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At least 631 records · Page 35
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DOE OSTI2015

Numerical simulation of hot accretion flows. III. Revisiting wind properties using the trajectory approach

Previous MHD simulations have shown that wind must exist in black hole hot accretion flows. In this paper, we continue our study by investigating the detailed properties of wind and the mechanism of wind production. For this aim, we make use of a 3D general relativistic MHD simulation of hot accretion flows around a Schwarzschild black hole. To distinguish real wind from turbulent outflows, we track the trajectories of the virtual Lagrangian particles from simulation data. We find two types of real outflows, i.e., a jet and a wind. The mass flux of wind is very significant, and its radial profile can be described by M-dot {sub wind}≈ M-dot {sub BH}(r/20 r{sub s}), with M-dot {sub BH} being the mass accretion rate at the black hole horizon and r{sub s} being the Schwarzschild radius. The poloidal wind speed almost remains constant once they are produced, but the flux-weighted wind speed roughly follows v{sub p,wind}(r)≈0.25v{sub k}(r), with v{sub k}(r) being the Keplerian speed at radius r. The mass flux of the jet is much lower, but the speed is much higher, v{sub p,jet} ∼ (0.3–0.4)c. Consequently, both the energy and momentum fluxes of the wind are much larger than those of the jet. The wind is produced and accelerated primarily by the combination of centrifugal force and magnetic pressure gradient, while the jet is mainly accelerated by the magnetic pressure gradient. Finally, we find that the wind production efficiency ϵ{sub wind}≡ E-dot {sub wind}/ M-dot {sub BH}c{sup 2}∼1/1000 is in good agreement with the value required from large-scale galaxy simulations with active galactic nucleus feedback.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE COSMIC BATTERY IN ASTROPHYSICAL ACCRETION DISKS

The aberrated radiation pressure at the inner edge of the accretion disk around an astrophysical black hole imparts a relative azimuthal velocity on the electrons with respect to the ions which gives rise to a ring electric current that generates large-scale poloidal magnetic field loops. This is the Cosmic Battery established by Contopoulos and Kazanas in 1998. In the present work we perform realistic numerical simulations of this important astrophysical mechanism in advection-dominated accretion flows, ADAFs. We confirm the original prediction that the inner parts of the loops are continuously advected toward the central black hole and contribute to the growth of the large-scale magnetic field, whereas the outer parts of the loops are continuously diffusing outward through the turbulent accretion flow. This process of inward advection of the axial field and outward diffusion of the return field proceeds all the way to equipartition, thus generating astrophysically significant magnetic fields on astrophysically relevant timescales. We confirm that there exists a critical value of the magnetic Prandtl number between unity and 10 in the outer disk above which the Cosmic Battery mechanism is suppressed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Variable protostellar accretion with episodic bursts

We present the latest development of the disk gravitational instability and fragmentation model, originally introduced by us to explain episodic accretion bursts in the early stages of star formation. Using our numerical hydrodynamics model with improved disk thermal balance and star-disk interaction, we computed the evolution of protostellar disks formed from the gravitational collapse of prestellar cores. In agreement with our previous studies, we find that cores of higher initial mass and angular momentum produce disks that are more favorable to gravitational instability and fragmentation, while a higher background irradiation and magnetic fields moderate the disk tendency to fragment. The protostellar accretion in our models is time-variable, thanks to the nonlinear interaction between different spiral modes in the gravitationally unstable disk, and can undergo episodic bursts when fragments migrate onto the star owing to the gravitational interaction with other fragments or spiral arms. Most bursts occur in the partly embedded Class I phase, with a smaller fraction taking place in the deeply embedded Class 0 phase and a few possible bursts in the optically visible Class II phase. The average burst duration and mean luminosity are found to be in good agreement with those inferred from observations of FUors. The model predicts the existence of two types of bursts: the isolated ones, showing well-defined luminosity peaks separated with prolonged periods (∼10{sup 4} yr) of quiescent accretion, and clustered ones, demonstrating several bursts occurring one after another during just a few hundred years. Finally, we estimate that 40%–70% of the star-forming cores can display bursts after forming a star-disk system.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

NEUTRINO CONSTRAINTS TO THE DIFFUSE GAMMA-RAY EMISSION FROM ACCRETION SHOCKS

Accretion of gas during the large-scale structure formation has been thought to give rise to shocks that can accelerate cosmic rays. This process then results in an isotropic extragalactic gamma-ray emission contributing to the extragalactic gamma-ray background (EGRB) observed by Fermi-LAT. Unfortunately, this emission has been difficult to constrain and thus presents an uncertain foreground to any attempts to extract a potential dark matter signal. Recently, IceCube has detected high-energy isotropic neutrino flux that could be of an extragalactic origin. In general, neutrinos can be linked to gamma rays since cosmic-ray interactions produce neutral and charged pions where neutral pions decay into gamma rays, while charged pions decay to give neutrinos. By assuming that isotropic high-energy IceCube neutrinos are entirely produced by cosmic rays accelerated in accretion shocks during the process of structure formation, we obtain the strongest constraint to the gamma-ray emission from large-scale structure formation (strong) shocks and find that they can make at best ∼20% of the EGRB, corresponding to neutrino flux with spectral index α{sub ν} = 2, or ∼10% for spectral index α{sub ν} = 2.46. Since typical objects where cosmic rays are accelerated in accretion shocks are galaxy clusters, observed high-energy neutrino fluxes can then be used to determine the gamma-ray emission of a dominant cluster type and constrain acceleration efficiency, and thus probe the process of large-scale structure formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Correlations among the jet, Accretion disk, and broad-line region of flat spectrum radio quasars

On the basis of the spectral energy distribution fits and emission line fluxes of 18 GeV flat spectrum radio quasars (FSRQs), we calculate their jet power (P{sub jet}), disk luminosity (L{sub disk}), and broad-line region luminosity (L{sub BLR}). No correlation between P{sub jet} with either L{sub disk} or L{sub BLR} is found, but L{sub BLR} is strongly correlated with L{sub disk}. On average, the broad-line region (BLR) covering factors of GeV-FSRQs in our sample are smaller than those of large samples of quasars. P{sub jet} of some GeV-FSRQs are higher than their L{sub disk}, but all of them are lower than the accretion power of black holes (BHs), which is estimated by M-dot c{sup 2}=L{sub disk}/0.1, indicating that the total accretion power of BHs is sufficient to drive the jets in these sources; however, lack of correlation between L{sub disk} and P{sub jet} of the GeV-FSRQs may suggest that their jets are launched by the Blandford–Znajek process via extracting the rotational energy of BHs. Using the L{sub BLR}–L{sub disk} relation of the GeV-FSRQs, we estimate L{sub disk} of a BL Lac sample with their L{sub BLR}. A comparison of L{sub BLR} and the Eddington ratio (L{sub disk}/L{sub Edd}) among BL Lacs, very radio-loud NLS1 galaxies, and FSRQs is also presented. It is found that along with the BL Lac–NLS1–FSRQ sequence L{sub BLR} and L{sub disk}/L{sub Edd} increase, which may correspond to the change of the accretion disk structure and the transformation of the dominant mechanism for jet launching. This is also consistent with the division of blazar parent populations, i.e., low/high-excitation radio galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Predictions for the reverberating spectral line from a newly formed black hole accretion disk: Case of tidal disruption flares

Tidal disruption events (TDEs) can be perfect probes of dormant supermassive black holes in normal galaxies. During the rising phase, the accretion luminosity can increase by orders of magnitude in several weeks, and the emergent ionizing radiation illuminates the fresh accretion flow. In this paper, we simulated the evolution of the expected spectral line profile of iron due to such a flare by using a ray-tracing code with effects of general relativity taken into account. We found that the time-dependent profile changes significantly with black hole spin, inclination angle with respect to the black hole equatorial plane, and the expansion velocity of the ionization front. At low values of spin, a “loop” feature appears in the line profile versus time plot when the inclination is no less than 30° and the expansion velocity v{sub exp} is no less than half the speed of light, owing to a shadow in the emission of the truncated disk. In the light curve two peaks occur depending on the inclination angle. At large v{sub exp}, a shallow “nose” feature may develop ahead of the loop; its duration depends on the expansion velocity and the inclination angle. We explore the entire interval of black hole spin parameter ranging from extreme prograde to extreme retrograde rotation, −1<a<1. In the prograde case, a low-energy tail appears to be more pronounced in the evolving centroid energy of the line. Our results demonstrate the importance of searching for X-ray spectral lines in the early phase of TDE flares in order to constrain black hole mass and spin, as well as properties of the innermost accretion flow.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ACCRETION DISK DYNAMO AS THE TRIGGER FOR X-RAY BINARY STATE TRANSITIONS

Magnetohydrodynamic accretion disk simulations suggest that much of the energy liberated by the magnetorotational instability (MRI) can be channeled into large-scale toroidal magnetic fields through dynamo action. Under certain conditions, this field can dominate over gas and radiation pressure in providing vertical support against gravity, even close to the midplane. Using a simple model for the creation of this field, its buoyant rise, and its coupling to the gas, we show how disks could be driven into this magnetically dominated state and deduce the resulting vertical pressure and density profiles. Applying an established criterion for MRI to operate in the presence of a toroidal field, we show that magnetically supported disks can have two distinct MRI-active regions, separated by a “dead zone” where local MRI is suppressed, but where magnetic energy continues to flow upward from the dynamo region below. We suggest that the relative strengths of the MRI zones, and the local poloidal flux, determine the spectral states of X-ray binaries. Specifically, “intermediate” and “hard” accretion states occur when MRI is triggered in the hot, upper zone of the corona, while disks in “soft” states do not develop the upper MRI zone. We discuss the conditions under which various transitions should take place and speculate on the relationship of dynamo activity to the various types of quasi-periodic oscillations that sometimes appear in the hard spectral components. The model also explains why luminous accretion disks in the “soft” state show no signs of the thermal/viscous instability predicted by standard α-models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Characterizing the mean-field dynamo in turbulent accretion disks

The formation and evolution of a wide class of astrophysical objects are governed by turbulent, magnetized accretion disks. Understanding their secular dynamics is of primary importance. Apart from enabling mass accretion via the transport of angular momentum, the turbulence affects the long-term evolution of the embedded magnetic flux, which in turn regulates the efficiency of the transport. In this paper, we take a comprehensive next step toward an effective mean-field model for turbulent astrophysical disks by systematically studying the key properties of magnetorotational turbulence in vertically stratified, isothermal shearing boxes. This allows us to infer emergent properties of the ensuing chaotic flow as a function of the shear parameter, as well as the amount of net vertical flux. Using the test-field method, we furthermore characterize the mean-field dynamo coefficients that describe the long-term evolution of large-scale fields. We simultaneously infer the vertical shape and the spectral scale dependence of these closure coefficients, with the latter describing nonlocal contributions to the turbulent electromotive force. Based on this, we infer a scale-separation ratio of about 10 for the large-scale dynamo. We finally determine scaling properties of the mean-field dynamo coefficients. The relevant component of the dynamo α effect is found to scale linearly with the shear rate, as does the corresponding turbulent diffusion, η. Together, these scalings allow us to predict, in a quantitative manner, the cycle period of the well-known butterfly diagram. This lends new support to the importance of the αΩ mechanism in determining the evolution of large-scale magnetic fields in turbulent accretion disks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ACCRETION KINEMATICS THROUGH THE WARPED TRANSITION DISK IN HD 142527 FROM RESOLVED CO(6–5) OBSERVATIONS

The finding of residual gas in the large central cavity of the HD 142527 disk motivates questions regarding the origin of its non-Keplerian kinematics and possible connections with planet formation. We aim to understand the physical structure that underlies the intra-cavity gaseous flows, guided by new molecular-line data in CO(6–5) with unprecedented angular resolutions. Given the warped structure inferred from the identification of scattered-light shadows cast on the outer disk, the kinematics are consistent, to first order, with axisymmetric accretion onto the inner disk occurring at all azimuths. A steady-state accretion profile, fixed at the stellar accretion rate, explains the depth of the cavity as traced in CO isotopologues. The abrupt warp and evidence for near free-fall radial flows in HD 142527 resemble theoretical models for disk tearing, which could be driven by the reported low-mass companion, whose orbit may be contained in the plane of the inner disk. The companion’s high inclination with respect to the massive outer disk could drive Kozai oscillations over long timescales; high-eccentricity periods may perhaps account for the large cavity. While shadowing by the tilted disk could imprint an azimuthal modulation in the molecular-line maps, further observations are required to ascertain the significance of azimuthal structure in the density field inside the cavity of HD 142527.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

RELATIVISTIC GLOBAL SOLUTIONS OF NEUTRINO-DOMINATED ACCRETION FLOWS

Neutrino-dominated accretion flows (NDAFs) around rotating stellar-mass black holes are plausible candidates for the central engines of gamma-ray bursts (GRBs). We investigate one-dimensional global solutions of NDAFs, taking into account general relativity in the Kerr metric, neutrino physics, and nucleosynthesis more precisely than previous works. We calculate 16 solutions with different characterized accretion rates and black hole spins to exhibit the radial distributions of various physical properties in NDAFs. We confirm that the electron degeneracy has important effects in NDAFs and we find that the electron fraction is about 0.46 in the outer region for all 16 solutions. From the perspective of the mass fraction, free nucleons, {sup 4}He, and {sup 5}6Fe dominate in the inner, middle, and outer regions, respectively. The influence of neutrino trapping on the annihilation is of importance for the superhigh accretion ( M-dot =10 M{sub sun} s{sup -1}) and most of the 16 solutions have an adequate annihilation luminosity for GRBs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Relativistic Bondi–Hoyle–LYttleton accretion onto a rotating black hole: density gradients

In this work, for the first time, we present a numerical study of Bondi–Hoyle accretion with density gradients in the fully relativistic regime. In this context, we consider the accretion onto a Kerr black hole (BH) of a supersonic ideal gas with density gradients perpendicular to the relative motion. The parameters of interest in this study are the Mach number, M, the spin of the BH, a, and the density-gradient parameter of the gas, ϵ{sub ρ}. We show that, unlike in the Newtonian case, all of the studied cases, especially those with a density gradient, approach a stationary flow pattern. To illustrate that the system reaches a steady state, we calculate the mass and angular momentum accretion rates on a spherical surface almost located at the event horizon. In the particular case of M=1, ϵ{sub ρ} = 0.5, and BH spin a = 0.5, we observe a disk-like configuration surrounding the BH. Finally, we present the gas morphology and some of its properties.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

Evolution of the spin parameter of accreting compact objects with non-Kerr quadrupole moment

There is robust observational evidence supporting the existence of 5−20 M{sub s}un compact bodies in X-ray binary systems and of 10{sup 5}−10{sup 9} M{sub s}un bodies at the center of many galaxies. All these objects are commonly interpreted as black holes, even is there is no direct evidence that they have an event horizon. A fundamental limit for a black hole in 4-dimensional general relativity is the Kerr bound |a{sub *}| ≤ 1, where a{sub *} is the spin parameter. This is just the condition for the existence of the event horizon. The accretion process can spin a black hole up to a{sub *} ≈ 0.998 and some super-massive objects in galactic nuclei could be rapidly rotating black holes with spin parameter close to this limit. However, if these super-massive objects are not black holes, the Kerr bound does not hold and the accretion process can spin them up to a{sub *} > 1. In this paper, I consider compact bodies with non-Kerr quadrupole moment. I study the evolution of the spin parameter due to accretion and I find its equilibrium value. Future experiments like the gravitational wave detector LISA will be able to test if the super-massive objects at the center of galaxies are the black holes predicted by general relativity. If they are not black holes, some of them may be super-spinning objects with a{sub *} > 1.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Super-spinning compact objects generated by thick accretion disks

If astrophysical black hole candidates are the Kerr black holes predicted by General Relativity, the value of their spin parameter must be subject to the theoretical bound |a{sub *}| ≤ 1. In this work, we consider the possibility that these objects are either non-Kerr black holes in an alternative theory of gravity or exotic compact objects in General Relativity. We study the accretion process when their accretion disk is geometrically thick with a simple version of the Polish doughnut model. The picture of the accretion process may be qualitatively different from the one around a Kerr black hole. The inner edge of the disk may not have the typical cusp on the equatorial plane any more, but there may be two cusps, respectively above and below the equatorial plane. We extend previous work on the evolution of the spin parameter and we estimate the maximum value of a{sub *} for the super-massive black hole candidates in galactic nuclei. Since measurements of the mean radiative efficiency of AGNs require η > 0.15, we infer the ''observational'' bound |a{sub *}|∼<1.3, which seems to be quite independent of the exact nature of these objects. Such a bound is only slightly weaker than |a{sub *}|∼<1.2 found in previous work for thin disks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Self-interacting dark scalar spikes around black holes via relativistic Bondi accretion

For this work, we consider the spike mass density profile in a dark halo by self-consistently solving the relativistic Bondi accretion of dark matter onto a non-spining black hole of mass M. We assume that the dominant component of the dark matter in the halo is a Standard model gauge-singlet scalar. Its mass m ≃ 10 -5 eV and quartic self-coupling λ ≲ 10 19 are constrained to be compatible with the properties of galactic dark halos. In the hydrodynamic limit, we find that the accretion rate is bounded from below, M min = 96πG 2 M 2 m 4 /λ$\hbar$ 3 . Therefore, for M = 10 6 M ⊙ we have M min ≃ 1.41 x 10 -9 M ⊙ yr -1 , which is subdominant compared to the Eddington accretion of baryons. The spike density profile ρ0(r) within the self-gravitating regime cannot be fitted well by a single-power law but a double-power one. Despite that, we can fit ρ 0 (r) piecewise and find that ρ 0 (r) ∝ r -1.20 near the sound horizon, ρ 0 (r) ∝ r -1.00 towards the Bondi radius and ρ 0 (r) ∝ r -1.08 for the region in between. This contrasts with more cuspy ρ 0 (r) ∝ r -1.75 for dark matter with Coulomb-like self-interaction.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

CHARACTERIZING THE TIME VARIABILITY IN MAGNETIZED NEUTRINO-COOLED ACCRETION DISKS: SIGNATURES OF THE GAMMA-RAY BURST CENTRAL ENGINE

The central engine of gamma-ray bursts (GRBs) is hidden from direct probing with photons mainly due to the high densities involved. Inferences on their properties are thus made from their cosmological setting, energetics, low-energy counterparts, and variability. If GRBs are powered by hypercritical accretion onto compact objects, on small spatial scales the flow will exhibit fluctuations, which could in principle be reflected in the power output of the central engine and ultimately in the high-energy prompt emission. Here, we address this issue by characterizing the variability in neutrino-cooled accretion flows through local shearing box simulations with magnetic fields, and then convolving them on a global scale with large-scale dynamical simulations of accretion disks. The resulting signature is characteristic and sensitive to the details of the cooling mechanism, providing in principle a discriminant for GRB central engine properties.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

THE H I MASS DENSITY IN GALACTIC HALOS, WINDS, AND COLD ACCRETION AS TRACED BY Mg II ABSORPTION

It is well established that Mg II absorption lines detected in background quasar spectra arise from gas structures associated with foreground galaxies. The degree to which galaxy evolution is driven by the gas cycling through halos is highly uncertain because their gas mass density is poorly constrained. Fitting the Mg II equivalent width (W) distribution with a Schechter function and applying the N(H I)-W correlation of Menard and Chelouche, we computed {Omega}(H I){sub MgII} {identical_to} {Omega}(H I){sub halo} = 1.41{sup +0.75}{sub -0.44} Multiplication-Sign 10{sup -4} for 0.4 {<=} z {<=} 1.4. We exclude damped Ly{alpha}'s (DLAs) from our calculations so that {Omega}(H I){sub halo} comprises accreting and/or outflowing halo gas not locked up in cold neutral clouds. We deduce that the cosmic H I gas mass density fraction in galactic halos traced by Mg II absorption is {Omega}(H I){sub halo}/{Omega}(H I){sub DLA} {approx_equal} 15% and {Omega}(H I){sub halo}/{Omega}{sub b} {approx_equal} 0.3%. Citing several lines of evidence, we propose that infall/accretion material is sampled by small W whereas outflow/winds are sampled by large W, and find that {Omega}(H I){sub infall} is consistent with {Omega}(H I){sub outflow} for bifurcation at W = 1.23{sup +0.15}{sub -0.28} Angstrom-Sign ; cold accretion would then comprise no more than {approx}7% of the total H I mass density. We discuss evidence that (1) the total H I mass cycling through halos remains fairly constant with cosmic time and that the accretion of H I gas sustains galaxy winds, and (2) evolution in the cosmic star formation rate depends primarily on the rate at which cool H I gas cycles through halos.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

NUMERICAL SIMULATIONS OF NATURALLY TILTED, RETROGRADELY PRECESSING, NODAL SUPERHUMPING ACCRETION DISKS

Accretion disks around black hole, neutron star, and white dwarf systems are thought to sometimes tilt, retrogradely precess, and produce hump-shaped modulations in light curves that have a period shorter than the orbital period. Although artificially rotating numerically simulated accretion disks out of the orbital plane and around the line of nodes generate these short-period superhumps and retrograde precession of the disk, no numerical code to date has been shown to produce a disk tilt naturally. In this work, we report the first naturally tilted disk in non-magnetic cataclysmic variables using three-dimensional smoothed particle hydrodynamics. Our simulations show that after many hundreds of orbital periods, the disk has tilted on its own and this disk tilt is without the aid of radiation sources or magnetic fields. As the system orbits, the accretion stream strikes the bright spot (which is on the rim of the tilted disk) and flows over and under the disk on different flow paths. These different flow paths suggest the lift force as a source to disk tilt. Our results confirm the disk shape, disk structure, and negative superhump period and support the source to disk tilt, source to retrograde precession, and location associated with X-ray and He II emission from the disk as suggested in previous works. Our results identify the fundamental negative superhump frequency as the indicator of disk tilt around the line of nodes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ACCELERATED ORBITAL EXPANSION AND SECULAR SPIN-DOWN OF THE ACCRETING MILLISECOND PULSAR SAX J1808.4-3658

The accreting millisecond pulsar SAX J1808.4-3658 has shown a peculiar orbital evolution in the past with an orbital expansion much faster than expected from standard binary evolutionary scenarios. Previous limits on the pulsar spin frequency derivative during transient accretion outbursts were smaller than predicted by standard magnetic accretion torque theory, while the spin evolution between outbursts was consistent with magnetic dipole spin-down. In this Letter, we present the results of a coherent timing analysis of the 2011 outburst observed by the Rossi X-Ray Timing Explorer and extend our previous long-term measurements of the orbital and spin evolution over a baseline of 13 years. We find that the expansion of the 2 hr orbit is accelerating at a rate of P-double dot{sub b} approx. = 1.6 x 10{sup -20} s s{sup -2} and we interpret this as the effect of short-term angular momentum exchange between the mass donor and the orbit. The gravitational quadrupole coupling due to variations in the oblateness of the companion can be a viable mechanism for explaining the observations. No significant spin frequency derivatives are detected during the 2011 outburst (|{nu}-dot| < or approx. 4 x 10{sup -13} Hz s{sup -1}) and the long-term spin-down remains stable over 13 years with {nu}-dot approx. = -10{sup -15} Hz s{sup -1}.

79 ASTRONOMY AND ASTROPHYSICS↗