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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
NASA NTRS2020

Spatial distribution of the X-ray-emitting plasma of U Geminorum in quiescence and outburst

We present our analysis of the Suzaku data of U Geminorum (U Gem) from 2012 both in quiescence and outburst. Unlike SS Cygni (SS Cyg), the hard X-ray flux of U Gem is known to increase at times of optical outburst. A sophisticated spectral model and reliable distance estimate now reveal that this can be attributed to the fact that the mass accretion rate onto the white dwarf (WD) does not exceed the critical rate that causes the optically thin to thick transition of the boundary layer. From comparison of the X-ray and optical light curves, the X-ray outburst peak seems to be retarded by 2.1 ± 0.5 d, although there remains uncertainty in the X-ray peak identification, due to short data coverage. The larger delay than SS Cyg (0.9–1.4 d) also supports the lower accretion rate in U Gem. A fluorescent iron 6.4 keV emission line bears significant information about the geometry of the X-ray-emitting hot plasma and the accretion disk (AD) that reflects the hard X-ray emission. Our reflection simulation has shown that the optically thick AD is truncated at a distance of 1.20–1.25 times the white dwarf radius (RWD) in quiescence, and the accreting matter in the disk turns into the optically thin hard-X-ray-emitting plasma at this radius. In outburst, on the other hand, our spectral analysis favors the picture that the optically thick disk reaches the WD surface, although disk truncation can take place in the region of <1.012 RWD. From the profile of the 6.4 keV line, we have also discovered that the accreting matter is heated up close to the maximum temperature immediately after the matter enters the boundary layer at the disk truncation radius. This is consistent with the fact that the hard X-ray spectra of dwarf novae, in general, can be well represented with the cooling flow model.

Mai Takeo↗
DOE OSTI2020

Kinetic Simulations of Radiative Magnetic Reconnection in the Coronae of Accreting Black Holes

We perform 2D and 3D particle-in-cell simulations of reconnection in magnetically dominated e ± plasmas subject to strong Compton cooling. Magnetic reconnection under such conditions can operate in accretion disk coronae around black holes, which produce hard X-rays through Comptonization. Our simulations show that most of the plasma in the reconnection layer is kept cold by Compton losses and locked in magnetically dominated plasmoids with a small thermal pressure. Compton drag clears cavities inside plasmoids and also affects their bulk motions. These effects, however, weakly change the reconnection rate and the plasmoid size distribution from those in nonradiative reconnection. This demonstrates that the reconnection dynamics is governed by similar magnetic stresses in both cases and weakly affected by thermal pressure. We examine the energy distribution of particles energized by radiative reconnection and observe two distinct components: (1) A mildly relativistic peak, which results from bulk motions of cooled plasmoids. This component receives most of the dissipated reconnection power and dominates the output X-ray emission. The peak has a quasi-Maxwellian shape with an effective temperature of ~100 keV. Thus, it mimics thermal Comptonization used previously to fit hard-state spectra of accreting black holes. (2) A high-energy tail, which receives ~20% of the dissipated reconnection power. It is populated by particles accelerated impulsively at X-points or "picked up" by fast outflows from X-points. The high-energy particles immediately cool, and their inverse Compton emission explains the MeV spectral tail detected in the hard state of Cyg X-1. Our first-principle simulations support magnetic reconnection as a mechanism powering hard X-ray emission from magnetically dominated regions of accreting black holes.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2016

Normalizing a Relativistic Model of X-Ray Reflection Definition of the Reflection Fraction and Its Implementation in relxill

Aims. The only relativistic reflection model that implements a parameter relating the intensity incident on an accretion disk to the observed intensity is relxill. The parameter used in earlier versions of this model, referred to as the reflection strength, is unsatisfactory; it has been superseded by a parameter that provides insight into the accretion geometry, namely the reflection fraction. The reflection fraction is defined as the ratio of the coronal intensity illuminating the disk to the coronal intensity that reaches the observer. Methods. The relxill model combines a general relativistic ray-tracing code and a photoionization code to compute the component of radiation reflected from an accretion that is illuminated by an external source. The reflection fraction is a particularly important parameter for relativistic models with well-defined geometry, such as the lamp post model, which is a focus of this paper. Results. Relativistic spectra are compared for three inclinations and for four values of the key parameter of the lamp post model,namely the height above the black hole of the illuminating, on-axis point source. In all cases, the strongest reflection is produced for low source heights and high spin. A low-spin black hole is shown to be incapable of producing enhanced relativistic reflection. Results for the relxill model are compared to those obtained with other models and a Monte Carlo simulation. Conclusions. Fitting data by using the relxill model and the recently implemented reflection fraction, the geometry of a system can be constrained. The reflection-fraction is independent of system parameters such as inclination and black hole spin. The reflection-fraction parameter was implemented with the name reflec_frac all flavours of the relxill model, and the non-relativistic reflection model xillver, in v0.4a (18 January 2016).

accretion↗
NASA NTRS2004

Black Hole Demographics in and Nuclear Properties of Nearby Low Luminosity Radio Galaxies; Connections to Radio Activity?

We combine the results of an HST STIS and WFPC study of a complete sample of 21 nearby UGC low luminosity radio galaxies with the results of a radio VLA and VLBA study of the same sample. We examine the relationship between the stellar and gaseous properties of the galaxies on tens to hundreds of parsec scale with the properties of the radio jets on the same scale. From the VLA and VLBA data we constrain the physics of the outflowing radio plasma from the tens of parsecs to hundreds of kiloparsec scales. From the WFPC2 H alpha and dust images and the STIS kinematics of the near nuclear gas we obtain constraints on the orientation of near nuclear disks of gas and measures of the nuclear stellar, continuum point source, and line emission fluxes. Under the statistically supported assumption that the radio jet issues perpendicular to the disk, we use the orientation of the optical (large scale accretion?) disks to constrain the three-dimensional orientation of the radio ejection. From HST/STIS spectroscopy of the near-nuclear emission line gas we obtain measures/limits on the black hole masses. We examine correlations between the VLBA and VLA-scale radio emission, the nuclear line emission, and the nuclear optical and radio continuum emission. Though our sample is relatively small, it is uniquely well defined, spans a narrow range in redshift and we have a consistent set of high resolution data with which to carefully examine these relationships. We use the combined radio and optical data to: 1) Constrain the orientation, physics, and bulk outflow speed of the radio plasma; 2) Put limits on the mass accretion rate and study the relationship between black hole mass, radio luminosity, and near nuclear gaseous content; 3) Provide insight into the relationship between BL Lac objects and low luminosity radio galaxies.

Baum, S. A.↗
DOE OSTI2025

An Exceptionally Powerful, Radiatively Driven Ultrafast Outflow in the Rapidly Accreting AGN RE J1034+396

We report the analysis of ∼1 Ms of XMM-Newton observations of the rapidly accreting active galactic nucleus RE J1034+396. The 0.3–9 keV EPIC-pn spectra are well described by a model consisting of steep continuum emission from the corona accompanied by relativistically blurred reflection from a highly ionized accretion disk. The source is known to exhibit strong excess soft X-ray emission, which we show is well represented by thermal disk photons Comptonized by a warm plasma spanning the inner accretion flow. Additionally, the EPIC-pn data provide compelling evidence (ΔC ∼ 60 for four additional parameters) for the presence of an ultrafast outflow (UFO) with a line-of-sight velocity v/c = 0.307$^{+0.001}_{-0.005}$, and an emission signature consistent with reflection of the corona from modestly ionized, outflowing gas. The simultaneous 0.5–2.5 keV RGS spectra show clear absorption lines. Modelling of these data confirms the presence of the UFO and constrains its equivalent hydrogen column density, log N H /(atom cm −2 ) = 21.7$^{+0.1}_{-0.2}$. The RGS data also reveal at least two warm absorber components with a modest outflow velocity (1680$^{+40}_{-50}$ km s -1 ). The measured properties and time evolution of the UFO in RE J1034+396 suggest that it is formed from collisionally ionized plasma, launched from the disk surface and accelerated by radiation pressure. The high terminal velocity and substantial absorbing column density imply that the outflow carries sufficient momentum and energy to transform its environment, being capable of driving out essentially all dust and gas it interacts with along the line of sight, even if the AGN were initially surrounded by a Compton-thick absorber.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1983

The relationship between the X-ray and optical luminosities for QSOs

A study of simple models for the optical and X-ray luminosities of QSOs shows that a thick accretion disk around a black hole is consistent with the observed dependence of the X-ray luminosity L(x) on the optical luminosity L(opt) to the bth power, with b roughly in the range 0.5-0.8. The disk is expected to be thick since the sources are accreting at rates near the Edington limit. Using the model of McMillan, Lightman, and Cohn (1981) for the gas supply rate, the evolution of L(opt) is predicted to be proportional to t/t(0) to the -8/3 and that of L(x) is predicted to be proportional to t/t(0) to the -2.

Tucker, W. H.↗
NASA NTRS1992

Rapid oscillations in cataclysmic variables. VIII - YY Draconis (=3A 1148 + 719)

A 16th magnitude cataclysmic variable presumably a dwarf nova, was detected near one of the positions permitted for the hard (2-10 keV) X-ray source 3A 1148 + 719. The optical and UV spectra of YY Dra are fairly typical of cataclysmic variables, except that the TiO absorption bands of an M dwarf can be seen for greater than 5000 A. This shows that the accretion disk is intrinsically faint in quiescence, suggesting a very low accretion rate. Yet the fairly blue continuum slope in the vacuum UV indicates the presence of a small hot object, presumably the white dwarf. The radial velocities at H-alpha indicate a 4.0-hr orbital period, and the IR light curve appears to show the expected 'double-humped' waveform from the distorted secondary. High-speed photometry in the U band reveals a stable periodicity of about 1-percent semiamplitude at a period of 275 s, with some power also in the subharmonic at about 550 s.

Patterson, J.↗
NASA NTRS2009

Accreting X-ray Pulsars

This presentation describes the behavior of matter in environments with extreme magnetic and gravitational fields, explains the instability/stability of accretion disks in certain systems, and discusses how emergent radiation affects accretion flow. Magnetic field measurements are obtained by measuring the lowest cyclotron absorption line energy, observing the cutoff of accretion due to centrifugal inhibition and measuring the spin-up rate at high luminosity.

Wilson-Hodge, Colleen A.↗
NASA NTRS1986

Nucleosynthesis in the neighborhood of a black hole

The preliminary results from simulations of nucleosynthesis inside a thick accretion disk around a black hole are discussed as a function of the accretion rate, the viscosity parameter, and the mass of the black hole. Results for the Bondi accretion case are also presented. Taking the case of a 10-solar mass and a 10 to the 6th-solar mass central Schwarzschild hole, detailed evolution of a representative element of matter as it accretes into the hole is presented in the case when the initial abundance (at the outer edge of the disk) is the same as the solar abundance. It is suggested that such studies may eventually shed light on the composition of the outgoing jets observed in the active galaxies and SS433.

Chakrabarti, Sandip K.↗
NASA NTRS1994

The stability of thick, self-gravitating disks in protostellar systems

Using numerical hydrodynamics techniques, we perform a nonlinear stability analysis of accretion disk systems that contain thick, self-gravitating disks. The systems are initially represented by a point mass M(sub c) at the center and a geometrically thick, axisymmetric disk of mass M(sub d) that supports uniform specific angular momentum and obeys an n = 3/2, polytropic equation of state. The equilibrium disk structure is uniquely defined upon the specification of two key dimensionless system parameters: M(sub d)/M(sub c) and T/absolute value of W (the ratio of rotational kinetic energy if the disk to the gravitational potential energy of the system). The focus of this work is on the identification of systems within this two-dimensional parameter space that are marginally unstable toward the development of nonaxisymmetric distortions. The geometric form and relative pattern speed of the disk's distortion as well as the likelihood of disk fragmentation as a result of such instabilities is examined, particularly in the context of protostellar systems. The value of T/absolute value of W at which thick disks first become dynamically unstable to nonaxisymmetric distortions is found to vary significantly with the mass ratio of the accertion disk system. Nonaxisymmetric eigenmodes with four distinctly different characters are identified in systems with mass ratios in the range 0.2 less than or equal to (M(sub d)/M(sub c)) less than or equal to 5.

Woodward, John W.↗
DOE OSTI2022

Contracting and Expanding Binary Black Holes in 3D Low-mass AGN Disks: The Importance of Separation

LIGO/Virgo has detected several binary black hole (BBH) merger events that may have originated in the accretion disks of active galactic nuclei (AGNs). These events require individual black hole masses that fall within the pair instability supernova mass gap, and therefore these black holes may have been grown from hierarchical mergers. AGN disks are a prime environment for hierarchical mergers and thus a potential location for the progenitors of BBH gravitational wave events. Understanding how a BBH embedded in an AGN disk interacts with the surrounding environment is thus crucial for determining whether this interaction can lead to its merger. However, there are few high-fidelity simulations of this process, and almost all are two-dimensional. We present the results from 3D, high-resolution, local shearing-box simulations of an embedded BBH interacting with an AGN disk. In these first simulations of their kind, we focus on determining the mass accretion rate and the orbital evolution rate at different BBH separations. We find that circular, equal-mass BBHs with separations greater than 10% of their Hill radius contract while accreting at a super-Eddington rate. At smaller separations, however, our 3D simulations find that BBHs expand their orbits. This result suggests that it may be difficult for an AGN disk to push a BBH to merger, but we discuss several mechanisms, including MHD turbulence and radiative and mechanical feedback, which could alleviate this difficulty.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1990

Spectropolarimetric test of the relativistic disk model for the broad emission lines of active galactic nuclei

Previously, it was claimed that the broad emission lines of the radio galaxy Arp 102B can be fitted by the line profile from a simple relativistic Keplerian thin disk. It was argued that the lines originating from the relativistic accretion disk could be polarized due to electron scattering, which is likely to be the dominant opacity in the line-emitting region of Arp 102B. In the present work, the expected polarization properties of these broad emission lines are calculated. The percentage of polarization depends strongly on the inclination angle. For some angles, the red peak of the polarized, double-peaked line profile can be higher than the blue peak. This is in contrast to the total line profile, in which the blue peak is always higher than the red one. Spectropolarimetric observations could, therefore, provide an independent test of the relativistic disk model for the broad emission lines of Arp 102B and other active galactic nuclei.

Chen, Kaiyou↗
NASA NTRS1993

Spectral variations of LMC X-3 observed with Ginga

The prime black hole candidate LMC X-3 was observed over three years with the Ginga satellite, and a characteristic spectral variation was found accompanying the periodic intensity variation of about 198 (or possibly about 99) days (Cowley et al., 1991). The energy spectrum of LMC X-3 consists of the soft, thermal component and the hard, power-law component, which are respectively dominant below and above about 9 keV. The soft component, which carries most of the X-ray intensity, shows a clear correlation between the intensity and the hardness, while the hard component varies independently of the soft component. It was found that the spectral variation of the soft component is well described by an optically thick accretion disk model with a remarkably constant innermost radius and variable mass accretion rate. The constancy of the innermost radius suggests it is related to the mass of the central object.

Ebisawa, Ken↗
NASA NTRS1999

Multi-Wavelength Superoutburst Observations of T Leonis

We present results of simultaneous superoutburst observations in the X-ray, Extreme Ultraviolet (EUV), optical, and IR bands of the tremendous outburst amplitude dwarf novae T Leonis. Near peak luminosity, a single blackbody represents a good fit to T Leo's observed continuum in the EUV spectral region, yielding a boundary layer temperature of 71,000 to 97,000K. Inclusion of the longer wavelength observations, UV to the IR, indicates that a blackbody fit is inappropriate. A single temperature fit to only the UV and red-ward data for T Leo works well, but yields a much lower temperature, near 28,000K. Using our own observations and previously obtained EUV, UV and optical (super)outburst observations for the dwarf novae U Gem and SS Cyg, the SU UMa star VW Hyi, and the TOADS, TV Crv, BC UMa, and SW UMa, we find that in all cases, high energy observations yield high temperature, small emitting regions, while fits to UV and red-ward data produce cooler temperatures from much larger emitting regions. These results are consistent with the idea that high energy data provide a direct measurement of the boundary layer while the lower energy data measure a much larger, multi-temperature region, likely to be dominated by the outburst heated inner accretion disk. High energy outburst observations show that the boundary layer temperature decreases with decreasing orbital period and UV outburst observations provide evidence for a missing or weak inner disk in the TOADs. We present a simple model of mass accretion onto the white dwarf during (super)outburst which can account for the observed correlation between temperature.

Howell, Steve B.↗
NASA NTRS1992

UV variability and the nature of the continuum source in NGC5548

The short-wave UV continuum of NGC5548 varied quasi-periodically during the IUE monitoring from December 1988 to August 1989. The spectral index of this SWP continuum varied in tight correlation with it, inviting comparison with models to search for clues to the physical nature of the continuum source. The observed covariation of spectral index and continuum are consistent with the model of a black body of constant area (that of a sphere the size of Neptune's orbit) but variable temperature ( 18,000K to 25,000 K). A thin accretion disk around a black hole may also be consistent with this data if the accretion rate can be variable on time scales of 100 days while remaining uniform across the disk. There is marginal evidence for hysteresis in which spectral index changes lead changes in the SWP flux, although the CCF peaks at zero time lag.

Stoner, R. E.↗
NASA NTRS1985

Evolution of the innermost stable orbits around accreting neutron stars

The surface of most neutron stars with 'soft' equations of state lies within the innermost stable circular orbit predicted by general relativity. In disk accretion onto a weakly magnetized neutron star, the disk will reach the stellar surface for 'stiff' equations of state, but for soft equations of state the matter will hit the surface in free fall at an angle of about 0.001 to 0.1 radians. All calculations are carried out through first order in the angular momentum of the star.

Kluzniak, W.↗
NASA NTRS1994

Disk-driven hydromagnetic winds as a key ingredient of active galactic nuclei unification schemes

Centrifugally driven winds from the surfaces of magnetized accretion disks have been recognized as an attractive mechanism of removing the angular momentum of the accreted matter and of producing the bipolar outflows and jets that are often associated with compact astronomical objects. As previously suggested in the context of young stellar objects, such winds have unique observational manifestations stemming from their highly stratified density and velocity structure and from their exposure to the strong continuum radiation field of the compact object. We have applied this scenario to active galactic nuclei (AGNs) and investigated the properties of hydromagnetic outflows that originate within approximately 10(M(sub 8)) pc of the central 10(exp 8)(M(sub 8)) solar mass black hole. On the basis of our results, we propose that hydromagnetic disk-driven winds may underlie the classification of broad-line and narrow-line AGNs (e.g., the Seyfert 1/Seyfert 2 dichotomy) as well as the apparent dearth of luminous Seyfert 2 galaxies. More generally, we demonstrate that such winds could strongly influence the spectral characteristics of Seyfert galaxies, QSOs, and BL Lac objects (BLOs). In our picture, the torus is identified with the outer regions of the wind where dust uplifted from the disk surfaces by gas-grain collisions is embedded in the outflow. Using an efficient radiative transfer code, we show that the infrared emission of Seyfert galaxies and QSOs can be attributed to the reprocessing of the UV/soft X-ray AGN continuum by the dust in the wind and the disk. We demonstrate that the radiation pressure force flattens the dust distribution in objects with comparatively high (but possibly sub-Eddington) bolometric luminosities, and we propose this as one likely reason for the apparent paucity of narrow-line objects among certain high-luminosity AGNs. Using the XSTAR photoionization code, we show that the inner regions of the wind could naturally account for the warm (greater than or approximately equal to 10(exp 5) K) and hot (greater than or approximately equal to 10(exp 6) K) gas components that have been inferred to exist on scales less than or approximately equal to 10(exp 2) pc in several Seyfert galaxies. We suggest that the partially ionized gas in the inner regions of the wind, rather than the dusty, neutral outflow that originates further out in the disk, could account for the bulk of the X-ray absorption in Seyferts observed at relatively small angles to their symmetry axes. Finally, we discuss the application of this model to the interpretation of the approximately 0.6 keV X-ray absorption feature reported in several BLOs.

Konigl, Arieh↗
DOE OSTI2025

Stratified wind from a super-Eddington X-ray binary is slower than expected

Accretion disks in strong gravity ubiquitously produce winds, seen as blueshifted absorption lines in the X-ray band of both stellar mass X-ray binaries (black holes and neutron stars) and supermassive black holes. Some of the most powerful winds (termed Eddington winds) are expected to arise from systems in which radiation pressure is sufficient to unbind material from the inner disk (L ≳ L Edd ). These winds should be extremely fast and carry a large amount of kinetic power, which, when associated with supermassive black holes, would make them a prime contender for the feedback mechanism linking the growth of those black holes with their host galaxies. Here we show the XRISM Resolve spectrum of the galactic neutron star X-ray binary, GX 13+1, which reveals one of the densest winds ever seen in absorption lines. This Compton-thick wind significantly attenuates the flux, making it appear faint, although it is intrinsically more luminous than usual (L ≳ L Edd ). However, the wind is extremely slow, more consistent with the predictions of thermal-radiative winds launched by X-ray irradiation of the outer disk than with the expected Eddington wind driven by radiation pressure from the inner disk. This puts new constraints on the origin of winds from bright accretion flows in binaries, but also highlights the very different origin required for the ultrafast (v ~ 0.3c) winds seen in recent Resolve observations of a supermassive black hole at a similarly high Eddington ratio.

Audard, Marc [Univ. of Geneva, Versoix (Switzerlan↗