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The study of ultraluminous X-ray sources (ULXs) has changed dramatically over the last decade. In this review we first describe the most important observations of ULXs in various wavebands, and across multiple scales in space and time. We discuss recent progress and current unanswered questions. We consider the range of current theories of ULX properties in the light of this observational progress. Applying these models to neutron-star ULXs offers particularly stringent tests, as this is the unique case where the mass of the accretor is effectively fixed., Comment: A preprint as close as possible to the open access article at: https://www.sciencedirect.com/science/article/pii/S1387647322000306?via%3Dihub

Trois lauréats partagent le prix Nobel de physique 2020 pour leurs découvertes sur l’un des phénomènes les plus exotiques de l’Univers, les trous noirs : le théoricien britannique Roger Penrose, l’astronome allemand Gerhard Genzel et l’astronome américaine Andrea Ghez.

We present high-speed optical photometry from SAAO and SALT on the black hole LMXB MAXI J1820+070 (ASSASN-18ey), some of it simultaneous with NICER, Swift, and Insight-HXMT X-ray coverage. We detect optical quasi-periodic oscillations (QPOs) that move to higher frequencies as the outburst progresses, tracking both the frequency and evolution of similar X-ray QPOs previously reported. Correlated X-ray/optical data reveal a complex pattern of lags, including an anticorrelation and a sub-second lag that evolve over the first few weeks of outburst. They also show correlated components separated by a lag equal to the QPO period roughly centered on zero lag, implying that the inter-band variability is strongly and consistently affected by these QPOs at a constant phase lag of roughly ±π. The synchronization of X-ray and optical QPOs indicates that they must be produced in regions physically very close to each other; we, thus, propose that they can be explained by a precessing jet model, based on analogies with V404 Cyg and MAXI J1348−630.

All ten LIGO/Virgo binary black hole (BH-BH) coalescences reported from the O1/O2 runs have near zero effective spins. There are only three potential explanations of this fact. If the BH spin magnitudes are large then (i) either both BH spin vectors must be nearly in the orbital plane or (ii) the spin angular momenta of the BHs must be oppositely directed and similar in magnitude. Or, (iii) the BH spin magnitudes are small. We test the third hypothesis within the framework of the classical isolated binary evolution scenario of the BH-BH merger formation. We test three models of angular momentum transport in massive stars: a mildly efficient transport by meridional currents (as employed in the Geneva code), an efficient transport by the Tayler-Spruit magnetic dynamo (as implemented in the MESA code), and a very-efficient transport (as proposed by Fuller et al.) to calculate natal BH spins. We allow for binary evolution to increase the BH spins through accretion and account for the potential spin-up of stars through tidal interactions. Additionally, we update the calculations of the stellar-origin BH masses, include revisions to the history of star formation and to the chemical evolution across cosmic time. We find that we can match simultaneously the observed BH-BH merger rate density, BH masses, and effective spins. Models with efficient angular momentum transport are favored. The updated stellar-mass weighted gas-phase metallicity evolution now used in our models appears to be a key in better reproducing the LIGO/Virgo merger rate estimate. Mass losses during the pair-instability pulsation supernova phase are likely overestimated if the merger GW170729 hosts a BH more massive than 50 Msun. We also estimate rate of BH-NS mergers from recent LIGO/Virgo observations. Our updated models of BH-BH, BH-NS and NS-NS mergers are now publicly available at www.syntheticuniverse.org., Comment: A&A accepted: revised comparison with LIGO/Virgo (41 pages)

Context. The Event Horizon Telescope (EHT) collaboration recently obtained first images of the surroundings of the supermassive compact object M87* at the center of the galaxy M87. Aims. We want to develop a simple analytic disk model for the accretion flow of M87*. Compared to general-relativistic magnetohydrodynamic (GRMHD) models, it has the advantage of being independent of the turbulent character of the flow, and controlled by only few easy-to-interpret, physically meaningful parameters. We want to use this model to predict the image of M87* assuming that it is either a Kerr black hole, or an alternative compact object. Methods. We compute the synchrotron emission from the disk model and propagate the resulting light rays to the far-away observer by means of relativistic ray tracing. Such computations are performed assuming different spacetimes (Kerr, Minkowski, non-rotating ultracompact star, rotating boson star or Lamy spinning wormhole). We perform numerical fits of these models to the EHT data. Results. We discuss the highly-lensed features of Kerr images and show that they are intrinsically linked to the accretion-flow properties, and not only to gravitation. This fact is illustrated by the notion of secondary ring that we introduce. Our model of spinning Kerr black hole predicts mass and orientation consistent with the EHT interpretation. The non-Kerr images result in similar quality of the numerical fits and may appear very similar to Kerr images, once blurred to the EHT resolution. This implies that a strong test of the Kerr spacetime may be out of reach with the current data. We notice that future developments of the EHT could alter this situation. Conclusions. Our results show the importance of studying alternatives to the Kerr spacetime in order to be able to test the Kerr paradigm unambiguously., 22 pages, 21 figures, accepted by A&A - see new Fig. 5

Although ultra-luminous X-ray sources (ULX) are important for astrophysics due to their extreme apparent super-Eddington luminosities, their nature is still poorly known. Theoretical and observational studies suggest that ULXs could be a diversified group of objects composed of low-mass X-ray binaries, high-mass X-ray binaries and marginally also systems containing intermediate-mass black holes, which is supported by their presence in a variety of environments. Observational data on the ULX donors could significantly boost our understanding of these systems, but only a few were detected. There are several candidates, mostly red supergiants (RSGs), but surveys are typically biased toward luminous near-infrared objects. Nevertheless, it is worth exploring if RSGs can be members of ULX binaries. In such systems matter accreted onto the compact body would have to be provided by the stellar wind of the companion, since a Roche-lobe overflow could be unstable for relevant mass-ratios. Here we present a comprehensive study of the evolution and population of wind-fed ULXs and provide a theoretical support for the link between RSGs and ULXs. Our estimated upper limit on contribution of wind-fed ULX to the overall ULX population is $\sim75$--$96\%$ for young ($2$ Gyr) ago. We show also that some wind-fed ULXs (up to $6\%$) may evolve into merging double compact objects (DCOs), but typical systems are not viable progenitors of such binaries because of their large separations. We demonstrate that, the exclusion of wind-fed ULXs from population studies of ULXs, might have lead to systematical errors in their conclusions., 14 pages, 9 figures, 2 tables, accepted to ApJ

The black-hole X-ray transient MAXI J1820+07 (=ASSASN-18ey) discovered in March 2018 was one of the optically brightest ever seen, which has resulted in very detailed optical outburst light-curves being obtained. We combine them here with X-ray and radio light-curves to show the major geometric changes the source undergoes. We present a detailed temporal analysis that reveals the presence of remarkably high amplitude (>0.5 mag) modulations, which evolve from the superhump (16.87 h) period towards the presumed orbital (16.45 h) period. These modulations appear ~87d after the outburst began, and follow the Swift/BAT hard X-ray light-curve, which peaks 4 days before the radio flare and jet ejection, when the source undergoes a rapid hard to soft state transition. The optical modulation then moves closer to the orbital period, with a light curve peak that drifts slowly in orbital phase from ~0.8 to ~0.3 during the soft state. We propose that the unprecedentedly large amplitude modulation requires a warp in the disc in order to provide a large enough radiating area, and for the warp to be irradiation-driven. Its sudden turn-on implies a change in the inner disc geometry that raises the hard X-ray emitting component to a height where it can illuminate the warped outer disc regions., Accepted to MNRAS

We show that ultraluminous X-ray sources (ULXs) with coherent X-ray pulsing (PULXs) probably have neutron-star spin axes significantly misaligned from their central accretion discs. Scattering in the funnels collimating their emission and producing their apparent super-Eddington luminosities is the most likely origin of the observed correlation between pulse fraction and X-ray photon energy. Pulsing is suppressed in systems with the neutron-star spin closely aligned to the inner disc, explaining why some ULXs show cyclotron features indicating strong magnetic fields, but do not pulse. We suggest that alignment (or conceivably, field suppression through accretion) generally occurs within a fairly short fraction of the ULX lifetime, so that most neutron-star ULXs become unpulsed. As a result we further suggest that almost all ULXs actually have neutron-star accretors, rather than black holes or white dwarfs, reflecting their progenitor high-mass X-ray binary and supersoft X-ray source populations., Comment: 6 pages, 2 figures, 2 tables. MNRAS in press

Context. Although the disc instability model is widely accepted as the explanation for dwarf nova outbursts, it is still necessary to confront its predictions to observations because much of the constraints on angular momentum transport in accretion discs are derived from the application of this model to real systems. Aims. We test the predictions of the model concerning the multicolour time evolution of outbursts for two well--observed systems, SS Cyg and VW Hyi. Methods. We calculate the multicolour evolution of dwarf nova outbursts using the disc instability model and taking into account the contribution from the irradiated secondary, the white dwarf and the hot spot. Results. Observations definitely show the existence of a hysteresis in the optical colour-magnitude diagram during the evolution of dwarf nova outbursts. We find that the disc instability model naturally explains the existence and the orientation of this hysteresis. For the specific cases of SS Cyg and VW Hyi, the colour and magnitude ranges covered during the evolution of the system are in reasonable agreement with observations. However, the observed colours are bluer than observed near the peak of the outbursts -- as in steady systems, and the amplitude of the hysteresis cycle is smaller than observed. The predicted colours significantly depend on the assumptions made for calculating the disc spectrum during rise, and on the magnitude of the secondary irradiation for the decaying part of the outburst. Conclusions. Improvements of the spectral disc models are strongly needed if one wishes to address the system evolution in the UV., Comment: Accepted for publication in Astronomy and Astrophysics

Magnetically gated accretion has emerged as a proposed mechanism for producing extremely short, repetitive bursts of accretion onto magnetized white dwarfs in intermediate polars (IPs), but this phenomenon has not been detected previously in a confirmed IP. We report the 27-day TESS light curve of V1025 Cen, an IP that shows a remarkable series of twelve bursts of accretion, each lasting for less than six hours. The extreme brevity of the bursts and their short recurrence times (~1-3 days) are incompatible with the dwarf-nova instability, but they are natural consequences of the magnetic gating mechanism developed by Spruit & Taam to explain the Type II bursts of the accreting neutron star known as the Rapid Burster. In this model, the accretion flow piles up at the magnetospheric boundary and presses inward until it couples with the star's magnetic field, producing an abrupt burst of accretion. After each burst, the reservoir of matter at the edge of the magnetosphere is replenished, leading to cyclical bursts of accretion. A pair of recent studies applied this instability to the suspected IPs MV Lyr and TW Pic, but the magnetic nature of these two systems has not been independently confirmed. In contrast, previous studies have unambiguously established the white dwarf in V1025 Cen to be significantly magnetized. The detection of magnetically gated bursts in a confirmed IP therefore validates the extension of the Spruit & Taam instability to magnetized white dwarfs., Accepted for publication in ApJL

The LIGO/Virgo collaboration has reported the detection of GW190412, a BH-BH merger with the most unequal masses to date: 24.4-34.7 Msun and 7.4-10.1 Msun (a mass ratio of q=0.21-0.41). Additionally, GW190412's effective spin was estimated to be Xeff=0.14-0.34, with the spin of the primary BH in the range a=0.17-0.59. Based on this and prior detections, about 10 percent of BH-BH mergers have q0.5). Here we test whether the classical isolated binary evolution channel can produce mergers resembling GW190412. We show that our standard binary evolution scenario, with the typical assumptions on input physics we have used in the past, produces such mergers (masses and spins). For this particular model of the input physics the overall BH-BH merger rate density in the local Universe (z=0) is: 73.5 Gpc^-3 yr^-1, while for systems with q0.5) then common envelope isolated binary BH-BH formation channel can be excluded as their origin., ApJ Letters: accepted

We present optical, UV and X-ray monitoring of the short orbital period black hole X-ray binary candidate Swift J1753.5-0127, focusing on the final stages of its 12$-$year long outburst that started in 2005. From September 2016 onward, the source started to fade and within three months, the optical flux almost reached the quiescent level. Soon after that, using a new proposed rebrightening classification method we recorded a mini-outburst and a reflare in the optical light curves, peaking in February (V$\rm\sim$17.0) and May (V$\rm\sim$17.9) 2017, respectively. Remarkably, the mini-outburst has a peak flux consistent with the extrapolation of the slow decay before the fading phase preceding it. The following reflare was fainter and shorter. We found from optical colors that the temperature of the outer disk was $\sim 11$,000 K when the source started to fade rapidly. According to the disk instability model, this is close to the critical temperature when a cooling wave is expected to form in the disk, shutting down the outburst. The optical color could be a useful tool to predict decay rates in some X-ray transients. We notice that all X-ray binaries that show mini-outbursts following a main outburst are short orbital period systems ($, Comment: 12 pages, 4 figures, accepted for publication in ApJ

We consider the current observed ensemble of pulsing ultraluminous X-ray sources (PULXs). We show that all of their observed properties (luminosity, spin period, and spinup rate) are consistent with emission from magnetic neutron stars with fields in the usual range $10^{11} - 10^{13}\, {\rm G}$, which is collimated (`beamed') by the outflow from an accretion disc supplied with mass at a super-Eddington rate, but ejecting the excess, in the way familiar for other (non-pulsing) ULXs. The observed properties are inconsistent with magnetar-strength fields in all cases. We point out that all proposed pictures of magnetar formation suggest that they are unlikely to be members of binary systems, in agreement with the observation that all confirmed magnetars are single. The presence of magnetars in ULXs is therefore improbable, in line with our conclusions above., 8 pages, 1 figure, MNRAS, accepted

The disc instability model accounts well for most of the observed properties of dwarf novae and soft X-ray transients, but the rebrightenings, reflares, and echoes occurring at the end of outbursts or shortly after in WZ Sge stars or soft X-ray transients have not yet been convincingly explained by any model. We determine the additional ingredients that must be added to the DIM to account for the observed rebrightenings. We analyse in detail a recently discovered system, TCP J21040470+4631129, which has shown very peculiar rebrightenings, model its light curve using our numerical code including mass transfer variations from the secondary, inner-disc truncation, disc irradiation by a hot white dwarf and, in some cases, the mass-transfer stream over(under)flow. We show that the luminosity in quiescence is dominated by a hot white dwarf that cools down on time scales of months. The mass transfer rate from the secondary has to increase by several orders of magnitudes during the initial superoutburst for a reason that remains elusive, slowly returning to its secular average, causing the observed succession of outbursts with increasing quiescence durations, until the disc can be steady, cold, and neutral; its inner parts being truncated either by the white dwarf magnetic field or by evaporation. The very short, quiescence phases between reflares are reproduced when the mass-transfer stream overflows the disc. Using similar additions to the DIM, we have also produced light curves close to those observed in two WZ Sge stars, the prototype and EG Cnc. Our model successfully explains the reflares observed in WZ Sge systems. It requires, however, the inner disc truncation in dwarf novae to be due not (only) to the white dwarf magnetic field but, as in X-ray binaries, rather to evaporation of the inner disc. A similar model could also explain reflares observed in soft X-ray transients., Comment: 13 pages, 11 figures. Accepted for publication in A&A

The phenomenological Disc Instability Model has been successful in reproducing the observed light curves of dwarf nova outbursts by invoking an enhanced Shakura-Sunyaev $\alpha$ parameter $\sim0.1-0.2$ in outburst compared to a low value $\sim0.01$ in quiescence. Recent thermodynamically consistent simulations of magnetorotational (MRI) turbulence with appropriate opacities and equation of state for dwarf nova accretion discs have found that thermal convection enhances $\alpha$ in discs in outburst, but only near the hydrogen ionization transition. At higher temperatures, convection no longer exists and $\alpha$ returns to the low value comparable to that in quiescence. In order to check whether this enhancement near the hydrogen ionization transition is sufficient to reproduce observed light curves, we incorporate this MRI-based variation in $\alpha$ into the Disc Instability Model, as well as simulation-based models of turbulent dissipation and convective transport. These MRI-based models can successfully reproduce observed outburst and quiescence durations, as well as outburst amplitudes, albeit with different parameters from the standard Disc Instability Models. The MRI-based model lightcurves exhibit reflares in the decay from outburst, which are not generally observed in dwarf novae. However, we highlight the problematic aspects of the quiescence physics in the Disc Instability Model and MRI simulations that are responsible for this behavior., Comment: 19 pages, 14 figures, 2 tables, accepted for publication in MNRAS

We consider ultraluminous X–ray systems (ULXs) where the accretor is a neutron star rather than a black hole. We show that the recently discovered example (M82 X–2) fits naturally into the simple picture of ULXs as beamed X–ray sources fed at super-Eddington rates, provided that its magnetic field is weaker (≃1011G) than a new-born X-ray pulsar, as expected if there has been mass gain. Continuing accretion is likely to weaken the field to the point that pulsing stops, and make the system indistinguishable from a ULX containing a black hole. Accordingly we suggest that a significant fraction of all ULXs may actually contain neutron star accretors rather than black holes, reflecting the neutron-star fraction among their X-ray binary progenitors. We emphasize that neutron-star ULXs are likely to have higher apparent luminosities than black hole ULXs for a given mass transfer rate, as their tighter beaming outweighs their lower Eddington luminosities. This further increases the likely proportion of neutron-star accretors among all ULXs. Cygnus X–2 is probably a typical descendant of neutron-star ULXs, which may therefore ultimately end as millisecond pulsar binaries with massive white dwarf companions.

We have analyzed how anisotropic emission of radiation affects the observed sample of ultraluminous X-ray sources (ULXs) by performing simulations of the evolution of stellar populations, employing recent developments in stellar and binary physics, and by utilizing a geometrical beaming model motivated by theory and observation. Whilst ULXs harboring black hole accretors (BH ULXs) are typically emitting isotropically, the majority of ULXs with neutron star accretors (NS ULXs) are found to be beamed. These findings confirm previous assertions that a significant fraction of ULXs are hidden from view due to a substantial misalignment of the emission beam and the line-of-sight. We find the total number of NS ULXs in regions with constant star formation, solar metallicity, and ages above ~1 Gyr to be higher than the BH ULXs, although observationally both populations are comparable. For lower metallicities BH ULX dominate both the total and observed ULX populations. As far as burst star-formation is concerned, young ULX populations are dominated by BH ULXs, but this changes as the population ages and, post star-formation, NS ULXs dominate both the observed and total population of ULXs. We also compare our simulation output to a previous analytical prediction for the relative ratio of BH to NS ULXs in idealized flux-limited observations and find broad agreement for all but the lowest metallicities. In so doing we find that in such surveys the observed ULX population should be heavily dominated by black-hole systems rather than by systems containing neutron stars., Comment: 11 pages, 8 figures, minor corrections, accepted to ApJ

The X-ray light-curves of the recurring outbursts observed in low-mass X-ray binaries provide strong test beds for constraining (still) poorly understood disc-accretion processes. These light-curves act as a powerful diagnostic to probe the physics behind the mechanisms driving mass inflow and outflow in these binary systems. We have thus developed an innovative methodology, combining a foundation of Bayesian statistics, observed X-ray light-curves, and accretion disc theory. With this methodology, we characterize the angular-momentum (and mass) transport processes in an accretion disc, as well as the properties of the X-ray irradiation-heating that regulates the decay from outburst maximum in low-mass X-ray transients. We recently applied our methodology to the Galactic black-hole low-mass X-ray binary population, deriving from their lightcurves the first-ever quantitative measurements of the $\alpha$-viscosity parameter in these systems \citep{tetarenko2018}. In this paper, we continue the study of these binaries, using Bayesian methods to investigate the X-ray irradiation of their discs during outbursts of strong accretion. We find that the predictions of the disc-instability model, assuming a source of X-ray irradiation proportional to the central accretion rate throughout outburst, do not adequately describe the later stages of BH-LMXB outburst light-curves. We postulate that the complex and varied light-curve morphology observed across the population is evidence for irradiation that varies in time and space within the disc, throughout individual transient outbursts. Lastly, we demonstrate the robustness of our methodology, by accurately reproducing the synthetic model light-curves computed from numerical codes built to simulate accretion flows in binary systems., Comment: 16 pages, 7 Figures, Accepted to MNRAS

Disc instability models predict that for X-ray binaries in quiescence, there should be a brightening of the optical flux prior to an X-ray outburst. Tracking the X-ray variations of X-ray binaries in quiescence is generally not possible, so optical monitoring provides the best means to measure the mass accretion rate variability between outbursts, and to identify the beginning stages of new outbursts. With our regular Faulkes Telescope/Las Cumbres Observatory (LCO) monitoring we are routinely detecting the optical rise of new X-ray binary outbursts before they are detected by X-ray all-sky monitors. We present examples of detections of an optical rise in X-ray binaries prior to X-ray detection. We also present initial optical monitoring of the new black hole transient MAXI J1820+070 (ASASSN-18ey) with the Faulkes, LCO telescopes and Al Sadeem Observatory in Abu Dhabi, UAE. Finally, we introduce our new real-time data analysis pipeline, the 'X-ray Binary New Early Warning System (XB-NEWS)' which aims to detect and announce new X-ray binary outbursts within a day of first optical detection. This will allow us to trigger X-ray and multi-wavelength campaigns during the very early stages of outbursts, to constrain the outburst triggering mechanism., Comment: Accepted to Astronomische Nachrichten/Astronomical Notes (proceedings of XMM-Newton workshop 'Time-Domain Astronomy: A High Energy View', ESAC, Madrid, Spain, 13-15 June 2018)

We study energy flows in geometrically thick accretion discs, both optically thick and thin, using general relativistic, three-dimensional simulations of black hole accretion flows. We find that for non-rotating black holes the efficiency of the total feedback from thick accretion discs is $3\%$ - roughly half of the thin disc efficiency. This amount of energy is ultimately distributed between outflow and radiation, the latter scaling weakly with the accretion rate for super-critical accretion rates, and returned to the interstellar medium. Accretion on to rotating black holes is more efficient because of the additional extraction of rotational energy. However, the jet component is collimated and likely to interact only weakly with the environment, whereas the outflow and radiation components cover a wide solid angle., Comment: 14 pages, 11 figures, published in MNRAS

Recurring outbursts associated with matter flowing onto compact stellar remnants (black-holes, neutron stars, white dwarfs) in close binary systems, provide strong test beds for constraining the poorly understood accretion process. The efficiency of angular momentum (and thus mass) transport in accretion discs, which has traditionally been encoded in the $\alpha$-viscosity parameter, shapes the light-curves of these outbursts. Numerical simulations of the magneto-rotational instability that is believed to be the physical mechanism behind this transport find values of $\alpha \sim 0.1-0.2$ as required from observations of accreting white dwarfs. Equivalent $\alpha$-viscosity parameters have never been estimated in discs around neutron stars or black holes. Here we report the results of an analysis of archival X-ray light-curves of twenty-one black hole X-ray binary outbursts. Applying a Bayesian approach for a model of accretion allows us to determine corresponding $\alpha$-viscosity parameters, directly from the light curves, to be $\alpha \sim$0.2--1. This result may be interpreted either as a strong intrinsic rate of angular momentum transport in the disc, which can only be sustained by the magneto-rotational instability if a large-scale magnetic field threads the disc, or as a direct indication that mass is being lost from the disc through substantial mass outflows strongly shaping the X-ray binary outburst. Furthermore, the lack of correlation between our estimates of $\alpha$-viscosity and accretion state implies that such outflows can remove a significant fraction of disc mass in all black hole X-ray binary accretion states, favouring magnetically-driven winds over thermally-driven winds that require specific radiative conditions., Comment: 28 pages, 4 Figures, Nature in press

The disk instability model attributes the outbursts of dwarf novae to a thermal-viscous instability of their accretion disk, an instability to which nova-like stars are not subject. We aim to test the fundamental prediction of the disk instability model: the separation of cataclysmic variables (CVs) into nova-likes and dwarf novae depending on orbital period and mass transfer rate from the companion. We analyse the lightcurves from a sample of ~130 CVs with a parallax distance in the Gaia DR2 catalogue to derive their average mass transfer rate. The method for converting optical magnitude to mass accretion rate is validated against theoretical lightcurves of dwarf novae. Dwarf novae (resp. nova-likes) are consistently placed in the unstable (resp. stable) region of the orbital period - mass transfer rate plane predicted by the disk instability model. None of the analyzed systems present a challenge to the model. These results are robust against the possible sources of error and bias that we investigated. Lightcurves from Kepler or, in the future, the LSST or Plato surveys, could alleviate a major source of uncertainty, the irregular sampling rate of the lightcurves, assuming good constraints can be set on the orbital parameters of the CVs that they happen to target. The disk instability model remains the solid base on which to construct the understanding of accretion processes in cataclysmic variables., 15 pages, 5 figures, 3 tables, accepted for publication by A&A

Using the disc instability model for dwarf novae and soft X-ray transients, we investigate the stability of accretion discs in long-period binary systems. We simulate outbursts due to this thermal-viscous instability for two symbiotic systems, RS~Ophiuchi and Z~Andromedae. The outburst properties deduced from our simulations suggest that, although the recurrent nova events observed in RS~Oph are due to a thermonuclear runaway at the white dwarf surface, these runaways are triggered by accretion disc instabilities. In quiescence, the disc builds up its mass and it is only during the disc-instability outburst that mass is accreted on to the white dwarf at rates comparable to or larger than the mass-transfer rate. For a mass-transfer rate in the range $10^{-8}$ to $10^{-7}~{\rm M}_{\odot}$ yr$^{-1}$, the accretion rate and the mass accreted are sufficient to lead to a thermonuclear runaway during one of a series of a few dwarf nova outbursts, barely visible in the optical, but easily detectable in X-rays. In the case of Z~And, persistent irradiation of the disc by the very hot white-dwarf surface strongly modifies the dwarf-nova outburst properties, making them significant only for very high mass-transfer rates, of the order of $10^{-6}~{\rm M}_{\odot}$ yr$^{-1}$, much higher than the expected secular mean in this system. It is thus likely that the so-called `combination nova' outburst observed in years 2000 to 2002 was triggered not by a dwarf-nova instability but by a mass-transfer enhancement from the giant companion, leading to an increase in nuclear burning at the accreting white-dwarf surface., Comment: 15 pages, 10 figures

Recently, several ultraluminous X-ray (ULX) sources were shown to host a neutron star (NS) accretor. We perform a suite of evolutionary calculations which show that, in fact, NSs are the dominant type of ULX accretor. Although black holes (BH) dominate early epochs after the star-formation burst, NSs outweigh them after a few 100 Myr and may appear as late as a few Gyr after the end of the star formation episode. If star formation is a prolonged and continuous event (i.e., not a relatively short burst), NS accretors dominate ULX population at any time in solar metallicity environment, whereas BH accretors dominate when the metallicity is sub-solar. Our results show a very clear (and testable) relation between the companion/donor evolutionary stage and the age of the system. A typical NS ULX consists of a $\sim1.3\,M_\odot$ NS and $\sim1.0\,M_\odot$ Red Giant. A typical BH ULX consist of a $\sim8\,M_\odot$ BH and $\sim6\,M_\odot$ main-sequence star. Additionally, we find that the very luminous ULXs ($L_X\gtrsim10^{41}$ erg/s) are predominantly BH systems ($\sim9\,M_\odot$) with Hertzsprung gap donors ($\sim2\,M_\odot$). Nevertheless, some NS ULX systems may also reach extremely high X-ray luminosities ($\gtrsim10^{41}$ erg/s)., 7 pages, 14 figures, published in ApJ

FO Aquarii, an asynchronous magnetic cataclysmic variable (intermediate polar) went into a low-state in 2016, from which it slowly and steadily recovered without showing dwarf nova outbursts. This requires explanation since in a low-state, the mass-transfer rate is in principle too low for the disc to be fully ionized and the disc should be subject to the standard thermal and viscous instability observed in dwarf novae. We investigate the conditions under which an accretion disc in an intermediate polar could exhibit a luminosity drop of 2 magnitudes in the optical band without showing outbursts. We use our numerical code for the time evolution of accretion discs, including other light sources from the system (primary, secondary, hot spot). We show that although it is marginally possible for the accretion disc in the low-state to stay on the hot stable branch, the required mass-transfer rate in the normal state would then have to be extremely high, of the order of 10$^{19}$ gs$^{-1}$ or even larger. This would make the system so intrinsically bright that its distance should be much larger than allowed by all estimates. We show that observations of FO Aqr are well accounted for by the same mechanism that we have suggested as explaining the absence of outbursts during low states of VY Scl stars: during the decay, the magnetospheric radius exceeds the circularization radius, so that the disc disappears before it enters the instability strip for dwarf nova outbursts. Our results are unaffected, and even reinforced, if accretion proceeds both via the accretion disc and directly via the stream during some intermediate stages; the detailed process through which the disc disappears still needs investigations., Comment: 6 pages, 5 figures. Accepted for publication in A&A

The disc instability model (DIM) has been very successful in explaining the dwarf nova outbursts observed in cataclysmic variables. When, as in intermediate polars (IP), the accreting white dwarf is magnetized, the disc is truncated at the magnetospheric radius, but for mass-transfer rates corresponding to the thermal-viscous instability such systems should still exhibit dwarf-nova outbursts. Yet, the majority of intermediate polars in which the magnetic field is not large enough to completely disrupt the accretion disc, seem to be stable, and the rare observed outbursts, in particular in systems with long orbital periods, are much shorter than normal dwarf-nova outbursts. We investigate the predictions of the disc instability model for intermediate polars in order to determine which of the observed properties of these systems can be explained by the DIM. We use our numerical code for the time evolution of accretion discs, modified to include the effects of the magnetic field, with constant or variable mass transfer from the secondary star. We show that intermediate polars have mass transfer low enough and magnetic fields large enough to keep the accretion disc stable on the cold equilibrium branch. We show that the infrequent and short outbursts observed in long period systems, such as e.g., TV Col, cannot be attributed to the thermal-viscous instability of the accretion disc, but instead have to be triggered by an enhanced mass-transfer from the secondary, or, more likely, by some instability coupling the white dwarf magnetic field with that generated by the magnetorotational instability operating in the accretion disc. Longer outbursts (a few days) could result from the disc instability., Comment: 7 pages, 5 figures; submitted to Astronomy & Astrophysics

Context. The disc instability model (DIM) successfully explains why many accreting compact binary systems exhibit outbursts, during which their luminosity increases by orders of magnitude. The DIM correctly predicts which systems should be transient and works regardless of whether the accretor is a black hole, a neutron star or a white dwarf. However, it has been known for some time that the outbursts of X-ray binaries (which contain neutron-star or black-hole accretors) exhibit hysteresis in the X-ray hardness-intensity diagram (HID). More recently, it has been shown that the outbursts of accreting white dwarfs also show hysteresis, but in a diagram combining optical, EUV and X-ray fluxes. Aims. We examine here the nature of the hysteresis observed in cataclysmic variables and low-mass X-ray binaries. Methods. We use the Hameury et al. (1998) code for modelling dwarf nova outbursts, and construct the hardness intensity diagram as predicted by the disc instability model. Results. We show explicitly that the standard DIM - modified only to account for disc truncation - can explain the hysteresis observed in accreting white dwarfs, but cannot explain that observed in X-ray binaries. Conclusions. The spectral evidence for the existence of different accretion regimes / components (disc, corona, jets, etc.) should be based only on wavebands that are specific to the innermost parts of the discs, i.e. EUV and X-rays, which is a difficult task because of interstellar absorption. The existing data, however, indicate that an EUV/X-ray hysteresis is present in SS Cyg., Accepted for publication in Astronomy and Astrophysics; 7 pages, 8 figures

Very faint X-ray binaries appear to be transient in many cases with peak luminosities much fainter than that of usual soft X-ray transients, but their nature still remains elusive. We investigate the possibility that this transient behaviour is due to the same thermal/viscous instability which is responsible for outbursts of bright soft X-ray transients, occurring in ultracompact binaries for adequately low mass-transfer rates. More generally, we investigate the observational consequences of this instability when it occurs in ultracompact binaries. We use our code for modelling the thermal-viscous instability of the accretion disc, assumed here to be hydrogen poor. We also take into account the effects of disc X-ray irradiation, and consider the impact of the mass-transfer rate on the outburst brightness. We find that one can reproduce the observed properties of both the very faint and the brighter short transients (peak luminosity, duration, recurrence times), provided that the viscosity parameter in quiescence is slightly smaller (typically a factor of between two and four) than in bright soft X-ray transients and normal dwarf nova outbursts, the viscosity in outburst being unchanged. This possibly reflects the impact of chemical composition on non-ideal MHD effects affecting magnetically driven turbulence in poorly ionized discs., 9 pages, 7 figures, Astronomy and Astrophysics, in press

At super-Eddington rates accretion flows onto black holes have been described as slim (aspect ratio $H/R \lesssim 1$) or thick (H/R >1) discs, also known as tori or (Polish) doughnuts. The relation between the two descriptions has never been established, but it was commonly believed that at sufficiently high accretion rates slim discs inflate, becoming thick. We wish to establish under what conditions slim accretion flows become thick. We use analytical equations, numerical 1+1 schemes, and numerical radiative MHD codes to describe and compare various accretion flow models at very high accretion rates.We find that the dominant effect of advection at high accretion rates precludes slim discs becoming thick. At super-Eddington rates accretion flows around black holes can always be considered slim rather than thick., 8 pages, 5 figures. Astronomy & Astrophysics, in press

Accretion disks in AGN should be subject to the same type of instability as in cataclysmic variables (CVs) or in low-mass X-ray binaries (LMXBs), which leads to dwarf nova and soft X-ray transient outbursts. It has been suggested that this thermal/viscous instability can account for the long term variability of AGNs. We test this assertion by presenting a systematic study of the application of the disk instability model (DIM) to AGNs. We are using the adaptative grid numerical code we have developed in the context of CVs, enabling us to fully resolve the radial structure of the disk. We show that, because in AGN disks the Mach numbers are very large, the heating and cooling fronts are so narrow that they cannot be resolved by the numerical codes that have been used until now. In addition, these fronts propagate on time scales much shorter than the viscous time. As a result, a sequence of heating and cooling fronts propagate back and forth in the disk, leading only to small variations of the accretion rate onto the black hole, with short quiescent states occurring for very low mass transfer rates only. Truncation of the inner part of the disk by e.g. an ADAF does not alter this result, but enables longer quiescent states. Finally we discuss the effects of irradiation by the central X-ray source, and show that, even for extremely high irradiation efficiencies, outbursts are not a natural outcome of the model., Astronomy & Astrophysics - in press

We consider evolutionary models for the population of short-period ( 0.35) donors at the RLOF. Longer period (0.5 - 1 day) SXT might descend from systems with initial donor mass about 1 solar and X_c < 0.35. It is unnecessary to invoke donors with almost hydrogen-depleted cores to explain the origin of LMBHB. Our models suggest that a very high efficiency of common-envelopes ejection is necessary to form LMBHB, unless currently commonly accepted empirical estimates of mass-loss rates by winds for pre-WR and WR-stars are significantly over-evaluated., 11 pages. To appear in New Astronomy Review, vol. 51, issues 10-12, Proceedings of "Jean-Pierre Lasota, X-ray binaries, accretion disks and compact stars" (October 2007); Ed. M. Abramowicz; v3: Eq. (8) for upper limit on mass-transfer rate and Figs. 4 and 7 corrected

This is an introduction to the models of accretion discs around black holes. After a presentation of the nonrelativistic equations describing the structure and evolution of geometrically thin accretion discs, we discuss their steady-state solutions and compare them to observation. Next, we describe in detail the thermal–viscous disc instability model and its application to dwarf novae for which it was designed and its X-ray irradiated disc version which explains the soft X-ray transients, i.e. outbursting black hole low-mass X-ray binaries. We then turn to the role of advection in accretion flows onto black holes illustrating its action and importance with a toy model describing both ADAFs and slim discs. We conclude with a presentation of the general-relativistic formalism describing accretion discs in the Kerr spacetime.

We investigate the conjecture by Sikora, Stawarz & Lasota (2007) that the observed AGN--radio-loudness bimodality can be explained by the morphology-related bimodality of black-hole spin distribution in the centers of galaxies: central black holes in giant elliptical galaxies may have (on average) much larger spins than black holes in spiral/disc galaxies. We study how accretion from a warped disc influences the evolution of black hole spins and conclude that within the cosmological framework, where the most massive BHs have grown in mass via merger driven accretion, one indeed expects most supermassive black holes in elliptical galaxies to have on average higher spin than black holes in spiral galaxies, where random, small accretion episodes (e.g. tidally disrupted stars, accretion of molecular clouds) might have played a more important role., Accepted for publication in the Astrophysical Journal

We investigate how the total radio luminosity of AGN-powered radio sources depends on their accretion luminosity and the central black hole mass. Our studies cover about seven orders of magnitude in accretion luminosity and the full range of AGN black hole masses. We find that AGNs form two distinct and well separated sequences on the radio-loudness -- Eddington-ratio plane. The `upper' sequence is formed by radio selected AGNs, the `lower' sequence contains mainly optically selected objects. Whereas an apparent `gap' between the two sequences may be an artifact of selection effects, the sequences themselves mark the real upper bounds of radio-loudness of two distinct populations of AGNs: those hosted respectively by elliptical and disk galaxies. Both sequences show the same dependence of the radio-loudness on the Eddington ratio (an increase with decreasing Eddington ratio), which suggests that the normalization of this dependence is determined by the black hole spin. This implies that central black holes in giant elliptical galaxies have (on average) much larger spins than black holes in spiral/disc galaxies. This galaxy-morphology related radio-dichotomy breaks down at high accretion rates where the dominant fraction of luminous quasars hosted by elliptical galaxies is radio quiet. This led to speculations in the literature that formation of powerful jets at high accretion rates is intermittent and related to switches between two disk accretion modes, as directly observed in some BH X-ray binaries. We argue that such intermittency can be reconciled with the spin paradigm, provided that successful formation of relativistic jets by rotating black holes requires collimation by MHD outflows from accretion disks., Comment: 33 pages, 4 figures and 5 tables included. Final version, accepted for publication in ApJ

Accretion disks in AGN should be subject to the same disk instability responsible for dwarf novae outbursts and soft X-ray transients in cataclysmic variables (CVs) and LMXBs. It has been suggested that this thermal/viscous instability can account for long term variability of AGNs. We analyze here the application of the DIM to the AGN case, using our adaptive grid numerical code developed in the context of CVs, enabling us to fully resolve the disk radial structure. We show that in AGNs, the width of heating and cooling fronts is so small that they cannot be resolved by standard codes, and that they propagate on time scales much shorter than the viscous time. As a result, transition fronts propagate back and forth in the disk, leading only to small luminosity variations. Truncation of the inner part of the disk by e.g. an ADAF will not alter this result, but enables the presence of quiescent states.

Outer radius variations play an important role in the disc structure and evolution. We consider theoretical and observational consequences of such variations in cataclysmic binaries and low-mass X-ray binaries. We find that the action of tidal torques must be important well inside the tidal radius. We also conclude that it is doubtful that the tidal–thermal instability is responsible for the superoutburst/superhump phenomena.

The formation of relativistic astrophysical jets is presumably mediated by magnetic fields threading accretion disks and central, rapidly rotating objects. As it is accelerated by magnetic stresses, the jet's kinetic energy flux grows at the expense of its Poynting flux. However, it is unclear how efficient is the conversion from magnetic to kinetic energy and whether there are any observational signatures of this process. We address this issue in the context of jets in quasars. Using data from all spatial scales, we demonstrate that in these objects the conversion from Poynting-flux-dominated to matter-dominated jets is very likely to take place closer to the black hole than the region where most of the Doppler boosted radiation observed in blazars is produced. We briefly discuss the possibility that blazar activity can be induced by global MHD instabilities, e.g., via the production of localized velocity gradients that lead to dissipative events such as shocks or magnetic reconnection, where acceleration of relativistic particles and production of non-thermal flares is taking place., Accepted by the Astrophysical Journal

Using XMM-Newton EPIC imaging data, we have detected 50 low-luminosity X-ray sources in the field of view of M 22, where 5 +/- 3 of these sources are likely to be related to the cluster. Using differential optical photometry, we have identified probable counterparts to those sources belonging to the cluster. Using X-ray spectroscopic and timing studies, supported by the optical colours, we propose that the most central X-ray sources in the cluster are cataclysmic variables, millisecond pulsars, active binaries and a blue straggler. We also identify a cluster of galaxies behind this globular cluster., 11 pages, 7 figures, accepted for publication in A&A

We show that the recently developed thermal model which successfully describes how jets are launched by young stellar objects, when applied to system containing disk-accreting white dwarfs naturally explain the otherwise surprising absence of jets in cataclysmic variable stars. Our main argument uses the crucial element of the thermal model, namely that the accreted material is strongly shocked due to large gradients of physical quantities in the boundary layer, and then cools on a time scale longer than its ejection time from the disk. In our scenario the magnetic fields are weak, and serve only to recollimate the outflow at large distances from the source, or to initiate the shock, but not as a jet-driving agent. Using two criteria in that model, for the shock formation and for the ejection of mass, we find the mass accretion rate above which jets could be blown from accretion disks around young stellar objects and white dwarfs. We find that these accretion mass rates are ~10^{-7} Mo/yr, and ~10^{-6} Mo/yr for young stellar objects and white dwarfs respectively. Considering the uncertainties of the model, these limits could overestimate the critical value by a factor of ~10., Comment: Submitted to Astronomy and Astrophysics

We show that the absence of outbursts during low states of VY Scl stars is easily explained if white dwarfs in these systems are weakly magnetized ($\mu \gta 5 10^{30}$ G cm$^{3}$). However, some of the VY Scl stars are observed to have very slow declines to minimum and similarly slow rises to maximum. The absence of outbursts during such intermediate (as opposed to low) states, which last much longer than typical disc's viscous times, can be explained only if accretion discs are absent when their temperatures would correspond to an unstable state. This requires magnetic fields stronger than those explaining outburst absence during low states, since white dwarfs in this sub-class of VY Scl stars should have magnetic moments $\mu \gta 1.5 10^{33}$ G cm$^{3}$ i.e. similar to those of Intermediate Polars. Since at maximum brightness several VY Scl stars are SW Sex stars, this conclusion is in agreement with recent claims about the magnetic nature of these systems.

The disc instability model which is supposed to describe outbursts of dwarf nova and low-mass X-ray binary transient systems is presented and reviewed in detail. Various deficiencies of the model are pointed out and various remedies and generalizations are presented and discussed., 102 pages, 36 figures (43 postscript files); New Astronomy Reviews, in press

We construct time-dependent models of accretion discs around black holes and neutron stars. We investigate the effect of evaporating the disc inner regions during quiescence on the predictions of the Disc Instability Model (DIM) for these systems. We do not include irradiation of the disc in the models. Removing the inner, most unstable parts of the accretion disc increases the predicted recurrence times. However, DIMs with values of the viscosity parameter alpha_hot ~ 0.1 and alpha_cold ~ 0.02 (typical of applications of the DIM to standard dwarf nova outbursts) fail to reproduce the long recurrence times of SXTs (unless we resort to fine-tuning of the parameters) independent of the evaporation strength. We show that models with evaporation and a smaller value of alpha_cold (~ 0.005) do reproduce the long recurrence times and the accretion rates at the level of the Eddington rate observed in outburst. The large difference between the values of alpha_hot and alpha_cold, if confirmed when disc irradiation is included, suggests that several viscosity mechanisms operate in these accretion discs. For some choices of parameters our models predict reflares during the decline from outburst. They are a physical property of the model and result from a heating front forming in the wake of an initial cooling front and subsequent, multiple front reflections. The reflares disappear in low-alpha models where front reflection can not occur., Comment: 14 pages, 8 figures; accepted for publication in MNRAS

We use the Shaviv and Wehrse (1991) code to model the vertical structure and the emission properties of quiescent dwarf nova discs. We find that in the case of HT Cas the quiescent disc must be optically thin, in contradiction with the requirements of the standard disc instability model. We find a viscosity parameter alpha > 1. Although this is much less than values (~ 100) obtained in isothermal slab models it is not consistent with the accretion disc model assumptions., To be published in Physics Reports, proceedings of the conference Astrophysical Fluids: From Atomic Nuclei to Stars and Galaxies; 11 pages, 7 figures

We review some of the properties of Advection-Dominated Accretion Flow (ADAF) models and show that they successfully describe many astrophysical systems. Despite these successful applications some fundamental problems still remain to be solved, the most important one being the physics of the transition between an ADAF and a geometrically thin Keplerian disc., Comment: To be published in Physics Reports, proceedings of the conference Astrophysical Fluids: From Atomic Nuclei to Stars and Galaxies; 10 pages, 3 figures

We calculate the effect of illumination of dwarf nova accretion discs by radiation from a hot, central, white dwarf. We show that only for very hot white dwarfs (Teff ~ 40 000$ K) the inner region of quiescent dwarf nova discs are partially depleted so that the delay between the rise to outburst of the optical and UV fluxes would be increased as suggested recently by King (1997). This depletion, however, must create several small outbursts between main outbursts, contrary to observations. Lower white dwarf temperatures may cause the outburts to be of the `inside-out' type removing the UV delay. We conclude that white dwarf irradiation of dwarf nova discs is not very efficient for example because the UV radiation from the hot white dwarf does not penetrate deep enough in the disc atmosphere. The total ablation of the inner disc by e.g. evaporation (possibly related to illumination) appears to be a very promising possibility, accounting for both the EUV delay and the general lightcurves properties., Comment: 6 pages, 8 figures; accepted for publication in MNRAS