我们对磁化吸积盘吸积到具有不同质量比（MR）的自旋双黑洞（BHBH）上的过程进行了广义相对论模拟。各个黑洞自旋的大小均为 χ=0.26，方向要么位于初始轨道平面内，要么相对该平面向上呈45°。我们在旋进、并合及并合后阶段演化这些系统，以确定黑洞自旋和质量比对任何喷流及其电磁（EM）信号的影响。我们发现，无论质量比如何，两个黑洞都会发射初始喷流，并且喷流沿自旋方向传播，只要漏斗内及其极点上方的无力参数 B²/(8πρ₀) 大于1即可。在远处，两股喷流合并为一股，这可能会阻碍对单个喷流的电磁探测。当吸积率在退耦前达到准稳态时，我们观察到外输坡印廷光度突然增强，且该增强依赖于质量比。并合之后，残余黑洞自旋方向相对于其前身黑洞自旋方向的突然改变导致喷流重新定向，从而驱动出一股单一、高速、向外传播的窄束；该窄束由紧密缠绕的螺旋形 B 场准直，而这又提升了坡印廷光度。这一效应几乎与质量比无关。在此过程中，B 场线中产生一个扭折，对喷流形成约束。该扭折沿喷流传播，但迅速衰减，不会留下自旋偏移的记忆。我们的结果表明，如果不考虑其他特征，低质量盘中未对准、低自旋 BHBH 的并合可能无法为解释 X 形射电星系提供可行情景。然而，并合时外输光度的突然变化可能有助于识别活动星系核中的黑洞并合，从而揭示黑洞增长机制以及观测到的其宿主星系的共同演化。
英文摘要：
We perform general relativistic simulations of magnetized, accreting disks onto spinning binary black holes (BHBHs) with different mass ratios (MRs). The magnitude of the individual BH spins are all $χ= 0.26$ and lie either along the initial orbital plane or $45^\circ$ above it. We evolve these systems throughout the inspiral, merger and postmerger phases to identify the impact of the BH spins and the MR on any jet and their electromagnetic (EM) signatures. We find that incipient jets are launched from both BHs regardless of the MR and along the spin directions as long as the force-free parameter $B^2/(8\,πρ_0)$ in the funnel and above their poles is larger than one. At large distances the two jets merge into a single one what may prevent the EM detection of individual jets. As the accretion rate reaches a quasistationary state during predecoupling, we observe a sudden amplification of the outgoing Poynting luminosity that depends on the MR. Following merger, the sudden change in the direction of the spin of the BH remnant with respect to the spins of its progenitors causes a reorientation of the jet, which drives a single, high-velocity, outward narrow beam collimated by a tightly wound, helical Bfield which, in turn, boosts the Poynting luminosity. This effect is nearly MR independent. During this process, a kink is produced in the Bfield lines, confining the jet. The kink propagates along the jet but rapidly decays, leaving no memory of the spin-shift. Our results suggest that the merger of misaligned, low-spinning, BHBH mergers in low-mass disks may not provide a viable scenario to explain X-shaped radio galaxies if other features are not taken into account. However, the sudden changes in the outgoing luminosity at merger may help to identify mergers of BHs in active galactic nuclei, shedding light on BH growth mechanisms and the observed co-evolution of their host galaxies.
We study the spatial and temporal behavior of fluid in fully three-dimensional, general relativistic, magnetohydrodynamical simulations of both tilted and untilted black hole accretion flows. We uncover characteristically greater variability in tilted simulations at frequencies similar to those predicted by the formalism of trapped modes, but ultimately conclude that its spatial structure is inconsistent with a modal interpretation. We find instead that previously identified, transient, over-dense clumps orbiting on roughly Keplerian trajectories appear generically in our global simulations, i…
We present new results of fully general relativistic magnetohydrodynamic simulations of binary neutron star (BNS) mergers performed with the Whisky code. All the models use a piecewise polytropic approximation of the APR4 equation of state for cold matter, together with a 'hybrid' part to incorporate thermal effects during the evolution. We consider both equal and unequal-mass models, with total masses such that either a supramassive NS or a black hole is formed after merger. Each model is evolved with and without a magnetic field initially confined to the stellar interior. We present the diff…
We conduct a systematic study of the properties of tilted accretion flows around spinning black holes, covering a range of tilt angles and black hole spins, using the general-relativistic magnetohydrodynamics code Athena++. The same initial magnetized torus is evolved around black holes with spins ranging from 0 to 0.9, with inclinations ranging from 0 degrees to 24 degrees. The tilted disks quickly reach a warped and twisted shape that rigidly precesses about the black hole spin axis with deformations in shape large enough to hinder the application of linear bending wave theory. Magnetized po…
Geometrically thick accretion flows may be present in black hole X-ray binaries observed in the low/hard state and in low-luminosity active galactic nuclei. Unlike in geometrically thin disks, the angular momentum axis in these sources is not expected to align with the black hole spin axis. We compute images from three-dimensional general relativistic magnetohydrodynamic simulations of misaligned (tilted) accretion flows using relativistic radiative transfer, and compare the estimated locations of the radiation edge with expectations from their aligned (untilted) counterparts. The radiation ed…
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 t…
We analyze the time dependence of fluid variables in general relativistic, magnetohydrodynamic simulations of accretion flows onto a black hole with dimensionless spin parameter a/M=0.9. We consider both the case where the angular momentum of the accretion material is aligned with the black hole spin axis (an untilted flow) and where it is misaligned by 15 degrees (a tilted flow). In comparison to the untilted simulation, the tilted simulation exhibits a clear excess of inertial variability, that is, variability at frequencies below the local radial epicyclic frequency. We further study the ra…
This paper presents our latest numerical simulations of accretion disks that are misaligned with respect to the rotation axis of a Kerr black hole. In this work we use a new, fully conservative version of the Cosmos++ general relativistic magnetohydrodynamics (GRMHD) code, coupled with an ad hoc cooling function designed to control the thickness of the disk. Together these allow us to simulate the thinnest tilted accretion disks ever using a GRMHD code. In this way, we are able to probe the regime where the dimensionless stress and scale height of the disk become comparable. We present results…
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 ($a\simeq0.9$) BH. While the disc and jets precess…
Using a combination of general-relativistic magnetohydrodynamics simulations and ray tracing of synchrotron emission, we study the effect of modest (24 degrees) misalignment between the black hole spin and plasma angular momentum, focusing on the variability of total flux, image centroids, and image sizes. We consider both millimeter and infrared (IR) observables motivated by Sagittarius A* (Sgr A*), though our results apply more generally to optically thin flows. For most quantities, tilted accretion is more variable, primarily due to a significantly hotter and denser "coronal" region well of…
Prevalent around luminous accreting black holes, thin discs are challenging to resolve in numerical simulations. When the disc and black hole angular momentum vectors are misaligned, the challenge becomes extreme, requiring adaptive meshes to follow the disc proper as it moves through the computational grid. With our new high-performance general relativistic magnetohydrodynamic (GRMHD) code H-AMR we have simulated the thinnest accretion disc to date, of aspect ratio H/R~0.03, around a rapidly spinning (a=0.9375) black hole, using a cooling function. Initially tilted at 10 degrees, the disc war…
Luminous active galactic nuclei (AGN) and X-Ray binaries (XRBs) often contain geometrically thin, radiatively cooled accretion discs. According to theory, these are -- in many cases -- initially highly misaligned with the black hole equator. In this work, we present the first general relativistic magnetohydrodynamic simulations of very thin (h/r~0.015-0.05) accretion discs around rapidly spinning (a~0.9) black holes and tilted by 45-65 degrees. We show that the inner regions of the discs with h/r<0.03 align with the black hole equator, though out to smaller radii than predicted by analytic wor…
We analyze two 3D general-relativistic magnetohydrodynamic accretion simulations in the context of how they would manifest in Event Horizon Telescope (EHT) observations of supermassive black holes. The two simulations differ only in whether the initial angular momentum of the plasma is aligned with the rapid (a = 0.9) spin of the black hole. Both have low net magnetic flux. Ray tracing is employed to generate resolved images of the synchrotron emission. When using parameters appropriate for Sgr A* and assuming a viewing angle aligned with the black hole spin, we find the most prominent differe…
Black hole accretion is one of nature's most efficient energy extraction processes. When gas falls in, a significant fraction of its gravitational binding energy is either converted into radiation or flows outwards in the form of black hole-driven jets and disk-driven winds. Recently, the Event Horizon Telescope (EHT), an Earth-size sub-millimetre radio interferometer, captured the first images of M87's black hole. These images were analysed and interpreted using general-relativistic magnetohydrodynamics (GRMHD) models of accretion disks with rotation axes aligned with the black hole spin axis…
We present 3D general relativistic magnetohydrodynamic(GRMHD) simulations of zero angular momentum accretion around a rapidly rotating black hole, modified by the presence of initially uniform magnetic fields. We consider serveral angles between the magnetic field direction and the black hole spin. In the resulting flows, the midplane dynamics are governed by magnetic reconnection-driven turbulence in a magnetically arrested (or a nearly arrested) state. Electromagnetic jets with outflow efficiencies ~10-200% occupy the polar regions, reaching several hundred gravitational radii before they di…
Black hole X-ray binaries (BHXRBs) display a wide range of variability phenomena, from long duration spectral state changes to short-term broadband variability and quasi-periodic oscillations (QPOs). A particularly puzzling aspect is the production of QPOs, which -- if properly understood -- could be used as a powerful diagnostic tool of black hole accretion and evolution. In this work we analyse a high resolution three-dimensional general relativistic magnetohydrodynamic simulation of a geometrically thin accretion disk which is tilted by $65^{\circ}$ with respect to the black hole spin axis.…
Turbulent high-energy astrophysical systems often feature asymmetric energy injection: for instance, Alfven waves propagating from an accretion disk into its corona. Such systems are "imbalanced": the energy fluxes parallel and anti-parallel to the large-scale magnetic field are unequal. In the past, numerical studies of imbalanced turbulence have focused on the magnetohydrodynamic regime. In the present study, we investigate externally-driven imbalanced turbulence in a collisionless, ultrarelativistically hot, magnetized pair plasma using three-dimensional particle-in-cell (PIC) simulations. …
Binary Neutron Star Merger Simulations with Microphysical Equation of State using Spritz
INFN, Sezione di Milano-Bicocca（意大利国家核物理研究所米兰比可卡分部）；The Grainger College of Engineering, Department of Physics & Illinois Center for Advanced Studies of the Universe, University of Illinois Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校格兰杰工程学院物理系及宇宙高级研究中心）；Dipartimento di Fisica G. Occhialini, Università di Milano-Bicocca（米兰比可卡大学G. Occhialini物理系）；INAF, Osservatorio Astronomico di Padova（意大利国家天体物理研究所帕多瓦天文台）；INFN, Sezione di Padova（意大利国家核物理研究所帕多瓦分部）；Dipartimento di Fisica, Università di Trento（特伦托大学物理系）；INFN-TIFPA, Trento Institute for Fundamental Physics and Applications（意大利国家核物理研究所-特伦托基础物理与应用研究所）
General-relativistic simulations of binary neutron star mergers with viscosity reveal a new outflow mechanism operating in unequal mass binaries on dynamical timescales and enabled by turbulent viscosity. These "viscous-dynamical" ejecta are launched during merger due to the thermalization of mass exchange streams between the secondary and the primary neutron star. They are characterized by asymptotic velocities extending up to ${\sim} 0.8\, c$, and have masses that depend on the efficiency of the viscous mechanism. Depending on the unknown strength of the effective viscosity arising from magn…
Black hole-neutron star binaries with high spins and large mass asymmetries: III. Properties of the ejected material and its electromagnetic signatures
University of Hamburg（汉堡大学）；Goethe Universität（法兰克福歌德大学）；University of Idaho（爱达荷大学）；INAF-Osservatorio di Astrofisica e Scienza dello Spazio（意大利国家天体物理研究所天文与空间科学观测站）；University of Ferrara（费拉拉大学）；INFN, Sezione di Ferrara（意大利国家核物理研究所费拉拉分部）；INAF - Osservatorio Astronomico d’Abruzzo（意大利国家天体物理研究所阿布鲁佐天文台）；New Uzbekistan University（新乌兹别克斯坦大学）；Trinity College（三一学院）
Black hole-neutron star (BHNS) mergers are one of the most promising targets for multimessenger astronomy. Using general relativistic magnetohydrodynamic simulations of BHNS undergoing merger we showed that a magnetically--driven jet can be launched by the remnant if the NS is endowed with a dipole B field extending from the interior into the exterior as in a radio pulsar. These self-consistent studies considered a BHNS system with mass ratio $q=3:1$, BH spin $a/M_{BH}=0.75$ aligned with the total orbital angular momentum (OAM), and a NS that is irrotational, threaded by an aligned B field, an…
Measurement of spin-orbit misalignment in binary black holes via the total spin
Northwestern University（西北大学）；University of California at Santa Barbara（加州大学圣塔芭芭拉分校）；California Institute of Technology（加州理工学院）；The University of Chicago（芝加哥大学）；Institute for Advanced Study（高等研究院）
General-relativistic magnetohydrodynamic (GRMHD) simulations have revolutionized our understanding of black hole accretion. Here, we present a graphics processing unit (GPU) accelerated GRMHD code \hammer{} with multi-faceted optimizations that, collectively, accelerate computation by 2-5 orders of magnitude for a wide range of applications. Firstly, it introduces a spherical grid with 3D adaptive mesh refinement that operates in each of the 3 dimensions independently. This allows us to circumvent the Courant condition near the polar singularity, which otherwise cripples high-resolution comput…
