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Register a new userA Phase Lag between Disk and Corona in GRMHD Simulations of Precessing Tilted Accretion Disks
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In the course of its evolution, a black hole (BH) accretes gas from a wide range of directions. Given a random accretion event, the typical angular momentum of an accretion disc would be tilted by $sim$60$^circ$ relative to the BH spin. Misalignment causes the disc to precess at a rate that increases with BH spin and tilt angle. We present the first general-relativistic magnetohydrodynamic (GRMHD) simulations spanning a full precession period of highly tilted (60$^circ$), moderately thin ($h/r=0.1$) accretion discs around a rapidly spinning ($asimeq0.9$) BH. While the disc and jets precess in phase, we find that the corona, sandwiched between the two, lags behind by $gtrsim 10^{circ}$. For spectral models of BH accretion, the implication is that hard non-thermal (corona) emission lags behind the softer (disc) emission, thus potentially explaining some properties of the hard energy lags seen in Type-C low frequency quasi-periodic oscillations in X-Ray binaries. While strong jets are unaffected by this disc-corona lag, weak jets stall when encountering the lagging corona at distances $r sim 100$ black hole radii. This interaction may quench large-scale jet formation.
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Luminous active galactic nuclei (AGN) and X-Ray binaries (XRBs) tend to be surrounded by geometrically thin, radiatively cooled accretion discs. According to both theory and observations, these are -- in many cases -- highly misaligned with the black hole spin axis. In this work we present the first general relativistic magnetohydrodynamic simulations of very thin ($h/r sim 0.015-0.05$) accretion discs around rapidly spinning ($a sim 0.9$) black holes and tilted by 45-65 degrees. We show that the inner regions of the discs with $h/r lesssim 0.03$ align with the black hole equator, though at smaller radii than predicted by theoretical work. The inner aligned and outer misaligned disc regions are separated by a sharp break in tilt angle accompanied by a sharp drop in density. We find that frame-dragging by the spinning black hole overpowers the disc viscosity, which is self-consistently produced by magnetized turbulence, tearing the disc apart and forming a rapidly precessing inner sub-disc surrounded by a slowly precessing outer sub-disc. We find that at all tilt values the system produces a pair of relativistic jets. At small distances the jets precess rapidly together with the inner sub-disc, whereas at large distances they partially align with the outer sub-disc and precess more slowly. If the tearing radius can be modeled accurately in future work, emission model independent measurements of black hole spin based on precession-driven quasi-periodic oscillations may become possible.
Global three dimensional magnetohydrodynamic (MHD) simulations of turbulent accretion disks are presented which start from fully equilibrium initial conditions in which the magnetic forces are accounted for and the induction equation is satisfied. The local linear theory of the magnetorotational instability (MRI) is used as a predictor of the growth of magnetic field perturbations in the global simulations. The linear growth estimates and global simulations diverge when non-linear motions - perhaps triggered by the onset of turbulence - upset the velocity perturbations used to excite the MRI. The saturated state is found to be independent of the initially excited MRI mode, showing that once the disk has expelled the initially net flux field and settled into quasi-periodic oscillations in the toroidal magnetic flux, the dynamo cycle regulates the global saturation stress level. Furthermore, time-averaged measures of converged turbulence, such as the ratio of magnetic energies, are found to be in agreement with previous works. In particular, the globally averaged stress normalized to the gas pressure, <alpha_{rm P}> = 0.034, with notably higher values achieved for simulations with higher azimuthal resolution. Supplementary tests are performed using different numerical algorithms and resolutions. Convergence with resolution during the initial linear MRI growth phase is found for 23-35 cells per scaleheight (in the vertical direction).
The episodic dynamics of the magnetic eruption of a spinning black hole (BH) accretion disks and its associated intense shapeup of their jets is studied via three-dimensional general-relativistic magnetohydrodynamics (GRMHD). The embedded magnetic fields in the disk get amplified by the magnetorotational instability (MRI) so large as to cause an eruption of magnetic field (recconection) and large chunks of matter episodically accrete toward the roots of the jets upon such an event. We also find that the eruption events produce intensive Alfven pulses, which propagate through the jets. After the eruption, the disk backs to the weakly magnetic states. Such disk activities cause short time variabilities in mass accretion rate at the event horizon as well as electromagnetic luminosity inside the jet. Since the dimensionless strength parameter $a_0=eE/m_e omega c$ of these Alfven wave pulses is extremely high for a substantial fraction of Eddington accretion rate accretion flow onto a supermassive black hole, the Alfven shocks turn into ultrarelativistic $(a_0gg 1)$ bow wake acceleration, manifesting into the ultra-high energy cosmic rays and electrons which finally emit gamma-rays. Since our GRMHD model has universality in its spatial and temporal scales, it is applicable to a wide range of astrophysical objects ranging from those of AGN (which is the primary target of this research), to micro-quasars. Properties such as time variabilities of blazar gamma-ray flares and spectrum observed by {it Fermi} Gamma-ray Observatory are well explained by linear acceleration of electrons by the bow wake.
Accretion is an essential physical process in black-hole X-ray binaries (BHXRBs) and active galactic nuclei. The properties of accretion flows and their radiation were originally considered to be uniquely determined by the mass accretion rate of the disk; however, the ``hysteresis effect observed during outbursts of nearly all BHXRBs seriously challenges this paradigm. The hysteresis effect is referred to that the hard-to-soft state transition in the fast-rise stage occurs at much higher luminosity than the soft-to-hard state transition in the slow-decay stage. That is, the same source can show different spectral/temporal properties at the same luminosity. Phenomenologically, this effect is also represented as the so-called ``q-shaped hardness-intensity diagram, which has been proposed as a unified scene for BHXRBs. However, there is still a lack of quantitative theoretical interpretation and observational understanding on the ``q-diagram. Here, we present a detailed time-lag analysis of a recently found BHXRB, MAXI J1348-630, intensively monitored by Insight-HXMT over a broad energy band (1-150 keV). We find the first observational evidence that the observed time-lag between radiations of the accretion disk and the corona leads naturally to the hysteresis effect and the ``q-diagram. Moreover, complemented by the quasi-simultaneous Swift data, we achieve a panorama of the accretion flow: the hard X-ray outburst from the corona heats and subsequently induces the optical brightening in the outer disk with nearly no lag; thereafter, the enhanced accretion in the outer disk propagates inward, generating the delayed soft X-ray outburst at the viscous timescale of ~ 8-12 days.
Gas falling into a black hole (BH) from large distances is unaware of BH spin direction, and misalignment between the accretion disc and BH spin is expected to be common. However, the physics of tilted discs (e.g., angular momentum transport and jet formation) is poorly understood. Using our new GPU-accelerated code H-AMR, we performed 3D general relativistic magnetohydrodynamic simulations of tilted thick accretion discs around rapidly spinning BHs, at the highest resolution to date. We explored the limit where disc thermal pressure dominates magnetic pressure, and showed for the first time that, for different magnetic field strengths on the BH, these flows launch magnetized relativistic jets propagating along the rotation axis of the tilted disc (rather than of the BH). If strong large-scale magnetic flux reaches the BH, it bends the inner few gravitational radii of the disc and jets into partial alignment with the BH spin. On longer time scales, the simulated disc-jet system as a whole undergoes Lense-Thirring precession and approaches alignment, demonstrating for the first time that jets can be used as probes of disc precession. When the disc turbulence is well-resolved, our isolated discs spread out, causing both the alignment and precession to slow down.
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