arXiv subjects

Nicholas Kaaz

Publications and source records attributed to Nicholas Kaaz.

12 recordsLinked to original sources

arXiv2025

Changing-Look AGN Powered By Disk Tearing

Changing-look active galactic nuclei (CLAGN) feature order-of-magnitude variability in both the continuum and broad line luminosities on months-to-years long timescales, and are currently unexplained. Simulations have demonstrated that rotating black holes sometimes tear apart tilted accretion disks. These tearing events violently restructure the disk on timescales much shorter than a viscous timescale, hinting at a connection to CLAGN. Here, we show that disk tearing can power changing-look events. We report synthetic observations of an extremely high resolution three-dimensional general-relativistic magnetohydrodynamic simulation of a geometrically thin, tilted accretion disk around a rapidly rotating, $10^8\,M_\odot$ black hole. We perform ray-tracing calculations that follow the disk light to both a line of sight camera and to a distribution of cameras in a prescribed torus-like broad line region. The continuum photoionizes the broad line region and we calculate the resulting spectrum. Both the continuum and line luminosities undergo order of magnitude swings on months-to-years long timescales. We find shorter, weeks long variability driven by the geometric precession of the inner disk and an intraday quasi-periodic oscillation driven by radial breathing of the inner disk. When the torn disk precesses, it causes asymmetric illumination of the broad line region, driving time-evolving red-to-blue asymmetries of the broad emission lines that may be a smoking gun for disk tearing. We also make predictions for future photometric observations from ULTRASAT and Vera Rubin Observatory, both of which may play an important role in detecting future changing-look events.

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arXiv2025

Zooming In On The Multi-Phase Structure of Magnetically-Dominated Quasar Disks: Radiation From Torus to ISCO Across Accretion Rates

Recent radiation-thermochemical-magnetohydrodynamic simulations resolved formation of quasar accretion disks from cosmological scales down to ~300 gravitational radii $R_{g}$, arguing they were 'hyper-magnetized' (plasma $\beta\ll1$ supported by toroidal magnetic fields) and distinct from traditional $\alpha$-disks. We extend these, refining to $\approx 3\,R_{g}$ around a $10^{7}\,{\rm M_{\odot}}$ BH with multi-channel radiation and thermochemistry, and exploring a factor of 1000 range of accretion rates ($\dot{m}\sim0.01-20$). At smaller scales, we see the disks maintain steady accretion, thermalize and self-ionize, and radiation pressure grows in importance, but large deviations from local thermodynamic equilibrium and single-phase equations of state are always present. Trans-Alfvenic and highly-supersonic turbulence persists in all cases, and leads to efficient vertical mixing, so radiation pressure saturates at levels comparable to fluctuating magnetic and turbulent pressures even for $\dot{m}\gg1$. The disks also become radiatively inefficient in the inner regions at high $\dot{m}$. The midplane magnetic field remains primarily toroidal at large radii, but at super-Eddington $\dot{m}$ we see occasional transitions to a poloidal-field dominated state associated with outflows and flares. Large-scale magnetocentrifugal and continuum radiation-pressure-driven outflows are weak at $\dot{m}<1$, but can be strong at $\dot{m}\gtrsim1$. In all cases there is a scattering photosphere above the disk extending to $\gtrsim 1000\,R_{g}$ at large $\dot{m}$, and the disk is thick and flared owing to magnetic support (with $H/R$ nearly independent of $\dot{m}$), so the outer disk is strongly illuminated by the inner disk and most of the inner disk continuum scatters or is reprocessed at larger scales, giving apparent emission region sizes as large as $\gtrsim 10^{16}\,{\rm cm}$.

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arXiv2024

H-AMR FORGE'd in FIRE I: Magnetic state transitions, jet launching and radiative emission in super-Eddington, highly magnetized quasar disks formed from cosmological initial conditions

Quasars are powered by supermassive black hole (SMBH) accretion disks, yet standard disk models are inconsistent with many quasar observations. Recently, Hopkins et al. (2024) simulated the formation of a quasar disk feeding a SMBH of mass $M=1.3\times10^7\,M_\odot$ in a host galaxy that evolved from cosmological initial conditions. The disk had surprisingly strong toroidal magnetic fields that supported it vertically from gravity and powered fast accretion. What radiation and feedback can such a system produce? To answer this, we must follow the gas to the event horizon. For this, we interpolated the accretion system onto the grid of the general-relativistic radiation magnetohydrodynamics code H-AMR and performed 3D simulations with BH spins $a=0$ and $a=0.9375$. This remapping generates spurious magnetic monopoles, which we erase using a novel divergence cleaning approach. Despite the toroidal magnetic field's dominance at large radii, vertical magnetic flux builds up at the event horizon. This causes a magnetic state transition within the inner $200$ gravitational radii of the disk, where net vertical magnetic flux begins dominating the accretion flow. This powers strong winds and, if the BH spins, relativistic jets that can spin-down the BH within $5-10\,{\rm Myrs}$. Sometimes, vertical magnetic fields of opposite sign reach the BH, causing polarity inversion events that briefly destroy the jets and, possibly, the X-ray corona. The disk powers accretion at rates $5\times$ the Eddington limit, which can double the BH mass in $5-10\,{\rm Myrs}$. When $a=0.9375$ ($a=0$), the energy in outflows and radiation equals about $60\%$ ($10\%$) and $100\%$ ($3\%$) of the accreted rest mass energy, respectively. Much of the light escapes in cool, extended $\gtrsim1300\,{\rm au}$ photospheres, consistent with quasar microlensing and the ``big blue bump'' seen in spectral energy distributions.

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arXiv2024

Extreme scale height variations and nozzle shocks in warped disks

Accretion disks around both stellar-mass and supermassive black holes are likely often warped. Whenever a disk is warped, its scale height varies with azimuth. Sufficiently strong warps cause extreme compressions of the scale height, which fluid parcels "bounce" off of twice per orbit to high latitudes. In this paper, we study the dynamics of such strong warps using two methods: (i) the nearly analytic "ring theory" of Fairbairn & Ogilvie (2021a), which we generalize to the Kerr metric; and (ii) 3D general-relativistic hydrodynamic simulations of tori ("rings") around black holes, using the H-AMR code. We initialize a ring with a warp and study the subsequent evolution on tens of orbital periods. The simulations agree excellently with the ring theory until the warp amplitude, $\psi$, reaches a critical value $\psi_{\rm c}$. When $\psi>\psi_{\rm c}$, the rings enter the bouncing regime. We analytically derive (and numerically validate) that $\psi_{\rm c}\approx (r/r_{\rm g})^{-1/2}$ in the non-Keplerian regime, where $r_{\rm g}=GM/c^2$ is the gravitational radius and $M$ is the mass of the central object. Whenever the scale height bounces, the vertical velocity becomes supersonic, which leads to a "nozzle shock" as the gas collides at the scale height minima. Nozzle shocks damp the warp within $\approx10-20$ orbits in the simulations; but, that damping is not captured by the ring theory. Nozzle shock dissipation leads to inflow timescales that are 1-2 orders of magnitude shorter than unwarped $\alpha$ disks which may result in rapid variability, such as in changing-look active galactic nuclei or in the soft state of X-ray binaries. We also propose that steady disks with strong enough warps may self-regulate to have amplitudes near $\psi_{\rm c}$.

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arXiv2023

Misaligned magnetized accretion flows onto spinning black holes: Magneto-spin alignment, outflow power, and intermittent jets

Magnetic fields regulate black hole (BH) accretion, governing both inflow and outflow dynamics. When a BH accumulates substantial vertical magnetic flux, it enters the magnetically arrested disk (MAD) state, where dynamically important fields power jets and trigger disk eruptions. We investigate MAD evolution when the BH spin and disk angular momentum are misaligned, a likely scenario in many BH systems. Using numerical simulations, we show that jets from rapidly spinning, prograde BHs realign the inner disk via the magneto-spin alignment mechanism for initial tilts up to $T \lesssim 60^\circ$. Larger tilts lead to intermittent jets that disrupt the disk out to $r\gtrsim100$ gravitational radii, creating hot cavities and magnetized filaments. These episodic jets form a mini$-$feedback loop and may explain quasiperiodic X-ray and radio flares observed in low-luminosity active galaxies. We also find that (i) BH spin and disk tilt influence the amount of magnetic flux accumulated at the horizon, and (ii) large-scale, thick, misaligned accretion flows do not exhibit sustained Lense$-$Thirring (LT) precession. This suggests that slowly accreting BHs ($\dot{M} \ll 10^{-3} \dot{M}_{\rm Edd}$) are unlikely to show lightcurve quasiperiodic oscillations from LT precession, consistent with observations. Instead, magnetic flux eruptions drive jet wobbling and lateral motion, offering an alternative explanation for phenomena such as the M87 jet's apparent precession and rapid swings in blazar jet orientation.

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arXiv2023

Winds and Disk Turbulence Exert Equal Torques on Thick Magnetically Arrested Disks

The conventional accretion disk lore is that magnetized turbulence is the principal angular momentum transport process that drives accretion. However, when dynamically important large-scale magnetic fields thread an accretion disk, they can produce mass and angular momentum outflows, known as winds, that also drive accretion. Yet, the relative importance of turbulent and wind-driven angular momentum transport is still poorly understood. To probe this question, we analyze a long-duration ($1.2 \times 10^5 r_{\rm g}/c$) simulation of a rapidly rotating ($a=0.9$) black hole feeding from a thick ($H/r\sim0.3$), adiabatic, magnetically arrested disk (MAD), whose dynamically important magnetic field regulates mass inflow and drives both uncollimated and collimated outflows (i.e., winds and jets, respectively). By carefully disentangling the various angular momentum transport processes within the system, we demonstrate the novel result that disk winds and disk turbulence both extract roughly equal amounts of angular momentum from the disk. We find cumulative angular momentum and mass accretion outflow rates of $\dot{L}\propto r^{0.9}$ and $\dot{M}\propto r^{0.4}$, respectively. This result suggests that understanding both turbulent and laminar stresses is key to understanding the evolution of systems where geometrically thick MADs can occur, such as the hard state of X-ray binaries, low-luminosity active galactic nuclei, some tidal disruption events, and possibly gamma-ray bursts.

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arXiv2022

Nozzle Shocks, Disk Tearing and Streamers Drive Rapid Accretion in 3D GRMHD Simulations of Warped Thin Disks

The angular momentum of gas feeding a black hole (BH) is typically misaligned with respect to the BH spin, resulting in a tilted accretion disk. Rotation of the BH drags the surrounding space-time, manifesting as Lense-Thirring torques that lead to disk precession and warping. We study these processes by simulating a thin ($H/r=0.02$), highly tilted ($\mathcal{T}=65^\circ$) accretion disk around a rapidly rotating ($a=0.9375$) BH at extremely high resolutions, which we performed using the general-relativistic magnetohydrodynamic (GRMHD) code H-AMR. The disk becomes significantly warped and continuously tears into two individually precessing sub-disks. We find that mass accretion rates far exceed the standard $\alpha$-viscosity expectations. We identify two novel dissipation mechanisms specific to warped disks that are the main drivers of accretion, distinct from the local turbulent stresses that are usually thought to drive accretion. In particular, we identify extreme scale height oscillations that occur twice an orbit throughout our disk. When the scale height compresses, `nozzle' shocks form, dissipating orbital energy and driving accretion. Separate from this phenomenon, there is also extreme dissipation at the location of the tear. This leads to the formation of low-angular momentum `streamers' that rain down onto the inner sub-disk, shocking it. The addition of low angular momentum gas to the inner sub-disk causes it to rapidly accrete, even when it is transiently aligned with the BH spin and thus unwarped. These mechanisms, if general, significantly modify the standard accretion paradigm. Additionally, they may drive structural changes on much shorter timescales than expected in $\alpha$-disks, potentially explaining some of the extreme variability observed in active galactic nuclei.

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arXiv2022

Jet Formation in 3D GRMHD Simulations of Bondi-Hoyle-Lyttleton Accretion

A black hole (BH) travelling through a uniform, gaseous medium is described by Bondi-Hoyle-Lyttleton (BHL) accretion. If the medium is magnetized, then the black hole can produce relativistic outflows. We performed the first 3D, general-relativistic magnetohydrodynamics simulations of BHL accretion onto rapidly rotating black holes using the code H-AMR, where we mainly varied the strength of a background magnetic field that threads the medium. We found that the ensuing accretion continuously drags to the BH the magnetic flux, which accumulates near the event horizon until it becomes dynamically important. Depending on the strength of the background magnetic field, the BHs can sometimes launch relativistic jets with high enough power to drill out of the inner accretion flow, become bent by the headwind, and escape to large distances. While for stronger background magnetic fields the jets are continuously powered, at weaker field strengths they are intermittent, turning on and off depending on the fluctuating gas and magnetic flux distributions near the event horizon. We find that our jets reach extremely high efficiencies of $\sim100-300\%$, even in the absence of an accretion disk. We also calculated the drag forces exerted by the gas onto to the BH, finding that the presence of magnetic fields causes drag forces to be much less efficient than in unmagnetized BHL accretion, and sometimes become negative, accelerating the BH rather than slowing it down. Our results extend classical BHL accretion to rotating BHs moving through magnetized media and demonstrate that accretion and drag are significantly altered in this environment.

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arXiv2021

The hydrodynamic evolution of binary black holes embedded within the vertically stratified disks of active galactic nuclei

Stellar-mass black holes can become embedded within the gaseous disks of active galactic nuclei (AGNs). Afterwards, their interactions are mediated by their gaseous surroundings. In this work, we study the evolution of stellar-mass binary black holes (BBHs) embedded within AGN disks using a combination of three-dimensional hydrodynamic simulations and analytic methods, focusing on environments in which the AGN disk scale height $H$ is $\gtrsim$ the BBH sphere of influence. We model the local surroundings of the embedded BBHs using a wind tunnel formalism and characterize different accretion regimes based on the local properties of the disk, which range from wind-dominated to quasi-spherical. We use our simulations to develop prescriptions for mass accretion and drag for embedded BBHs. We use these prescriptions, along with AGN disk models that can represent the Toomre-unstable outer regions of AGN disks, to study the long-term evolution of the BBHs as they migrate through the disk. We find that BBHs typically merge within $\lesssim 5-30\,{\rm Myr}$, increasing their mass significantly in the process, allowing BBHs to enter (or cross) the pair-instability supernova mass gap. The rate at which gas is supplied to these BBHs often exceeds the Eddington limit, sometimes by several orders of magnitude. We conclude that most embedded BBHs will merge before migrating significantly in the disk. Depending on the conditions of the ambient gas and the distance to the system, LISA can detect the transition between the gas-dominated and gravitational wave dominated regime for inspiraling BBHs that are formed sufficiently close to the AGN ($\lesssim$ 0.1 pc). We also discuss possible electromagnetic signatures during and following the inspiral, finding that it is generally unlikely but not inconceivable for the bolometric luminosity of the BBH to exceed that of the host AGN.

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arXiv2021

Illuminating black hole subsystems in young star clusters

There is increasing evidence that globular clusters retain sizeable black hole populations at present day. This is supported by dynamical simulations of cluster evolution, which have unveiled the spatial distribution and mass spectrum of black holes in clusters across cosmic age. However, black hole populations of young, high metallicity clusters remain unconstrained. Black holes hosted by these clusters mass segregate early in their evolutionary history, forming central subsystems of hundreds to thousands of black holes. We argue that after supernova feedback has subsided ($\gtrsim 50\,{\rm Myr}$), the host cluster can accumulate gas from its dense surroundings, from which the black hole subsystem accretes at highly enhanced rates. The collective accretion luminosity can be substantial and provides a novel observational constraint for young massive clusters. We test this hypothesis by performing 3D hydrodynamic simulations where we embed discretized potentials, representing our black holes, within the potential of a massive cluster. This system moves supersonically with respect to a gaseous medium from which it accretes. We study the accretion of this black hole subsystem for different subsystem populations and determine the integrated accretion luminosity of the black hole subsystem. We apply our results to the young massive clusters of the Antennae Galaxies and find that a typical subsystem accretion luminosity should be in excess of $\approx 10^{40}\,{\rm ergs\,\,s^{-1}}$. We argue that no strong candidates of this luminous signal have been observed and constrain the subsystem population of a typical cluster in the Antennae Galaxies to $\lesssim10-2\times10^2$ $10\,M_\odot$ black holes, given that feedback doesn't significantly impede accretion and that the gas remains optically thin.

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arXiv2019

Bondi-Hoyle-Lyttleton Accretion onto Star Clusters

An isolated star moving supersonically through a uniform gas accretes material from its gravitationally-induced wake. The rate of accretion is set by the accretion radius of the star and is well-described by classical Bondi-Hoyle-Lyttleton theory. Stars, however, are not born in isolation. They form in clusters where they accrete material that is influenced by all the stars in the cluster. We perform three-dimensional hydrodynamic simulations of clusters of individual accretors embedded in a uniform-density wind in order to study how the accretion rates experienced by individual cluster members are altered by the properties of the ambient gas and the cluster itself. We study accretion as a function of number of cluster members, mean separation between them, and size of their individual accretion radii. We determine the effect of these key parameters on the aggregate and individual accretion rates, which we compare to analytic predictions. We show that when the accretion radii of the individual objects in the cluster substantially overlap, the surrounding gas is effectively accreted into the collective potential of the cluster prior to being accreted onto the individual stars. We find that individual cluster members can accrete drastically more than they would in isolation, in particular when the flow is able to cool efficiently. This effect could potentially modify the luminosity of accreting compact objects in star clusters and could lead to the rejuvenation of young star clusters as well as globular clusters with low-inclination and low-eccentricity.

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arXiv2018

Lessons from HAWC PWNe observations: the diffusion constant is not a constant; Pulsars remain the likeliest sources of the anomalous positron fraction; Cosmic rays are trapped for long periods of time in pockets of inefficient diffusion

Recent TeV observations of nearby pulsars with the HAWC telescope have been interpreted as evidence that the diffusion of high-energy electrons and positrons within pulsar wind nebulae is highly inefficient compared to the rest of the interstellar medium. If the diffusion coefficient well outside the nebula is close to the value inferred for the region inside the nebula, high-energy electrons and positrons produced by the two observed pulsars could not contribute significantly to the local measured cosmic-ray flux. The HAWC Collaboration thus concluded that, under the assumption of isotropic and homogeneous diffusion, the two pulsars are ruled out as sources of the anomalous high-energy positron flux. Here, we argue that since the diffusion coefficient is likely not spatially homogeneous, the assumption leading to this conclusion is flawed. We solve the diffusion equation with a radially dependent diffusion coefficient, and show that the pulsars observed by HAWC produce potentially perfect matches to the observed high-energy positron fluxes. We also study the implications of inefficient diffusion within pulsar wind nebulae on Galactic scales, and show that cosmic rays are likely to have very long residence times in regions of inefficient diffusion. We describe how this prediction can be tested with studies of the diffuse Galactic emission.

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