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VY Scl binaries are a sub-class of cataclysmic variable (CV) which show extended low states, but do not show outbursts which are seen in other classes of CV. To better determine how often these systems spend in low states and to resolve the state transitions we have analysed Zwicky Transient Facility (ZTF) data on eight systems and Transiting Exoplanet Survey Satellite (TESS) data on six systems. Half of the sample spent most of the time in a high state; three show a broad range and one spends roughly half the time transitioning between high and low states. Using the ZTF data, we explore the colour variation as a function of brightness. In KR Aur, we identify a series of repeating outburst events whose brightness appears to increase over time. Using TESS data, we searched for periods other than the orbital. In LN UMa, we find evidence for a peak whose period varies between 3 and 6 d. We outline the current models which aim to explain the observed properties of VY Scl systems which includes disc irradiation and a white dwarf having a significant magnetic field. [ABSTRACT FROM AUTHOR]

Context. TT Arietis (TT Ari) is a nova-like cataclysmic variable of the VY Scl subtype with light-curve variations on multiple timescales. In addition to the superhump modulation, quasi-periodic oscillations (QPOs) have been found. Aims. Our aim is to determine the occurrence, strength, and variability of QPOs in TT Ari based on more complete data than in previous works. Methods. The data were obtained during the high state of TT Ari in October 2012 by the MOST space telescope, covering a total of 361.2 hours of continuous observation. We searched for frequencies over subsets of time using a Fourier-like power spectrum and then added the frequencies together, forming groups. Results. Our method has revealed QPOs that occur in 'frequency groups', which are events with a continuous oscillation of similar, constant or slowly variable frequency. We report a total of 160 frequency groups in the period range between 14 and 53 minutes (27 and 98 days−1), with two peaks in the power spectrum at 18.5 and 33.8 minutes (42.5 and 77.5 days−1). The duration of these frequency groups varies between 0.72 and 7.5 hours (average 2.8 hours) revealing between 3 and 18 complete cycles in the light curve. Most of them show significant frequency variations over the course of their duration. Sometimes two frequency groups occur simultaneously. An analysis with randomised data confirms that stochastic processes can only explain a fraction of the QPOs found. The occurrence of QPOs appears not to be related to the superhump phase. [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]

The spectral variation in the optical region of a peculiar variable star V Sagittae was monitored in the recent three decades. The main aim of this article is to report several interesting phenomena discovered in the monitoring; we briefly discuss interpretations of them. Emission lines of H  i and He  ii were decomposed into narrow components and wide wings. The radial velocities of the wings were related to the orbital motion of the hot primary, but did not reflect it directly. Widely red-shifted sinusoidal variations according to the orbital period were detected in 1993–2005 (e.g. |$K = 195$|  km s |$^{-1}$| and |$\gamma = +252$|  km s |$^{-1}$| for He  ii 4686). Sporadic blue-shifts were also observed. The mode of the radial velocity variation drastically changed between 2005 and 2007. Red-shifted sinusoidal variations (⁠|$K = 224$|  km s |$^{-1}$| and |$\gamma = +260$|  km s |$^{-1}$| idem) or blue-shifted ones (⁠|$K =206$|  km s |$^{-1}$| and |$\gamma = -289$|  km s |$^{-1}$| idem) appeared alternately in 2007–2019. One red- or blue-shift state lasted several years. The radial velocities of O  iii 5592 and the doublet C  iv 5805 exhibited sinusoidal variations without large red- or blue-shifts (e.g. |$K =215$|  km s |$^{-1}$| and |$\gamma = -18$|  km s |$^{-1}$| for C  iv 5805). The widths of emission lines He  ii 5412 and C  iv in the doublet 5805 suggest that helium and carbon ions contained nearly the same kinetic energy, because the former line was wider than the latter by a factor of about |$\surd {3}$| on most spectra. It is probable that there was no large-scale mass motion in such a thermal-equilibrium-like condition. The C  iv lines and He  ii 5412 had roughly same widths, like the spectra of Wolf–Rayet stars, only three times in 2007–2019 on which there were likely large-scale mass outflows. [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]

Aims. Cataclysmic variables (CVs) are compact binary systems in which a white dwarf accretes matter from a Roche-lobe-filling companion star. For this study we searched for new CVs in the Milky Way in the Chandra Source Catalog v2.0, cross-matched with Gaia Data Release 3 (DR3). Methods. We identified new CV candidates by combining X-ray and optical data in a color-color diagram called the X-ray main sequence. We used two different cuts in this diagram to compile pure and optically variable samples of CV candidates. We undertook optical spectroscopic follow-up observations with the Keck and Palomar Observatories to confirm the nature of these sources. Results. We assembled a sample of 25 887 Galactic X-ray sources and found 14 new CV candidates. Seven objects show X-ray and/or optical variability. All sources show X-ray luminosity in the 1029 − 1032 erg s−1 range, and their X-ray spectra can be approximated by a power-law model with photon indices in the Γ ∼ 1 − 3 range or an optically thin thermal emission model in the kT ∼ 1 − 70 keV range. We spectroscopically confirmed four CVs, discovering two new polars, one low accretion rate polar and a WZ Sge-like low accretion rate CV. X-ray and optical properties of the other nine objects suggest that they are also CVs (likely magnetic or dwarf novae), and one other object could be an eclipsing binary, but revealing their true nature requires further observations. Conclusions. These results show that a joint X-ray and optical analysis can be a powerful tool for finding new CVs in large X-ray and optical catalogs. X-ray observations such as those by Chandra are particularly efficient at discovering magnetic and low accretion rate CVs, which could be missed by purely optical surveys. [ABSTRACT FROM AUTHOR]

We present a comparative analysis of photometric observations of the cataclysmic variable TT Ari in its bright state obtained by the TESS orbital observatory in 2021 and 2023 and by ground-based amateur telescopes in 2022. The light curves from 2021 and 2022 are dominated by modulations with a period slightly shorter than the orbital one (negative superhumps), 0.13292 and 0.13273 d, respectively. From the data obtained in 2023, we see much stronger modulations appearing on a much longer timescale of a few days with an amplitude of up to 0.5 mag, compared to 0.2 mag in 2021. We also find the negative superhump variability with the period of 0.1338 d in the 2023 observations, but the significance of these negative superhumps is much lower than in the previous seasons. We detect less significant additional modulations with a period exceeding the orbital one (positive superhumps) in the observations from 2021 and 2022. Their periods are 0.15106 and 0.1523 d, respectively. We also find a previously unnoticed periodic signal corresponding to the orbital period of 0.13755 d in the TESS observations in 2021. Theoretical models of tidal precession of an elliptical disk predict a decrease in the precession period (and an increase in the period of the positive superhumps) with increasing disk radius, which is consistent with the observed photometric behavior of the system. This enables us to estimate the mass ratio q of the components in TT Ari to be in the range of 0.24–0.29. The tilted disk precession model predicts a period of nodal precession whose value is in general agreement with observations. [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]

Context. The transition zone between the white dwarf (WD) envelope and a circumstellar accretion disk in classical novae, the boundary layer, is a region of strong dissipation and intense vorticity. In this strongly sheared layer, the hydrogen-rich accreted gas is expected to mix with the underlying WD outermost layers so the conditions for the onset of the thermonuclear runaway (TNR) in classical nova will be different from the standard treatment of the onset and subsequent mixing. Aims. We applied the critical layer instability (CLI) to the boundary between a disk-accreted H/He zone and the C/O- or O/Ne – rich outer layers of a mass-accreting WD in a cataclysmic binary and then used the resulting structure as input to one-dimensional nuclear-hydrodynamic simulations of the nova outburst. Methods. We simulated the subsonic mixing process in two dimensions for conditions appropriate for the inner disk and a CO 0.8 M⊙ and CO and ONe 1.25 M⊙ WDs using the compressible hydrodynamics code PLUTO. The resulting compositional profile was then imported into the one-dimensional nuclear-hydrodynamics code SHIVA to simulate the triggering and growth rate for the TNR and subsequent envelope ejection. Results. We find that the deep shear driven mixing changes the triggering and development of the TNR. In particular, the time to reach peak temperature is significantly shorter, and the ejected mass and maximum velocity of the ejecta substantially greater, than the current treatment. The 7Li yield is reduced by about an order of magnitude relative to the current treatments. [ABSTRACT FROM AUTHOR]

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

The Tsinghua University–Ma Huateng Telescopes for Survey (TMTS) started to monitor the LAMOST plates in 2020, leading to the discovery of numerous short-period eclipsing binaries, peculiar pulsators, flare stars, and other variable objects. Here, we present the uninterrupted light curves for a sample of 64 cataclysmic variables (CVs) observed/discovered using the TMTS during its first three-year observations, and we introduce new CVs and new light-variation periods (from known CVs) revealed through the TMTS observations. Thanks to the high-cadence observations of TMTS, diverse light variations, including superhumps, quasi-periodic oscillations, large-amplitude orbital modulations, and rotational modulations, are able to be detected in our CV samples, providing key observational clues for understanding the fast-developing physical processes in various CVs. All of these short-timescale light-curve features help further classify the subtypes of CV systems. We highlight the light-curve features observed in our CV sample and discuss further implications of minute-cadence light curves for CV identifications and classifications. Moreover, we examine the Hα emission lines in the spectra from our nonmagnetic CV samples (i.e., dwarf novae and nova-like subclasses) and find that the distribution of Hα emission strength shows significant differences between the sources with orbital periods above and below the period gap, which agrees with the trend seen from the SDSS nonmagnetic CV sample. [ABSTRACT FROM AUTHOR]

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

The eruption of Nova V2891 Cygni in 2019 offers a rare opportunity to explore the shock-induced processes in novae ejecta. The spectral evolution shows noticeable differences in the evolution of various oxygen emission lines such as O  i 7773 Å, O  i 8446 Å, O  i 1.1286  μ m, O  i 1.3164  μ m, etc. Here, we use spectral synthesis code cloudy to study the temporal evolution of these oxygen emission lines. Our photoionization model requires the introduction of a component with a very high density (⁠|$n \sim 10^{11}$| cm |$^{-3}$|⁠) and an enhanced oxygen abundance (O/H |$\sim$| 28) to produce the O  i 7773 Å emission line, suggesting a stratification of material with high oxygen abundance within the ejecta. An important outcome is the behaviour of the O  i 1.3164  μ m line, which could only be generated by invoking the collisional ionization models in cloudy. Our phenomenological analysis suggests that O  i 1.3164  μ m emission originates from a thin, dense shell characterized by a high density of about |$10^{12.5}\!-\!10^{12.8}$| cm |$^{-3}$|⁠ , which is most likely formed due to the strong internal collisions. If such is the case, the O  i 1.3164  μ m emission presents itself as a tracer of shock-induced dust formation in V2891 Cyg. The collisional ionization models have also been successful in creating the high-temperature conditions (⁠|$\sim 7.07\!-\!7.49 \times 10^5$|  K) required to reproduce the observed high ionization potential coronal lines, which coincide with the epoch of dust formation and evolution of the O  i 1.3164 μ m emission line. [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]

We review the evolution of our understanding of the planetary nebulae phenomenon and their place in the scheme of stellar evolution. The historical steps leading to our current understanding of central star evolution and nebular formation are discussed. Recent optical imaging, X-ray, ultraviolet, infrared, millimeter wave, and radio observations have led to a much more complex picture of the structure of planetary nebulae. The optically bright regions have multiple shell structures (rims, shells, crowns, and haloes), which can be understood within the interacting winds framework. However, the physical mechanism responsible for bipolar and multipolar structures that emerged during the proto-planetary nebulae phase is yet to be identified. Our morphological classifications of planetary nebulae are hampered by the effects of sensitivity, orientation, and field-of-view coverage, and the fraction of bipolar or multipolar nebulae may be much higher than commonly assumed. The optically bright bipolar lobes may represent low-density, ionization-bounded cavities carved out of a neutral envelope by collimated fast winds. Planetary nebulae are sites of active synthesis of complex organic compounds, suggesting that planetary nebulae play a major role in the chemical enrichment of the Galaxy. Possible avenues of future advancement are discussed. [ABSTRACT FROM AUTHOR]

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

Accretion discs are fundamental to much of astronomy. They can occur around stars, both young and old, around compact objects they provide a window into the extremes of physics, and around supermassive black holes in galaxy centres, they generate spectacular luminosities that can outshine the entire galaxy. However, our understanding of the inner workings of accretion discs remains far from complete. Here, we revisit a conundrum in the observations of some of the simplest accreting systems: the cataclysmic variables (CVs). The high-accretion-rate states of (non-magnetic) CVs can be divided into the short-lived outbursts (∼a week), typical of dwarf novae (DNe), and the long-lived (and sometimes perpetual) high states of nova-like (NL) CVs. Since both sorts of high-state occur in approximately steady-state accretion discs with similar properties and accretors, we would expect them to display similar spectral energy distributions. However, previous analyses based on UV spectra from the International Ultraviolet Explorer have shown that their spectral energy distributions are different. We perform a re-analysis of the data using up to date calibrations and distance (and thus dereddening) estimates to test whether this difference persists and whether it is statistically significant over the sample. We find that it does persist, and it is statistically significant. We propose routes to investigating this discrepancy further and discuss the implications this has for other accreting systems, such as X-ray binaries, active galactic nuclei, and protoplanetary discs. [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 optical follow-up observations of an X-ray source initially detected by the Einstein Probe mission. Our investigations categorize the source as an intermediate polar, a class of magnetic cataclysmic variables, exhibiting an orbital period of 3.7614(4) h and a white dwarf spin period of 3.97 min. The orbital period was identified through TESS observations, while our high-speed photometric data, obtained using the 1.9m and Lesedi 1.0m telescopes at the South African Astronomical Observatory, revealed both the spin and beat periods. Additionally, we present orbitally phase-resolved spectroscopic observations using the 1.9m telescope, specifically centred on the H β emission line, which reveal two emission components that exhibit Doppler variations throughout the orbital cycle. [ABSTRACT FROM AUTHOR]

We present high‐resolution (0.06 Å px −1$$ {}^{-1} $$) spectroscopic observations of the recurrent nova T Coronae Borealis obtained during the last 1.5 years (September 2022–January 2024), with the 2.0‐m RCC telescope of the Rozhen National Astronomical Observatory, Bulgaria. Double‐peaked emission is visible in the Hα$$ {H}_{\alpha } $$ line after the end of the superactive state. We subtract the red giant contribution and measure the distance between the peaks (Δva$$ \Delta {v}_a $$) of the line. For the period July 2023–January 2024, we find that Δva$$ \Delta {v}_a $$ is in range 90<Δva<120$$ 90<\Delta {v}_a<120 $$ km s −1$$ {}^{-1} $$. Assuming that the emission is from the accretion disc around the white dwarf, we find average radius of the accretion disc Rdisc=89±19$$ {R}_{\mathrm{disc}}=89\pm 19 $$ R ⊙$$ {}_{\odot } $$, which is approximately equal to the Roche lobe size of the white dwarf. Our results indicate that tidal torque plays an important role but that the disc can extend up to the Roche lobe of the accreting star. [ABSTRACT FROM AUTHOR]

We present archival and ground-based infrared observations of the γ-ray-emitting nova V959 Mon, covering the period 100–4205 d after the 2012 eruption. We use these data to determine that the secondary in the nova system is a G5 main sequence star. Data from the NEOWISE survey reveal a significant increase in the emission at 3.4 and 4.6  |$\, \mu$| m at late (≳600 d) times, which we interpret as emission by dust. Other interpretations are considered but cannot be reconciled with the data. The presence of such late dust emission, and in particular its variation with time, are unprecedented in the context of novae. The behaviour of the dust emission suggests a qualitative interpretation in which ejecta from the 2012 eruption encounter denser pre-eruption circumbinary material, giving rise to Rayleigh–Taylor instabilities that cause clumps of dust-bearing material to fall back towards the central binary, the dust undergoing destruction by chemisputtering as it does so. The observed rise in the dust temperature, the decline in the nova-dust distance, and in the dust mass are consistent with this interpretation. Not all novae are expected to show this behaviour, but inspection of resources such as NEOWISE might reveal other novae post-eruption that do. [ABSTRACT FROM AUTHOR]

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

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

We report the results of an analysis of X-ray and γ-ray data from the nova FM Cir taken by Swift and Fermi –LAT. The γ-ray emission from FM Cir can be identified with a significance level of ∼3σ within ∼40 d after the nova eruption (2018 January 19) when we bin the light curve per day. The significance can exceed the 4σ confidence level if we accumulate a longer time (i.e. 20 d) to bin the light curve. The γ-ray counterpart could be identified with a Test Statistic (TS) above 4 until ∼180 d after the eruption. The duration of the γ-ray detection is longer than those reported in previous studies of other novae detected in the GeV range. Significant X-ray emission was observed after the γ-ray flux level fell below the sensitivity limit of Fermi –LAT. The hardness ratio of the X-ray emission decreased rapidly with time, and the spectra were dominated by blackbody radiation from the hot white dwarf. Except for the longer duration of the γ-ray emission, the multiwavelength properties of FM Cir closely resemble those of other novae detected in the GeV range. [ABSTRACT FROM AUTHOR]

We present a systematic search for periodic X-ray sources in the bulge of M31, using ∼2 Ms of archival Chandra observations spanning a temporal baseline of 16 yr. Utilizing the Gregory–Loredo algorithm that is designed for photon-counting, phase-folded light curves, we detect seven periodic X-ray sources, among which four are newly discovered. Three of these sources are novae, the identified periods of which range between 1.3 and 2.0 h and are most likely the orbital period. The other four sources are low-mass X-ray binaries, the identified periods of which range between 0.13 and 19.3 h and are also likely orbital due to a clear eclipsing/dipping behaviour in the light curve. We address implications on the X-ray binaries population of the M31 bulge. Our study demonstrates the potential of using archival X-ray observations to systematically identify periodic X-ray sources in external galaxies, which would provide valuable information about the underlying exotic stellar populations. [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]

We analyse a Chandra observation of the rich globular cluster NGC 362, finding 33 X-ray sources within 1 arcmin (1.2 half-mass radii) of the cluster centre. Spectral analysis of the brightest source (X1) shows blackbody-like emission, indicating it is likely a quiescent low-mass X-ray binary; we find a possible counterpart that falls in the sub-subgiant region. We use Hubble Space Telescope ultraviolet (UV) Globular Cluster Survey photometry to identify 15 potential optical/UV counterparts to these X-ray sources, including two background active galactic nuclei. We identify no likely cataclysmic variables (CVs), probably due to crowding in optical filters in the core, though we predict of order 8 CVs among the detected X-ray sources. We identify three other sub-subgiants and two red straggler counterparts, which are likely powered by coronal activity, along with five other potential coronally active binary counterparts to three X-ray sources. Finally, we note two unusual counterpart candidates that lie to the red of the red giant branch in V 606 − I 814, and shift well to the blue of the red giant branch in ultraviolet colour–magnitude diagrams. These systems seem to contain a red giant with a distorted evolutionary history, plus a bright blue light source, either a blue straggler star (an Algol-like system) or an accreting white dwarf (a long-period CV, or a symbiotic star). [ABSTRACT FROM AUTHOR]

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

We present a catalogue of point sources detected with the Mikhail Pavlinsky ART-XC telescope onboard the SRG observatory during a wide-field survey of the Galactic Bulge that was conducted as part of the mission's calibration and performance verification phase in 2019. The survey consisted of nearly 18 d of consecutive scanning observations of the sky region | l | < 6 deg, | b | < 2.5 deg with the median sensitivity of 4 × 10−13 erg s−1 cm−2 in the 4–12 keV energy band, which allows the detection of sources down to a luminosity of 3 × 1033 erg s−1 near the Galactic Centre. Using a maximum-likelihood-based algorithm, 172 sources were detected. Of these, 153 are registered on the average 4–12 keV map and 18 sources are either extremely hard (detected only at energies above 7 keV) or highly variable (detected only in individual scans shorter than a day). For 121 sources, there are plausible counterparts in other X-ray source catalogues, including 43 with known classification. The remaining 51 sources are previously unknown objects, discovered by ART-XC during the Galactic Bulge survey. [ABSTRACT FROM AUTHOR]

We present our optical photometric observations of the 2022 eruption of the recurrent nova U Scorpii (U Sco) using 49152 data points over 70 d following the optical peak. We have also analyzed its soft X-ray (0.3–1 keV) light curve by the Neil Gehrels Swift Observatory. During the 2022 eruption, the optical plateau stage started 13.8–15.0 d and ended 23.8–25.0 d after the optical peak. The soft X-ray stage started 14.6–15.3 d and ended 38.7–39.5 d after the optical peak. Both stages started later and had shorter durations, and the soft X-ray light curve peaked earlier and was less luminous compared to those during the U Sco 2010 eruption. These points suggest that there were differences in the envelope mass between the different cycles of the nova eruption. Furthermore, we have analyzed the optical eclipses during the 2022 eruption. The primary eclipse was first observed 10.4–11.6 d after the optical peak, earlier than the beginning of the optical plateau stage. This sequence of events can be explained by the receding ejecta photosphere associated with the expanding nova ejecta. We have determined the ingress and egress phases of the primary eclipses and estimated the outer radius of the optical light source centered at the white dwarf (WD). During the optical plateau stage, the source radius remained ∼1.2 times larger than the Roche volume radius of the primary WD, being close to the L1 point. When the optical plateau stage ended, the source radius drastically shrank to the tidal truncation radius within a few orbital periods. This previously unresolved phenomenon can be interpreted as a structural change in U Sco where the temporarily expanded accretion disk due to the nova wind returned to a steady state. [ABSTRACT FROM AUTHOR]

The classical nova CP Puppis has been observed to have particularly puzzling and peculiar properties. In particular, this classical nova displays occasional bursts in its long-term ASAS-SN light curve. Here, we report on five sectors of TESS data displaying two of these rapid bursts, lasting ∼1 d. Based on the estimated lower energy limits of the bursts, we discuss whether the bursts may be examples of micronovae resulting from localized thermonuclear explosion. Furthermore, its orbital period remains uncertain, with several inconsistent periodic signals appearing in spectroscopic and photometric observations at various wavelengths. Although we cannot unambiguously unravel the physical origin of the signals, the previously suggested nature of CP Puppis as a long orbital period system may be a viable explanation. The recurrence time of the bursts in CP Puppis, together with the unexplained variable modulations make it a prime candidate for intense monitoring. [ABSTRACT FROM AUTHOR]

Just 10 recurrent novae (RNe) – which erupt repeatedly on time-scales shorter than one century – are known in our Galaxy. The most extreme RN known (located in the Andromeda galaxy), M31N 2008-12a, undergoes a nova eruption every year, and is surrounded by a vast nova 'super-remnant', 134 pc in extent. Simulations predict that all RNe should be surrounded by similar vast shells, but previous searches have failed to detect them. KT Eri has recently been suggested to be a RN, and we have used the Condor Array Telescope to image its environs through multiple narrow-band filters. We report the existence of a large (∼50-pc diameter), H  |$\, \alpha$| -bright shell centred on KT Eri, exactly as predicted. This strongly supports the claim that KT Eri is the 11th Galactic recurrent nova, and only the second nova known to be surrounded by a super-remnant. SALT spectra of the super-remnant demonstrate that its velocity width is consistent with that of M31-2008-12a. [ABSTRACT FROM AUTHOR]

A nova super-remnant (NSR) is an immense structure associated with a nova that forms when frequent and recurrent nova (RN) eruptions sweep up surrounding interstellar medium (ISM) into a high-density and distant shell. The prototypical NSR, measuring over 100 pc across, was discovered in 2014 around the annually erupting nova M 31N 2008-12a. Hydrodynamical simulations demonstrated that the creation of a dynamic NSR by repeated eruptions transporting large quantities of ISM is not only feasible but that these structures should exist around all novae, whether the white dwarf (WD) is increasing or decreasing in mass. But it is only the RN with the highest WD masses and accretion rates that should host observable NSRs. KT Eridani is, potentially, the eleventh RNe recorded in the Galaxy and is also surrounded by a recently unveiled H α shell tens of parsecs across, consistent with an NSR. Through modelling the nova ejecta from KT Eri, we demonstrate that such an observable NSR could form in approximately 50 000 yr, which fits with the proper motion history of the nova. We compute the expected H α emission from the KT Eri NSR and predict that the structure might be accessible to wide-field X-ray facilities. [ABSTRACT FROM AUTHOR]

The problem caused by the inconsistency between the age and spin-down time scale of the white dwarfs in the systems AR Sco and AE Aqr is discussed within the hypothesis of their accretion-driven spin-up during the previous epoch. Considering a scenario of accretion from a Keplerian disk, we conclude that the white dwarfs in these systems could be spun-up to the currently observed periods provided the radius of their magnetospheres at the spin-up phase was substantially smaller than the Alfven radius. We show that the required magnetospheric radius value can be reached if the accretion flow penetrates the magnetic field of the white dwarf at its magnetospheric boundary due to anomalous (Bohm) diffusion. In this case the spin-up of the white dwarfs in the systems under consideration can be described based on a model of stationary accretion from the Keplerian disk at an average rate of yr without any additional assumptions about the evolution of the intrinsic magnetic field of the white dwarfs during the spin-up phase. [ABSTRACT FROM AUTHOR]

The prototypical nova super-remnant (NSR) was uncovered around the most rapidly recurring nova (RN), M31N 2008-12a. Simulations of the growth of NSRs revealed that these large structures should exist around all novae, whether classical or recurrent. NSRs consist of large shell-like structures surrounding excavated cavities. Predictions, informed by these simulations, led to the discovery of an extended cavity coincident with the Galactic RN, RS Ophiuchi (RS Oph), in far-infrared archival IRAS images. We propose that this cavity is associated with RS Oph and is therefore evidence of another NSR to be uncovered. [ABSTRACT FROM AUTHOR]

We present the discoveries of two of AM CVn systems, Gaia14aae and SDSS J080449.49+161624.8, which show X-ray pulsations at their orbital periods, indicative of magnetically collimated accretion. Both also show indications of higher rates of mass transfer relative to the expectations from binary evolution driven purely by gravitational radiation, based on existing optical data for Gaia14aae, which show a hotter white dwarf temperature than expected from standard evolutionary models, and X-ray data for SDSS J080449.49+161624.8 which show a luminosity 10−100 times higher than those for other AM CVn at similar orbital periods. The higher mass transfer rates could be driven by magnetic braking from the disc wind interacting with the magnetosphere of the tidally locked accretor. We discuss implications of this additional angular momentum transport mechanism for evolution and gravitational wave detectability of AM CVn objects. [ABSTRACT FROM AUTHOR]

The Andromeda Galaxy is home to the annually erupting recurrent nova (RN) M 31N 2008-12a (12a); the first nova found to host a nova super-remnant (NSR). An NSR is an immense structure surrounding a RN, created from many millions of eruptions sweeping up material in the local environment to form a shell tens of parsecs across. Theory has demonstrated that NSRs should be found around all recurrent novae (RNe), even those systems with long periods between eruptions. Befittingly, the second NSR was found around the Galactic classical (and long suspected recurrent) nova, KT Eridani. In this Paper, we aim to find more of these phenomena through conducting the first ever survey for NSRs in M 31 and the Large Magellanic Cloud (LMC). We find that the surroundings of fourteen RNe in M 31 as well as the surroundings of the four RNe in the LMC do not show any evidence of vast parsec-scale structures in narrow-band (H α and |$[{\rm S\, {\small II}}]$|⁠) images, unlike the one clearly seen around 12a, and therefore conclude that observable NSRs are either rare structures, or they are too faint (or small) to be detected in our existing data sets. Yet, the NSR surrounding 12a would also likely to have been overlooked in our study if it were approximately one magnitude fainter. Searches for NSRs around other RNe 'masquerading' as classical novae may prove to be fruitful as would whole surveys of other Local Group galaxies. [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]

Nova Her 2021 or V1674 Her was one of the fastest novae to be observed so far. We report here the results from our timing and spectral studies of the source observed at multiple epochs with AstroSat. We report the detection of a periodicity in the source in soft X-rays at a period of 501.4–501.5 s which was detected with high significance after the peak of the super-soft phase, but was not detected in the far ultraviolet (FUV) band of AstroSat. The shape of the phase-folded X-ray light curves has varied significantly as the nova evolved. The phase-resolved spectral studies reveal the likely presence of various absorption features in the soft X-ray band of 0.5–2 keV, and suggest that the optical depth of these absorption features may be marginally dependent on the pulse phase. Strong emission lines from Si, N, and O are detected in the FUV, and their strength declined continuously as the nova evolved and went through a bright X-ray state. [ABSTRACT FROM AUTHOR]

Using a 12 ks archival Chandra X-ray Observatory ACIS-S observation on the massive globular cluster (GC) M14, we detect a total of 7 faint X-ray sources within its half-light radius at a |$0.5{\small --}7\, \mathrm{keV}$| depth of |$2.5\times 10^{31}\, \mathrm{erg s^{-1}}$|⁠. We cross-match the X-ray source positions with a catalogue of the Very Large Array radio point sources and a Hubble Space Telescope (HST) UV/optical/near-IR photometry catalogue, revealing radio counterparts to 2 and HST counterparts to 6 of the X-ray sources. In addition, we also identify a radio source with the recently discovered millisecond pulsar PSR 1737−0314A. The brightest X-ray source, CX1, appears to be consistent with the nominal position of the classic nova Ophiuchi 1938 (Oph 1938), and both Oph 1938 and CX1 are consistent with a UV-bright variable HST counterpart, which we argue to be the source of the nova eruption in 1938. This makes Oph 1938 the second classic nova recovered in a Galactic GC since Nova T Scorpii in M80. CX2 is consistent with the steep-spectrum radio source VLA8, which unambiguously matches a faint blue source; the steepness of VLA8 is suggestive of a pulsar nature, possibly a transitional millisecond pulsar with a late K dwarf companion, though an active galactic nucleus (AGN) cannot be ruled out. The other counterparts to the X-ray sources are all suggestive of chromospherically active binaries or background AGNs, so their nature requires further membership information. [ABSTRACT FROM AUTHOR]

Observations of the dwarf nova SS Cyg were made in 2019–2021 at different brightness levels () both at the brightness decay stage after the outburst maximum, and in the quite state between outbursts. Data were obtained in ( observations, 3 sets) and V ( points, 22 sets) bands. The value of the system's orbital period obtained in 2019–2021 () used in this study is 0.4% less than the value obtained more than a quarter of a century ago (1983–1996). The time resolution between two successive measurements is 6–14 s depending on the equipment used. An extensive database of new observational data allowed us to perform a quantitative analysis of observations. Analysis of the data after taking into account orbital variability and other trends associated with changes in the system's emission flux during the night showed the presence of cyclic fluctuations in brightness, usually 4–10 events per orbital cycle – flickering. For most series of observations, the Lafleur-Kinman method determined such a value of the oscillation period at which convolution of observations with it showed a single wave. The obtained values of the characteristic flickering times and their amplitudes show their dependence on the average brightness level of the system. With increasing luminosity of the system, both of these quantities decreased linearly. From the size ratios of the binary system components of SS Cyg, it was shown that the source of flickering is located in the interaction region of the gas flow with the near-disk halo: only this region in the SS Cyg system with parameters ( , , ), defined by the authors earlier, can be eclipsed at large radii disk, and is clearly visible in all other orbital phases of the system. [ABSTRACT FROM AUTHOR]

Disc winds are a common feature in accreting astrophysical systems on all scales. In active galactic nuclei (AGNs) and accreting white dwarfs (AWDs), specifically, radiation pressure mediated by spectral lines is a promising mechanism for driving these outflows. Previous hydrodynamical simulations have largely supported this idea, but relied on highly approximate treatments of ionization and radiative transfer. Given the sensitivity of line driving to the ionization state and radiation field in the outflow, here, we present a new method for carrying out 2.5D radiation hydrodynamic simulations that takes full account of the frequency-dependent radiative transfer through the wind, the corresponding ionization state, and the resulting radiative accelerations. Applying our method to AWDs, we find that it is much harder to drive a powerful line-driven outflow when the interaction between matter and radiation is treated self-consistently. This conclusion is robust to changes in the adopted system parameters. The fundamental difficulty is that discs luminous enough to drive such a wind are also hot enough to overionize it. As a result, the mass-loss rates in our simulations are much lower than those found in earlier, more approximate calculations. We also show that the ultraviolet spectra produced by our simulations do not match those observed in AWDs. We conclude that, unless the overionization problem can be mitigated (e.g. by subgrid clumping or a softer-than-expected radiation field), line driving may not be a promising mechanism for powering the outflows from AWDs. These conclusions are likely to have significant implications for disc winds in AGN also. [ABSTRACT FROM AUTHOR]

We present a systematic search for periodic X-ray sources in 10 Galactic globular clusters (GCs) utilizing deep archival Chandra observations. By applying the Gregory–Loredo algorithm, we detect 28 periodic signals among 27 independent X-ray sources in 6 GCs, which include 21 newly discovered ones in the X-ray band. The remaining 4 GCs exhibit no periodic X-ray sources, mainly due to a relatively lower sensitivity of the data. Through analysis of their X-ray timing and spectral properties, complemented with available optical and ultraviolet information, we identify 21 of these periodic sources as cataclysmic variables (CVs). Combining with 11 periodic CVs in 47 Tuc similarly identified in the X-ray band, we compile the most comprehensive sample to date of GC CVs with a probable orbital period. The scarcity of old, short-period CVs in GCs compared to the Galactic inner bulge and solar neighbourhood, can be attributed to both a selection effect favouring younger, dynamically formed systems and the hindrance of CV formation through primordial binary evolution by stellar dynamical interactions common to the GC environment. Additionally, we identify a significant fraction of the GC CVs, most with an orbital period below or within the CV period gap, as probable magnetic CVs, but in the meantime there is a deficiency of luminous intermediate polars in the GC sample compared to the solar neighbourhood. [ABSTRACT FROM AUTHOR]

We conduct numerical simulations of multiple nova eruptions in detached, widely separated symbiotic systems that include an asymptotic giant branch (AGB) companion to investigate the impact of white dwarf (WD) mass and binary separation on the evolution of the system. The accretion rate is determined using the Bondi–Hoyle–Lyttleton method, incorporating orbital momentum loss caused by factors such as gravitational radiation, magnetic braking, and drag. The WD in such a system accretes matter coming from the strong wind of an AGB companion until it finishes shedding its envelope. This occurs on an evolutionary time-scale of ≈3 × 105 yr. Throughout all simulations, we use a consistent AGB model with an initial mass of 1.0 M⊙ while varying the WD mass and binary separation, as they are the critical factors influencing nova eruption behaviour. We find that the accretion rate fluctuates between high and low rates during the evolutionary period, significantly impacted by the AGB's mass loss rate. We show that unlike novae in cataclysmic variables, the orbital period may either increase or decrease during evolution, depending on the model, while the separation consistently decreases. Furthermore, we have identified cases in which the WDs produce weak, non-ejective novae and experience mass gain. This suggests that provided the accretion efficiency can be achieved by a more massive WD and maintained for long enough, they could potentially serve as progenitors for type Ia supernovae. [ABSTRACT FROM AUTHOR]

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

Context. Recently, large and homogeneous samples of cataclysmic variables identified by the Sloan Digital Sky Survey (SDSS) were published. In these samples, the famous orbital period gap, which is a dearth of systems in the orbital period range ∼2 − 3 h and the defining feature of most evolutionary models for cataclysmic variables, has been claimed not to be clearly present. If true, this finding would completely change our picture of cataclysmic variable evolution. Aims. In this Letter we focus on potential differences with respect to the orbital period gap between cataclysmic variables in which the magnetic field of the white dwarf is strong enough to connect with that of the donor star, so-called polars, and non-polar cataclysmic variables as the white dwarf magnetic field in polars has been predicted to reduce the strength of angular momentum loss through magnetic braking. Methods. We separated the SDSS I–IV sample of cataclysmic variables into polars and non-polar systems and performed statistical tests to evaluate whether the period distributions are bimodal as predicted by the standard model for cataclysmic variable evolution or not. We also compared the SDSS I–IV period distribution of non-polars to that of other samples of cataclysmic variables. Results. We confirm the existence of a period gap in the SDSS I–IV sample of non-polar cataclysmic variables with > 98% confidence. The boundaries of the orbital period gap are 147 and 191 min, with the lower boundary being different to previously published values (129 min). The orbital period distribution of polars from SDSS I–IV is clearly different and does not show a similar period gap. Conclusions. The SDSS samples as well as previous samples of cataclysmic variables are consistent with the standard theory of cataclysmic variable evolution. Magnetic braking does indeed seem get disrupted around the fully convective boundary, which causes a detached phase during cataclysmic variable evolution. In polars, the white dwarf magnetic field reduces the strength of magnetic braking and consequently the orbital period distribution of polars does not display an equally profound and extended period gap as non-polars. It remains unclear why the breaking rates derived from the rotation of single stars in open clusters favour prescriptions that are unable to explain the orbital period distribution of cataclysmic variables. [ABSTRACT FROM AUTHOR]