Search

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]

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]

IW And-type dwarf novae are anomalous Z Cam stars featured with outbursts happening during standstill states, which are not expected in the standard disc instability model. The physical mechanisms for these variations remain unclear. In this study, we report the discovery of a new candidate IW And-type dwarf nova J0652+2436, identified with its frequent outbursts from the slowly rising standstill states. Luckily, the Transiting Exoplanet Survey Satellite observations during a long standstill state and the earlier K2 observations give a chance to find the orbital and negative superhump period in the light curve of J0652+2436, allowing the measurement of its mass ratio of 0.366. This mass ratio is marginally possible for the tidal instability to set in according to previous smoothed particle hydrodynamics simulations. Thus, we propose that the outbursts in J0652+2436 are likely to be caused by the growing accretion disc during standstills, in favour of the previous hypothesis of the mechanisms lying in all IW And stars. We conclude that J0652+2436 might be the first IW And star with both a precessing tilted disc and tidal instability, which will be an important laboratory for studying the accretion disc dynamics and help understand the IW And phenomenon. [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]

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 the light curve of the 2020 outburst of PQ Andromedae, which was only the fourth confirmed on record, with a mean interval of 27 years. The outburst was detected at magnitude 10.5, representing an amplitude of -8.6 magnitudes above mean quiescence of 19.1. The main part of the outburst lasted about 53 days, although it is likely that the first part was missed. The system underwent five dramatic rebrightening episodes during the decline phase, the longest appearing as an eight-day plateau in the light curve. Long photometry runs were not possible due to the short observing window. Humps were detected during one run which resembled superhumps. The strongest signal in the photometry was at 17.835(35) cycles/d, or 0.05607(11) d. The system was still slightly above quiescence more than 800 days after the outburst started. The observed characteristics of PQ And are consistent with it being a member of the WZ Sge subclass. Moreover, in terms of outburst frequency and amplitude, orbital period, and light curve characteristics, PQ And appears to be a close sibling of WZ Sge itself. [ABSTRACT FROM AUTHOR]

The existence of a vast nova shell surrounding the prototypical dwarf nova Z Camelopardalis (Z Cam) proves that some old novae undergo metamorphosis to appear as dwarf novae thousands of years after a nova eruption. The expansion rates of ancient nova shells offer a way to constrain both the time between nova eruptions and the time for post-nova mass transfer rates to decrease significantly, simultaneously testing nova thermonuclear runaway models and hibernation theory. Previous limits on the expansion rate of part of the Z Cam shell constrain the inter-eruption time between Z Cam nova events to be >1300 yr. Deeper narrow-band imaging of the ejecta of Z Cam with the Condor Array Telescope now reveals very low surface brightness areas of the remainder of the shell. A second, even fainter shell is also detected, concentric with and nearly three times the size of the 'inner' shell. This is the first observational support of the prediction that concentric shells must surround the frequently erupting novae of relatively massive white dwarfs. The Condor images extend our Z Cam imaging baseline to 15 yr, yielding the inner shell's expansion rate as v  = 83 ± 37 km s−1 at 23 deg south of west, in excellent agreement with our 2012 prediction. This velocity corresponds to an approximate age of |$t = 2672^{-817}_{+2102}$| yr. While consistent with the suggestion that the most recent nova eruption of Z Cam was the transient recorded by Chinese imperial astrologers in the year 77 bce , the age uncertainty is still too large to support or disprove a connection with Z Cam. [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]

Cataclysmic variables (CV) encompass a diverse array of accreting white dwarf binary systems. Each class of CV represents a snapshot along an evolutionary journey, one with the potential to trigger a type Ia supernova event. The study of CVs offers valuable insights into binary evolution and accretion physics, with the rarest examples potentially providing the deepest insights. However, the escalating number of detected transients, coupled with our limited capacity to investigate them all, poses challenges in identifying such rarities. Machine learning (ML) plays a pivotal role in addressing this issue by facilitating the categorization of each detected transient into its respective transient class. Leveraging these techniques, we have developed a two-stage pipeline tailored to the Zwicky Transient Facility transient alert stream. The first stage is alerts filter aimed at removing non-CVs, while the latter is an ML classifier produced using Extreme Gradient Boosting, achieving a macro average area under the curve score of 0.92 for distinguishing between CV classes. By utilizing the generative topographic mapping algorithm with classifier posterior probabilities as input, we obtain representations indicating that CV evolutionary factors play a role in classifier performance, while the associated feature maps present a potent tool for identifying the features deemed most relevant for distinguishing between classes. Implementation of the pipeline in 2023 June yielded 51 intriguing candidates that are yet to be reported as CVs or classified with further granularity. Our classifier represents a significant step in the discovery and classification of different CV classes, a domain of research still in its infancy. [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]

The population of optical transients evolving within a time-scale of a few hours or a day (so-called fast optical transients, FOTs) has recently been debated extensively. In particular, our understanding of extragalactic FOTs and their rates is limited. We present a search for extragalactic FOTs with the Tomo-e Gozen high-cadence survey. Using the data taken from 2019 August to 2022 June, we obtain 113 FOT candidates. Through light curve analysis and cross-matching with other survey data, we find that most of these candidates are in fact supernovae, variable quasars, and Galactic dwarf novae that were partially observed around their peak brightness. We find no promising candidate of extragalactic FOTs. From this non-detection, we obtain upper limits on the event rate of extragalactic FOTs as a function of their time-scale. For a very luminous event (absolute magnitude M < −26 mag), we obtain the upper limits of 4.4 × 10−9 Mpc−3 yr−1 for a time-scale of 4 h, and 7.4 × 10−10 Mpc−3 yr−1 for a time-scale of 1 d. Thanks to our wide (although shallow) surveying strategy, our data are less affected by the cosmological effects, and thus, give one of the more stringent limits to the event rate of intrinsically luminous transients with a time-scale of <1 d. [ABSTRACT FROM AUTHOR]

Observations, mainly of outbursts in dwarf novae, imply that the anomalous viscosity in highly ionized accretion discs is magnetic in origin and requires that the plasma ${\beta \sim 1}$. Until now, most simulations of the magnetic dynamo in accretion discs have used a local approximation (known as the shearing box). While these simulations demonstrate the possibility of a self-sustaining dynamo, the magnetic activity generated in these models saturates at $\beta \gg 1$. This long-standing discrepancy has previously been attributed to the local approximation itself. There have been recent attempts at simulating magnetic activity in global accretion discs with parameters relevant to the dwarf novae. These too find values of $\beta \gg 1$. We speculate that the tension between these models and the observations may be caused by numerical magnetic diffusivity. As a pedagogical example, we present exact time-dependent solutions for the evolution of weak magnetic fields in an incompressible fluid subject to linear shear and magnetic diffusivity. We find that the maximum factor by which the initial magnetic energy can be increased depends on the magnetic Reynolds number as ${\mathcal {R}}_{m}^{2/3}$. We estimate that current global numerical simulations of dwarf nova discs have numerical magnetic Reynolds numbers around six orders of magnitude less than the physical value found in dwarf nova discs of ${\mathcal {R}}_{m} \sim 10^{10}$. We suggest that, given the current limitations on computing power, expecting to be able to compute realistic dynamo action in observable accretion discs using numerical MHD is, for the time being, a step too far. [ABSTRACT FROM AUTHOR]

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

Using the Catalina Sky Survey and Gaia alert data bases, we have carried out a systematic search for long-term variability of the brightness of dwarf novae in quiescence. A small, but significant portion of them revealed variability on time-scales between 2 and 14 yr which can be fitted, in some of the targets, by linear slopes of 0.14 mag yr−1 on average and up to 0.6 mag yr−1 in extreme cases. Another group of dwarf novae showed quasi-periodic variations in their quiescent brightness, with periods of 3.7 yr on average and up to 13 yr in extreme cases, and mean amplitudes of 0.5 mag, reaching up to 1.3 mag. The observed slopes could be understood as portions of a quasi-sinusoidal variability with larger periods, not covered by our data. SU UMa stars have, on average, smaller slope values and longer periods, compared to other dwarf novae. These variations could be interpreted as caused by magnetic activity of the secondary companion star, similar to the 11-yr solar cycle, but one could consider also other origins of this phenomenon. [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]

Recent advances in time-domain astronomy have led to fresh observational insights into intermediate polars, a subtype of magnetic cataclysmic variables generally accreting via a partial accretion disc. These new discoveries include detections of superhumps, low states, and outbursts. However, these studies have largely relied on relative photometry. Here, we tabulate the absolute G magnitudes of confirmed intermediate polars, plot them against their orbital periods, and compare the results to similar studies of dwarf novae during quiescence and in outburst. This exercise suggests the presence of two distinct luminosity classes of intermediate polars, with practical and physical implications for the studies of low states and outbursts. In particular, we point out that two of the optically luminous systems showing short outbursts are also seen to exhibit superhumps, suggesting that they may be caused by the same underlying mechanism. [ABSTRACT FROM AUTHOR]

We present photometry of the 2021 outburst of LL And, which was only the fourth confirmed on record, with a mean interval between outbursts of 14.0 years. The outburst amplitude was -7.8mag, reaching magnitude 12.8 at maximum, and the main part lasted about 30 days, although the object was still slightly above quiescence some 70 days after the outburst started. Super umps appeared nine days into the outburst, confirming that this was a super outburst. The super umps grew to a maximum peak-to-peak amplitude of 0.32mag and a period of 0.056633(180)d. We estimate the mass ratio of the secondary to the white-dwarf primary as q = 0.13. The photometric behavior, short orbital period and long interval between outbursts are consistent with LL And being a member of the WZ Sge subclass of dwarf novae. [ABSTRACT FROM AUTHOR]

The disc instability mechanism (DIM) is widely accepted to account for the transient behaviour of dwarf novae (DNe), which experience short outbursts separated by long quiescence. The duty cycle (the ratio between the outburst duration and the recurrence time) determines the amount of accreted mass by the white dwarf (WDs) during outbursts, thus playing an important role in the long-term binary evolution. Employing the code of Modules for Experiments in Stellar Astrophysics, we systemically investigate the influence of the duty cycles on the evolution of DNe and the mass growth of accreting carbon-oxygen (CO) WDs. Our calculations show that, while the DIM can considerably influence the accretion process, efficient WD-mass growth requires a particular range of the duty cycle. For WDs with the initial masses of 0.6, 0.7, and 1.1 M⊙, these duty cycles are 0.006 |$\, \le$| d |$\, \le$| 0.007, d   =  0.005, and , d   = 0.003 and the accumulated mass of the WDs can reach 0.1, 0.13, and 0.21 M⊙, respectively. In all of our simulations, no CO WDs can grow their masses to the explosion mass of Type Ia supernovae of about 1.38 M⊙. Because of a much short time-scale of the outburst state, the final donor-star masses and orbital periods are insensitive to the duty cycles. Therefore, we propose that the DIM in DNe could alleviate the WD mass problem to some extent. [ABSTRACT FROM AUTHOR]

Disc accretion rate onto low mass protostar FU Ori suddenly increased hundreds of times 85 yr ago and remains elevated to this day. We show that the sum of historic and recent observations challenges existing FU Ori models. We build a theory of a new process, Extreme Evaporation (EE) of young gas giant planets in discs with midplane temperatures of ≳ 30 000 K. Such temperatures are reached in the inner 0.1 AU during thermal instability bursts. In our 1D time-dependent code the disc and an embedded planet interact through gravity, heat, and mass exchange. We use disc viscosity constrained by simulations and observations of dwarf novae instabilities, and we constrain planet properties with a stellar evolution code. We show that dusty gas giants born in the outer self-gravitating disc reach the innermost disc in a ∼ O (104) yr with radius of ∼10 R J. We show that their EE rates are |$\gtrsim 10^{-5} {\rm {\rm M}_{\odot }}$|  yr−1; if this exceeds the background disc accretion activity then the system enters a planet-sourced mode. Like a stellar secondary in mass-transferring binaries, the planet becomes the dominant source of matter for the star, albeit for ∼ O (100) yr. We find that a ∼6 Jupiter mass planet evaporating in a disc fed at a time-averaged rate of |$\sim 10^{-6} {\rm {\rm M}_{\odot }}$|  yr−1 appears to explain all that we currently know about FU Ori accretion outburst. More massive planets and/or planets in older less massive discs do not experience EE process. Future FUOR modelling may constrain planet internal structure and evolution of the earliest discs. [ABSTRACT FROM AUTHOR]

White dwarfs that accrete from non-degenerate companions show anomalous carbon and nitrogen abundances in the photospheres of their stellar components have been postulated to be descendants of supersoft X-ray binaries. Measuring the carbon-to-nitrogen abundance ratio may provide constraints on their past evolution. We fit far-ultraviolet spectroscopy of the cataclysmic variable HS 0218 + 3229 taken with the Cosmic Origins Spectrograph using Markov chain Monte Carlo methods, and found the carbon-to-nitrogen ratio is about one tenth of the Solar value |$(\rm{\log \mathrm{[C/N]}}=-0.56\pm 0.15)$|⁠. We also provide estimates of the silicon and aluminium abundances, and upper limits for iron and oxygen. Using the parameters we derived for HS 0218 + 3229 we reconstruct its past. We calculated a grid of mesa models and implemented Gaussian process fits in order to determine its most likely initial binary configuration. We found that an initial mass of the donor of |$M_{\rm donor;i}=0.90-0.98,\rm{\mathrm{M}_{\odot }}$| and an initial orbital period of P orb; i = 2.88 d (P orb; i = 3.12–3.16 d) for an assumed initial white dwarf mass of |$\rm{M_{\mathrm{WD}}}_\mathrm{;i}=0.83\, \rm{\mathrm{M}_{\odot }}$| |$(\rm{M_{\mathrm{WD}}}_{\rm ;i}=0.60\, \rm{\mathrm{M}_{\odot }})$| can replicate the measured parameters. The low mass ratio, |$M_{\rm donor;i} / \rm{M_{\mathrm{WD}}}_{\rm ;i} =1.08-1.18\, (1.5-1.63)$|⁠ , suggests that the system did not go through a phase of hydrogen-burning on the white dwarf's surface. However, we can not exclude a phase of thermal time-scale mass transfer in the past. We predict that HS 0218 + 3229 will evolve below the ≃ 76.2 ± 1 min period minimum for normal cataclysmic variables. [ABSTRACT FROM AUTHOR]

The 4U 1820-30 is an ultra-compact low-mass X-ray binary (LMXB) near the center of the globular cluster NGC 6624. Its negative orbital period derivative, observed from the phase evolution of its sinusoidal-like orbital variation, contradicts the positive value that was obtained from the theoretical prediction. In this paper, we present the analysis of the 4U 1820-30 orbital modulation from light curves obtained from the Neutron star Interior Composition ExploreR (NICER) observations from 2017 to mid-2022. Combined with historical records, the orbital derivative is measured from the orbital phase evolution between 1976 and 2002 is P ̇ / P = (− 5.21 ± 0.13) × 10 − 8 yr−1. No significant second order orbital period derivative is detected with a 2 σ upper limit of ∣ P ̈ ∣ < 5.48 × 10 − 22 s s−2. We discuss the possible intrinsic orbital period derivative of 4U 1820-30 and suggest that this binary system may have a significant mass outflow similar to some other LMXBs. In addition, a periodic modulation with a period of 691.6 ± 0.7 s, which is consistent with the superhump period discovered in the far-ultraviolet band of the Hubble Space Telescope, was also detected in the X-ray light curves that were collected by NICER. We conclude that this modulation is probably caused by a period of 0.8 ± 0.1 day apsidal precession of the accretion disk, similar to the SU UMa type dwarf novae and some LMXBs. However we cannot exclude the possibility that it is induced by a hierarchical third star that orbits around the binary system. [ABSTRACT FROM AUTHOR]

We present a new study of the Z Cam-type eclipsing cataclysmic variable AY Piscium with the aim of determining the fundamental parameters of the system and the structure of the accretion flow therein. We use time-resolved photometric observations supplemented by spectroscopy in the standstill, to which we applied our light-curve modeling techniques and the Doppler tomography method, to update system parameters. We found that the system has a massive white dwarf M WD = 0.90(4) M ☉, a mass ratio q = 0.50(3), and the effective temperature of a secondary T 2 = 4100(50) K. The system inclination is i = 74.°8(7). The orbital period of the system P orb = 0.217320523(8) day is continuously increasing at a rate of P ̇ orb = + 7.6 (5) × 10 − 9 day yr−1. The mass-transfer rate varies between 2.4 × 10−10 M ⊙ yr−1 in quiescence up to 1.36 × 10−8 M ⊙ yr−1 in outburst. The accretion disk transitions from the cooler, flared, steady-state disk to a warmer state with a practically constant and relatively high disk height. The mass-transfer rate is about 1.6 × 10−9 M ⊙ yr−1 in the standstill. The Balmer emission lines show a multicomponent structure similar to that observed in long-orbital-period nova-like systems. Out of standstill, the system exhibits outburst bimodality, with long outbursts being more prominent. We conclude that the Balmer emission lines in AY Psc are formed by the combination of radiation from the irradiated surface of the secondary, from the outflow zone, and from winds originating in the bright spot and the disk's inner part. [ABSTRACT FROM AUTHOR]

We present a new study of the eclipsing cataclysmic variable SDSS J154453.60+255348.8 to determine the object's nature and its system parameters together with the probe of the accretion flow structure in the system. Based on analyses of new simultaneous time-resolved photometric and spectroscopic observations of SDSS J154453.60+255348.8 and using our light-curve modelling techniques and the Doppler tomography method, we found that the system contains a white dwarf with the mass of M WD = 0.62(7) M⊙ and an evolved red dwarf as a secondary. The system inclination is close to 90° and the mass ratio is q = 0.49(2). The secondary has an effective temperature T 2 = 3400(40) K and a radius about 1.35(15) times larger than a zero-age main-sequence star with similar mass. From observation of the system in high- and low-brightness states, we conclude that SDSS J154453.60+255348.8 is a long-orbital period VY Scl-type system. The accretion disc in the high state is about two times less than the truncation radius, and is completely missing during the low state of the system. [ABSTRACT FROM AUTHOR]

Photometric studies of the dwarf nova ASASSN-19fy in the "period gap" were carried out in 2020-2021 over 24 nights at the Crimean Astrophysical Observatory of the Russian Academy of Sciences and 3 nights at the Sanglokh International Astronomical Observatory of the Institute of Astrophysics, National Academy of Sciences of Tajikistan. The observations covered a superoutburst, two successive rebrightenings, and a slow return to the pre-outburst (quiescent) state. During this time, superhumps were observed, in the evolution of which the stages of developed superhumps "B," their damping "C," and a transition between them were identified. The average period of the superhumps in stage "B" was 0.09278(13) days and it was found its increase during this stage at a rate of (dP / dT) / P = 10·10–5. In stage "C" the period of the superhumps was equal to 0.092289(15) days. It is shown that ASASSN-19fy is the twelfth object to join the group of long-period dwarf novae similar to WZ Sge-type stars. [ABSTRACT FROM AUTHOR]

Superoutbursts in WZ Sge-type dwarf novae (DNe) are characterized by both early superhumps and ordinary superhumps originating from the 2 : 1 and 3 : 1 resonances, respectively. However, some WZ Sge-type DNe show a superoutburst lacking early superhumps; it is not well established how these differ from superoutbursts with an early superhump phase. We report time-resolved photometric observations of the WZ Sge-type DN V627 Peg during its 2021 superoutburst. The detection of ordinary superhumps before the superoutburst peak highlights that this 2021 superoutburst of V627 Peg, like that in 2014, did not feature an early superhump phase. The duration of stage B superhumps was slightly longer in the 2010 superoutburst accompanied by early superhumps than that in the 2014 and 2021 superoutbursts, which lacked early superhumps. This result suggests that an accretion disk experiencing the 2 : 1 resonance may have a larger mass at the inner part of the disk and hence needs more time for the inner disk to become eccentric. The presence of a precursor outburst in the 2021 superoutburst suggests that the maximum disk radius should be smaller than that of the 2014 superoutburst, even though the duration of quiescence was longer than that before the 2021 superoutburst. This could be accomplished if the 2021 superoutburst was triggered as an inside-out outburst or if the mass transfer rate in quiescence changes by a factor of two, suggesting that the outburst mechanism and quiescence state of WZ Sge-type DNe may have more variety than ever thought. [ABSTRACT FROM AUTHOR]

We present the results of model analysis of the optical spectra of SU UMa and WZ Sge-type dwarf novae TY Psc, FL Psc, and V455 And. Spectroscopic observations were carried out at the SAO RAS 6-m BTA telescope in 2013. The selection criterion for the systems under study is their existence in a quiescent state with an optically thin accretion disk and dominance of white dwarf radiation. The fulfillment of the criterion was verified by observing the H I Balmer series broad absorption lines simultaneously with two-peak H I, He I, and Fe II emissions. A technique for automated fitting of model spectra of white dwarfs and observed spectra of dwarf novae based on quantitative criteria for their matching is proposed and implemented. The atmospheric parameters (effective temperature and surface gravity ) of accretors in TY Psc, FL Psc, and V455 And were determined applying it and used further to obtain their masses and radii. Due to the technique proposed here we obtained the estimates of the fundamental parameters of the secondaries using the theoretical "mass–radius" relationships for brown and red main-sequence dwarfs. As a result, we demonstrate the possibility in principle of determining all the main parameters of the SU UMa and WZ Sge dwarf novae based on modeling and the analysis of a small number of their spectra in the quiescent state. [ABSTRACT FROM AUTHOR]

During the British Astronomical Association (BAA) 2022 campaign, 27,436 photometric observations of the dwarf nova (DN) CG Draconis were made, with 106 eclipses recorded. This work summarizes the new data available and provides updated ephemeris and commentary on the observed eclipse profiles. The orbital period found is Porb=4h31m38s±1s$$ {P}_{orb}={4}^{\mathrm{h}}{31}^{\mathrm{m}}{38}^{\mathrm{s}}\pm {1}^{\mathrm{s}} $$. Two types of quasi‐periodic outbursts are identified: normal outbursts, of ΔV≈1.25$$ \Delta V\approx 1.25 $$ mag amplitude, and bright, of ΔV≈1.5$$ \Delta V\approx 1.5 $$ mag. The pattern resembles superoutbursts of SU UMa‐type DNe, however, no presence of superhumps characterizing these DNe was found. Given CG Dra is located above the period gap, it may represent a new intermediary subtype between SS Cyg and SU UMa‐type stars, or provide support to superoutburst models that do not rely on eccentric accretion disks. [ABSTRACT FROM AUTHOR]

EX Draconis is an eclipsing dwarf nova that shows outbursts with moderate amplitude (≃2 mag) and a recurrence timescale of ≃20–30 days. Dwarf novae outbursts are explained in terms of either a thermal-viscous instability in the disc or an instability in the mass-transfer rate of the donor star (MTIM). We developed simulations of the response of accretion discs to events of enhanced mass transfer, in the context of the MTIM, and applied them to model the light curve and variations in the radius of the EX Dra disc throughout the outburst. We obtain the first modeling of a dwarf nova outburst by using χ ^2 to select, from a grid of simulations, the best-fit parameters to the observed EX Dra outbursts. The observed time evolution of the system brightness and the changes in the radius of the outer disc along the outburst cycle are satisfactorily reproduced by a model of the response of an accretion disc with a viscosity parameter α = 4.0 and a quiescent mass-transfer rate ${\dot{M}}_{2}({\rm{quiescence}})=4.0\times {10}^{16}$ g s ^−1 to an event of width Δ t = 6.0 × 10 ^5 s (∼7 days) where the mass-transfer rate increases to ${\dot{M}}_{2}({\rm{outburst}})=1.5\times {10}^{18}$ g s ^−1 .

Tilted disk precession exists in different objects. Negative superhumps (NSHs) in cataclysmic variable stars are believed to arise from the interaction between the reverse precession of a tilted disk and the streams from the secondary star. Utilizing Transiting Exoplanet Survey Satellite photometry, we present a comprehensive investigation into the tilted disk precession and NSHs in the dwarf nova (DN) HS 2325+8205, employing eclipse minima, eclipse depths, NSH frequencies, and NSH amplitudes and the correlation between them as the windows. We identified NSHs with a period of 0.185671(17) day in HS 2325+8205. The NSH frequency exhibits variability with a period of 3.943(9) days, akin to the tilted disk precession period validated in nova-like stars (SDSS J0812) and intermediate polars (IPs; TV Col). The O − C of the eclipse minima were similarly found to vary cyclically in a period of 4.135(5) days, characterized by a faster rise than fall. Furthermore, the NSH amplitude exhibits complex and diverse variations, which may be linked to changes in the disk radius, the mass transfer rate, and the apparent area of the hot spot. For the first time in DNe, we observe biperiodic variations in eclipse depth ( P _1 = 4.131(4) days and P _2 = 2.065(2) days ≈ P _prec /2) resembling those seen in IPs, suggesting that variations with P _2 are not attributable to an accretion curtain, as previously suspected. Moreover, NSH amplitude and eclipse depth decrease with increasing NSH frequency, while NSH amplitude correlates positively with eclipse depth. These complex variations observed across multiple observational windows provide substantial evidence for the understanding of tilted disk precession and NSHs.

We present a photometric study of EX Dra, a dwarf nova that has been extensively observed by the Transiting Exoplanet Survey Satellite. The data reveal the occurrence of 20 complete outbursts, exhibiting several intriguing and rare characteristics. The light curves exhibit a distinct superorbital signal with a period of approximately P _sor ∼ 4.39(7) days, along with a negative superhump showing an approximate period of P _nsh ∼ 4.805(1) hr, indicating that the accretion disk is tilted and undergoing precession with the period of P _sor . In addition, the time-varying nature of P _sor suggests that the precession period is fluctuating.The eclipsing light minima O – C analysis during quiescence shows an oscillation with period of 3.9(5) days, which is a little shorter than the superorbital period. We contend that this is unlikely to be a sudden alteration of the orbital period, but rather, it is influenced by the tilt and precession of the accretion disk. Notably, we found an amplitude shift in the outburst behavior from 3.5 mag with a periodicity of about 26 days to an amplitude of around 2.5 mag with a periodicity of about 12 days, which persisted for 14 yr before reverting. Furthermore, we have extracted quasiperiodic oscillations in the plateau at the noneclipsed phases, characterized by periods ranging between 37 and 40 minutes.

We present the discovery of a peculiar dwarf nova KSP-OT-201712a using high-cadence, multicolor observations made with the Korea Microlensing Telescope Network. KSP-OT-201712a exhibits a rare presence of outbursts during standstills, as well as strong H α emission for a dwarf nova below the period minimum with an orbital period of 58.75 ± 0.02 minutes. The outburst cycles are ∼6.6 days within standstills but increase to ∼15 days outside of them. Both B − V and V − I colors become bluer and redder as the outburst luminosities increase and decrease, respectively, for the outburst within standstill, while they evolve in the opposite directions outside of the standstills. The presence of strong double-peaked H α and weak He i emission lines with He/H flux ratio of 0.27, together with absorption lines of Mg b and Na D in the source, leads to the estimation T _eff ≃ 4570 ± 40 K, [Fe/H] ≃ 0.06 ± 0.15 dex, and log g ≃ 4.5 ± 0.1 for its secondary. KSP-OT-201712a is the second He-deficient dwarf nova below the period minimum, while the temperature of the secondary is measured for the first time in such objects. We identify it to be an ER UMa-type dwarf nova, suggesting that the evolution of dwarf novae across the period minimum is accompanied by large mass transfers. The high temperature of the secondary indicates that the system started its mass transfer when the secondary was about 93% of its main-sequence age. The system will evolve to a helium cataclysmic variable or to AM CVn once its hydrogen envelope is exhausted before it explodes as a Type Ia supernova.

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

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

We investigate K2BS5, an optical transient that we identified in Campaign 13 of the Kepler/K2 archives by the K2 Background Survey, and classify it as a new SU UMa-type dwarf nova. Using the light curve generated from Keplers long-cadence observation mode, we analyze the dwarf nova during quiescence and superoutburst. Following 20 days of quiescence at the start of the observation, the system entered a superoutburst lasting 12 days, after which it experienced at least one rebrightening. K2BS5 clearly meets the criteria for an SU UMa star, but at the peak of the superoutburst, it also shows double-wave oscillations consistent with the spectroscopic orbital period, a phenomenon that closely resembles early superhumps in WZ Sge stars. While we do not classify K2BS5 as a WZ Sge system, we discuss how this phenomenon could complicate efforts to use the suspected detection of early superhumps to distinguish SU UMa-type dwarf novae from the recently recognized class of long-orbital-period WZ Sge systems.

The document discusses the importance of colloquia and seminars in research facilities as a way to discuss current research and trends. These sessions have traditionally been held in person but have now transitioned to hybrid sessions or webinars due to the COVID-19 pandemic. The document provides information on upcoming colloquia and seminars, including topics such as dark energy, optical outflow signatures in dwarf novae, and tracing neutral hydrogen across cosmic time. The speakers come from various institutions around the world, highlighting the global nature of astronomical research. [Extracted from the article]

Superhumps are among the abundant variable phenomena observed in the light curves of cataclysmic variables (CVs). They come in two flavours as positive and negative superhumps, distinguished by periods slightly longer or shorter, respectively, than the orbital periods of these interacting binary systems. Positive superhumps are ubiquitous in superoutbursting short-period dwarf novae of the SU UMa type but are less common in longer period systems with accretion discs in a permanent bright state, such as novalike variables and most old novae. Negative superhumps do not seem to have a preference for a particular type of CV. Here, I take advantage of the long high-cadence light curves provided by TESS for huge number of stars, selecting all old novae and novalike variables with past reported superhumps for which TESS light curves are available and have not yet been analysed in previous publications in order to study their superhump behaviour. In combination with information taken from the literature, the results enable to compile the most complete census of superhumps in these stars so far. As a corollary, for the eclipsing systems in the present sample of objects, eclipse epochs derived from the TESS light curves and in some cases from archival light curves are listed and used to update orbital ephemeris and to discuss period changes. [ABSTRACT FROM AUTHOR]

The prototype dwarf nova SS Cyg unexpectedly exhibited an anomalous event in its light curve in the early few months of 2021 in which regular dwarf nova-type outbursts stopped, and only small-amplitude fluctuations occurred. Inspired by this event, we have performed numerical simulations of light curves of SS Cyg by varying mass transfer rates and varying viscosity parameters in the cool disk. We have also studied the effect of gas-stream overflows beyond the outer disk edge in the light curve simulations. We have confirmed that the enhanced mass transfer is not likely to be responsible for the 2021 anomalous event or its forerunner. We have found that the enhancement of the viscosity in the disk may reproduce the forerunner of that event but may not be sufficient to explain the 2021 anomalous event, although the latter result might be particular to the thermal equilibrium curve we used. Within our simulations, a model of the gas-stream overflow with a slightly higher mass transfer rate than that of our standard model reproduces light curves similar to the 2021 anomalous event. We suggest that the gas-stream overflow is necessary to reproduce that event. The gas-stream overflow may also be responsible for the abnormally high X-ray flux during the normal quiescent state in SS Cyg. [ABSTRACT FROM AUTHOR]

HS 2325+8205 is a long-period eclipsing dwarf nova with an orbital period above the period gap (P orb > 3 h) and is reported to be a Z Cam-type dwarf nova. We study the light variation and the quasi-periodic oscillations (QPOs) of HS 2325+8205 based on the photometry of the Transiting Exoplanet Survey Satellite (TESS). Using a continuous wavelet transform, a Lomb–Scargle periodogram (LSP), and sine fitting methods, we find for the first time that there is a QPO of ∼2160 s in the long outburst top light curves of HS 2325+8205. Moreover, we find that the oscillation intensity of the QPOs of HS 2325+8205 is related to the orbital phase, and the intensity in orbital phases 0.5–0.9 is stronger than in orbital phases 0.1–0.5. Therefore, the relationship between the oscillation intensity of QPOs and the orbital phase may become a topic of research into the origin of QPOs. In addition, we use the LSP to correct the orbital period of HS 2325+8205 to 0.19433475(6) d. [ABSTRACT FROM AUTHOR]

I report on a large-scale search for the orbital periods (P) of most known nova systems, by looking for significant, coherent, and stable optical photometric modulation in two or more independent light curves taken mostly from the large surveys of TESS ,  Kepler, AAVSO, SMARTS, OGLE, ASAS, and ZTF. I have discovered 31 new orbital periods. Further, I have measured new periods for 18 novae with evolved companions, to 30 per cent accuracy, as based on their spectral energy distribution. Also, I have confirmed, improved, and rejected prior claims for P in 46 novae. (As part of this effort, I recognize that 5 novae display 1–3 coherent, significant, and transient periodicities 0.12–4.1 d, with these being mysterious as not being the orbital, spin, or superhump periods). In all, I have compiled a comprehensive list of 156 reliable P values for novae. The histogram of nova periods shows a minimum P at 0.059 h (85 min), and a Period Gap from 0.071 to 0.111 d (1.70–2.66 h). The upper edge of the Period Gap is significantly different between novae (0.111 d), nova-like systems (0.131 d), and dwarf novae (0.141 d). A further issue from the histogram is that 31 per cent of nova systems have evolved companions, for which there has been no models or understanding for their current state or evolution. For the novae with red giant companions, 15-out-of-20 are in the bulge population, despite novae with main-sequence and subgiant companions having bulge fractions near 0.11–0.32. [ABSTRACT FROM AUTHOR]

We present the analysis of existing optical photometry and new optical spectroscopy of the candidate cataclysmic variable star OGLE-BLG504.12.201843. As was shown previously, this object has an orbital period of 0.523419 d and exhibits year-long outbursts with a mean period of 973 d. Using digitized photographic archives, we show that the earliest recorded outburst occurred in 1910. We propose that this object is a U Gem-type dwarf nova (DN) with extreme properties. The orbital variability of the system in outburst shows clear signs of an accretion disc, from which the outburst likely originates. During quiescence, the object slowly brightens by up to 0.75 mag in the I  band over 600 d before the outburst and exhibits small flares with amplitude ≲0.2 mag in the I  band. We interpret the gradual brightening as an increase in the luminosity and temperature of the accretion disc, which is theoretically predicted but only rarely seen in DNe. The origin of small flares remains unexplained. The spectra show Balmer absorption lines both in quiescence and outburst, which can be associated with a bright secondary star or a cold accretion disc. During outbursts, emission lines with full width at half-maximum of about 450 km s−1 appear; however, they lack typical double-peaked profiles. We suggest that either these lines originate in the disc winds or the orbital inclination is low, the latter being consistent with constrains obtained from the orbital variability of the system. Due to its extreme properties and peculiarities, OGLE-BLG504.12.201843 is an excellent object for further follow-up studies. [ABSTRACT FROM AUTHOR]

We present the light curve of the dwarf nova DDE 39 (= 1 RXS ]040253.8+435553) between 2011 October and 2021 January, which reveals two bright outbursts: one in 2018 January and the one in 2020 December which is discussed in this paper. There is also evidence of two fainter outbursts: one in 2012 january and another in 2020 October, reaching magnitude 16.3 and 16.0 respectively. We report digital photometry of the 2020 December outburst. At its brightest, the system reached magnitude 15.1 - about four magnitudes above quiescence - and the event lasted at least 11 days. Superhumps with peak-to-peak amplitude of up to 0.4 magnitudes were present during the outburst, thereby establishing it to be a member of the SU UMa family. The mean superhump period, based on a linear analysis of superhump maxima, was Psh = 0·08176(235)d Period analysis of the superhump signal yields Psh = 0·08199(24)d. [ABSTRACT FROM AUTHOR]

In the first days of WZ Sge-type dwarf nova (DN) outbursts, the 2 : 1 resonance induces a spiral arm structure in the accretion disk, which is observed as early superhumps in optical light curves. We reports on our optical observations of an eclipsing WZ Sge-type DN PNV J00444033+4113068 during its 2021 superoutburst using the 3.8 m Seimei telescope and through the Variable Star Network collaboration. The eclipse analysis showed that its orbital period was 0.055425534(1) d. Our observations confirmed early superhumps with an amplitude of 0.7 mag, the largest amplitude among known WZ Sge-type DNe. More interestingly, its early superhumps became the reddest around their secondary minimum, whereas other WZ Sge-type DNe show the reddest color around the early superhump maximum. The spectrum around the peak of the outburst showed two double-peaked emission lines of He  ii  4686 Å and Hα with a peak separation of ≥700 km s−1, supporting a very high-inclination system. With the early superhump mapping, the unique profile and color of the early superhump are successfully reproduced by an accretion disk with a vertically extended double arm structure. Therefore, a large amplitude and a unique color behavior of the early superhumps in PNV J00444033+4113068 can be explained by the 2 : 1 resonance model along with other WZ Sge-type DNe. [ABSTRACT FROM AUTHOR]

We present the results of the participation of the MASTER Global Robotic Network in the program to support the aLIGO (O1) and LIGO/Virgo (O2 and O3) gravitational-wave experiments in the electromagnetic channel. This study covers the first observing run O1 from September 2015 to January 2016, the second observing run O2 from November 2016 to August 2017 (only LIGO in January–July, joint LIGO/VIRGO (LVC) in August) and the third observing run O3 from April 2019 to April 2020. The main goal of these observations was to, for the first time in the history of astronomy, perform precise localization of gravitational-wave sources, which successfully resulted in the independent discovery of the kilonova using MASTER telescopes in the process of searching for the source of the GW170817 event. In many other events, dozens of optical transients unrelated to gravitational waves have been detected. Nevertheless, the experience of optical localization of gravitational waves is of exceptional importance for the development of a future successful strategy for the localization of gravitational wave events involving relativistic stars. In addition, objects detected in the analysis of huge areas in the sky, determined by the localization errors of the GW source, have been studied in particular detail by telescopes around the world. Objects such as supernovae, novae, active galactic nuclei, dwarf novae and other explosive phenomena in the Universe have been found and analyzed. More than 220 000 deg2 within the region of the most probable localization of a gravitational wave source have been explored by the MASTER Global Robotic Network. This paper reports on the observations of the MA-STER Global Robotic Network for all alert events from the O1, O2, and O3 observing runs. [ABSTRACT FROM AUTHOR]

T CrB is a symbiotic recurrent nova that exhibits quiescent and active phases between its classical nova eruptions. The statistical properties of these active phases have been poorly studied thus far. Because of that their nature remained unknown. Here we study statistical properties of the active phases and show that they are consistent with outburst and superoutbursts observed in SU UMa–type dwarf novae. The recurrence time of these outbursts is consistent with theoretical predictions for similar systems. Moreover, the visual and X-ray evolution of the last active phase is consistent with a superoutburst. This suggests that T CrB is a dwarf nova with an extremely long orbital period, closely related to SU UMa dwarf novae. The similarities between the last superoutburst and the reported activity preceding the 1946 nova eruption may suggest that the next classical nova eruption in T CrB could be indeed soon expected.

We present a detailed study of the 2019 outburst of the cataclysmic variable V1047 Cen, which hosted a classical nova eruption in 2005. The peculiar outburst occurred 14 yr after the classical nova event and lasted for more than 400 days, reaching an amplitude of around 6 magnitudes in the optical. Early spectral follow-up revealed what could be a dwarf nova (accretion disk instability) outburst. However, the outburst duration, high-velocity (>2000 km sâˆ'1) features in the optical line profiles, luminous optical emission, and presence of prominent long-lasting radio emission together suggest a phenomenon more exotic and energetic than a dwarf nova outburst. The outburst amplitude, radiated energy, and spectral evolution are also not consistent with a classical nova eruption. There are similarities between V1047 Cen’s 2019 outburst and those of classical symbiotic stars, but pre-2005 images of the field of V1047 Cen indicate that the system likely hosts a dwarf companion, implying a typical cataclysmic variable system. Based on our multiwavelength observations, we suggest that the outburst may have started with a brightening of the disk due to enhanced mass transfer or disk instability, possibly leading to enhanced nuclear shell burning on the white dwarf, which was already experiencing some level of quasi-steady shell burning. This eventually led to the generation of a wind and/or bipolar, collimated outflows. The 2019 outburst of V1047 Cen appears to be unique, and nothing similar has been observed in a typical cataclysmic variable system before, hinting at a potentially new astrophysical phenomenon. [ABSTRACT FROM AUTHOR]

We present a new method to derive binary inclinations in quiescent black hole (BH) X-ray transients (XRTs), based on the depth of the trough (T) from double-peaked H α emission profiles arising in accretion discs. We find that the inclination angle (i) is linearly correlated with T in phase-averaged spectra with sufficient orbital coverage (≳50 per cent) and spectral resolution, following i (deg) = 93.5 × T + 23.7. The correlation is caused by a combination of line opacity and local broadening, where a leading (excess broadening) component scales with the deprojected velocity of the outer disc. Interestingly, such scaling allows to estimate the fundamental ratio M 1/ P orb by simply resolving the intrinsic width of the double-peak profile. We apply the T – i correlation to derive binary inclinations for GRO J0422+32 and Swift J1357−0933, two BH XRTs where strong flickering activity has hindered determining their values through ellipsoidal fits to photometric light curves. Remarkably, the inclination derived for GRO J0422+32 (i = 55.6 ± 4.1○) implies a BH mass of |$2.7^{+0.7}_{-0.5}$| M⊙ thus placing it within the gap that separates BHs from neutron stars. This result proves that low-mass BHs exist in nature and strongly suggests that the so-called 'mass gap' is mainly produced by low number statistics and possibly observational biases. On the other hand, we find that Swift J1357−0933 contains a |$10.9^{+1.7}_{-1.6}$| M⊙ BH, seen nearly edge on (⁠|$i=87.4^{+2.6}_{-5.6}$| deg). Such extreme inclination, however, should be treated with caution since it relies on extrapolating the T – i correlation beyond i ≳ 75○, where it has not yet been tested. [ABSTRACT FROM AUTHOR]