ABSTRACT
We present the results of nine simulations of radiatively inefficient magnetically arrested discs (MADs) across different values of the black hole spin parameter a*: −0.9, −0.7, −0.5, −0.3, ...0, 0.3, 0.5, 0.7, and 0.9. Each simulation was run up to $t \gtrsim 100\, 000\, GM/c^3$ to ensure disc inflow equilibrium out to large radii. We find that the saturated magnetic flux level, and consequently also jet power, of MAD discs depends strongly on the black hole spin, confirming previous results. Prograde discs saturate at a much higher relative magnetic flux and have more powerful jets than their retrograde counterparts. MADs with spinning black holes naturally launch jets with generalized parabolic profiles whose widths vary as a power of distance from the black hole. For distances up to 100GM/c2, the power-law index is k ≈ 0.27–0.42. There is a strong correlation between the disc–jet geometry and the dimensionless magnetic flux, resulting in prograde systems displaying thinner equatorial accretion flows near the black hole and wider jets, compared to retrograde systems. Prograde and retrograde MADs also exhibit different trends in disc variability: accretion rate variability increases with increasing spin for a* > 0 and remains almost constant for a* ≲ 0, while magnetic flux variability shows the opposite trend. Jets in the MAD state remove more angular momentum from black holes than is accreted, effectively spinning down the black hole. If powerful jets from MAD systems in Nature are persistent, this loss of angular momentum will notably reduce the black hole spin over cosmic time.
The chemical industry contributes to 6% of global anthropogenic greenhouse gas (GHG) emissions. A handful of chemical processes (ammonia, nitric acid, methanol, olefins, aromatics, and chlor-alkali) ...account for 65% of those emissions. Decarbonization of the chemical industry will depend on addressing the intermittency of renewable electricity possibly via low-carbon hydrogen production using water electrolysis. A low-carbon power grid, which could happen in the next decade, would enable the chemical industry to reduce its GHG emissions by at least 35 percent. The remaining heat-based and direct emissions could be addressed by direct use of low-carbon electricity for heat or by generating hydrogen that can be used as a fuel and reducing agent coupled with CO
2
capture and utilization efforts. Herein, we discuss how materials innovations could enable the transition to a lower carbon future when based on first-principles and economic realities.
Graphical Abstract
Abstract
We present general relativistic radiation magnetohydrodynamics simulations of super-Eddington accretion on a 10 M⊙ black hole. We consider a range of mass accretion rates, black hole spins ...and magnetic field configurations. We compute the spectra and images of the models as a function of viewing angle and compare them with the observed properties of ultraluminous X-ray sources (ULXs). The models easily produce apparent luminosities in excess of 1040 erg s−1 for pole-on observers. However, the angle-integrated radiative luminosities rarely exceed 2.5 × 1039 erg s−1 even for mass accretion rates of tens of Eddington. The systems are thus radiatively inefficient, though they are energetically efficient when the energy output in winds and jets is also counted. The simulated models reproduce the main empirical types of spectra – disc-like, supersoft, soft, hard – observed in ultraluminous X-ray sources (ULXs). The magnetic field configuration, whether ‘standard and normal evolution’ (SANE) or ’magnetically arrested disc’ (MAD), has a strong effect on the results. In SANE models, the X-ray spectral hardness is almost independent of accretion rate, but decreases steeply with increasing inclination. MAD models with non-spinning black holes produce significantly softer spectra at higher values of
$\dot{M}$
, even at low inclinations. MAD models with rapidly spinning black holes are unique. They are radiatively efficient (efficiency factor ∼10–20 per cent), superefficient when the mechanical energy output is also included (70 per cent) and produce hard blazar-like spectra. In all models, the emission shows strong geometrical beaming, which disagrees with the more isotropic illumination favoured by observations of ULX bubbles.
We explore a simple spherical model of optically thin accretion on a Schwarzschild black hole, and study the properties of the image as seen by a distant observer. We show that a dark circular region ...in the center-a shadow-is always present. The outer edge of the shadow is located at the photon ring radius , where is the gravitational radius of the accreting mass M. The location of the shadow edge is independent of the inner radius at which the accreting gas stops radiating. The size of the observed shadow is thus a signature of the spacetime geometry and it is hardly influenced by accretion details. We briefly discuss the relevance of these results for the Event Horizon Telescope image of the supermassive black hole in M87.
ABSTRACT
We describe global, 3D, time‐dependent, non‐radiative, general‐relativistic, magnetohydrodynamic simulations of accreting black holes (BHs). The simulations are designed to transport a large ...amount of magnetic flux to the centre, more than the accreting gas can force into the BH. The excess magnetic flux remains outside the BH, impedes accretion, and leads to a magnetically arrested disc. We find powerful outflows. For a BH with spin parameter a = 0.5, the efficiency with which the accretion system generates outflowing energy in jets and winds is η≈ 30 per cent. For a = 0.99, we find η≈ 140 per cent, which means that more energy flows out of the BH than flows in. The only way this can happen is by extracting spin energy from the BH. Thus the a = 0.99 simulation represents an unambiguous demonstration, within an astrophysically plausible scenario, of the extraction of net energy from a spinning BH via the Penrose–Blandford–Znajek mechanism. We suggest that magnetically arrested accretion might explain observations of active galactic nuclei with apparent η≈ few × 100 per cent.
Hot collisionless accretion flows, such as the one in Sgr A* at our Galactic center, provide a unique setting for the investigation of magnetic reconnection. Here protons are nonrelativistic, while ...electrons can be ultrarelativistic. By means of 2D particle-in-cell simulations, we investigate electron and proton heating in the outflows of transrelativistic reconnection (i.e., w ∼ 0.1 - 1 , where the magnetization w is the ratio of magnetic energy density to enthalpy density). For both electrons and protons, we find that heating at high β i (here β i is the ratio of proton thermal pressure to magnetic pressure) is dominated by adiabatic compression ("adiabatic heating"), while at low β i it is accompanied by a genuine increase in entropy ("irreversible heating"). For our fiducial w = 0.1 , the irreversible heating efficiency at β i 1 is nearly independent of the electron-to-proton temperature ratio T e T i (which we vary from 0.1 up to 1), and it asymptotes to ∼ 2 % of the inflowing magnetic energy in the low- β i limit. Protons are heated more efficiently than electrons at low and moderate β i (by a factor of ∼7), whereas the electron and proton heating efficiencies become comparable at β i ∼ 2 if T e T i = 1 , when both species start already relativistically hot. We find comparable heating efficiencies between the two species also in the limit of relativistic reconnection ( w 1 ). Our results have important implications for the two-temperature nature of collisionless accretion flows and may provide the subgrid physics needed in general relativistic MHD simulations.
Magnetic fields appear to be present in all galaxies and galaxy clusters. Recent measurements indicate that a weak magnetic field may be present even in the smooth low density intergalactic medium. ...One explanation for these observations is that a seed magnetic field was generated by some unknown mechanism early in the life of the Universe, and was later amplified by various dynamos in nonlinear objects like galaxies and clusters. We show that a primordial magnetic field is expected to be generated in the early Universe on purely linear scales through vorticity induced by scale-dependent temperature fluctuations, or equivalently, a spatially varying speed of sound of the gas. Residual free electrons left over after recombination tap into this vorticity to generate magnetic field via the Biermann battery process. Although the battery operates even in the absence of any relative velocity between dark matter and gas at the time of recombination, the presence of such a relative velocity modifies the predicted spatial power spectrum of the magnetic field. At redshifts of order a few tens, we estimate a root mean square field strength of order 10(-25)-10(-24) G on comoving scales ~10 kpc. This field, which is generated purely from linear perturbations, is expected to be amplified significantly after reionization, and to be further boosted by dynamo processes during nonlinear structure formation.
A new general relativistic radiation magnetohydrodynamical code koral is described, which employs the M1 scheme to close the radiation moment equations. The code has been successfully verified ...against a number of tests. Axisymmetric simulations of super-critical magnetized accretion on non-rotating (a
* = 0.0) and spinning (a
* = 0.9) black holes are presented. The accretion rates in the two models are
. These first general relativistic simulations of super-critical black hole accretion are potentially relevant to tidal disruption events and hyper-accreting supermassive black holes in the early Universe. Both simulated models are optically and geometrically thick, and have funnels through which energy escapes in the form of relativistic gas, Poynting flux and radiative flux. The jet is significantly more powerful in the a
* = 0.9 run. The net energy outflow rate in the two runs correspond to efficiencies of 5 per cent (a
* = 0) and 33 per cent (a
* = 0.9), as measured with respect to the mass accretion rate at the black hole. These efficiencies agree well with those measured in previous simulations of non-radiative geometrically thick discs. Furthermore, in the a
* = 0.9 run, the outflow power appears to originate in the spinning black hole, suggesting that the associated physics is again similar in non-radiative and super-critical accretion flows. While the two simulations are efficient in terms of total energy outflow, both runs are radiatively inefficient. Their luminosities are only ∼1-10L
Edd, which corresponds to a radiative efficiency ∼0.1 per cent. Interestingly, most of the radiative luminosity emerges through the funnels where the local radiative flux is highly super-Eddington.
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