AN ACCRETION DISK-OUTFLOW MODEL FOR HYSTERETIC STATE TRANSITION IN X-RAY BINARIES

Luminosity of X-ray spectral state transitions in black hole and neutron star X-ray binaries can put constraint on the critical mass accretion rate between accretion regimes. Previous studies indicate that the hard-to-soft spectral state transitions in some ultracompact neutron star LMXBs have the lowest luminosity. With X-ray monitoring observations in the past decade, we were able to identify state transitions towards the lowest luminosity limit in 4U 0614+091, 2S 0918-549 and 4U 1246-588. By analysing corresponding X-ray pointed observations with the Swift/XRT and the RXTE/PCA, we found no hysteresis of state transitions in these sources, and determined the critical mass accretion rate in the range of 0.002 - 0.04 $\dot{\rm M}_{\rm Edd}$ and 0.003 - 0.05 $\dot{\rm M}_{\rm Edd}$ for the hard-to-soft and the soft-to-hard transition, respectively, by assuming a neutron star mass of 1.4 solar masses. This range is comparable to the lowest transition luminosity measured in black hole X-ray binaries, indicating the critical mass accretion rate is not affected by the nature of the surface of the compact stars. Our result does not support the Advection-Dominated Accretion Flow (ADAF) model which predicts that the critical mass accretion rate in neutron star systems is an order of magnitude lower if same viscosity parameters are taken. The low transition luminosity and insignificant hysteresis in these ultracompact X-ray binaries provide further evidence that the transition luminosity is likely related to the mass in the disc.

Context. The hard-to-soft state transition of the outbursts in X-ray binaries (XRBs) is triggered by the rising of the mass accretion rate as a result of the disk instability. The hard X-ray transition luminosity is found to be tightly correlated to the soft X-ray peak luminosity in the soft state, the physical origin of which is still a mystery. Aims. In order to explain the observed correlation between the hard X-ray transition luminosity and the soft X-ray peak luminosity in the soft state, we construct a magnetic disk-outflow model for the state transition in XRBs. Methods. We assumed that the large-scale magnetic field in the outer thin disk is formed through an inverse cascade of the field generated by the small-scale dynamo, which is then advected by the inner advection-dominated accretion flow (ADAF). The advected field accelerates a fraction of the gas in the ADAF into the outflows. We calculated the transition luminosity of an ADAF that is driven by these magnetic outflows, which vary with the mass accretion rate of the outer disk. Results. During the outbursts, the heating front moves inward, and the field strength at the heating front of the outer disk is proportional to the accretion rate of the disk. Much angular angular momentum of the inner ADAF is carried away by the outflows for a stronger magnetic field, which leads to a high radial velocity of the ADAF. This increases the critical mass accretion rate of the ADAF with the field strength, and it therefore leads to a correlation between transition luminosity and the peak luminosity in the thermal state. We found that the values of the viscosity parameter α of the neutron star XRBs are systematically higher for those of the black hole (BH) XRBs (α ∼ 0.05−0.15 for BHs, and α ∼ 0.15−0.4 for neutron stars). Our model predicts that the transition luminosity may be higher than the peak luminosity provided α is sufficiently high, which is able to explain a substantial fraction of outbursts in BHXRBs that do not reach the thermally dominant accretion state.

Dwarf nova outbursts are nonlinear phenomena, and a time-dependent disk model is necessary to account for observations in detail. However, it is also necessary to elaborate a simpler steady-state fit to interpret observations. To know in what condition the outburst is initiated, understanding of the dwarf nova outburst is important. The parameterized, steady-state fitting formulae are suggested by Smak (Acta Astron. 52, 429 (2002); ibid 60, 83 (2010)) for the critical disk temperature and mass accretion rate above which the disk becomes thermally unstable. The fits give a single-valued temperature and accretion rate and are radius-independent whereas the observations show that the outbursts are radius-dependent phenomena of the ionizaton propagating in the disk. The fits have been tested to account for the observed outbursts only for systems with orbital periods shorter than a half day. Therefore, we examine the fits for orbital period as long as 2 days and compare the fits to the time-dependent model of a long-period dwarf nova GK Per. The fits are not much different from the time-dependent result for the critical temperature. However, the fits for the critical mass accretion rate above which the disk enters the hot state overestimate the time-dependent model for a long-period system like GK Per. The critical mass accretion rate in the intermediate state is consistent with that from the time-dependent disk model. However, the fit value should be treated as a maximum possible value below which the disk maintains the intermediate state, which is consistent with an interpretation for the observations of the Z Cam stars.

The truncation of an optically thick, geometrically thin accretion disk is\ninvestigated in the context of low luminosity AGN (LLAGN). We generalize the\ndisk evaporation model used in the interpretative framework of black hole X-ray\nbinaries by including the effect of a magnetic field in accretion disks\nsurrounding supermassive black holes. The critical transition mass accretion\nrate for which the disk is truncated is found to be insensitive to magnetic\neffects, but its inclusion leads to a smaller truncation radius in comparison\nto a model without its consideration. That is, a thin viscous disk is truncated\nfor LLAGN at an Eddington ratio less than 0.03 for a standard viscosity\nparameter ($\\alpha = 0.3$). An increase of the viscosity parameter results in a\nhigher critical transition mass accretion rate and a correspondingly smaller\ntruncation distance, the latter accentuated by greater magnetic energy\ndensities in the disk. Based on these results, the truncation radii inferred\nfrom spectral fits of LLAGN published in the literature are consistent with the\ndisk evaporation model. The infrared emission arising from the truncated\ngeometrically thin accretion disks may be responsible for the red bump seen in\nsuch LLAGN.\n

All high temperature accretion solutions including ADAF are physically thick, so outgoing radiation interacts with the incoming flow, sharing as much or more resemblance with classical spherical accretion flows as with disk flows. We examine this interaction for the popular ADAF case. We find that without allowance for Compton preheating, a very restricted domain of ADAF solution is permitted and with Compton preheating included a new high temperature PADAF branch appears in the solution space. In the absence of preheating, high temperature flows do not exist when the mass accretion rate mdot == Mdot c^2 / L_E >~ 10^-1.5. Below this mass accretion rate, a roughly conical region around the hole cannot sustain high temperature ions and electrons for all flows having mdot >~ 10^-4, which may lead to a funnel possibly filled with a tenuous hot outgoing wind. If the flow starts at large radii with the usual equilibrium temperature ~10^4 K, the critical mass accretion rate is much lower, mdot \~10^-3.7 above which level no self-consistent ADAF (without preheating) can exist. However, above this critical mass accretion rate, the flow can be self-consistently maintained at high temperature if Compton preheating is considered. These solutions constitute a new branch of solutions as in spherical accretion flows. High temperature PADAF flows can exist above the critical mass accretion rate in addition to the usual cold thin disk solutions. We also find solutions where the flow near the equatorial plane accretes normally while the flow near the pole is overheated by Compton preheating, possibly becoming, a polar wind, solutions which we designate WADAF.

Recent studies of different X-ray binaries (XRBs) have shown a clear correlation between the radio and X-ray emission. We present evidence of a close relationship found between the radio and X-ray emission at different epochs for GRS 1915+105, using observations from the Ryle Telescope and Rossi X-ray Timing Explorer satellite. The strongest correlation was found during the hard state (also known as the "plateau" state), where a steady AU-scale jet is known to exist. Both the radio and X-ray emission were found to decay from the start of most plateau states, with the radio emission decaying faster. An empirical relationship of S radio. S. X-ray was then fitted to data taken only during the plateau state, resulting in a power-law index of xi similar to 1.7 +/- 0.3, which is significantly higher than in other black hole XRBs in a similar state. An advection-flow model was then fitted to this relationship and compared to the universal XRB relationship as described by Gallo et al. (2003, MNRAS, 344, 60). We conclude that either (I) the accretion disk in this source is radiatively efficient, even during the continuous outflow of a compact jet, which could also suggest a universal turn-over from radiatively inefficient to efficient for all stellar-mass black holes at a critical mass accretion rate (m(c) approximate to 10(18.5) g/s); or (II) the X-rays in the plateau state are dominated by emission from the base of the jet and not the accretion disk (e. g. via inverse Compton scattering from the outflow).

It is suggested that the variation of the mass accretion rate in the accretion disk may be responsible for the occurrence of most changing-look active galactic nuclei (CL AGNs). However, the viscous timescale of a thin disk is far longer than the observed timescale of CL AGNs. Though this problem can be resolved by introducing the large-scale magnetic field, the mechanism for radio-quiet CL AGNs with a weak/absent large-scale magnetic field remains a mystery. In this work, we assume that the thin accretion disk is collapsed from the inner advection-dominated accretion flow (ADAF) instead of being formed from the outer thin disk through advection. This idea is tested by comparing the cooling timescale (t cool) of an ADAF with the observed timescale (t tran) of turn-on CL AGNs. We compile a sample of 102 turn-on CL AGNs from the archived data and calculate the cooling timescale of an ADAF with the critical mass accretion rate based on some conventional assumptions. It is found that t cool is much shorter than t tran in most of the CL AGNs, which validates our assumption, though t cool is not consistent with t tran (t cool < t tran). However, this is reasonable since most of the CL AGNs were observed only two times, indicating that the observed timescale t tran is the maximum value because the changing-look behavior can indeed happen before the second observation.

We studied the frequency and critical mass accretion rate of millihertz quasi-periodic oscillations (mHz QPOs) using a one-zone X-ray burst model. The surface gravity is specified by two kinds of equation of states: neutron star (NS) and strange star (SS). The base flux, Qb, is set in the range of 0–2 MeV nucleon−1. It is found that the frequency of mHz QPO is positively correlated to the surface gravity but negatively to the base heating. The helium mass fraction has a significant influence on the oscillation frequency and luminosity. The observed 7–9 mHz QPOs can be either explained by a heavy NS/light SS with a small base flux or a heavy SS with a large base flux. As base flux increases, the critical mass accretion rate for marginally stable burning is found to be lower. Meanwhile, the impact of metallicity on the properties of mHz QPOs was investigated using one-zone model. It shows that both the frequency and critical mass accretion rate decrease as metallicity increases. An accreted NS/SS with a higher base flux and metallicity, combined with a lower surface gravity and helium mass fraction, could be responsible for the observed critical mass accretion rate ($\dot{m}\simeq 0.3\dot{m}_{\rm Edd}$). The accreted fuel would be in stable burning if base flux is over than ∼2 MeV nucleon−1. This finding suggests that the accreting NSs/SSs in low-mass X-ray binaries showing no type I X-ray bursts possibly have a strong base heating.

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 large-scale magnetic field threading an accretion disk plays an important role in launching jets/outflows. The field may probably be advected inward by the plasma in the accretion disk from the ambient environment (interstellar medium or a companion star). It has been suggested that the external field can be efficiently dragged inward in a thin disk with magnetic outflows. We construct a self-consistent global disk-outflow model in which the large-scale field is formed by the advection of the external field in the disk. The outflows are accelerated by this field corotating with the disk, which carries away most of the angular momentum of the disk and causes its structure to become significantly different from the conventional viscous disk structure. We find that the magnetic field strength in the inner region of the disk can be several orders of magnitude higher than the external field strength for a geometrically thin disk with H/R ∼0.1 if the ratio of the gas to magnetic pressure β out ∼ 102 at the outer edge of the disk. The outflow velocity shows a layer-like structure, i.e., it decreases with radius where it is launched. The outflow can be accelerated up to ∼0.2–0.3c from the inner region of the disk, and the mass-loss rate in the outflows is ∼10%–70% of the mass accretion rate at the outer radius of the disk, which may account for the fast outflows that are observed in some active galactic nuclei.

Large-scale magnetic field is believed to play a key role in launching and collimating jets/outflows. It was found that advection of the external field by a geometrically thin disk is rather inefficient, while the external weak field may be dragged inwards by fast radially moving tenuous and/or hot gas above the thin disk. We investigate the field advection in a thin (cold) accretion disk covered with hot corona, in which turbulence is responsible for the angular momentum transfer of the gas in the disk and corona. The radial velocity of the gas in the corona is significantly higher than that in the thin disk. Our calculations show that the external magnetic flux is efficiently transported inwards by the corona, and the field line is strongly inclined toward the disk surface, which helps to launch outflows. The field configurations are consistent with those observed in the numerical simulations. The strength of the field is substantially enhanced in the inner region of the disk (usually several orders of magnitude higher than the external field strength), which is able to drive a fraction of gas in the corona into outflows. This mechanism may be useful in explaining the observational features in X-ray binaries and active galactic nuclei. Our results may help in understanding the physics of the magnetohydrodynamic simulations.

We reexamine the hypothesis that the optical/UV/soft X-ray continuum of Active Galactic Nuclei is thermal emission from an accretion disk. Previous studies have shown that fitting the spectra with the standard, optically thick and geometrically thin accretion disk models often led to luminosities which contradict the basic assumptions adopted in the standard model. There is no known reason why the accretion rates in AGN should not be larger than the thin disk limit. In fact, more general, slim accretion disk models are self-consistent even for moderately super-Eddington luminosities. We calculate here spectra from a set of thin and slim, optically thick accretion disks. We discuss the differences between the thin and slim disk models, stressing the implications of these differences for the interpretation of the observed properties of AGN. We found that the spectra can be fitted not only by models with a high mass and a low accretion rate (as in the case of thin disk fitting) but also by models with a low mass and a high accretion rate. In the first case fitting the observed spectra in various redshift categories gives black hole masses around 10^9 solar masses for a wide range of redshifts, and for accretion rates ranging from 0.4 to 8 solar masses/year. In the second case the accretion rate is around 10^2 solar masses/year for all AGN and the mass ranges from 3*10^6 to 10^8 solar masses. Unlike the disks with a low accretion rate, the spectra of the high-accretion-rate disks extend into the soft X-rays. A comparison with observations shows that such disks could produce the soft X-ray excesses claimed in some AGNs. We show also that the sequence of our models with fixed mass and different accretion rates can explain the time evolution of the observed spectra in Fairall 9.

Stellar mass black holes in X-ray binaries (XRBs) are known to display different states characterized by different spectral and timing properties, understood in the framework of a hot corona coexisting with a thin accretion disc whose inner edge is truncated. There are several open questions related to the nature and properties of the corona, the thin disc, and dynamics behind the hard state. This motivated us to perform 2D hydrodynamical simulations of accretion flows onto a $10 \, \mathrm{M}_\odot$ black hole. We consider a two-temperature plasma, incorporate radiative cooling with bremmstrahlung, synchrotron, and Comptonization losses and approximate the Schwarzschild space–time via a pseudo-Newtonian potential. We varied the mass accretion rate in the range $0.02 \le \dot{M}/\dot{M}_{\rm Edd} \le 0.35$. Our simulations show the natural emergence of a colder truncated thin disc embedded in a hot corona, as required to explain the hard state of XRBs. We found that as $\dot{M}$ increases, the corona contracts and the inner edge of the thin disc gets closer to the event horizon. At a critical accretion rate $0.02 \le \dot{M}_{\text{crit }}/\dot{M}_{\rm Edd} \le 0.06$, the thin disc disappears entirely. We discuss how our simulations compare with XRB observations in the hard state.

For all sources in which the phenomenon of kilohertz quasi-periodic oscillation (kHz QPO) is observed, the QPOs disappear abruptly when the inferred mass accretion rate exceeds a certain threshold. Although the threshold cannot at present be accurately determined (or even quantified) observationally, it is clearly higher for bright Z sources than for faint atoll sources. Here we propose that the observational manifestation of kHz QPOs requires direct interaction between the neutron star magnetosphere and the Keplerian accretion disk and that the cessation of kHz QPOs at a high accretion rate is due to the lack of such an interaction when the Keplerian disk terminates at the last stable orbit and yet the magnetosphere is pushed farther inward. The threshold is therefore dependent on the magnetic field strength-the stronger the magnetic field, the higher the threshold. This is certainly in agreement with the atoll/Z paradigm, but we argue that it is also generally true, even for individual sources within each (atoll or Z) category. For atoll sources, the kHz QPOs also seem to vanish at a low accretion rate. Perhaps the "disengagement" between the magnetosphere and the Keplerian disk also takes place under such circumstances because of, for instance, the presence of quasi-spherical advection-dominated accretion flow (ADAF) close to the neutron star. Unfortunately, in this case, the estimation of the accretion rate threshold would require a knowledge of the physical mechanisms that cause the disengagement. If the ADAF is responsible, the threshold is likely dependent on the magnetic field of the neutron star.

Black hole accretion flows can be divided into two broad classes: cold and hot. Whereas cold accretion flows consist of cool optically thick gas and are found at relatively high mass accretion rates, hot accretion flows, the topic of this review, are virially hot and optically thin, and occur at lower mass accretion rates. They are described by accretion solutions such as the advection-dominated accretion flow and luminous hot accretion flow. Because of energy advection, the radiative efficiency of these flows is in general lower than that of a standard thin accretion disk. Moreover, the efficiency decreases with decreasing mass accretion rate. Observations show that hot accretion flows are associated with jets. In addition, theoretical arguments suggest that hot flows should produce strong winds. Hot accretion flows are believed to be present in low-luminosity active galactic nuclei and in black hole X-ray binaries in the hard and quiescent states. The prototype is Sgr A*, the ultralow-luminosity supermassive black hole at our Galactic center. The jet, wind, and radiation from a supermassive black hole with a hot accretion flow can interact with the external interstellar medium and modify the evolution of the host galaxy.