Three‐dimensional Magnetohydrodynamic Simulations of Radiatively Inefficient Accretion Flows

Igumenshchev, I. V.; Narayan, Ramesh; Abramowicz, Marek A.

doi:10.1086/375769

Cited by 454 publications

(483 citation statements)

References 49 publications

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“…In particular, the magnetic field equipartition fraction as well as the bias field strength in the case of magnetic field reversals (as expected for a turbulent flow) are unknown. Variations in the magnetic field structure as well as the field strength with respect to equipartition (Marrone et al 2007;Igumenshchev et al 2003) can result in higher densities at distances of only a few Schwarzschild radii from the central SMBH.…”

Section: Resultsmentioning

confidence: 99%

Millimeter to X-ray flares from Sagittarius A*

Eckart

García-Marín

Vogel

et al. 2012

A&A

146

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Context. We report on new simultaneous observations and modeling of the millimeter, near-infrared, and X-ray flare emission of the source Sagittarius A* (SgrA*) associated with the super-massive (4 × 10 6 M ) black hole at the Galactic center. Aims. We study the applicability of the adiabatic synchrotron source expansion model and study physical processes giving rise to the variable emission of SgrA* from the radio to the X-ray domain. Methods. Our observations were carried out on 18 May 2009 using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope, the ACIS-I instrument aboard the Chandra X-ray Observatory, the LABOCA bolometer at the Atacama Pathfinder EXperiment (APEX), and the CARMA mm telescope array at Cedar Flat, California. Results. The X-ray flare had an excess 2−8 keV luminosity between 6 and 12×10 33 erg s −1 . The observations reveal flaring activity in all wavelength bands that can be modeled as the signal from an adiabatically expanding synchrotron self-Compton (SSC) component.Modeling of the light curves shows that the sub-mm follows the NIR emission with a delay of about three-quarters of an hour with an expansion velocity of about v exp ∼ 0.009c. We find source component sizes of around one Schwarzschild radius, flux densities of a few Janskys, and spectral indices α of about +1 (S (ν) ∝ ν −α ). At the start of the flare, the spectra of the two main components peak just short of 1 THz. To statistically explain the observed variability of the (sub-)mm spectrum of SgrA*, we use a sample of simultaneous NIR/X-ray flare peaks and model the flares using a synchrotron and SSC mechanism. Conclusions. These parameters suggest that either the adiabatically expanding source components have a bulk motion larger than v exp or the expanding material contributes to a corona or disk, confined to the immediate surroundings of SgrA*. For the bulk of the synchrotron and SSC models, we find synchrotron turnover frequencies in the range of 300−400 GHz. For the pure synchrotron models, this results in densities of relativistic particles of the order of 10 6.5 cm −3 and for the SSC models, the median densities are about one order of magnitude higher. However, to obtain a realistic description of the frequency-dependent variability amplitude of SgrA*, models with higher turnover frequencies and even higher densities are required.

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Section: Resultsmentioning

confidence: 99%

Millimeter to X-ray flares from Sagittarius A*

Eckart

García-Marín

Vogel

et al. 2012

A&A

146

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“…The magnetic pressure can dominate over the gas pressure in the inner region of the disk, and it becomes a magnetic arrested disk (Igumenshchev et al 2003). For a given external field, the ADAF is magnetically arrested only if the accretion rate is lower than a certain rate (Cao 2011).…”

Section: ¢ = ¢mentioning

confidence: 99%

An Accretion Disk-Outflow Model for Hysteretic State Transition in X-Ray Binaries

Cao

2016

ApJ

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We suggest a model of the advection-dominated accretion flow (ADAF) with magnetically driven outflows to explain the hysteretic state transition observed in X-ray binaries (XRBs). The transition from a thin disk to an ADAF occurs when the mass accretion rate is below a critical value. The critical mass accretion rate for the ADAF can be estimated by equating the equilibration timescale to the accretion timescale of the ADAF, which is sensitive to its radial velocity. The radial velocity of thin disks is very small, which leads to the advection of the external field in thin disks becoming very inefficient. ADAFs are present in the low/hard states of XRBs, and their radial velocity is large compared with the thin disk. The external field can be dragged inward efficiently by the ADAF, so a strong large-scale magnetic field threading the ADAF can be formed, which may accelerate a fraction of gas in the ADAF into the outflows. Such outflows may carry away a large amount of angular momentum from the ADAF, which significantly increases the radial velocity of the ADAF. This leads to a high critical mass accretion rate, below which an ADAF with magnetic outflows can survive. Our calculations show that the critical luminosity of the ADAF with magnetic outflows can be one order of magnitude higher than that for a conventional ADAF, if the ratio of gas to magnetic pressure 4 b~in the disk. This can naturally explain the hysteretic state transition observed in XRBs.

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“…Here β 0 = β(r G ) = 8πρ 0 c 2 so /B 2 f0 is the ratio of thermal to magnetic pressure in the captured matter at the Bondi radius, c so = c s (r G ) is the sound speed and B f0 = B f (r G ) is the magnetic field in the accreting matter. The flow at this radius is converted into a non-Keplerian ML-disc in which the accreting matter is confined by its own magnetic field (for discussion see Bisnovatyi-Kogan & Ruzmaikin 1976;Igumenshchev, Narayan, & Abramowicz 2003). The accretion inside the ML-disc proceeds on the time-scale of the magnetic flux dissipation and, in the general case, can be treated in a diffusion approximation.…”

Section: Magnetic Levitation Accretionmentioning

confidence: 99%

On the magnetic fields of Be/X-ray pulsars in the Small Magellanic Cloud: Figure 1.

Ikhsanov¹,

Mereghetti²

2015

Mon. Not. R. Astron. Soc.

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We explore the possibility of explaining the properties of the Be/X-ray pulsars observed in the Small Magellanic Cloud (SMC) within the magnetic levitation accretion scenario. This implies that their X-ray emission is powered by a wind-fed accretion on to a neutron star (NS) which captures matter from a magnetized stellar wind. The NS in this case is accreting matter from a non-Keplerian magnetically levitating disc which is surrounding its magnetosphere. This allows us to explain the observed periods of the pulsars in terms of spin equilibrium without the need of invoking dipole magnetic fields outside the usual range ∼ 10 11 − 10 13 G inferred from cyclotron features of Galactic high-mass X-ray binaries. We find that the equilibrium period of a NS, under certain conditions, depends strongly on the magnetization of the stellar wind of its massive companion and, correspondingly, on the magnetic field of the massive companion itself. This may help to explain why similar NSs in binaries with similar properties rotate with different periods yielding a large scatter of periods of the accretionpowered pulsar observed in SMC and our galaxy.

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Three‐dimensional Magnetohydrodynamic Simulations of Radiatively Inefficient Accretion Flows

Cited by 454 publications

References 49 publications

Millimeter to X-ray flares from Sagittarius A*

Millimeter to X-ray flares from Sagittarius A*

An Accretion Disk-Outflow Model for Hysteretic State Transition in X-Ray Binaries

On the magnetic fields of Be/X-ray pulsars in the Small Magellanic Cloud: Figure 1.

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