Accretion Regime Research Articles

Black hole spin evolution affected by magnetic field decay

Black holes are spun up by accreting matter and possibly spun-down by magnetic fields. In our work we consider the effect on black hole rotation of the two electromagnetic processes, Blandford-Znajek and Direct Magnetic Link, that differ in their magnetic field configuration. The efficiency of these processes varies with mass accretion rate and accretion regime and generally result in an equilibrium spin parameter in the range from 0.35 to ~0.98. Magnetic field loses its energy while being accreted that may lead to an increase in equilibrium Kerr parameter for the case of advection-dominated disc. We find magnetic field decay decay can decrease electromagnetic term significantly thus increasing the Kerr parameter. We have performed Monte-Carlo simulations for a supermassive black hole population. Our simulations show broad distributions in Kerr parameter (0.1< a <0.98) with a peak at a~0.6. To explain the high observational Kerr parameter values of a>0.9, episodes of supercritical accretion are required. This implication does not however take into account black hole mergers (that play an important role for supermassive black hole evolution).

THE NATURE OF TRANSITION BLAZARS

Blazars are classically divided into the BL Lac (BLL) and Flat-Spectrum Radio Quasar (FSRQ) subclasses, corresponding to radiatively inefficient and efficient accretion regimes, respectively, largely based on the equivalent width (EW) of their optical broad emission lines (BEL). However, EW-based classification criteria are not physically motivated, and a few blazars have previously 'transitioned' from one subclass to the other. We present the first systematic search for these transition blazars in a sample of 602 unique pairs of repeat spectra of 354 blazars in SDSS, finding six clear cases. These transition blazars have bolometric Eddington ratios of ~0.3 and low-frequency synchrotron peaks, and are thus FSRQ-like. We show that the strong EW variability (up to an unprecedented factor of >60) is due to swamping of the BELs from variability in jet continuum emission, which is stronger in amplitude and shorter in timescale than typical blazars. Although these transition blazars appear to switch between FSRQ and BLL according to the phenomenologically-based EW scheme, we show that they are most likely rare cases of FSRQs with radiatively efficient accretion flows and especially strongly-beamed jets. These results have implications for the decrease of the apparent BLL population at high-redshifts, and may add credence to claims of a negative BLL redshift evolution.

A deficit of ultraluminous X-ray sources in luminous infrared galaxies

We present results from a Chandra study of ultraluminous X-ray sources (ULXs) in a sample of 17 nearby (D_L<60 Mpc) luminous infrared galaxies (LIRGs), selected to have star formation rates (SFRs) in excess of 7 M_sun yr^-1 and low foreground Galactic column densities (N_H < 5*10^20 cm^-2). A total of 53 ULXs were detected and we confirm that this is a complete catalogue of ULXs for the LIRG sample. We examine the evolution of ULX spectra with luminosity by stacking the spectra of individual objects in three luminosity bins, finding a distinct change in spectral index at luminosity ~2 *10^39 erg s^-1. This may be a change in spectrum as 10 M_sun black holes transit from a ~Eddington to a super-Eddington accretion regime, and is supported by a plausible detection of partially-ionised absorption imprinted on the spectrum of the luminous ULX (L_X ~5*10^39 erg s^-1) CXOU J024238.9-000055 in NGC 1068, consistent with the highly ionised massive wind that we would expect to see driven by a super-Eddington accretion flow. This sample shows a large deficit in the number of ULXs detected per unit SFR (0.2 ULXs M_sun^-1 yr^-1). This deficit also manifests itself as a lower differential X-ray luminosity function normalisation for the LIRG sample than for samples of other star forming galaxies. We show that it is unlikely that this deficit is a purely observational effect. Part of this deficit might be attributable to the high metallicity of the LIRGs impeding the production efficiency of ULXs and/or a lag between the star formation starting and the production of ULXs; however, we argue that the evidence -- including very low N_ULX/L_FIR, and an even lower ULX incidence in the central regions of the LIRGs -- shows that the main culprit for this deficit is likely to be the high column of gas and dust in these galaxies, that fuels the high SFR but also acts to obscure many ULXs from our view.

Mathematical modeling of ice body formation on the wing airfoil surface

The main models and methods for investigating the process of icing of aerodynamic surfaces, the meteorological parameters of the icing, the cloud types, the types of the ice coating, and the systems of protection against icing are analyzed. A software is developed which makes it possible to model the icing on the leading edge of the wing airfoil. The motion of the carrying medium is described on the basis of the Navier-Stokes equations for a compressible gas, together with the Spalart-Allmaras turbulence model. A delayed model is applied to describe the motion of supercooled water droplets. The numerical modeling of the ice accretion process is based on the control volume method which consists in the solution of the differential equations of the mass, momentum, and energy conservation for any surface element using a model which allows for the surface moisture behavior on macro- and microlevels. With reference to the example of the NACA 0012 airfoil in two-dimensional viscous compressible two-phase flow the main shapes of the ice bodies are reproduced with account for the ice accretion regime, the form of the moisture existence on the airfoil, the surface heating, and the effect of the variation in the airfoil geometry on the external flow and the aerodynamic characteristics.

He-accreting white dwarfs: accretion regimes and final outcomes

The behaviour of carbon-oxygen white dwarfs (WDs) subject to direct helium accretion is extensively studied. We aim to analyze the thermal response of the accreting WD to mass deposition at different time scales. The analysis has been performed for initial WDs masses and accretion rates in the range (0.60 - 1.02) Msun and 1.e-9 - 1.e-5 Msun/yr, respectively. Thermal regimes in the parameters space M_{WD} - dot{M}_{He}, leading to formation of red-giant-like structure, steady burning of He, mild, strong and dynamical flashes have been identified and the transition between those regimes has been studied in detail. In particular, the physical properties of WDs experiencing the He-flash accretion regime have been investigated in order to determine the mass retention efficiency as a function of the accretor total mass and accretion rate. We also discuss to what extent the building-up of a He-rich layer via H-burning could be described according to the behaviour of models accreting He-rich matter directly. Polynomial fits to the obtained results are provided for use in binary population synthesis computations. Several applications for close binary systems with He-rich donors and CO WD accretors are considered and the relevance of the results for the interpretation of He-novae is discussed.

The jet–disc connection in AGN

We present our latest results on the connection between accretion rate and relativistic jet power in AGN, by using a large sample which includes mostly blazars, but contains also some radio--galaxies. The jet power can be traced by $\gamma$--ray luminosity in the case of blazars, and radio luminosity for both classes. The accretion disc luminosity is instead traced by the broad emission lines. Among blazars, we find a correlation between broad line emission and the $\gamma$--ray or radio luminosities, suggesting a direct tight connection between jet power and accretion rate. We confirm that the observational differences between blazar subclasses reflect differences in the accretion regime, but with blazars only we cannot properly access the low--accretion regime. By introducing radio--galaxies, we succeed in observing the fingerprint of the transition between radiatively efficient and inefficient accretion discs in the jetted AGN family. The transition occurs at the standard critical value $L_{\rm d}/L_{\rm Edd}\sim10^{-2}$ and it appears smooth. Below this value, the ionizing luminosity emitted by the accretion structure drops significantly.

Ultra‐Luminous X‐ray sources and intermediate‐mass black holes

AbstractThe extremely high X‐ray luminosity of non‐nuclear Ultra Luminous X‐ray sources (ULXs) may be evidence of the existence of black holes with masses intermediate between those produced by stellar evolution and those encountered in active galactic nuclei. Alternatively, they may reflect the existence of stellar mass black holes undergoing steady super‐Eddington accretion. In this short review, we will describe the main X‐ray and optical observational properties of these ULXs and discuss their likely accretion regimes. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Radio-mode feedback in local AGNs: dependence on the central black hole parameters

Radio mode feedback, in which most of the energy of an active galactic nucleus (AGN) is released in a kinetic form via radio-emitting jets, is thought to play an important role in the maintenance of massive galaxies in the present-day Universe. We study the link between radio emission and the properties of the central black hole in a large sample of local radio galaxies drawn from the Sloan Digital Sky Survey (SDSS), based on the catalogue of Best and Heckman (2012). Our sample is mainly dominated by massive black holes (mostly in the range $10^8-10^9 M_{\odot}$) accreting at very low Eddington ratios (typically $\lambda < 0.01$). In broad agreement with previously reported trends, we find that radio galaxies are preferentially associated with the more massive black holes, and that the radio loudness parameter seems to increase with decreasing Eddington ratio. We compare our results with previous studies in the literature, noting potential biases. The majority of the local radio galaxies in our sample are currently in a radiatively inefficient accretion regime, where kinetic feedback dominates over radiative feedback. We discuss possible physical interpretations of the observed trends in the context of a two-stage feedback process involving a transition in the underlying accretion modes.

FERMIBUBBLES INFLATED BY WINDS LAUNCHED FROM THE HOT ACCRETION FLOW IN SGR A*

A pair of giant gamma-ray bubbles have been revealed by the {\it Fermi} LAT. In this paper we investigate their formation mechanism. Observations have indicated that the activity of the supermassive black hole located at the Galactic center, Sgr A*, was much stronger than the present time. Specifically, one possibility is that while Sgr A* was also in the hot accretion regime, the accretion rate should be $10^3-10^4$ times higher during the past $\sim 10^7$ yr. On the other hand, recent MHD numerical simulations of hot accretion flows have unambiguously shown the existence of strong winds and obtained their properties. Based on these knowledge, by performing three-dimensional hydrodynamical simulations, we show in this paper that the Fermi bubbles could be inflated by winds launched from the ``past' hot accretion flow in Sgr A*. In our model, the active phase of Sgr A* is required to last for about 10 million years and it was quenched no more than 0.2 million years ago. The Central Molecular Zone (CMZ) is included and it collimates the wind orientation towards the Galactic poles. Viscosity suppresses the Rayleigh-Taylor and Kelvin-Helmholtz instabilities and results in the smoothness of the bubble edge. The main observational features of the bubbles can be well explained. Specifically, the {\it ROSAT} X-ray features are interpreted by the shocked interstellar medium and the interaction region between winds and CMZ gas. The thermal pressure and temperature obtained in our model are in good consistency with the recent {\it Suzaku} observations.

Hiccup accretion in the swinging pulsar IGR J18245–2452

IGR J18245-2452 is the fifteenth discovered accreting millisecond X-ray pulsar and the first source of this class showing direct evidence for transition between accretion and rotational powered emission states. These swing provided the strongest confirmation of the pulsar recycling scenario available so far. During the two XMM-Newton observations that were carried out while the source was in outburst in April 2013, IGR J18245-2452 displayed a unique and peculiar variability of its X-ray emission. In this work, we report on a detailed analysis of the XMM- Newton data and focus in particular on the timing and spectral variability of the source. IGR J18245-2452 continuously switches between lower and higher intensity states, with typical variations in flux up to a factor of about 500 in time scales as short as few seconds. These variations in the source intensity are sometimes associated to a dramatic spectral hardening, during which the power-law photon index of the source changes from Gamma=1.7 to Gamma=0.9. The pulse profiles extracted at different count rates and energies show a complex variability. These phenomena are not usually observed in accreting millisecond X-ray pulsars, at least not on such a short time scale. Fast variability was also found in the ATCA radio observations carried out for about 6 hours during the outburst at a frequency of 5.5 and 9 GHz. We interpret the variability observed from IGR J18245-2452 in terms of a "hiccup" accretion phase, during which the accretion of material from the inner boundary of the Keplerian disk is reduced by the onset of centrifugal inhibition of accretion, possibly causing the launch of strong outflows. Changes across accretion and propeller regimes have been long predicted and reproduced by MHD simulations of accreting millisecond X-ray pulsars but never observed to produce an extreme variability as that shown by IGR J18245-2452.

The X-ray emission of the high-mass X-ray binary IGR J17200−3116

The source IGR J17200-3116 was discovered in the hard X-ray band by INTEGRAL. A periodic X-ray modulation at ~326 s was detected in its Swift light curves by our group (and subsequently confirmed by a Swift campaign). In this paper, we report on the analysis of all the Swift observations, which were collected between 2005 and 2011, and of a ~20 ks XMM-Newton pointing that was carried out in 2013 September. During the years covered by the Swift and XMM-Newton observations, the 1-10 keV fluxes range from ~1.5 to 4E-11 erg/cm^2/s. IGR J17200-3116 displays spectral variability as a function of the pulse phase and its light curves show at least one short (a few hundreds of seconds) dip, during which the flux dropped at 20-30% of the average level. Overall, the timing and spectral characteristics of IGR J17200-3116 point to an accreting neutron star in a high-mass system but, while the pulse-phase spectral variability can be accounted for by assuming a variable local absorbing column density, the origin of the dip is unclear. We discuss different possible explanations for this feature, favouring a transition to an ineffective accretion regime, instead of an enhanced absorption along the line of sight.

The influence of mass-transfer variability on the growth of white dwarfs, and the implications for Type Ia supernova rates

White dwarfs (WDs) can increase their mass by accretion from companion stars, provided the mass-accretion rate is high enough to avoid nova eruptions. The accretion regimes that allow growth of the WDs are usually calculated assuming constant mass-transfer rates. However, it is possible that these systems are influenced by effects that cause the rate to fluctuate on various time-scales. We investigate how long-term mass-transfer variability affects accreting WDs systems. We show that, if such variability is present, it expands the parameter space of binaries where the WD can effectively increase its mass. Furthermore, we find that the Type Ia supernova (SNIa) rate is enhanced by a factor 2–2.5 to a rate that is comparable with the lower limit of the observed rates. The changes in the delay-time distribution allow for more SNIae in stellar populations with ages of a few Gyr. Thus, mass-transfer variability gives rise to a new formation channel of SNIa events that can significantly contribute to the SNIa rate. Mass-transfer variability is also likely to affect other binary populations through enhanced WD growth. For example, it may explain why WDs in cataclysmic variables are observed to be more massive than single WDs, on average.

Properties of stellar populations in isolated lenticular galaxies

In this paper we present the results of long-slit spectral observations for a sample of isolated lenticular galaxies, made with the SCORPIO and SCORPIO-2 spectrographs of the 6-meter BTA telescope of the SAO RAS. By applying full spectral fitting technique using the stellar population evolutionary synthesis models, we have measured the radial profiles of the stellar line-of-sight velocity as well as the velocity dispersion, SSP-equivalent age and SSP-equivalent metallicity of stars along the radius in 12 targets. The resulting averaged ages of the stellar population in bulges and discs cover an entire range of possible values from 1.5 to 15 Gyr which indicates the absence of a certain formation epoch for the structural components in the isolated lenticular galaxies, unlike in the members of clusters and rich groups: they could have been formed at a redshift of $z>2$ as well as only a few billion years ago. Unlike S0 galaxies in more dense environments, the isolated galaxies typically have the same age of stars in the bulges and discs. The disc-embedded lenses and rings of increased stellar brightness, identified from the photometry in 7 of 11 galaxies, do not differ strongly from the stellar discs as concerning the properties of stellar populations and stellar velocity dispersion. We conclude that the final shaping of the morphological type of a lenticular galaxy in complete isolation is critically dependent on the possible regimes of cold-gas accretion from outside.

Ultraluminous X-ray sources: a deeper insight into their spectral evolution

We select a sample of nearby Ultraluminous X-ray sources with long XMM-Newton observations and analyse all the available XMM-Newton data using both X-ray spectral fitting techniques and hardness-intensity diagrams. The sample includes IC 342 X-1, NGC 5204 X-1, NGC 5408 X-1, Holmberg IX X-1, Holmberg II X-1, NGC 1313 X-1, NGC 1313 X-2 and NGC 253 X-1. We found that, although a common reference model can be used to describe the X-ray spectra, the sources show different spectral evolutions, phenomenologically described in terms of variations in the properties of a soft component and a high energy tail. Variations at low energies are accounted for (mostly) by changes in the normalization of the soft component and/or in the column density to the source, while variations in the high energy tail by changes in the parameters of an optically thick corona. This spectral variability is rather well characterized on a colour-colour and hardness-intensity diagram in terms of suitably defined hardness ratios. We suggest the existence of a variability pattern on the hardness-intensity diagram and we interpret it in terms of the switch between a near-Eddington and a super-Eddington accretion regime. The transition between the two regimes seems to be driven mostly by changes in the contribution of the soft component, which can be explained in terms of the increasing importance of wind emission. The analysis is complemented by an investigation of the short-term time variability of all the sources. In general, no clear correlation between the spectral and temporal properties is found.

Theory of wind accretion

A review of wind accretion in high-mass X-ray binaries is presented. We focus attention to different regimes of quasi-spherical accretion onto the neutron star: the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically toward NS magnetospghere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. Two regimes of accretion are separated by an X-ray luminosity of about $4\times10^{36}$ erg/s. In the subsonic case, which sets in at low luminosities, a hot quasi-spherical shell must be formed around the magnetosphere, and the actual accretion rate onto NS is determined by ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. We calculate the rate of plasma entry the magnetopshere and the angular momentum transfer in the shell due to turbulent viscosity appearing in the convective differentially rotating shell. We also discuss and calculate the structure of the magnetospheric boundary layer where the angular momentum between the rotating magnetosphere and the base of the differentially rotating quasi-spherical shell takes place. We show how observations of equilibrium X-ray pulsars Vela X-1 and GX 301-2 can be used to estimate dimensionless parameters of the subsonic settling accretion theory, and obtain the width of the magnetospheric boundary layer for these pulsars.

MHD Simulations of Magnetized Stars in the Propeller Regime of Accretion

Accreting magnetized stars may be in the propeller regime of disc accretion in which the angular velocity of the stellar magnetosphere exceeds that of the inner disc. In these systems, the stellar magnetosphere acts as a centrifugal barrier and inhibits matter accretion onto the rapidly rotating star. Instead, the matter accreting through the disc accumulates at the disc-magnetosphere interface where it picks up angular momentum and is ejected from the system as a wide-angled outflow which gradually collimates at larger distances from the star. If the ejection rate is lower than the accretion rate, the matter will accumulate at the boundary faster than it can be ejected; in this case, accretion onto the star proceeds through an episodic accretion instability in which the episodes of matter accumulation are followed by a brief episode of simultaneous ejection and accretion of matter onto the star. In addition to the matter dominated wind component, the propeller outflow also exhibits a well-collimated, magnetically-dominated Poynting jet which transports energy and angular momentum away from the star. The propeller mechanism may explain some of the weakly-collimated jets and winds observed around some T Tauri stars as well as the episodic variability present in their light curves. It may also explain some of the quasi-periodic variability observed in cataclysmic variables, millisecond pulsars and other magnetized stars.

The reflection component in NS LMXBs

Thanks to the good spectral resolution and large effective area of the EPIC/PN instrument on board of XMM-Newton, we have at hand a large number of observations of accreting low-mass X-ray binaries, that allow for the fist time a comprehensive view on the characteristics of the reflection component at different accretion regimes and to probe the effects of a magnetosphere on its formation. We focus here on a comparative analysis of the reflection component from a series of spectroscopic studies on selected sources: 4U 1705-44, observed both in the soft and hard state, the pulsating ms pulsars SAX J1808.4- 3658 and IGR J17511-3057, and the intermittent pulsar HETE J1900-2455. Although the sources can present very similar accretion rates and continuum shapes, the reflection parameters do not generally result the same, moreover the effect of a magnetosphere on the formation of the reflection component appears elusive.

崇明东滩盐沼植被扩散格局及其形成机制研究

长江河口盐沼植被的形成和演化是生物与其生长环境相互作用的结果。以崇明东滩盐沼植被典型扩散前沿为研究对象，2011至2012年期间调查了盐沼植被扩散前沿实生苗扩散、定居以及形成的扩散格局，同时测定了盐沼植被扩散前沿的潮滩冲淤动态和水文动力条件。研究结果表明，崇明东滩盐沼植被在扩散前沿形成了互花米草-光滩（<em>Spartina alterniflora </em>-Mudflat，SM）和互花米草-海三棱藨草-光滩（<em>Spartina alterniflora</em>-<em>Scirpus mariqueter </em>-Mudflat，SSM）两种典型的扩散格局。冲淤动态和水文动力条件是影响盐沼植被扩散格局的重要因子，尤其是在4-6月盐沼植物实生苗传播和定居的关键阶段。在此基础上，分析了东滩盐沼植被扩散前沿的生物-物理相互作用以及盐沼植被扩散格局的形成机制。研究结果不仅有助于理解长江河口地区盐沼植被扩散的生物物理过程，并对全球气候变化和海平面上升条件下滨海生态系统动态预测与湿地保护与管理具有重要的意义。;The range expansion of saltmarshes in the Yangtze Estuary depends on accretion of dynamic mudflats resulting from the large amount of silt brought down the Yangtze River. The formation and succession of saltmarsh vegetation is a result of the interaction between saltmarsh plants and their living environment. Over the last 15 years, rapid invasion of large areas of the saltmarshes of Chongming Dongtan by an exotic plant<em> Spartina alterniflora</em> has occurred. Two types of advancing fronts are found: the <em>S. alterniflora</em>-mudflat (SM) front and the <em>S. alterniflora</em>-<em>Scirpus mariqueter </em>-mudflat (SSM) front. The dynamic and natural characteristics of the coastal wetlands at Chongming Dongtan, both in terms of sediment deposition, salt marsh succession and biological invasion, make it an ideal place for studying the interaction between the range expansion of saltmarsh vegetation and the hydrodynamic regimes. In this study, we invested the seedling dispersal, establishment and formation of expansion pattern at the advancing fronts during the years of 2011 and 2012. At the same time, the accretion/erosion regime and hydrodynamic conditions at the saltmarsh expansion fronts were measured. The research aims were to: 1) test whether and how the hydrodynamic regimes govern the spatial structure of range expansion of saltmarshes at these two advancing fronts; 2) determine the interactions between the range expansion of saltmarsh and hydrodynamic regimes, and 3) understand the mechanisms underlying the range expansion of saltmarshes and their implications for the spatial structure of tidal saltmarshes. The results showed that the accretion/erosion regime and hydrodynamic condition, especially in the critical stage of seedling establishment during April-June, were the most important factors responsible for the formation of these two typical expansion patterns. The rate of range expansion at the SM front was much faster than the SSM front. The different colonization behaviors at these two types of advancing fronts could be related to the differences in hydrodynamic regimes. At the site with a regime of autumn/winter erosion and spring/summer accretion, the sediment accretion and seedling recruitment of <em>S. alterniflora</em> were much higher, while the wave energy and current velocity were relatively gentle during the critical period of seedling establishment. At such site, the original pioneer species of <em>S. mariqueter</em> was replaced by<em> S. alterniflora</em> and a SM pattern of range expansion developed. In contrast, at the site with a relatively stable autumn/winter accretion regime and relatively strong wave energy and current velocity during the critical period of seedling establishment, the original pioneer species of <em>S. mariqueter</em> could remain within the advancing front and a SSM pattern of range expansion developed. Based on these results, the impact of abiotic and biotic factors governing the range expansion of and its implications on the spatial structure of tidal saltmarshes are discussed. The results from this study can provide a valuable insight into the bio-physical interactions on the formation of expansion patterns and the mechanisms underlying the range expansion of saltmarshes in the Yangtze Estuary, which could be important for the maintenance of coastal saltmarsh resource and biodiversity as well as the dynamics prediction of coastal saltmarshes under the impact of climate change.

The magnetospheric accretion/ejection process in young stellar objects: open issues and perspectives

This summary talk aims at highlighting some of the remaining open issues regarding the physics of the magnetospheric accretion/ejection process in young stellar objects (YSOs). It lists a number of questions related to YSOs magnetic fields and accretion regimes, the structure and properties of accretion shocks, the origin of multiple outflow components, and the impact of the star-disk magnetic interaction onto early angular momentum evolution.

Boundary Between Stable and Unstable Regimes of Accretion

We investigated the boundary between stable and unstable regimes of accretion and its dependence on different parameters. Simulations were performed using a "cubed sphere" code with high grid resolution (244 grid points in the azimuthal direction), which is twice as high as that used in our earlier studies. We chose a very low viscosity value, with alpha-parameter alpha=0.02. We observed from the simulations that the boundary strongly depends on the ratio between magnetospheric radius r_m (where the magnetic stress in the magnetosphere matches the matter stress in the disk) and corotation radius r_cor (where the Keplerian velocity in the disk is equal to the angular velocity of the star). For a small misalignment angle of the dipole field, Theta=5 degrees, accretion is unstable if r_cor/r_m>1.35, and is stable otherwise. In cases of a larger misalignment angle of the dipole, Theta=20 degrees, instability occurs at slightly larger values, r_cor/r_m>1.41.