enrichment occurs in the donors' cores via the NeNa cycle near the end of their main-sequence evolution. Na-enhanced material is exposed when the binaries reach orbital periods of a few hours. Donors with higher initial masses are predicted to have higher Na abundances at fixed orbital period owing to their higher core temperatures during main-sequence evolution. The observed [Na/H] values are on average ≈0.3 dex higher than predicted by the models. Surface abundances of evolved CV donors provide a unique opportunity to study nuclear burning products in the cores of intermediate-mass stars. [ABSTRACT FROM AUTHOR]

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 yr of data from the Near Earth Object WISE (NEOWISE) mid-infrared (MIR) survey. Operating at 3.4 and 4.6 µm where interstellar extinction is negligible, the 6-month cadence NEOWISE data set 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 confirms 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. [ABSTRACT FROM AUTHOR]

CSS1603+19 is a cataclysmic variable (CV) with an orbital period of 81.96 min, near the minimal period of CVs. It is unusual in having a strong mid-infrared excess inconsistent with thermal emission from a brown dwarf companion. Here, we present time-resolved multiwavelength observations of this system. WISE photometry indicates that the mid-infrared excess displays a one-magnitude eclipsing-like variability during the orbit. We obtained near-infrared and optical spectroscopy using Gemini, MDM, and APO telescopes. Near-infrared spectra show possible cyclotron features indicating that the white dwarf has a magnetic field of about 5 MG. Optical and near-infrared spectra display double-peaked emission lines, with both components showing strong radial velocity variations during the orbital period and with the broad component leading the narrow component stably by about 0.2 of the orbital phase. We construct a physical model informed by existing observations of the system and determine that one component likely originates from the accretion column on to the magnetized white dwarf in synchronous rotation with the orbital motion and the other from the Roche overflow point. This allows us to constrain the masses of the binary components to be M 1 > 0.24 M⊙ for the white dwarf accretor and M 2 = 0.0644 ± 0.0074 M⊙ for the donor. We classify the system as an AM Herculis star, or a polar. It has likely completed its stint on the period gap, but has not yet gone through the period bounce. [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]

We model cataclysmic variables (CVs) with solar metallicity donors (X  = 0.7 and  Z  = 0.02) that evolve to form AM CVn stars through the evolved CV formation channel using various angular momentum loss mechanisms by magnetic braking (AMLMB). We find that the time-scale for AMLMB in our double-dynamo (DD) model is shorter than that of previously used empirical formulae. Owing to the shorter time-scales, a larger parameter space of initial conditions evolves to form AM CVn stars with the DD model than with other models. We perform an analysis of the expected number of AM CVn stars formed through the Evolved CV channel and find about 3 times as many AM CVn stars as reported before. We evolve these systems in detail with the Cambridge stellar evolution code (stars) and show that evolved CVs populate a region with orbital period |$P_\mathrm{orb}\ge 5.5\, \mathrm{hr}$|⁠. We evolve our donors beyond their orbital period minimum and find that a significant number become extremely H-exhausted systems. This makes them indistinguishable from systems evolved from the He-star and the White Dwarf (WD) channels in terms of the absence of H in their spectra. We also compare the masses, mass-transfer rates of the donor, and the orbital period with observations. We find that the state of the donor and the absence of H in systems such as YZ LMi and V396 Hya match with our modelled trajectories, while systems such as CR Boo and HP Lib match with our modelled tracks if their actual donor mass lies on the lower-end of the observed mass range. [ABSTRACT FROM AUTHOR]

Aims. We performed a systematic analysis of X-ray point sources within 1.°5 of the Galactic centre using archival XMM-Newton data. While doing so, we discovered Fe Kα complex emission and pulsation in two highly variable sources, 4XMM J174917.7–283329 and 4XMM J174954.6–294336. In this work we report the findings of the X-ray spectral and timing studies. Methods. We performed detailed spectral modelling of the sources and searched for pulsation in the light curves using Fourier timing analysis. We also searched for multi-wavelength counterparts for the characterization of the sources. Results. The X-ray spectrum of 4XMM J174917.7–283329 shows the presence of complex Fe K emission in the 6–7 keV band. The equivalent widths of the 6.4 and 6.7 keV lines are 99−72+84 and 220−140+160 eV, respectively. The continuum is fitted by a partially absorbed apec model with a plasma temperature of kT = 13−2+10 keV. The inferred mass of the white dwarf (WD) is 0.9−0.2+0.3 M⊙. We detected pulsations with a period of 1212 ± 3 s and a pulsed fraction of 26 ± 6%. The light curves of 4XMM J174954.6–294336 display an asymmetric eclipse and dipping behaviour. To date, this is only the second known intermediate polar to show a total eclipse in X-rays. The spectrum of the sources is characterized by a power-law model with photon index Γ = 0.4 ± 0.2. The equivalent widths of the iron fluorescent (6.4 keV) and Fe XXV (6.7 keV) lines are 171−79+99 and 136−81+89 eV, respectively. The continuum is described by emission from optically thin plasma with a temperature of kT ∼ 35 keV. The inferred mass of the WD is 1.1−0.3+0.2 M⊙. We detect coherent pulsations from the source with a period of 1002 ± 2 s. The pulsed fraction is 66 ± 15%. Conclusions. The spectral modelling indicates the presence of intervening clouds with a high absorbing column density in front of both sources. The detected periodic modulations in the light curves are likely associated with the spin period of WDs in magnetic cataclysmic variables. The measured spin period, hard photon index, and equivalent width of the fluorescent Fe Kα line are consistent with the values found in intermediate polars. 4XMM J174954.6–294336 has already been classified as an intermediate polar, and we suggest that 4XMM J174917.7–283329 is a new intermediate polar. The X-ray eclipses in 4XMM J174954.6–294336 are most likely caused by a low-mass companion star obscuring the central X-ray source. The asymmetry in the eclipse is likely caused by a thick bulge that intercepts the line of sight during the ingress phase but not during the egress phase located behind the WD along the line of sight. [ABSTRACT FROM AUTHOR]

We model helium-rich stars with solar metallicity (X  = 0.7, Z  = 0.02) progenitors that evolve to form AM Canum Venaticorum systems through a helium-star formation channel, with the aim to explain the observed properties of Gaia14aae and ZTFJ1637+49. We show that semidegenerate, H-exhausted (X ≤ 10−5), He-rich (Y ≈ 0.98) donors can be formed after a common envelope evolution (CEE) phase if either additional sources of energy are used to eject the common envelope, or a different formalism of CEE is implemented. We follow the evolution of such binary systems after the CEE phase using the Cambridge stellar evolution code when they consist of a He-star and a white dwarf accretor, and report that the mass, radius, and mass-transfer rate of the donor, the orbital period of the system, and the lack of hydrogen in the spectrum of Gaia14aae and ZTFJ1637+49 match well with our modelled trajectories wherein, after the CEE phase Roche lobe overflow is governed not only by the angular momentum loss (AML) owing to gravitational wave radiation (AMLGR) but also an additional AML owing to α–Ω dynamos in the donor. This additional AML is modelled with our double-dynamo (DD) model of magnetic braking in the donor star. We explain that this additional AML is just a consequence of extending the DD model from canonical cataclysmic variable donors to evolved donors. We show that none of our modelled trajectories match with Gaia14aae or ZTFJ1637+49 if the systems are modelled only with AMLGR. [ABSTRACT FROM AUTHOR]

This article is a presentation of a project which is a statistical study of white dwarfs’ properties carried out based on the sample of the First Byurakan Survey Blue Stellar Objects (FBS BSOs) and the Catalogue of White Dwarfs. Using these results, we have searched and will continue to search for new white dwarfs by means of Gaia accurate astrometry. The discovery and study of new White Dwarfs (WDs) is extremely important for understanding the kinematics and dynamics of the local stellar population. They are evolutionary signatures of the Milky Way, as most of stars pass through this evolutionary stage. We have carried out a statistical analysis of the White Dwarfs Catalogue to establish their average physical properties. Based on these data, we have searched for new WDs using Gaia EDR3 accurate astrometry. It provides huge amount of data to confirm WDs and reveal many more such objects. Gaia’s astrometric accuracy allows more detailed studies of WDs and many other stars. Using Gaia accurate data in combination with SDSS and DFBS low-dispersion spectra and additional multiwavelength data, we have revealed many new WDs. Many of them are subject for studies on binarity and variability (including a number of subtypes, e.g. ZZ Ceti stars (pulsating WDs), cataclysmic variables, etc.). WDs among FBS blue stellar objects at high galactic latitudes have been searched and revealed. Search for high galactic latitude WDs in DFBS have also been carried out. Using the revealed and calculated characteristics, a statistical study of WD properties is being done.

Intermediate polars are a class of cataclysmic variables in which a white dwarf accretes material from a companion star. The intermediate polar nature confirmation usually derives from the detection of two periods in both X -ray and optical photometry. In this respect, the high-energy signal is often characterized by modulations on the white dwarf spin and the orbital period. However, noting that the periodograms may be characterized by strong features also at the synodic period and/or other sidebands, the timing analysis of the X -ray signal may offer the unique possibility to firmly discover an intermediate polar candidate. Here, we concentrate on a sample of five cataclysmic variable binary candidates: i.e. SAXJ1748.2-2808, 1RXS J211336.1 + 542226, CXOGC J174622.7-285218, CXOGC J174517.4-290650, and V381 Vel, listed in the IPHome catalogue. Our main aim is to confirm if they belong to the intermediate polar class or not. The results of our analysis show that we can safely assess the intermediate polar nature of all the considered sources, apart for the case of V381 Vel which instead behaves like a cataclysmic variable of the polar subclass. Moreover, the source SAXJ1748.2-2808, previously classified as an HMXB, appears to be, most likely, an intermediate polar variable. [ABSTRACT FROM AUTHOR]

Fluctuations during a prolonged maximum have been observed in several nova eruptions, although it is not clear, and cannot be deduced directly from observations, whether the phenomenon is an actual physical reaction to some mechanism originating in the erupting white dwarf, whether it is occurring in the expanding ejected shell, or whether it is a form of interaction with the red dwarf companion. A handful of erupting nova models are investigated in this work, in order to assess the possibility of this sort of feature being an actual part of the eruption itself. The results explain that the mechanism that may produce these fluctuations is the repeated approach and recession of the convective front from the surface. The efficiency of this mechanism, being dependent on the mass of the WD (white dwarf) envelope and the time-scale of the nova cycle, favours low-mass WDs and long accretion phases. [ABSTRACT FROM AUTHOR]

We give an updated version of the analytical equation of state used in the Cambridge stellar evolution code (stars) as a free to use open-source package that we have used to model cool white dwarfs (WDs) down to temperatures |$\log _{10}(T_\mathrm{eff}/\mathrm{K})\, =\, 3$|⁠. With this update in the stars code, we model the secular evolution of cataclysmic variable (CV) stars using a double dynamo model wherein there is an interplay between two α − Ω dynamos, one in the convective envelope and the other at the boundary of a slowly rotating shrinking radiative core and the growing convective envelope. We confirm that this model provides a physical formalism for the interrupted magnetic braking paradigm. In addition, our model also provides a mechanism for extra angular momentum loss below the period gap. We construct the relative probability distribution of orbital periods P orb using the mass distribution of WDs in CVs and find that our model excellently reproduces the period gap and the observed period minimum spike in CV distribution. We also compare the evolutionary trajectories from our model with those of other empirical models and find agreement between the two. We also report good agreement between our modelled systems and observational data. [ABSTRACT FROM AUTHOR]

AM CVn systems are ultra-compact, hydrogen-depleted, and helium-rich, accreting binaries with degenerate or semidegenerate donors. We report the discovery of five new eclipsing AM CVn systems with orbital periods of 61.5, 55.5, 53.3, 37.4, and 35.4 min. These systems were discovered by searching for deep eclipses in the Zwicky Transient Facility (ZTF) light curves of white dwarfs selected using Gaia parallaxes. We obtained phase-resolved spectroscopy to confirm that all systems are AM CVn binaries, and we obtained high-speed photometry to confirm the eclipse and characterize the systems. The spectra show double-peaked H e lines but also show metals, including K and Zn, elements that have never been detected in AM CVn systems before. By modelling the high-speed photometry, we measured the mass and radius of the donor star, potentially constraining the evolutionary channel that formed these AM CVn systems. We determined that the average mass of the accreting white dwarf is ≈0.8 M⊙, and that the white dwarfs in long-period systems are hotter than predicted by recently updated theoretical models. The donors have a high entropy and are a factor of ≈2 more massive compared to zero-entropy donors at the same orbital period. The large donor radius is most consistent with H e-star progenitors, although the observed spectral features seem to contradict this. The discovery of five new eclipsing AM CVn systems is consistent with the known observed AM CVn space density and estimated ZTF recovery efficiency. [ABSTRACT FROM AUTHOR]

We present a simultaneous broad-band analysis of X-ray data obtained with the XMM – Newton and the Nuclear Spectroscopic Telescope Array (NuSTAR) for the asynchronous polar source, CD Ind. The spin-folded light curve in the soft 0.3–3.0 keV band shows a single broad hump-like structure superimposed with occasional narrow dips, indicating a single-pole accretion model with a complex intrinsic absorber. Lack of strong modulation in the folded light curve above 3 keV reveals that emission from the corresponding zone of the post-shock region (PSR) remains in view throughout the spin phase. The broad-band spectrum is modelled with a three-component absorbed plasma emission model and absorbed isobaric cooling flow model, both of which fit the data well with similar statistical significance. The presence of a partial covering absorber is evident in the spectra with equivalent column density |${\sim}7\times 10^{22}\, \text{cm}^{-2}$| and a covering fraction of ~25 per cent. Strong ionized oxygen Kα line emission is detected in the spectra. We notice spectral variability during the spin phase 0.75–1.05, when there is a considerable increase in the column density of the overall absorber (from |${\sim}1 \times 10^{20}\, \text{cm}^{-2}$| to |${\sim}9 \times 10^{20}\, \text{cm}^{-2}$|⁠). We required at least three plasma temperatures to describe the multitemperature nature of the PSR. The shock temperature |${\sim}43.3_{-3.4}^{+3.8}$| keV, represented by the upper temperature of the cooling flow model, implies a white dwarf mass of |${\sim}0.87^{+0.04}_{-0.03}\, \mathrm{M}_{\odot }$|⁠. The iron Kα line complex show strong He-like and weak neutral fluorescence lines. We could not unambiguously detect the presence of Compton reflection in the spectra, which is probably very small and signifies a tall shock height. [ABSTRACT FROM AUTHOR]

Many new and unidentified Galactic sources have recently been revealed by ongoing hard X-ray surveys. A significant fraction of these have been shown to be the type of accreting white dwarfs known as cataclysmic variables (CVs). Follow-up observations are often required to categorize and classify these sources, and may also identify potentially unique or interesting cases. One such case is IGR J18007−4146, which is likely a CV based on follow-up Chandra observations and constraints from optical/IR catalogues. Utilizing simultaneous XMM–Newton and NuSTAR observations, as well as the available optical/IR data, we confirm the nature of IGR J18007−4146 as an intermediate polar type CV. Timing analysis of the XMM data reveals a periodic signal at 424.4 ± 0.7 s that we interpret as the spin period of the white dwarf. Modelling the 0.3–78 keV spectrum, we use a thermal bremsstrahlung continuum but require intrinsic absorption as well as a soft component and strong Fe lines between 6 and 7 keV. We model the soft component using a single-temperature blackbody with |$kT = 73^{+8}_{-6}$|  eV. From the X-ray spectrum, we are able to measure the mass of the white dwarf to be |$1.06^{+0.19}_{-0.10}$|   |$\mathrm{ M}_{\mathord \odot }$|⁠ , which means IGR J18007−4146 is more massive than the average for magnetic CVs. [ABSTRACT FROM AUTHOR]

RU Pegasi is a dwarf nova (DN) of the U Geminorum type. This analysis of its long-term optical activity uses data from the data base of the American Association of Variable Star Observers (AAVSO). It concentrates on investigating the properties of the individual outbursts and the time evolution of the ensemble of these events. No significant irradiation of the disc by the white dwarf was detected. In the interpretation, a variable steepness of the rising branches of the individual outbursts shows that the start of outbursts of RU Peg can occur at various distances from the disc centre without remarkable changes of the outburst recurrence time T C. The disc overflow of the inflowing mass stream, varying with time, could contribute to the changes in the starting position of the heating front, and hence the variations of the outburst types. A typical length of T C was 90 d. The segments of the relatively stable length of T C were accompanied by the primarily little variable and small values of the fluence (the energy radiated in the optical band in the individual outbursts). Jumps of T C, accompanied by the big scatter of the fluences, sometimes replaced them. In the interpretation, a combination of variations of T C with the unstable properties of the outbursts, including an unstable mass transfer rate between the components, shows the influence of several mechanisms on the state of the disc in various time segments. [ABSTRACT FROM AUTHOR]

The metallicity of a star affects its evolution in a variety of ways, changing stellar radii, luminosities, lifetimes, and remnant properties. In this work, we use the population synthesis code binary_c to study how metallicity affects novae in the context of binary stellar evolution. We compute a 16-point grid of metallicities ranging from Z  = 10−4 to 0.03, presenting distributions of nova white dwarf masses, accretion rates, delay-times, and initial system properties at the two extremes of our 16-point metallicity grid. We find a clear anticorrelation between metallicity and the number of novae produced, with the number of novae at Z  = 0.03 roughly half that at Z  = 10−4. The white dwarf mass distribution has a strong systematic variation with metallicity, while the shape of the accretion rate distribution is relatively insensitive. We compute a current nova rate of approximately 33 novae per year for the Milky Way, a result consistent with observational estimates relying on extra-Galactic novae but an under-prediction relative to observational estimates relying on Galactic novae. However, the shape of our predicted Galactic white dwarf mass distribution differs significantly to existing observationally derived distributions, likely due to our underlying physical assumptions. In M31, we compute a current nova rate of approximately 36 novae per year, under-predicting the most recent observational estimate of |$65^{+15}_{-16}$|⁠. Finally, we conclude that when making predictions about currently observable nova rates in spiral galaxies, or stellar environments where star formation has ceased in the distant past, metallicity can likely be considered of secondary importance compared to uncertainties in binary stellar evolution. [ABSTRACT FROM AUTHOR]

We present a systematic survey for mass-transferring and recently detached cataclysmic variables (CVs) with evolved secondaries, which are progenitors of extremely low mass white dwarfs (ELM WDs), AM CVn systems, and detached ultracompact binaries. We select targets below the main sequence in the Gaia colour–magnitude diagram with ZTF light curves showing large-amplitude ellipsoidal variability and orbital period P orb < 6 h. This yields 51 candidates brighter than G  = 18, of which we have obtained many-epoch spectra for 21. We confirm all 21 to be completely or nearly Roche lobe filling close binaries. Thirteen show evidence of ongoing mass transfer, which has likely just ceased in the other eight. Most of the secondaries are hotter than any previously known CV donors, with temperatures 4700 < T eff/K < 8000. Remarkably, all secondaries with |$T_{\rm eff} \gtrsim 7000\, \rm K$| appear to be detached, while all cooler secondaries are still mass-transferring. This transition likely marks the temperature where magnetic braking becomes inefficient due to loss of the donor's convective envelope. Most of the proto-WD secondaries have masses near |$0.15\, \rm M_{\odot }$|⁠ ; their companions have masses near |$0.8\, \rm M_{\odot }$|⁠. We infer a space density of |$\sim 60\, \rm kpc^{-3}$|⁠ , roughly 80 times lower than that of normal CVs and three times lower than that of ELM WDs. The implied Galactic birth rate, |$\mathcal {R}\sim 60\, \rm Myr^{-1}$|⁠ , is half that of AM CVn binaries. Most systems are well-described by mesa models for CVs in which mass transfer begins only as the donor leaves the main sequence. All are predicted to reach minimum periods 5 ≲ P orb min−1 ≲ 30 within a Hubble time, where they will become AM CVn binaries or merge. This sample triples the known evolved CV population and offers broad opportunities for improving understanding of the compact binary population. [ABSTRACT FROM AUTHOR]

The intermediate Palomar Transient Factory (iPTF) detection of the most recent outburst of the recurrent nova (RN) system RX J0045.4+4154 in the Andromeda galaxy has enabled the unprecedented study of a massive (M > 1.3 M) accreting white dwarf (WD). We detected this nova as part of the near-daily iPTF monitoring of M31 to a depth of R 21 mag and triggered optical photometry, spectroscopy and soft X-ray monitoring of the outburst. Peaking at an absolute magnitude of MR= -6.6 mag, and with a decay time of 1 mag per day, it is a faint and very fast nova. It shows optical emission lines of He/N and expansion velocities of 1900-2600 km s-1 1-4 days after the optical peak. The Swift monitoring of the X-ray evolution revealed a supersoft source (SSS) with kT eff 90-110 eV that appeared within 5 days after the optical peak, and lasted only 12 days. Most remarkably, this is not the first event from this system, rather it is an RN with a time between outbursts of approximately 1 yr, the shortest known. Recurrent X-ray emission from this binary was detected by ROSAT in 1992 and 1993, and the source was well characterized as a M > 1.3 M WD SSS. Based on the observed recurrence time between different outbursts, the duration and effective temperature of the SS phase, MESA models of accreting WDs allow us to constrain the accretion rate to and WD mass >1.30 M. If the WD keeps 30% of the accreted material, it will take less than a Myr to reach core densities high enough for carbon ignition (if made of C/O) or electron capture (if made of O/Ne) to end the binary evolution. © 2014. The American Astronomical Society. All rights reserved..

Nova outbursts play an important role in the chemical evolution of Galaxy, especially they are the main source of synthetic |$^{13}\rm C$|⁠ , |$^{15}\rm N$|⁠ , |$^{17}\rm O$|⁠ , and some radioactive isotopes like |$^{22}\rm Na$| and |$^{26}\rm Al$|⁠. The enrichment of He in nova ejecta indicates that the accreted material may mix with the He-shell (He-mixing). The purpose of this work is to investigate how the He-mixing affects the nova outbursts in a systematic way. We evolved a series of accreting WD models, and found that the mass fraction of H and He in nova ejecta can be influenced by different He-mixing fractions significantly. We also found that both the nova cycle duration and ejected mass increase with the He-mixing fractions. Meanwhile, the nuclear energy generation rate of p – p chains decreases with the He-mixing fraction during the nova outbursts, whereas the CNO-cycle increases. This work can reproduce the chemical abundances in the ejecta of some representative novae, such as GQ Mus, ASASSN-18fv, HR Del, T Aur, and V443 Sct. This implies that the He-mixing process cannot be neglected when studying nova outbursts. This study also develops a He-mixing meter (i.e. |$\rm He/H$|⁠) that can be used to estimate the He-mixing fraction in classical nova systems. [ABSTRACT FROM AUTHOR]

We present LAMOST J0140355 + 392651 (hereafter J0140), a close (P orb = 3.81 h) binary containing a bloated, low-mass (⁠|$M \approx 0.15 \, \mathrm{M}_{\odot }$|⁠) proto-white dwarf (WD) and a massive (⁠|$M \approx 0.95\, \mathrm{M}_{\odot }$|⁠) WD companion. The system's optical light curve is dominated by large-amplitude ellipsoidal variability but also exhibits additional scatter, likely driven by pulsations. The proto-WD is cooler (T eff = 6800 ± 100 K) and more puffy (⁠|$\log \left[g/\left({\rm cm\, s^{-2}}\right)\right]=4.74\pm 0.07$|⁠) than any known extremely low-mass (ELM) WD, but hotter than any known cataclysmic variable (CV) donor. It either completely or very nearly fills its Roche lobe (⁠|$R/R_{{\rm Roche\, lobe}}=0.99\pm 0.01$|⁠), suggesting ongoing or recently terminated mass transfer. No dwarf nova-like outbursts have been observed. The spectrum is dominated by the proto-WD but shows tentative hints of H α emission, perhaps due to accretion on to the massive WD. The properties of the system are well-matched by mesa binary evolution models of CVs with donors that underwent significant nuclear evolution before the onset of mass transfer. In these models, the bloated proto-WD is either still losing mass via stable Roche lobe overflow or was doing so until very recently. In either case, it is evolving towards higher temperatures at near-constant luminosity to become an ELM WD. If the system is detached, mass transfer likely ended when the donor became too hot for magnetic braking to remain efficient. Evolutionary models predict that the binary will shrink to P orb ≲ 10 min within a few Gyr, when it will either merge or become an AM CVn binary. J0140 provides an observational link between the formation channels of CVs, ELM WDs, detached ultracompact WD binaries, and AM CVn systems. [ABSTRACT FROM AUTHOR]

Intermediate polars (IPs) are cataclysmic variables with mildly magnetized white dwarfs (WDs). This analysis of the long-term optical activity of five examples of IPs with accretion discs used data from the Catalina Real-time Transient Survey, Digital Access to a Sky Century @ Harvard (DASCH) and the American Association of Variable Star Observers (AAVSO). It is shown that each of these IPs had their most preferred value of absolute magnitude M opt, even if it significantly varied on the superorbital time-scale. The values of M opt of these IPs were in the zone of thermal-viscous instability (TVI) of the disc most of the time. The properties of a series of outbursts of V426 Oph can be explained by an intermittently operating TVI. The activity of TV Col and DW Cnc is interpreted as caused by a gradually variable mass inflow rate from the secondary to a cool disc. The mass transfer rate from the secondary varied on a well-determined time-scale. It is shown that M opt of EI UMa, close to the peaks of outbursts of non-magnetic dwarf novae, fluctuated on the time-scale of days; it also produced short flares, ascribed to the bursts of matter from the donor. HY Leo, with a presumably cool disc, fluctuated between its high and low states. A temporary brightening from an extended low state is ascribed to a short, intense burst of matter from the donor to the remaining cool disc or torus. [ABSTRACT FROM AUTHOR]

We report on multiwavelength observations during quiescence and of the first detected outburst of the ≈60 min orbital period AM CVn SDSS J113732+405458. Using X-ray and UV observations, we determined an upper limit duration of the event of about 1 yr. The amplitude of the outburst was remarkably small, of around 1 mag in r and 0.5 mag in g. We have also investigated the colour variations of SDSS J113732+405458 and other long-period AM CVns in outbursts and identified a track on the colour–magnitude diagram that is not compatible with the predictions of the disc instability model, suggesting that some outbursts in long-period AM CVns are caused by enhanced mass-transfer. To our knowledge, these are the first studies of the colour evolution in AM CVns. During quiescence we measured an X-ray luminosity for SDSS J113732+405458 of ≈3 × 1029 erg s−1 in the 0.5–10 keV band. This indicates a very low accretion rate, in agreement with the disc instability model for long-period systems. However, such a model predicts stable discs at somewhat long periods. The discovery of this system outburst, along with similarities to the long-period system SDSS J080710+485259 with a comparably long, weak outburst, indicates that these enhanced mass-transfer events may be more common in long-period AM CVns. A larger sample would be needed to determine empirically at what period, if any, the disc instability stops functioning entirely. Finally, we identified an infrared excess in the quiescence spectrum attributable to the donor. This makes SDSS J113732+405458 the second AM CVn to have a directly detected donor. [ABSTRACT FROM AUTHOR]

Novae are some of the most commonly detected optical transients and have the potential to provide valuable information about binary evolution. Binary population synthesis codes have emerged as the most effective tool for modelling populations of binary systems, but such codes have traditionally employed greatly simplified nova physics, precluding detailed study. In this work, we implement a model treating H and He novae as individual events into the binary population synthesis code binary_c. This treatment of novae represents a significant improvement on the 'averaging' treatment currently employed in modern population synthesis codes. We discuss the evolutionary pathways leading to these phenomena and present nova event rates and distributions of several important physical parameters. Most novae are produced on massive white dwarfs, with approximately 70 and 55 per cent of nova events occurring on O/Ne white dwarfs for H and He novae, respectively. Only 15 per cent of H-nova systems undergo a common-envelope phase, but these systems are responsible for the majority of H nova events. All He-accreting He-nova systems are considered post-common-envelope systems, and almost all will merge with their donor star in a gravitational-wave-driven inspiral. We estimate the current annual rate of novae in M31 (Andromeda) to be approximately 41 ± 4 for H novae, underpredicting the current observational estimate of |$65^{+15}_{-16}$|⁠ , and 0.14 ± 0.015 for He novae. When varying common-envelope parameters, the H nova rate varies between 20 and 80 events per year. [ABSTRACT FROM AUTHOR]

We present and analyse optical photometry and high-resolution Southern African Large Telescope spectra of the symbiotic recurrent nova V3890 Sgr at quiescence. The orbital period, P = 747.6 d, has been derived from both photometric and spectroscopic data. Our double-line spectroscopic orbits indicate that the mass ratio is q = M g/ M WD = 0.78 ± 0.05, and that the component masses are M WD ≈ 1.35 ± 0.13 M⊙ and M g ≈ 1.05 ± 0.11 M⊙. The orbit inclination is ≈67−69○. The red giant is filling (or nearly filling) its Roche lobe, and the distance set by its Roche lobe radius, d ≈ 9 kpc, is consistent with that resulting from the giant pulsation period. The outburst magnitude of V3890 Sgr is then very similar to those of RNe in the Large Magellanic Cloud. V3890 Sgr shows remarkable photometric and spectroscopic activity between the nova eruptions with time-scales similar to those observed in the symbiotic recurrent novae T CrB and RS Oph and Z And-type symbiotic systems. The active source has a double-temperature structure that we have associated with the presence of an accretion disc. The activity would be then caused by changes in the accretion rate. We also provide evidence that V3890 Sgr contains a CO WD accreting at a high, ∼a few× 10−8 to 10−7 M⊙ yr−1, rate. The WD is growing in mass, and should give rise to a Type Ia supernova within ≲106 yr – the expected lifetime of the red giant. [ABSTRACT FROM AUTHOR]

It was recently proposed that the cataclysmic variable (CV) LAMOST J024048.51+195226.9 may be a twin to the unique magnetic propeller system AE Aqr. If this is the case, two predictions are that it should display a short period white dwarf spin modulation, and that it should be a bright radio source. We obtained follow-up optical and radio observations of this CV, in order to see if this holds true. Our optical high-speed photometry does not reveal a white dwarf spin signal, but lacks the sensitivity to detect a modulation similar to the 33 s spin signal seen in AE Aqr. We detect the source in the radio, and measure a radio luminosity similar to that of AE Aqr and close to the highest so far reported for a CV. We also find good evidence for radio variability on a time-scale of tens of minutes. Optical polarimetric observations produce no detection of linear or circular polarization. While we are not able to provide compelling evidence, our observations are all consistent with this object being a propeller system. [ABSTRACT FROM AUTHOR]

The hard X-ray spectrum of magnetic cataclysmic variables can be modelled to provide a measurement of white dwarf mass. This method is complementary to radial velocity measurements, which depend on the (typically rather uncertain) binary inclination. Here, we present results from a Legacy Survey of 19 magnetic cataclysmic variables with NuSTAR. We fit accretion column models to their 20–78 keV spectra and derive the white dwarf masses, finding a weighted average |$\bar{M}_{\rm WD}=0.77\pm 0.02$|   M ⊙, with a standard deviation σ = 0.10  M ⊙, when we include the masses derived from previous NuSTAR observations of seven additional magnetic cataclysmic variables. We find that the mass distribution of accreting magnetic white dwarfs is consistent with that of white dwarfs in non-magnetic cataclysmic variables. Both peak at a higher mass than the distributions of isolated white dwarfs and post-common-envelope binaries. We speculate as to why this might be the case, proposing that consequential angular momentum losses may play a role in accreting magnetic white dwarfs and/or that our knowledge of how the white dwarf mass changes over accretion–nova cycles may also be incomplete. [ABSTRACT FROM AUTHOR]

We report on the XMM – Newton observation of HP Cet and Swift J0820.6–2805, two X-ray photon sources that are candidates to be members of the intermediate polar class of cataclysmic variables. If the historical optical light curve of HP Cet shows a periodic feature at ≃96 min, a clear identification of such a signature in the high energy band (apart for a variability on a time-scale of ≃8 min as detected by the ROSAT satellite) is lacking. By using XMM – Newton archive data, we clearly identify a feature (at ≃88 min) which is marginally consistent with one of the binary system orbital periods reported in the literature. We also found a signature of a periodic features on the time-scale of ≃5.6 min. In the case of Swift J0820.6–2805, the intermediate polar nature was previously unclear and the orbital and the white dwarf spin periods were unknown. Here, the 0.3–10 keV data undoubtedly reveal an orbital period and a white dwarf spin of ≃87.5 and ≃27.9 min, respectively. The spectral analysis showed that both HP Cet and Swift J0820.6–280 are members of the underluminous IP subclass since their luminosity in the 0.3–10 keV band is estimated to be ≃5 × 1030 and ≃3.8 × 1029 erg s−1, respectively. [ABSTRACT FROM AUTHOR]

Shocks in γ-ray emitting classical novae are expected to produce bright thermal and non-thermal X-rays. We test this prediction with simultaneous NuSTAR and Fermi /LAT observations of nova V906 Car, which exhibited the brightest GeV γ-ray emission to date. The nova is detected in hard X-rays while it is still γ-ray bright, but contrary to simple theoretical expectations, the detected 3.5–78 keV emission of V906 Car is much weaker than the simultaneously observed >100 MeV emission. No non-thermal X-ray emission is detected, and our deep limits imply that the γ-rays are likely hadronic. After correcting for substantial absorption (N H ≈ 2 × 1023 cm−2), the thermal X-ray luminosity (from a 9 keV optically thin plasma) is just ∼2 per cent of the γ-ray luminosity. We consider possible explanations for the low thermal X-ray luminosity, including the X-rays being suppressed by corrugated, radiative shock fronts or the X-rays from the γ-ray producing shock are hidden behind an even larger absorbing column (N H > 1025 cm−2). Adding XMM–Newton and Swift /XRT observations to our analysis, we find that the evolution of the intrinsic X-ray absorption requires the nova shell to be expelled 24 d after the outburst onset. The X-ray spectra show that the ejecta are enhanced in nitrogen and oxygen, and the nova occurred on the surface of a CO-type white dwarf. We see no indication of a distinct supersoft phase in the X-ray light curve, which, after considering the absorption effects, may point to a low mass of the white dwarf hosting the nova. [ABSTRACT FROM AUTHOR]

Cataclysmic Variables (CVs) are binaries in which a white dwarf accretes from a low-mass companion star. CVs are the best-suited laboratories to test our understanding of the evolution of compact, interacting binaries as they are numerous, relatively bright, and both stellar components are structurally simple. Nonetheless, while a large fraction (≃ 40 - 80 %) of the present-day Galactic CV population is expected to be old, highly evolved and to host brown dwarf companions, yet very few of these so-called "period bouncers" have been identified so far. The lack of these systems in the observed Galactic CV population possibly suggests that the physical mechanisms driving CV evolution (such as the common envelope phase, the mechanisms of angular momentum loss and/or the response of the companions to the mass loss) are still not completely understood. The Compact binary HIgh CAdence Survey (CHiCaS) is a high cadence photometric survey performing three hours of uninterrupted time series photometry at one minute cadence over 136 square degrees of sky with JAST/T80Cam. CHiCaS aims to unambiguously identify the predicted large population of period-bounce CVs via detection of binary eclipses, thus providing an observational support for the current evolutionary models of all kind of compact binaries, such as black hole binaries, X-ray transients, double degenerates or SN Ia progenitors. Moreover, CHiCaS will deliver high cadence light curves along with full-colour information for about 2.5 million sources, thus identifying several hundred thousand variable stars, including eclipsing and contact binaries, pulsating and flaring stars, which will provide a significant legacy value. [ABSTRACT FROM AUTHOR]

Cataclysmic Variables (CVs) are compact binaries with white dwarf (WD) primaries. CVs and other accreting WD binaries (AWBs) are useful laboratories for studying accretion flows, gas dynamics, outflows, transient outbursts, and explosive nuclear burning under different astrophysical plasma conditions. They have been studied over decades and are important for population studies of galactic X-ray sources. Recent space- and ground-based high resolution spectral and timing studies, along with recent surveys indicate that we still have observational and theoretical complexities yet to answer. I review accretion in nonmagnetic AWBs in the light of X-ray observations. I present X-ray diagnostics of accretion in dwarf novae and the disk outbursts, the nova-like systems, and the state of the research on the disk winds and outflows in the nonmagnetic CVs together with comparisons and relations to classical and recurrent nova systems, AM CVns and Symbiotic systems. I discuss how the advective hot accretion flows (ADAF-like) in the inner regions of accretion disks (merged with boundary layer zones) in nonmagnetic CVs explain most of the discrepancies and complexities that have been encountered in the X-ray observations. I stress how flickering variability studies from optical to X-rays can be probes to determine accretion history and disk structure together with how the temporal and spectral variability of CVs are related to that of LMXBs and AGNs. Finally, I discuss the nature of accretion in nonmagnetic WDs in terms of ADAF-like accretion flows, and elaborate on the solutions it brings and its complications, constructing an observational framework to motivate new theoretical calculations that introduce this flow-type in disks, outflow and wind models together with disk-instability models of outbursts and nova outbursts in AWBs and WD physics, in general. [ABSTRACT FROM AUTHOR]

Direct determination of WD masses in CVs is extremely difficult, which is the reason very few direct determinations exist. We present a method for calculating an estimate of WD masses in CVs and of the rate at which they accrete matter from their companion, by combining results of nova simulations with cataloged observations of 92 novae. We present observed distributions of the WD mass ( M WD ) and accretion rate ( M ̇ ), and corrections of these distributions while taking into account the observation frequency of each nova. We show that WDs in RN systems are typically more massive than WDs in CN systems and their M ̇ is higher. If the M ̇ is sufficiently high, the WD will eject only a fraction of the accreted mass, resulting in the secular growth of the WD mass. We have followed the evolution of WDs of various initial masses and accretion rates through long series of nova cycles, focusing on the WD mass change, considering both hydrogen and helium accretion and addressing the consequences of helium flashes. We show the circumstances under which WDs can grow up to the Chandrasekhar mass, to become progenitors of type Ia Supernovae. [ABSTRACT FROM AUTHOR]

We present the results of MeerKAT radio observations of 11 nearby nova-like cataclysmic variables (CVs). We have detected radio emission from IM Eri, RW Sex, V3885 Sgr, and V603 Aql. While RW Sex, V3885 Sgr, and V603 Aql had been previously detected, this is the first reported radio detection of IM Eri. Our observations have doubled the sample of non-magnetic CVs with sensitive radio data. We observe that at our radio detection limits, a specific optical luminosity |${\gtrsim}2.2\times 10^{18}\,$|  erg s−1 Hz−1 (corresponding to MV  ≲ 6.0) is required to produce a radio detection. We also observe that the X-ray and radio luminosities of our detected nova-like CVs are on an extension of the |$L_X\propto L_R^{\sim 0.7}$| power law originally proposed for non-pulsating neutron star low-mass X-ray binaries. We find no other correlations between the radio emission and emission in other wavebands or any other system parameters for the existing sample of radio-detected non-magnetic CVs. We measure in-band (0.9–1.7 GHz) radio spectral indices that are consistent with reports from earlier work. Finally, we constructed broad spectral energy distributions for our sample from published multiwavelength data, and use them to place constraints on the mass transfer rates of these systems. [ABSTRACT FROM AUTHOR]

Ultracompact accreting binary systems each consist of a stellar remnant accreting helium-enriched material from a compact donor star. Such binaries include two related sub-classes, AM CVn-type binaries and helium cataclysmic variables, in both of which the central star is a white dwarf. We present a spectroscopic and photometric study of six accreting binaries with orbital periods in the range of 40–70 min, including phase-resolved VLT spectroscopy and high-speed ULTRACAM photometry. Four of these are AM CVn systems and two are helium cataclysmic variables. For four of these binaries we are able to identify orbital periods (of which three are spectroscopic). SDSS J1505+0659 has an orbital period of 67.8 min, significantly longer than previously believed, and longer than any other known AM CVn binary. We identify a Wide-field Infrared Survey Explorer (WISE) infrared excess in SDSS J1505+0659 that we believe to be the first direct detection of an AM CVn donor star in a non-direct impacting binary. The mass ratio of SDSS J1505+0659 is consistent with a white dwarf donor. CRTS J1028–0819 has an orbital period of 52.1 min, the shortest period of any helium cataclysmic variable. MOA 2010-BLG-087 is co-aligned with a K-class star that dominates its spectrum. ASASSN-14ei and ASASSN-14mv both show a remarkable number of echo outbursts following superoutbursts (13 and 10 echo outbursts respectively). ASASSN-14ei shows an increased outburst rate over the years following its superoutburst, perhaps resulting from an increased accretion rate. [ABSTRACT FROM AUTHOR]

The spectra of disc-dominated cataclysmic variables (CVs) often deviate from the spectra of accretion disc models; in particular, the Balmer jump and absorption lines are found to be shallower in the observations than in the models. We carried out a combined ultraviolet–optical spectral analysis of two dwarf novae (DNe): UZ Ser in outburst, decline, and quiescence, and CY Lyr on the rise to outburst and in outburst. We fit the Balmer jump and absorption lines, the continuum flux level and slope by adjusting the accretion rate, inclination, and disc outer radius. For both systems, we find an accretion rate |$\dot{M} \approx 8 \times 10^{-9}\,\mathrm{ M}_\odot\,\mathrm{ yr}^{-1}$| in outburst, and |$\dot{M} \approx 2-3 \times 10^{-9}\,\mathrm{ M}_\odot\,\mathrm{ yr}^{-1}$| for the rise and decline phases. The outer disc radius we derive is smaller than expected (R disc ≈ 0.2 a , where a is the binary separation), except during late rise (for CY Lyr) where R disc = 0.3 a. UZ Ser also reveals a 60 000 K white dwarf. These results show that during a DN cycle the radius of the disc is the largest just before the peak of the outburst, in qualitative agreement with the disc instability model for DN outbursts. We suspect that an additional emitting component (e.g. disc wind) is also at work to reduce the slope of the continuum and size of the Balmer jump and absorption lines. We stress that both the outer disc radius and disc wind need to be taken into account for more realistic disc modelling of CVs. [ABSTRACT FROM AUTHOR]

We present extensive radial velocity observations of the intermediate polar DW Cnc during its 2018–2019 low state. We show that the 86 min signal associated with the orbital period is strong in our radial velocity analysis, power spectrum search, and in our Doppler tomography. However, we find that the velocity modulation associated with the 70 min beat period and the 38 min spin cycle is dramatically weaker than that previously observed. We put forward two interpretations for this change. The first is that a sudden drop into a low state detected in 2018–2019 caused an episode of low mass transfer from the companion, thus inhibiting the lighthouse effect produced by the rebound emission. The second is that this is a consequence of a rare outburst detected in 2007 by Crawford et al. (2008). We find this post-outburst hypothesis to be less likely. If the first scenario is correct, we predict that DW Cnc  will recover its intermediate polar characteristics. A new ephemeris is presented by combining Patterson et al. (2004) radial velocities with ours. [ABSTRACT FROM AUTHOR]