ABSTRACT Rapid, large amplitude variability at optical to X-ray wavelengths is now seen in an increasing number of Seyfert galaxies and luminous quasars. The variations imply a global change in ...accretion power, but are too rapid to be communicated by inflow through a standard thin accretion disc. Such discs are long known to have difficulty explaining the observed optical/UV emission from active galactic nuclei. Here we show that alternative models developed to explain these observations have larger scale heights and shorter inflow times. Accretion discs supported by magnetic pressure in particular are geometrically thick at all luminosities, with inflow times as short as the observed few year time-scales in extreme variability events to date. Future time-resolved, multiwavelength observations can distinguish between inflow through a geometrically thick disc as proposed here, and alternative scenarios of extreme reprocessing of a central source or instability-driven limit cycles.
We reformulate the adiabatic inflow-outflow solution (ADIOS) model for radiatively inefficient accretion flows, treating the inflow and outflow zones on an equal footing. For purely adiabatic flows ...(i.e. with no radiative losses), we show that the mass flux in each zone must satisfy
with n= 1, in contrast to previous work in which 0 < n < 1 is a free parameter but in rough agreement with numerical simulations. We also demonstrate that the resulting two-zone ADIOS models are not dynamically self-consistent without the introduction of an energy source close in to the central regions of the flow; we identify this with the energy liberated by accretion. We explore the parameter space of non-radiative models and show that both powerful winds and gentle breezes are possible. When small radiative losses (with fixed efficiency) are included, any centrally injected energy flux is radiated away and the system reverts to a power-law behaviour with n≲ 1, where n falls in a small range determined by the fractional level of radiative losses. We also present an ADIOS model for hypercritical (super-Eddington) disc accretion, in which the radiative losses are closely related to the flow geometry. We suggest that hyperaccretion can lead to either winds or breezes.
What really makes an accretion disc MAD Begelman, Mitchell C; Scepi, Nicolas; Dexter, Jason
Monthly notices of the Royal Astronomical Society,
02/2022, Letnik:
511, Številka:
2
Journal Article
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ABSTRACT
Magnetically arrested accretion discs (MADs) around black holes (BHs) have the potential to stimulate the production of powerful jets and account for recent ultra-high-resolution ...observations of BH environments. Their main properties are usually attributed to the accumulation of dynamically significant net magnetic (vertical) flux throughout the arrested region, which is then regulated by interchange instabilities. Here, we propose instead that it is mainly a dynamically important toroidal field – the result of dynamo action triggered by the significant but still relatively weak vertical field – that defines and regulates the properties of MADs. We suggest that rapid convection-like instabilities, involving interchange of toroidal flux tubes and operating concurrently with the magnetorotational instability (MRI), can regulate the structure of the disc and the escape of net flux. We generalize the convective stability criteria and disc structure equations to include the effects of a strong toroidal field and show that convective flows could be driven towards two distinct marginally stable states, one of which we associate with MADs. We confirm the plausibility of our theoretical model by comparing its quantitative predictions to simulations of both MAD and SANE (standard and normal evolution; strongly magnetized but not ‘arrested’) discs, and suggest a set of criteria that could help to distinguish MADs from other accretion states. Contrary to previous claims in the literature, we argue that MRI is not suppressed in MADs and is probably responsible for the existence of the strong toroidal field.
Supermassive stars, with masses ≳106 M⊙, are possible progenitors of supermassive black holes in galactic nuclei. Because of their short nuclear burning time-scales, such objects can be formed only ...when matter is able to accumulate at a rate exceeding ∼1 M⊙ yr−1. Here we revisit the structure and evolution of rotationally stabilized supermassive stars, taking into account their continuous accumulation of mass and their thermal relaxation. We show that the outer layers of a supermassive star are not thermally relaxed during much of the star's main-sequence lifetime. As a result, they do not resemble n= 3 polytropes, as assumed in previous literature, but rather consist of convective (polytropic) cores surrounded by convectively stable envelopes that contain most of the mass. We compute the structures of these envelopes, in which the equation of state obeys P/ρ4/3∝M2/3(R), where M(R) is the mass enclosed within radius R. By matching the envelope solutions to convective cores, we calculate the core mass as a function of time. We estimate the initial black hole masses formed as a result of core-collapse, and their subsequent growth via accretion from the bloated envelopes (‘quasi-stars’) that result. The seed black holes formed in this way could have typical masses in the range ∼104–105 M⊙, considerably larger than the remnants thought to be left by the demise of Population III stars. Supermassive black holes therefore could have been seeded during an epoch of rapid infall considerably later than the era of Population III star formation.
ABSTRACT
Sgr A* exhibits flares in the near-infrared and X-ray bands, with the luminosity in these bands increasing by factors of 10–100 for ≈60 min. One of the models proposed to explain these ...flares is synchrotron emission of non-thermal particles accelerated by magnetic reconnection events in the accretion flow. We use the results from particle-in-cell simulations of magnetic reconnection to post-process 3D two-temperature GRMHD simulations of a magnetically arrested disc (MAD). We identify current sheets, retrieve their properties, estimate their potential to accelerate non-thermal particles, and compute the expected non-thermal synchrotron emission. We find that the flux eruptions of MADs can provide suitable conditions for accelerating non-thermal particles to energies γe ≲ 106 and producing simultaneous X-ray and near-infrared flares. For a suitable choice of current-sheet parameters and a simplified synchrotron cooling prescription, the model can simultaneously reproduce the quiescent and flaring X-ray luminosities as well as the X-ray spectral shape. While the near-infrared flares are mainly due to an increase in the temperature near the black hole during the MAD flux eruptions, the X-ray emission comes from narrow current sheets bordering highly magnetized, low-density regions near the black hole, and equatorial current sheets where the flux on the black hole reconnects. As a result, not all infrared flares are accompanied by X-ray ones. The non-thermal flaring emission can extend to very hard (≲ 100 keV) X-ray energies.
We argue that the magnetic flux threading the black hole (BH), rather than BH spin or Eddington ratio, is the dominant factor in launching powerful jets and thus determining the radio loudness of ...active galactic nuclei (AGNs). Most AGNs are radio quiet because the thin accretion disks that feed them are inefficient in depositing magnetic flux close to the BH. Flux accumulation is more likely to occur during a hot accretion (or thick disk) phase, and we argue that radio-loud quasars and strong emission-line radio galaxies occur only when a massive, cold accretion event follows an episode of hot accretion. Such an event might be triggered by the merger of a giant elliptical galaxy with a disk galaxy. This picture supports the idea that flux accumulation can lead to the formation of a so-called magnetically choked accretion flow. The large observed range in radio loudness reflects not only the magnitude of the flux pressed against the BH, but also the decrease in UV flux from the disk, due to its disruption by the "magnetosphere" associated with the accumulated flux. While the strongest jets result from the secular accumulation of flux, moderate jet activity can also be triggered by fluctuations in the magnetic flux deposited by turbulent, hot inner regions of otherwise thin accretion disks, or by the dissipation of turbulent fields in accretion disk coronae. These processes could be responsible for jet production in Seyferts and low-luminosity AGNs, as well as jets associated with X-ray binaries.
ABSTRACT
We argue that the changing-look event in the active galactic nucleus (AGN) 1ES 1927+654, followed by a dip of three orders of magnitude in the X-ray luminosity, is controlled by a change in ...the accretion rate and an inversion of magnetic flux in a magnetically arrested disc (MAD). Before the changing-look event, strong magnetic flux on the black hole powers X-ray emission via the Blandford–Znajek process, while the UV emission is produced by a radiatively inefflcient magnetized disc. An advection event, bringing flux of the opposite polarity, propagates inward leading, first, to a rise in the UV/optical luminosity and, then, to a dip in the X-ray luminosity. We find that the observed time-scale between the beginning of the changing-look event and the minimum in the X-ray luminosity, ≈200 d, is in agreement with the time needed to cancel the magnetic flux in a MAD extending to ≈180 rg. Although flux inversion events might be rare due to the large ratio of flux-to-mass that is needed, we argue that AGN showing an unusually high ratio of X-ray to UV luminosity are prime candidates for such events. We suggest that similar events may lead to jet interruptions in radio-loud objects.
Electron and ion energization (i.e., heating and nonthermal acceleration) is a fundamental, but poorly understood, outcome of plasma turbulence. In this work, we present new results on this topic ...from particle-in-cell simulations of driven turbulence in collisionless, relativistic electron-ion plasma. We focus on temperatures such that ions (protons) are subrelativistic and electrons are ultrarelativistic, a regime relevant for high-energy astrophysical systems such as hot accretion flows onto black holes. We find that ions tend to be preferentially heated, gaining up to an order of magnitude more energy than electrons, and propose a simple empirical formula to describe the electron-ion energy partition as a function of the ratio of electron-to-ion gyroradii (which in turn is a function of initial temperatures and plasma beta). We also find that while efficient nonthermal particle acceleration occurs for both species in the ultrarelativistic regime, nonthermal electron populations are diminished with decreasing temperature whereas nonthermal ion populations are essentially unchanged. These results have implications for modeling and interpreting observations of hot accretion flows.
We study the redshift evolution of the dynamical properties of ~180,000 massive galaxies from SDSS-III/BOSS combined with a local early-type galaxy sample from SDSS-II in the redshift range 0.1 < or ...=, slant z < or =, slant 0.6. We analyze the evolution of the galaxy parameters effective radius, stellar velocity dispersion, and the dynamical to stellar mass ratio with redshift. We further apply a correction for progenitor bias to build a sample which consists of a coeval, passively evolving population. Systematic errors due to size correction and the calculation of dynamical mass are assessed through Monte Carlo simulations. At fixed stellar or dynamical mass, we find moderate evolution in galaxy size and stellar velocity dispersion, in agreement with previous studies. We show that this results in a decrease of the dynamical to stellar mass ratio with redshift at > 2sigma significance.
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