Nova Systems Research Articles

A census of the Sun’s ancestors and their contributions to the Solar System chemical composition

In this work, we compute the rates and numbers of different types of stars and phenomena (supernovae, novae, white dwarfs, merging neutron stars, black holes) that contributed to the chemical composition of the Solar System. During the Big Bang, only light elements formed, while all the heavy ones, from carbon to uranium and beyond, have since been created inside stars. Stars die and release the newly formed elements into the interstellar gas. This process is called ‘chemical evolution’. In particular, we analyse the death rates of stars of all masses, whether they die quiescently or explosively. These rates and total star numbers are computed in the context of a revised version of the two-infall model for the chemical evolution of the Milky Way, which reproduces the observed abundance patterns of several chemical species, the global solar metallicity, and the current gas, stellar, and total surface mass densities relatively well. We also compute the total number of stars ever born and still alive as well as the number of stars born up to the formation of the Solar System with mass and metallicity like those of the Sun. This latter number accounts for all the possible existing Solar systems that can host life in the solar vicinity. We conclude that, among all the stars (from 0.8 to 100 M⊙) that were born and died from the Big Bang up until the Solar System formation epoch and that contributed to its chemical composition, 93.00% were stars that died as single white dwarfs (without interacting significantly with a companion star) and originated in the mass range of 0.8–8 M⊙, while 5.24% were neutron stars and 0.73% were black holes, both originating from core-collapse supernovae (M > 8 M⊙); 0.64% were Type Ia supernovae and 0.40% were nova systems, both originating from the same mass range as the white dwarfs. The number of stars similar to the Sun born from the Big Bang up until the formation of the Solar System, with metallicity in the range 12+log(Fe/H)= 7.50 ± 0.04 dex, is ~31•107, and in particular our Sun is the ~2.61• 107-th star of this kind.

Following the Pulsations in the Long-term Cooling of GW Librae and V386 Serpentis

GW Lib and V386 Ser are dwarf novae systems containing pulsating white dwarfs that underwent large 8–9 mag amplitude outbursts in 2007 and 2019, respectively. Following the pulsation periods in these systems after the outburst provides a means to view the heating and cooling effects of mass accretion on the instability region of the white dwarf. Follow-up optical observations during 2021–2023 for these two systems are reported, resulting in a time span coverage of 16 yr for GW Lib and 4 yr for V386 Ser after their outbursts that reveal large differences in behavior as their white dwarfs returned to quiescence. GW Lib showed intermittent longer periods at 19 minutes, 1.4, 2, and 4 hr during the first 16 yr after the outburst, before finally showing, at 14 yr post-outburst, two of the three short-period modes apparent during preoutburst quiescence. In contrast, V386 Ser appeared to follow theoretical expectations, showing a shorter-period pulsation soon after the outburst, and progressively longer periods as it cooled to its quiescent state. While the optical light returns to quiescent value within 2 yr, it is apparent that the white dwarf takes much longer to recover to its quiescent state. Theoretical work is needed to explain the large differences in otherwise similar systems.

Exploring the Archives: A Search for Novae in UVIT Snapshots of M31

Extensive multiwavelength studies of novae have been carried out in our galaxy and in M31 for decades. However, UV studies of extragalactic novae are limited, especially those in quiescence. For the first time, we present a UV catalog of novae in M31 using the archival AstroSat Ultraviolet Imaging Telescope (UVIT) imaging data. We used two image subtraction techniques to retrieve objects located deep into the M31 central region. We have found 42 novae in total in the UVIT images, 15 of which have been detected in multiple filters in the far-ultraviolet (FUV) and near-ultraviolet. The novae detected at quiescence show signatures of accretion disk from their UV spectral energy distributions, whereas those in the outburst phase show signatures of a pseudo-photosphere. A few novae were also detected in multiple epochs. Some show a near-constant FUV magnitude at quiescence, while others caught near the outburst reveal pre-eruption dips in their light curves. We conclude with a discussion on the significance of UV surveys in illuminating theoretical predictions for novae systems, including detecting the elusive early UV flash.

Evolutionary Period Changes for 52 Cataclysmic Variables, and the Failure for the Most-fundamental Prediction of the Magnetic Braking Model

The evolution of cataclysmic variables (CVs) is driven by period changes ( Ṗ ), for which the long-venerable consensus is the magnetic braking model (MBM). The MBM has its only distinctive assumption being a power-law “recipe” describing the angular momentum loss (AML) in the binary, producing a single unique evolutionary track with Ṗ as a function of the orbital period. This required prediction can be used to test the most-fundamental assumption of MBM, but it has never been tested previously. In this paper, I collect Ṗ measures for 52 CVs of all types. First, 44% of the CVs have positive Ṗ , with such being impossible in MBM. Second, even among the CVs with negative Ṗ , their Ṗ measures are always more negative than required by MBM, with an average deviation of 110×. Third, three CVs have large chaotic variations in Ṗ that are impossible for MBM, proving that some unknown mechanism exists and is operating that dominates for these systems. Fourth, the MBM does not account for the long-term effects on evolution arising from the large sudden period decreases seen across many nova events, with this unaccounted effect dominating for the majority of nova systems and changing the sign of the overall evolutionary Ṗ . Fifth, three recurrent novae are observed to suddenly change Ṗ by an order of magnitude across a nova event, with this being impossible in the MBM. In all, the required MBM Ṗ predictions all fail for my 52 CVs, usually by orders of magnitude, so the MBM AML-recipe is wrong by orders of magnitude.

Orbital period changes for 14 novae and the critical failures of the predictions of standard theories, the Hibernation Model, and the Magnetic Braking Model

ABSTRACT The evolution of novae and cataclysmic variables (CVs) is driven by changes in the binary orbital periods. In a direct and critical test for various evolution models and their physical mechanisms, I measure the sudden changes in the period (ΔP) across 14 nova eruptions, and I measure the steady period change during quiescence (Ṗ) for 20 inter-eruption intervals. The standard theory for ΔP is dominated by the mechanism of mass loss, and this fails completely for the five novae with negative values, and it fails to permit the ΔP for U Sco eruptions to change by one order of magnitude eruption to eruption. The Hibernation Model of evolution is refuted because all the ΔP measures are orders of magnitude too small to cause any significant drop in accretion luminosity, and indeed, near half of the nova have negative ΔP as the opposite of the required mechanism for any hibernation state. As for the Magnetic Braking Model, this fails by many orders of magnitude in its predictions of the required Ṗ for 9 out of 13 novae. The observed Ṗ values scatter, both positively and negatively, over a range of ±10−9, while the predicted values are from −10−13 to −10−11. This huge scatter is not possible with standard theory, and there must be some currently unknown mechanism to be added in, with this new mechanism 100–10 000× larger in effect than the current theory allows. In all, these failed predictions demonstrate that nova systems must have unknown physical mechanisms for both ΔP and Ṗ that dominate over all other effects.

Chemical evolution of 26Al and 60Fe in the Milky Way

ABSTRACT We present theoretical mass estimates of 26Al and 60Fe throughout the Galaxy, performed with a numerical chemical evolution model including detailed nucleosynthesis prescriptions for stable and radioactive nuclides. We compared the results for several sets of stellar yields taken from the literature, for massive, low and intermediate mass stars, nova systems (only for 26Al) and supernovae Type Ia. We then computed the total masses of 26Al and 60Fe in the Galaxy. We studied the bulge and the disc of the Galaxy in a Galactocentric radius range of 0–22 kpc. We assumed that the bulge region (within 2 kpc) evolved quickly suffering a strong star formation burst, while the disc formed more slowly and inside–out. We compared our results with the 26Al mass observed by the gamma-ray surveys COMPTEL and INTEGRAL to select the best model. Concerning 60Fe, we do not have any observed mass value so we just performed a theoretical prediction for future observations. In conclusion, low, intermediate mass stars, and Type Ia supernovae contribute negligibly to the two isotopes, while massive stars are the dominant source. The nova contribution is, however, necessary to reproduce the observations of 26Al. Our best model predicts 2.12 M⊙ of 26Al, in agreement with observations, while for 60Fe our best mass estimate is ∼1.05 M⊙. We also predicted the present injection rate of 26Al and 60Fe in the Galaxy and compared it with previous results, and we found a larger present time injection rate along the disc.

Informing the Cataclysmic Variable Sequence from Gaia Data: The Orbital-period–Color–Absolute-magnitude Relationship

The orbital-period (P orb) gap in the population of cataclysmic variables (CVs) informs the theoretical narrative of CV evolution, yet a complete understanding of the driving angular momentum loss mechanisms above and below this gap remains elusive. Here we identify, for standard CVs, a new, apparently monotonic relationship between quiescent color (G BP–G RP), absolute magnitude (M G ), and P orb (between 70 minutes and 8 hr) revealed in Gaia DR2 and EDR3. We show that P orb increases in the color–absolute-magnitude diagram roughly orthogonally to the white dwarf and main sequences. We find the orbital-period–color–absolute-magnitude relationship to be stable across different CV subtypes: dwarf novae, intermediate polars, polars, and novalike systems. We place our findings in context with the known semiempirical donor sequence for CVs and find a dependence between color and M G for a given P orb specifically for dwarf novae and intermediate polars above the period gap. These relations have the potential to inform a more complete picture of CV evolution.

Comprehensive listing of 156 reliable orbital periods for novae, including 49 new periods

ABSTRACT 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.

AT 2019qyl in NGC 300: Internal Collisions in the Early Outflow from a Very Fast Nova in a Symbiotic Binary* †

Abstract Nova eruptions, thermonuclear explosions on the surfaces of white dwarfs (WDs), are now recognized to be among the most common shock-powered astrophysical transients. We present the early discovery and rapid ultraviolet (UV), optical, and infrared (IR) temporal development of AT 2019qyl, a recent nova in the nearby Sculptor Group galaxy NGC 300. The light curve shows a rapid rise lasting ≲1 day, reaching a peak absolute magnitude of M V = −9.2 mag and a very fast decline, fading by 2 mag over 3.5 days. A steep dropoff in the light curves after 71 days and the rapid decline timescale suggest a low-mass ejection from a massive WD with M WD ≳ 1.2 M ⊙. We present an unprecedented view of the early spectroscopic evolution of such an event. Three spectra prior to the peak reveal a complex, multicomponent outflow giving rise to internal collisions and shocks in the ejecta of an He/N-class nova. We identify a coincident IR-variable counterpart in the extensive preeruption coverage of the transient location and infer the presence of a symbiotic progenitor system with an O-rich asymptotic-giant-branch donor star, as well as evidence for an earlier UV-bright outburst in 2014. We suggest that AT 2019qyl is analogous to the subset of Galactic recurrent novae with red-giant companions such as RS Oph and other embedded nova systems like V407 Cyg. Our observations provide new evidence that internal shocks between multiple, distinct outflow components likely contribute to the generation of the shock-powered emission from such systems.

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Helium enrichment during classical nova outbursts

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

Magnetic outflows from turbulent accretion disks

Context. Astrophysical disks are likely embedded in an ambient vertical magnetic field generated by its environment. This ambient field is known to drive magneto-rotational turbulence in the disk bulk, but it is also responsible for launching magnetised outflows at the origin of astrophysical jets. Yet, the interplay between turbulence and outflows is not understood. In particular, the vertical structure and long-term (secular) evolution of such a system lack quantitative predictions. It is, nevertheless, this secular evolution which is proposed to explain time variability in many accreting systems such as FuOr, X-ray binaries, and novae like systems. Aims. We seek to constraint the structure and long-term evolution of turbulent astrophysical disks subject to magnetised outflows in the non-relativistic regime. More specifically we aim to characterise the mechanism driving accretion, the dynamics of the disk atmosphere, the role played by the outflow, and the long-term evolution of mass and magnetic flux distributions. Methods. We computed and analysed global 3D ideal magnetohydrynamic (MHD) simulations of an accretion disk threaded by a large-scale magnetic field. We measured the turbulent state of the system by Reynolds averaging the ideal MHD equations and evaluate the role of the turbulent terms in the equilibrium of the system. We then computed the transport of mass, angular momentum, and magnetic fields in the disk to characterise its secular evolution. Finally, we performed a parameter exploration survey in order to characterise how the transport properties depend on the disk properties. Results. We find that weakly magnetised disks drive jets that carry a small fraction of the disk angular momentum away. The mass-weighted accretion speed remains subsonic, although there is always an upper turbulent atmospheric region where transsonic accretion takes place. We show that this turbulence is driven by a strongly magnetised version of the magneto-rotational instability. The internal disk structure therefore appears drastically different from the conventional hydrostatic picture. We expect that the turbulent atmosphere region will lead to non-thermal features in the emission spectra from compact objects. In addition, we show that the disk is subject to a secular viscous-type instability, which leads to the formation of long-lived ring-like structures in the disk surface density distribution. This instability is likely connected to the magnetic field transport. Finally, we show that for all of the parameters explored, the ambient magnetic field is always dragged inward in the disk at a velocity which increases with the disk magnetisation. Beyond a threshold on the latter, the disk undergoes a profound radial readjustment. It leads to the formation of an inner accretion-ejection region with a supersonic mass-weighted accretion speed and where the magnetic field distribution becomes steady and reaches a magnitude near equipartition with the thermal pressure. This inner structure shares many properties with the jet emitting disk model. Overall, these results pave the way for quantitative self-consistent secular disk models.

Consumption of ultra-processed foods and relationship between nutrient intake and obesity among participants undergoing specific health checkups provided by National Health Insurance

Objective Obesity associated with an increased consumption of ultra-processed foods (UPF) has been reported by studies abroad using the NOVA food classification system, an international framework for classifying food according to the degree of processing. However, no such study has been conducted in Japan. In this study, we evaluated the association between UPF consumption, nutrient intake, and obesity using the NOVA system.Method In September 2017, we conducted a survey using a questionnaire of patients who had attended routine health checkups in Manazuru Town, Kanagawa Prefecture and collected their three-day dietary records as well as medical examination results. The final analysis included 169 patients (66 men, 103 women). Food consumed by the respondents were classified into four groups of NOVA systems. The dietary share of UPF (UPF energy ratio) was compared to total energy intake (except alcoholic beverages and eating out). Nutrient intake and obesity risk were compared across the UPF energy ratio tertile (low, middle, and high intake). Covariance and logistic regression analysis were conducted and adjusted based on age, sex, household structure, education, income, and total energy intake (except energy-providing nutrients, macronutrients, and protein intake per body weight) to analyze the association between UPF consumption and nutrient intake. Finally, for obesity, we adjusted for physical activity and smoking status using a similar analysis of the relationship between UPF consumption and energy providing nutrients and macronutrients.Result Approximately 75% of patients investigated were older than 65 years. The average (standard deviation) UPF energy ratio was 29.7% (15.0). There were no significant differences in socioeconomic status among groups according to the UPF energy ratio. The high-UPF group had a significantly greater total energy intake. However, the protein energy ratio, protein intake per body weight, dietary fiber, and vitamins A, E, K, B1, B6, C, niacin, folic acid, potassium, magnesium, and iron intake were significantly lower in the high-UPF group. Moreover, the high-UPF group had a significantly higher BMI. The odds ratio for obesity was higher (4.51[1.50-13.57]) in the high-UPF group than in the low-UPF group (1.00).Conclusion Those who consumed more UPF had lower protein intake and suffered from multiple vitamin and mineral deficiencies. Furthermore, their energy intake was greater, and the odds ratio for obesity was significantly higher. It has been suggested that excessive UPF consumption warrants further attention.

Accretion flows in nonmagnetic white dwarf binaries as observed in X-rays

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

A photoionization model grid for novae: estimation of physical parameters

We present here a method to estimate physical parameters of novae systems using an extensive grid of photoionization models for novae. We use the photoionization code CLOUDY to construct grid of models covering a wide range of different parameters, e.g. total hydrogen density ($n_H$), source temperature ($T_{BB}$) and luminosity ($L$), inner radius ($R_{in}$) and thickness of ejecta ($\Delta R$), keeping other elements at solar metallicity. In this way, a total of 1792 models have been generated. From the model generated spectra which cover a wide wavelength region from ultra-violet to infrared, we calculate ratios of hydrogen and helium emission lines fluxes which are generally strong in novae spectra. We show that physical parameters associated with novae system could be estimated by comparing these line ratios with those obtained from observed spectra. We elaborate the idea with examples and estimate the parameter values in case of few other novae. The results of the grid model are available online.

Digitalisation as a critical tool in resource asset valuations, acquisitions and dispositions

Digital twins and digital ‘solutions’ have become increasingly common in recent years, as companies recognise the benefit of greater visibility and control into their operations. When built and implemented properly, a digital twin (DT) offers the potential for significant cost savings, coupled with attractive add-on value in safety, operational integrity and predictive maintenance, all of which can also result in increased production. DTs are also critical for effective portfolio management, in allowing a full understanding of asset or field value, upside, and long-term potential. By identifying areas for optimisation, a DT can help the operator prioritise investment, and accurately understand which assets to keep or monetise. Through building DTs for operating properties, Nova Systems and PTC have gained a deep understanding of the upside potential inherent in the technology. This also means that DT technology can be invaluable in asset acquisitions. During merger and acquisition (M&A) analysis, a company typically looks for technical, operational and financial leverage which can unlock potential not seen by the current owner. These can be commercial (commodity contract, price deck and agreements), technical (e.g. engineering approach and models, and subsurface), operating synergies (common services, facilities and duplication) and digital solutions. While most companies utilise traditional and tested approaches to asset valuation, new digital solutions offer the opportunity for break-out higher valuations which can drive an entirely new approach to M&A growth. In today’s competitive marketplace, the company with an innovative digital solutions skillset will have the advantage.

CONSTRAINED LINERS AND DUAL MOBILITY SYSTEMS FOR PREVENTION OF INSTABILITY IN REVISION HIP ARTHROPLASTY

Purpose. Instability is a challenging complication of revision hip arthroplasty and a frequent cause of repeat revisions. Constrained liners and dual mobility systems have gained major attention among the options of dislocation prophylaxis. The aim of this study is to compare the outcomes of revision hip arthroplasy with use of constrained liners and dual mobility systems. Materials and Methods. We used DePuy Duraloc (inner diameter 28 mm) and Zimmer Trilogy (inner diameter 32 mm) systems in the constrained liners group (N 78, mean follow-up — 66.2 month, 54-82), Serf Novae and Biomet Avantage systems in the dual mobility group (N 58, mean follow-up — 17.8 month, 10-41). The two groups were comparable in age, sex and different potential dislocation risk factors, however, dual mobility cups were used more frequently in revisions due to unreduced and recurrent dislocations and in patients with the history of instability following total hip arthroplasty. There were also differences in the structure of primary diagnosis. Results. We observed 14 (17.9%) dislocations, 10 (12.8%) of which occurred within 2 years after surgery, and also 3 (3.8%) cases of aseptic loosening of the acetabular component, 4 (5.1%) cases of locking mechanism damage without dislocation and 8 (10.3%) cases of deep infection in the constrained liners group. In the dual mobility group we observed 3 (5.17%) large articulation dislocations, 1 (1.7%) case of aseptic loosening of the acetabular component and 4 (6.9%) cases of deep infection. The difference in dislocation rates in two groups was significant (p0.05). The analysis of the constrained liners group revealed an increased risk of dislocation in cases when a constrained system was implanted into a retained acetabular component compared to cases with acetabular shell revision (p0.01; RR = 7.2, 95% CI: 2.05-25.26), as well as a trend for increased risk of dislocation in cases when DePuy Duraloc liners (inner diameter 28 mm) were used compared to Zimmer Trilogy (inner diameter 32 mm) (p = 0.07; RR = 4.97, 95% CI: 1.03-24.04). Conclusion. Dual mobility systems proved to be more effective than constrained liners in revision hip arthroplasty although being used more frequently as a treatment rather than prophylaxis of instability. Constrained liners bear a higher risk of dislocations when implanted into retained acetabular components and when used with heads of lesser diameter.

Evolution of broad-band SED during outburst rise in NS X-ray Nova Aql X-1

The observed evolution of the broad-band spectral energy distribution (SED) in NS X-ray Nova Aql X-1 during the rise phase of a bright FRED-type outburst in 2013 can be understood in the framework of thermal emission from unstationary accretion disc with temperature radial distribution transforming from a single-temperature blackbody emitting ring into the multi-colour irradiated accretion disc. SED evolution during the hard to soft X-ray state transition looks curious, as it can not be repro- duced by the standard disc irradiation model with a single irradiation parameter for NUV, Optical and NIR spectral bands. NIR (NUV) band is correlated with soft (hard) X-ray flux changes during the state transition interval, respectively. In our interpreta- tion, at the moment of X-ray state transition UV-emitting parts of the accretion disc are screened from direct X-ray illumination from the central source and are heated primary by hard X-rays (E > 10 keV), scattered in the hot corona or wind possibly formed above the optically-thick outer accretion flow; the outer edge of multi-colour disc, which emits in Optical-NIR, can be heated primary by direct X-ray illumination. We point out that future simultaneous multi-wavelength observations of X-ray Nova systems during the fast X-ray state transition interval are of great importance, as it can serve as 'X-ray tomograph' to study physical conditions in outer regions of accretion flow. This can provide an effective tool to directly test the energy-dependent X-ray heating efficiency, vertical structure and accretion flow geometry in transient LMXBs.

The first gravitational-wave burst GW150914, as predicted by the scenario machine

The Advanced LIGO observatory recently reported (Abbott et al., 2016a) the first direct detection of gravitational waves predicted by Einstein (1916). The detection of this event was predicted in 1997 on the basis of the Scenario Machine population synthesis calculations (Lipunov et al., 1997b) Now we discuss the parameters of binary black holes and event rates predicted by different scenarios of binary evolution. We give a simple explanation of the big difference between detected black hole masses and the mean black hole masses observed in of X-ray Nova systems. The proximity of the masses of the components of GW150914 is in good agreement with the observed initial mass ratio distribution in massive binary systems, as is used in Scenario Machine calculations for massive binaries.

Active phases and flickering of a symbiotic recurrent nova T CrB

T CrB is a symbiotic recurrent nova known to exhibit active phases, characterised by apparent increases in the hot component temperature and the appearance of flickering, i.e. changes in the observed flux on the time-scale of minutes. Historical UV observations have ruled out orbital variability as an explanation for flickering and instead suggest flickering is caused by variable mass transfer. We have analysed optical and X-ray observations to investigate the nature of the flickering as well as the active phases in T CrB. The spectroscopic and photometric observations confirm that the active phases follow two periods of ~1000d and ~5000d. Flickering in the X-rays is detected and follows an amplitude-flux relationship similar to that observed in the optical. The flickering is most prominent at harder X-ray energies, suggesting that it originates in the boundary layer between the accretion disc and the white dwarf. The X-ray radiation from the boundary layer is then reprocessed by a thick accretion disc or a nebula into UV radiation. A more detailed understanding of flickering would benefit from long-term simultaneous X-ray and optical monitoring of the phenomena in symbiotic recurrent novae and related systems such as Z And type symbiotic stars.