Critical Accretion Rate Research Articles

Constraints on Torque‐reversing Accretion‐powered X‐Ray Pulsars

The observed abrupt torque reversals in X-ray pulsars, 4U 1626-67, GX 1+4, and OAO 1657—415, can be explained by a transition in accretion-flow rotation from Keplerian to sub-Keplerian, which takes place at a critical accretion rate, ~1016-1017gs-1. When a pulsar system spins up near equilibrium spin before the transition, the system goes into spin-down after transition to sub-Keplerian. If a system is well into the spin-up regime, the transition can cause a sharp decrease in spin-up rate but not a sudden spin-down. These observable types of abrupt torque change are distinguished from the smooth torque variation caused by a change of in the Keplerian flow. The observed abrupt torque reversal is expected when the pulsar magnetic field B* ~5 × 1011b−1/2p L P1/2*,10 G, where the magnetic pitch parameter bp is of the order of a few, Lx,36 is the X-ray luminosity in 1036 ergs s-1, and P*,10 is the pulsar spin period in 10 s. Observed quasi-periodic oscillation (QPO) periods tightly constrain the model. For 4U 1626-67, ≈ 2.7 × 1016 g s-1 with b1/2p B* ≈ 2 × 1012 G. We estimate ~6 × 1016 g s-1 and b1/2p B* ~5 × 1013 G for GX 1+4, and ~1 × 1017 g s-1 and b1/2p B* ~2 × 1013G for OAO 1657-415. Reliable detection of QPOs before and after torque reversal could directly test the model. We discuss some outstanding uncertainties and difficulties in the present model.

Torque Reversal in Accretion-powered X-Ray Pulsars

Accretion-powered X-ray pulsars 4U 1626-67, GX 1+4, and OAO 1657-415 have recently shown puzzling torque reversals. These reversals are characterized by short timescales, on the order of days, nearly identical spin-up and spin-down rates, and very small changes in X-ray luminosity. We propose that this phenomenon is the result of sudden dynamical changes in the accretion disks triggered by a gradual variation of mass accretion rates. These sudden torque reversals may occur at a critical accretion rate ~1015-1016 g s-1 when the system makes a transition from (to) a primarily Keplerian flow to (from) a substantially sub-Keplerian, radial advective flow in the inner disk. For systems near spin equilibrium, the spin-up torques in the Keplerian state are slightly larger than the spin-down torques in the advective state, in agreement with observation. The abrupt reversals could be a signature of pulsar systems near spin equilibrium with the mass accretion rates modulated on a timescale of a year or longer near the critical accretion rate. It is interesting that cataclysmic variables and black hole soft X-ray transients change their X-ray emission properties at accretion rates similar to the pulsars' critical rate. We speculate that the dynamical change in pulsar systems shares a common physical origin with white dwarf and black hole accretion disk systems.

Scaling Laws for Advection‐dominated Flows: Applications to Low‐Luminosity Galactic Nuclei

We present analytical scaling laws for self-similar advection dominated flows. The spectra from these systems range from 10$^{8}$ - 10$^{20}$ Hz, and are determined by considering cooling of electrons through synchrotron, bremsstrahlung, and Compton processes. We show that the spectra can be quite accurately reproduced without detailed numerical calculations, and that there is a strong testable correlation between the radio and X-ray fluxes from these systems. We describe how different regions of the spectrum scale with the mass of the accreting black hole, $M$, the accretion rate of the gas, $\dot{M}$, and the equilibrium temperature of the electrons, $T_e$. We show that the universal radio spectral index of 1/3 observed in most elliptical galaxies (Slee et al. 1994) is a natural consequence of self-absorbed synchrotron radiation from these flows. We also give expressions for the total luminosity of these flows, and the critical accretion rate, $\dot{M}_{crit}$, above which the advection solutions cease to exist. We find that for most cases of interest the equilibrium electron temperature is fairly insensitive to $M$, $\dot{M}$, and parameters in the model. We apply these results to low luminosity black holes in galactic nuclei. We show that the problem posed by Fabian & Canizares (1988) of whether bright elliptical galaxies host dead quasars is resolved, as pointed out recently by Fabian & Rees (1995), by considering advection-dominated flows.

The Nature of the Giant Outbursts in the Bursting Pulsar GRO J1744−28

We investigate the possible role of an accretion disk instability in producing the giant outbursts seen in GRO J1744-28. Specifically, we study the global, time-dependent evolution of the Lightman-Eardley instability, which can develop near the inner edge of an accretion disk when the radiation pressure becomes comparable to the gas pressure. Broadly speaking, our results are compatible with earlier works by Taam & Lin and by Lasota & Pelat. The uniqueness of GRO J1744-28 appears to be associated with the constraint that, in order for outbursts to occur, the rate of accretion at the inner edge must be within a narrow range just above the critical accretion rate at which radiation pressure is beginning to become significant.

Advection-dominated Models of Luminous Accreting Black Holes

view Abstract Citations (136) References (54) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Advection-dominated Models of Luminous Accreting Black Holes Narayan, Ramesh Abstract It has been found that a class of optically thin two-temperature advection-dominated accretion solutions explains many observations of low-luminosity accreting black holes. Here it is shown that these models give a satisfactory description also of higher luminosity systems, provided the viscosity parameter α is large. The models reproduce the spectra of black hole X-ray binaries in the low state and explain the transition from the low state to the high state at a critical mass accretion rate. The models also show that the X-ray/γ-ray spectra of X-ray binaries and active galactic nuclei should be similar, as confirmed by observations. Publication: The Astrophysical Journal Pub Date: May 1996 DOI: 10.1086/177136 arXiv: arXiv:astro-ph/9510028 Bibcode: 1996ApJ...462..136N Keywords: ACCRETION; ACCRETION DISKS; BLACK HOLE PHYSICS; GALAXIES: ACTIVE; X-RAYS: STARS; Astrophysics E-Print: 12 pages compressed uuencoded postscript, including 2 figures. To appear in Part 1 of The Astrophysical Journal full text sources arXiv | ADS | data products SIMBAD (9)

The Effect of Deep Hydrogen Burning in the Accreted Envelope of a Neutron Star on the Properties of X-Ray Bursts

The thermal and compositional evolution of a neutron star has been numerically followed to determine the long-term properties of X-ray bursts produced by thermonuclear shell flashes in its accreted hydrogen-rich envelope. Uniform burning over the entire neutron star surface is assumed and mass accretion rates greater than {approximately}0.1 {dot {ital M}}{sub Edd} (where {dot {ital M}}{sub Edd} is the critical mass accretion rate for which the accretion luminosity is equal to the Eddington luminosity) are considered. Specific attention is focused on the consequences of electron capture initiated burning of hydrogen at high densities ({approx_gt}10{sup 7} gcm{sup {minus}3}). The degree of heating associated with the burning of the residual hydrogen (i.e., the matter which is not completely processed in the outburst) is a function of the mass accretion rate and the composition of the accreted matter. Heating of the neutron star envelope is found to be more important for greater mass accretion rates and for greater residual hydrogen abundances. Because of the higher envelope temperatures, the resulting bursts are weaker and recur more frequently, for a given mass accretion rate, than in situations where the deep hydrogen burning does not occur. The mass accretion rate, which delineates strong X-ray bursts (wheremore » the ratio of the peak burst luminosity to the quiescent level of emission is greater than {approximately}3) from weak X-ray bursts, lies in the range of 0.1{endash}0.2 times the Eddington value. Weak burst activity is found for accretion rates extending to about the Eddington limit provided that the helium content of the accreted matter is greater than {approximately}0.23. The implications of our results with regard to the absence of regular, periodic X-ray bursting activity in the bright low-mass X-ray binary sources are briefly discussed. {copyright} {ital 1996 The American Astronomical Society.}« less

On the Nature of Core-Collapse Supernova Explosions

We investigate in this paper the core-collapse supernova explosion mechanism in both one and two dimensions. With a radiation/hydrodynamic code based upon the PPM algorithm, we verify the usefulness of neutrino-driven overturn (\convection") between the shock and the neutrinosphere in igniting the supernova explosion. The 2-D simulation of the core of a 15M star that we present here indicates that the breaking of spherical symmetry may be central to the explosion itself and that a multitude of bent and broken ngers is a common feature of the ejecta. As in one-dimension, the explosion seems to be a mathematically critical phenomenon, evolving from a steady-state to explosion after a critical mass accretion rate through the stalled shock has been reached. In the 2-D simulation we show here, the pre-explosion convective phase lasted 30 overturns ( 100 milliseconds) before exploding. The pre-explosion steady-state in 2-D is similar to that achieved in 1-D, but, in 2-D, due to the higher dwell time of matter in the overturning region, the average entropy achieved behind the stalled shock is larger. In addition, the entropy gradient in the convecting region is atter. These e ects, together with the dynamical pressure of the buoyant plumes, serve to increase the steady-state shock radius (Rs) over its value in 1-D by 30%{100%. A large Rs enlarges the volume of the gain region, puts shocked matter lower in the gravitational potential well, and lowers the accretion ram pressure at the shock for a given _ M. The critical condition for explosion is thereby relaxed. Since the \escape" temperature (Tesc) decreases with radius faster than the actual matter temperature (T ) behind the shock, a larger Rs puts a larger fraction of the shocked material above its local escape temperature. T > Tesc is the condition for a thermally-driven corona to lift o of a star. In one, two, or three dimensions, since supernovae are driven by

Optically thin accretion disks in the Kerr metric

We calculate the structure of an effectively optically thin and geometrically thin accretion disk in the Kerr geometry, including electron-positron pairs. We show that the properties of the disk solutions are strongly dependent on the angular momentum of the central black hole. We find that close to a rapidly rotating hole there can be an appreciable pair density even for modest accretion rates. Pair critical accretion rates recently discovered in Newtonian disk models are also shown to be present in the general relativistic models, and we show that the geometrically thin disk approximations easily break down for rapidly rotating holes.

Hot accretion disks with pairs: Effects of magnetic field and thermal cyclocsynchrotron radiation

We show the effects of thermal cyclosynchrotron radiation and magnetic viscosity on the structure of hot, two-temperature accretion disks. Magnetic field, B, is assumed to be randomly oriented and the ratio of magnetic pressure to either gas pressure, alpha = P(sub mag)/P(sub gas), or the sum of the gas and radiation pressures, alpha = (P(sub mag)/P(sub gas) + P(sub rad)), is fixed. We find those effects do not change the qualitative properties of the disks, i.e., there are still two critical accretion rates related to production of e(sup +/-) pairs, (M dot)((sup U)(sub cr)) and (M dot)((sup L)(sub cr)), that affect the number of local and global disk solutions, as recently found by Bjoernsson and Svensson for the case with B = 0. However, a critical value of the alpha-viscosity parameter above which those critical accretion rates disappear becomes smaller than alpha(sub cr) = 1 found in the case of B = 0, for P(sub mag) = alpha(P(sub gas) + P(sub rad)). If P(sub mag) = alpha P(sub gas), on the other hand, alpha(sub cr) is still about unity. Moreover, when Comptonized cyclosynchrotron radiation dominates Comptonized bremsstrahlung, radiation from the disk obeys a power law with the energy spectral index of approximately 0.5, in a qualitative agreement with X-ray observations of active galactic nuclei (AGNS) and Galactic black hole candidates. We also extend the hot disk solutions for P(sub mag) = alpha(P(sub gas) + P(sub rad)) to the effectively optically thick region, where they merge with the standard cold disk solutions. We find that the mapping method by Bjoernsson and Svensson gives a good approximation to the disk structure in the hot region and show where it breaks in the transition region. Finally, we find a region in the disk parameter space with no solutions due to the inability of Coulomb heating to supply enough energy to electrons.

Hot Accretion Disks with Magnetic Field and Thermal Cyclo-Synchrotron Radiation

We study the structure of hot, two-temperature accretion disks around black holes, including the effects of thermal cyclo-synchrotron radiation and magnetic viscosity. This work is an extension of previous work by Björnsson & Svensson (1991a, b, 1992) and Kusunose & Mineshige (1992), which did not include those effects. Magnetic field, B, is assumed to be randomly oriented and determined by prescribing the ratio α = Pmag/Pgas or α = Pmag/(Pgas + Prad), where Pmag, Pgas, and Prad are the pressures of magnetic field, gas, and radiation, respectively. We find those effects do not change the qualitative properties of the disks, i.e., there are two critical accretion rates related to production of e± pairs, ṀcrU and ṀcrL that affect the number of local and global disk solutions, as recently found for the case with B = 0 (Björnsson & Svensson 1991a, b, 1992). However, a critical value of the α-viscosity parameter above which those critical accretion rates disappear becomes smaller than αcr = 1 found in the case of B = 0, for Pmag = α(Pgas + Prad). If Pmag = αPgas, on the other hand, αcr is still about unity. Moreover, when Comptonized cyclo-synchrotron radiation dominates Comptonized bremsstrahlung, radiation from the disk obeys a power law with the energy spectral index of ∼ 0.5, in a qualitative agreement with X-ray observations of AGNs and Galactic black hole candidates. The spectral index is weakly dependent on the mass accretion rate.

Two-temperature accretion disks with electron—positron pairs

We investigate two-temperature disks that contain electron-positron pairs generated by nonthermal cascading. We show the existence of a critical accretion rate both for bremsstrahlung and comptonized bremsstrahlung disks. Below the critical value, there is a unique stationary solution for the disk structure; above the critical value, there is none such. For the comptonized disk, we have derived the relations between critical accretion rate and the Shakura-Sunyaev viscosity parameter α and the energy-generation parameter ε e. For the bremsstrahlung disk, the effect of electron pairs can be neglected; for the comptonized disk, the electron-pairs can have densities as high as 10 −2 ∼ 10 0, and exert much influence on the disk structure and spectrum.

Stability of slim accretion discs – Effects of central mass and viscosity

Slim accretion discs have a total luminosity of the order |$L/L_{E}=\dot{m}\sim 1$|⁠, where LE is the Eddington luminosity and |$\dot{m}=\dot{M}/\dot{M}_{c}$|⁠, where Me is a critical accretion rate, related to the Eddington one. The local stability properties of such discs are examined, in the three-dimensional parameter space spanned by the (α, μ, m) axes, where α and μ are two viscosity parameters, and m = M/M⊙ the central mass. We suggest that various types of observed quasi-periodic behaviour may be connected with slim disc instabilities. If this turns out to be correct, the so-called normal and horizontal branch oscillations could be due to unstable thermal and acoustic modes, respectively. It is subsequently shown that some of the observed short-term (quasi-periodic) variability in active galactic nuclei may also originate from short-wavelength acoustic modes in the innermost region of the disc. Consequently, observational characteristics, in connection with stability theory, may yield estimates of basic accretion parameters. In the case of the Seyfert galaxy NGC6814, this process seems to favour |$(\alpha ,\mu ,m,\dot{m})\simeq (0.5,0,{10}^{6},{10}^{-2})$|⁠. We finally conclude that this line of work may provide additional evidence for both accretion discs and black holes, in various compact sources.

Instabilities and time evolution of hot accretion disks with electron-positron pairs

view Abstract Citations (20) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Instabilities and Time Evolution of Hot Accretion Disks with Electron-Positron Pairs White, Timothy R. ; Lightman, Alan P. Abstract Since the 1970s, hot, optically thin, thermal accretion disks have been proposed in some models of galactic and extragalactic X-ray sources. Recent investigations of such disks, including electron-positron pairs, have found that in disks cooled by internally produced photons, there exists a critical accretion rate above which equilibrium is not possible in an annular region around r = 10GM/c^2^, where M is the mass of the central object. (The critical accretion rate corresponds to a luminosity of a few percent of the Eddington limit.) Below the critical rate, there are two self-consistent disk structures at each radius, one with a low density of pairs and one with a high density of pairs. We consider the stability and time-dependent behavior of such disks. At accretion rates below the critical value, the high pair density equilibrium is unstable to pair runaway. Such disks, when perturbed, produce pairs rampantly, then cool, and collapse toward cool, optically thick disks without pairs on a couple of hydrodynamical time scales (~ 10 hr at r = 10GM/c^2^ for M = 10^8^ M_sun_). This behavior should be accompanied by the reduction or elimination of all hard X-ray emission and an increase in UV emission. Optically thin disks subject to accretion rates above the critical rate would behave in a similar way. Because the inner regions of the optically thick disks are known to be secularly unstable, such a disk may fluctuate between optically thick and optically thin states, with an accompanying change in the spectrum. The absence of such fluctuations would suggest that the dominant X-ray and gamma-ray emission of these objects is not produced by a thermal plasma, unless the plasma is cooled by an external source of soft photons. When the spectral and luminosity fluctuations are present, their time scale provides an estimate of the mass of the central black hole. Publication: The Astrophysical Journal Pub Date: April 1990 DOI: 10.1086/168553 Bibcode: 1990ApJ...352..495W Keywords: Accretion Disks; Electron-Positron Pairs; Galactic Nuclei; Galactic Radiation; Optical Thickness; X Ray Sources; Compton Effect; Digital Simulation; Electron Energy; Elementary Particles; Astrophysics; ELEMENTARY PARTICLES; GALAXIES: NUCLEI; RADIATION MECHANISMS full text sources ADS |

Hot accretion disks with electron-positron pairs

view Abstract Citations (75) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hot Accretion Disks with Electron-Positron Pairs White, Timothy R. ; Lightman, Alan P. Abstract The hot thermal accretion disks of the 1970s are studied and consideration is given to the effects of electron-positron pairs, which were originally neglected. It is found that disks cooled by internally produced photons have a critical accretion rate above which equilibrium is not possible in a radial annulus centered around r = 10 GM/c-squared, where M is the mass of the central object. This confirms and extends previous work by Kusunose and Takahara. Above the critical rate, pairs are created more rapidly than they can be destroyed. Below the critical rate, there are two solutions to the disk structure, one with a high pair density and one with a low pair density. Depending on the strength of the viscosity, the critical accretion rate corresponds to a critical luminosity of about 3-10 percent of the Eddington limit. Publication: The Astrophysical Journal Pub Date: May 1989 DOI: 10.1086/167455 Bibcode: 1989ApJ...340.1024W Keywords: Accretion Disks; Bremsstrahlung; Compton Effect; Electron-Positron Pairs; Nuclear Astrophysics; Luminosity; Pair Production; X Ray Sources; Astrophysics; ELEMENTARY PARTICLES; STARS: ACCRETION full text sources ADS |

Surface conditions in accreting neutron stars

The structure of the boundary layer where the accretion disk interacts with an accreting neutron star surface is studied using a one-zone approximation to observe the effects of the accretion on the thermal structure of the surface layers and on the progress of shell burning. The kinematic viscosity is parameterized by means of the critical Reynolds number, R/sub cr/. It is found that for higher accretion rate M/M/sub Edd/>>1/R/sub cr/ (M/sub Edd/ being the critical accretion rate corresponding to the Eddington luminosity), an optically thick boundary layer is formed, where radiation pressure is dominant. As long as it remains optically thick, the observable properties of the boundary layer are rather insensitive to the assumption of viscosity and are determined solely by the accretion rate. The internal structure, however, depends strongly on assumed magnitude of the viscosity as well as on the accretion rate; the pressure and temperature increase rapidly with R/sub cr/ and also with M. For large R/sub cr/, therefore, the boundary layer lies well inside the stellar photosphere, where the dissipation of kinetic energy plays a critical role in heating the envelope and in leading to the ignition of shell flashes. Results are discussed on the possible modificationsmore » of the X-ray burst model from that constructed under spherical symmetry and on their relevance to X-ray observations of low-mass binary systems.« less

Time-dependent accretion onto magnetized white dwarfs

view Abstract Citations (117) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Time-dependent accretion onto magnetized white dwarfs. Langer, S. H. ; Chanmugam, C. ; Shaviv, G. Abstract Time-dependent accretion onto magnetized white dwarfs is considered. A detailed description of a numerical method of solution to the hydrodynamical equations is given. The postshock flow is cooled by optically thin bremsstrahlung and is thermally unstable. As a result the shock height undergoes periodic oscillations. The properties of this oscillation are considered as a function of the accretion rate and of the mass and radius of the white dwarf. The structure of the accretion flow depends on a single scaling parameter. Below a critical accretion rate, which depends on the particular white dwarf, the nature of the accretion flow changes and the shock propagates up the accretion column indefinitely. Publication: The Astrophysical Journal Pub Date: July 1982 DOI: 10.1086/160079 Bibcode: 1982ApJ...258..289L Keywords: Binary Stars; Hydrodynamic Equations; Magnetic Stars; Stellar Mass Accretion; Time Dependence; White Dwarf Stars; Bremsstrahlung; Cyclotron Radiation; Shock Wave Propagation; Stellar Oscillations; X Ray Sources; Astrophysics full text sources ADS | data products SIMBAD (4)

Choking of optically thin spherical accretion by dissipative heating

Dissipative heating can be sufficient to reduce the Mach number of supersonic spherical accretion to unity in the optically thin part of the flow: at a radius of order 10/sup 2/--10/sup 3/ Schwarzchild radii. If the flow at a larger radius is forced to be supersonic and cold, by some cooling process like collisional excitation of line radiation, the flow cannot be time-independent. The critical accretion rates below which accretion flows either are forced to be time dependent, or become optically thick before the minimum in the Mach number is reached, are determined. The implication for the time variability of quasars and active galactic nuclei is briefly discussed.

Formation of Luminous Contact Binaries by Rapid Accretion onto White Dwarfs

During the evolution of a close binary system, there is a phase of mass exchange between its component stars. When the mass-losing star is an evolved star off the main sequence, the mass exchange is very rapid. Its rate may well exceed the critical accretion rate corresponding to the Eddington limit of the mass accreting star. Effects of such a rapid accretion were studied for the mass-accreting main-sequence stars (Flannery and Ulrich 1977; Kippenhahn and Mayer-Hofmeister 1977; Neo et al. 1977). The mass-accreting stars become quickly overluminous and their radii increase even to a red-giant size as mass is accumulated. Then the both component stars will fill their Roche lobes to form a contact binary system.