Magnetosonic Waves as a Potential Driver of Observed Temperature Fluctuation Patterns in AGN Accretion Disks

The Astrophysical Journal · 2025-09-29

Abstract Recent observations have revealed slow, coherent temperature fluctuations in AGN disks that propagate both inward and outward at velocities of ∼0.01–0.1 c , a kind of variability that is distinct from reverberation (mediated by the reprocessing of light) between different regions of the disk. We investigate the origin and nature of these fluctuations using global 3D radiation–magnetohydrodynamic simulations of radiation- and magnetic-pressure-dominated AGN accretion disks. Disks with a significant turbulent Maxwell stress component exhibit wave-like temperature perturbations, most evident close to the midplane, whose propagation speeds exactly match the local fast magnetosonic speed and are consistent with the speeds inferred in observations. These fluctuations have amplitudes of 2%–4% in gas temperature, which are also consistent with observational constraints. Disks that are dominated by mean-field Maxwell stresses do not exhibit such waves. While waves may be present in the body of the disk, we do not find them to be present in the photosphere. Although this may in part be due to low numerical resolution in the photosphere region, we discuss the physical challenges that must be overcome for the waves to manifest there. In particular, the fact that such waves are observed implies that the disk photospheres must be magnetically dominated, since radiative damping from photon diffusion smooths out radiation pressure fluctuations. Furthermore, the gas and radiation fluctuations must be out of local thermodynamic equilibrium.

Radiation and Magnetic Pressure Support in Accretion Disks Around Supermassive Black Holes and the Physical Origin of the Extreme-ultraviolet to Soft X-Ray Spectrum

The Astrophysical Journal · 2025-07-11 · 6 citations

Abstract We present the results of four 3D radiation magnetohydrodynamic simulations of accretion disks around a 10 8 solar mass black hole, which produce the far-ultraviolet spectrum peak and demonstrate a robust physical mechanism for producing the extreme-ultraviolet to soft X-ray power-law continuum component. The disks are fed from rotating tori and reach accretion rates ranging from 0.03 to 4 times the Eddington value. The disks become radiation pressure or magnetic pressure dominated, depending on the relative timescales of radiative cooling and gas inflow. Magnetic pressure supported disks can form with or without net poloidal magnetic fields, as long as the inflowing gas can cool quickly enough, which can typically happen when the accretion rate is low. We calculate the emerging spectra from these disks using multigroup radiation transport with realistic opacities and find that they typically peak around 10 eV. At accretion rates close to or above the Eddington limit, a power-law component can appear for photon energies between 10 eV and 1 keV, with a spectral slope varying between L ν ∝ ν −1 and ν −2 , comparable to what is observed in radio-quiet quasars. A disk with a 3% Eddington accretion rate does not exhibit this component. These high-energy photons are produced in an optically thick region ≈30 ∘ –45 ∘ from the disk midplane, by compressible bulk Comptonization within the converging accretion flow. Strongly magnetized disks that have a very small surface density will produce a spectrum that is very different from what is observed.

Thermal Spectra of Warped and Broken Accretion Disks

The Astrophysical Journal · 2025-02-10 · 1 citations

Abstract Black holes may accrete gas with angular momentum vectors misaligned with the black hole spin axis. The resulting accretion disks are subject to Lense–Thirring precession, and hence torque. Analytical calculations and simulations show that Lense–Thirring precession will warp, and, for large misalignments, fracture the disk. In GRMHD simulations, the warping or breaking occurs at ≲10 r s , where r s is the Schwarzschild radius. Considering that accretion disk spectra in the soft state of stellar-mass black holes are generally well modeled as multicolor blackbodies, the question arises as to how consistent warped and broken disks are with observations. Here, we analytically calculate thermal spectra of warped and broken disks with a warp or break radius at 10 r s for various disk inclinations. Due to self-irradiation and the projected area of the inclined disk regions, the spectra of inclined disks significantly deviate from multicolor blackbodies and do not follow the multicolor blackbody relation νL ν ∝ ν γ = ν 4/3 at low frequencies ν . The power-law indices at low frequencies of the inclined disks vary with viewing angle; when viewed face-on, they vary between γ ≈ 0.91 and 1.26 for the warped disks and γ ≈ 1.37–1.54 for the broken disks depending on the inclination angle. The differences decrease when moving the location of the disk warp and break to larger radii; for inclined disks to emit as multicolor blackbodies, they must warp or break at radii ≥50 r s . Our results imply that accretion disks around black holes in the soft state warp or break at larger radii than suggested in GRHMD simulations.

Thermal Spectra of Warped and Broken Accretion Disks

ArXiv.org · 2025-01-24

Black holes may accrete gas with angular momentum vectors misaligned with the black hole spin axis. The resulting accretion disks are subject to Lense-Thirring precession, and hence torque. Analytical calculations and simulations show that Lense-Thirring precession will warp, and, for large misalignments, fracture the disk. In GRMHD simulations, the warping or breaking occurs at $\lesssim10r_s$, where $r_s$ is the Schwarzschild radius. Considering that accretion disk spectra in the soft state of stellar-mass black holes are generally well modeled as multicolor blackbodies, the question arises as to how consistent warped and broken disks are with observations. Here, we analytically calculate thermal spectra of warped and broken disks with a warp or break radius at $10r_s$ for various disk inclinations. Due to self-irradiation and the projected area of the inclined disk regions, the spectra of inclined disks significantly deviate from multicolor blackbodies and do not follow the multicolor blackbody relation $νL_ν\proptoν^γ=ν^{4/3}$ at low frequencies $ν$. The power-law indices at low frequencies of the inclined disks vary with viewing angle; when viewed face-on, they vary between $γ\approx0.91-1.26$ for the warped disks and $γ\approx1.37-1.54$ for the broken disks depending on the inclination angle. The differences decrease when moving the location of the disk warp and break to larger radii; for inclined disks to emit as multicolor blackbodies, they must warp or break at radii $\geq50r_s$. Our results imply that accretion disks around black holes in the soft state warp or break at larger radii than suggested in GRHMD simulations.

A Parameter Survey of Neutron Star Accretion Column Simulations

ArXiv.org · 2025-06-02

We conduct a parameter survey of neutron star accretion column simulations by solving the relativistic radiation MHD equations with opacities that account for strong magnetic fields and pair production. We study how column properties depend on accretion rate, magnetic field strength, and accretion flow geometry. All the simulated accretion columns exhibit kHz oscillatory behavior, consistent with our previous findings. We show how the predicted oscillation properties depend on the column parameters. At higher accretion rates for fixed magnetic field, the column height increases, reducing the local field strength and leading to an anti-correlation between the observed cyclotron line energy and luminosity. We estimate the line energy from the simulations and find agreement with the observed trend. Downward scattering in the free-fall zone plays a key role in shaping sideways emission properties and column height. Strong downward scattering not only re-injects heat back into the column, increasing its height, but also compresses sideways emission, potentially smearing out shock oscillation signals. When the pair-production regime is reached at the base of the column, the system quickly readjusts to a force balance between gravity and radiative support. The high opacity in the pair-production region raises the radiation energy density, enhancing sideways emission through a large horizontal gradient. This shifts the sideways fan-beam radiation toward lower altitudes. In a hollow column geometry, both pencil- and fan-beam radiation emission occurs. Self-illumination across the hollow region increases the height and stabilizes the inner wall of the column, while shock oscillations persist in the outer regions.

Radiation and Magnetic Pressure Support in Accretion Disks around Supermassive Black Holes and The Physical Origin of the Extreme Ultraviolet to Soft X-ray Spectrum

ArXiv.org · 2025-05-14

We present the results of four three-dimensional radiation magnetohydrodynamic simulations of accretion disks around a $10^8$ solar mass black hole, which produce the far ultraviolet spectrum peak and demonstrate a robust physical mechanism to produce the extreme ultraviolet to soft X-ray power-law continuum component. The disks are fed from rotating tori and reach accretion rates ranging from $0.03$ to $4$ times the Eddington value. The disks become radiation pressure or magnetic pressure dominated depending on the relative timescales of radiative cooling and gas inflow. Magnetic pressure supported disks can form with or without net poloidal magnetic fields as long as the inflowing gas can cool quickly enough, which can typically happen when the accretion rate is low. We calculate the emerging spectra from these disks using multi-group radiation transport with realistic opacities and find that they typically peak around $10$ eV. At accretion rates close to or above the Eddington limit, a power-law component can appear for photon energies between $10$ eV and 1 keV with a spectral slope varying between $L_ν\proptoν^{-1}$ and $ν^{-2}$, comparable to what is observed in radio quiet quasars. The disk with $3\%$ Eddington accretion rate does not exhibit this component. These high energy photons are produced in an optically thick region $\approx 30^{\circ}-45^{\circ}$ from the disk midplane by compressible bulk Comptonization within the converging accretion flow. Strongly magnetized disks that have a very small surface density will produce a spectrum that is very different from what is observed.

Non-Stationary Discs and Instabilities

Space Science Reviews · 2025-11-28 · 1 citations

Abstract We review our current knowledge of thermal and viscous instabilities in accretion discs around compact objects. We begin with classical disc models based on analytic viscosity prescriptions, discussing physical uncertainties and exploring time-dependent solutions of disc evolution. We also review the ionization instability responsible for outbursting dwarf nova and X-ray binary systems, including some detailed comparisons between alpha-based models and the observed characteristics of these systems. We then review modern theoretical work based on ideas around angular momentum transport mediated by magnetic fields, focusing in particular on knowledge gained through local and global computer simulations of MHD processes in discs. We discuss how magnetohydrodynamics (MHD) may alter our understanding of outbursts in white dwarf and X-ray binary systems. Finally, we turn to the putative thermal/viscous instabilities that were predicted to exist in the inner, radiation pressure-dominated regions of black hole and neutron star discs, in apparent contradiction to the observed stability of the high/soft state in black hole X-ray binaries.

Simulations of Eccentricity Growth in Compact Binary Accretion Disks with Magnetohydrodynamic Turbulence

The Astrophysical Journal · 2025-01-22 · 3 citations

Abstract We present the results of four magnetohydrodynamic simulations and one alpha-disk simulation of accretion disks in a compact binary system, neglecting vertical stratification and assuming a locally isothermal equation of state. We demonstrate that in the presence of a net vertical field, disks that extend out to the 3:1 mean-motion resonance grow eccentricity in full MHD in much the same way as in hydrodynamical disks. Hence, turbulence due to the magnetorotational instability (MRI) does not impede the tidally driven growth of eccentricity in any meaningful way. However, we find two important differences with alpha-disk theory. First, in MHD, eccentricity builds up in the inner disk with a series of episodes of radial disk-breaking into two misaligned eccentric disks, separated by a region of circular orbits. Standing eccentric waves are often present in the inner eccentric disk. Second, the successful spreading of an accretion disk with MRI turbulence out to the resonant radius is nontrivial—and much harder than spreading an alpha disk. This is due to the tendency to develop overdense rings in which tidal torques overwhelm MRI transport and truncate the disk too early. We believe that the inability to spread the disk sufficiently was the reason why our previous attempt to excite eccentricity via the 3:1 mean-motion resonance with MHD failed. Exactly how MHD disks successfully spread outward in compact binary systems is an important problem that has not yet been understood.

Magnetosonic Waves as a Driver of Observed Temperature Fluctuation Patterns in AGN Accretion Disks

ArXiv.org · 2025-06-12

Recent observations have revealed slow, coherent temperature fluctuations in AGN disks that propagate both inward and outward at velocities of $\sim 0.01 - 0.1c$, a kind of variability that is distinct from reverberation (mediated by the reprocessing of light) between different regions of the disk. We investigate the origin and nature of these fluctuations using global 3D radiation-magnetohydrodynamic simulations of radiation and magnetic pressure-dominated AGN accretion disks. Disks with a significant turbulent Maxwell stress component exhibit wave-like temperature perturbations, most evident close to the midplane, whose propagation speeds exactly match the local fast magnetosonic speed and are consistent with the speeds inferred in observations. These fluctuations have amplitudes of $2 - 4\%$ in gas temperature, which are also consistent with observational constraints. Disks that are dominated by mean-field Maxwell stresses do not exhibit such waves. While waves may be present in the body of the disk, we do not find them to be present in the photosphere. Although this may in part be due to low numerical resolution in the photosphere region, we discuss the physical challenges that must be overcome for the waves to manifest there. In particular, the fact that such waves are observed implies that the disk photospheres must be magnetically dominated, since radiative damping from photon diffusion smooths out radiation pressure fluctuations. Furthermore, the gas and radiation fluctuations must be out of local thermodynamic equilibrium.

A parameter survey of neutron star accretion column simulations

Monthly Notices of the Royal Astronomical Society · 2025-06-14 · 2 citations

ABSTRACT We conduct a parameter survey of neutron star accretion column simulations by solving the relativistic radiation magnetohydrodynamics equations with opacities that account for strong magnetic fields and pair production. We study how column properties depend on accretion rate, magnetic field strength, and accretion flow geometry. All the simulated accretion columns exhibit kHz oscillatory behaviour, consistent with our previous findings. We show how the predicted oscillation properties depend on the column parameters. At higher accretion rates for fixed magnetic field, the column height increases, reducing the local field strength and leading to an anticorrelation between the observed cyclotron line energy and luminosity. We estimate the line energy from the simulations and find agreement with the observed trend. Downward scattering in the free-fall zone plays a key role in shaping sideways emission properties and column height. Strong downward scattering not only re-injects heat back into the column, increasing its height, but also compresses sideways emission, potentially smearing out shock oscillation signals. When the pair-production regime is reached at the base of the column, the system quickly readjusts to a force balance between gravity and radiative support. The high opacity in the pair-production region raises the radiation energy density, enhancing sideways emission through a large horizontal gradient. This shifts the sideways fan-beam radiation towards lower altitudes. In a hollow column geometry, both pencil- and fan-beam radiation emission occurs. Self-illumination across the hollow region increases the height and stabilizes the inner wall of the column, while shock oscillations persist in the outer regions.