arXiv subjects

Yan-Fei Jiang

Publications and source records attributed to Yan-Fei Jiang.

At least 37 records · Page 2Linked to original sources

arXiv2025

Super-Eddington Chimneys: On the Cooling Evolution of Tidal Disruption Event Envelopes

The formation of a compact accretion disk following a tidal disruption event (TDE) requires that the shocked stellar debris cool efficiently as it settles toward the black hole. While recent simulations suggest that stream dissipation occurs rapidly, how the weakly bound debris subsequently loses its thermal energy to assemble a compact disk near the circularization radius remains uncertain. We investigate this cooling process using axisymmetric radiation-hydrodynamic simulations of quasi-hydrostatic 'TDE envelopes', initialized with the total mass, angular momentum, and binding energy expected from a complete stellar disruption. The envelopes, supported by radiation pressure on large scales and rotation near the circularization radius, evolve through a combination of radiative diffusion, turbulent mixing, and polar outflows. In our fiducial model, a quasi-steady state is achieved in which a polar outflow radiates and expels matter at several times the Eddington luminosity. This enables the envelope to cool and contract, forming a dense, rotationally supported ring near the circularization radius, but on a timescale roughly ten times shorter than the naive photon-diffusion timescale. Comparative models without radiation transport confirm that cooling, not purely adiabatic evolution, is essential to driving this rapid inflow. Nevertheless, across a range of envelope masses, the effective envelope cooling time scales only weakly with its optical depth, implying that advective and wind-driven energy transport dominate over diffusion. Our results demonstrate the cooling-induced contraction, even absent viscosity and associated black hole accretion, can produce luminosities and large photosphere radii consistent with early UV/optical TDE emission. However, more quantitative light-curve predictions must incorporate self-consistent formation and feeding of the envelope by fall-back accretion.

astro-ph.HE↗

arXiv2025

Too Quiet for Comfort: Local Little Red Dots Lack Variability over Decades

Several local ($z\lesssim 0.2$) metal-poor dwarf AGNs have remarkably similar properties to those of high-redshift Little Red Dots (LRDs), and are recently proposed to be local analogs of LRDs. We use long-term photometric and spectroscopic observations of three local LRDs spanning $\sim 20$ years to measure variability in their rest-frame optical continuum and broad H$α$ emission lines. Using ZTF light curves over a rest-frame $\sim 5$ yr baseline, the $r$-band intrinsic rms variability is $(9\times 10^{-5})_{\rm -9E-5}^{+0.014}$ mag (J1022), $0.025\pm0.004$ mag (J1025) and $0.020\pm0.005$ mag (J1047), indicating low intrinsic variability ($<3-4\%$ at 3$σ$). These rms variability amplitudes are much lower than those for dwarf AGNs and more massive quasars. There is little structure in the optical variability structure functions for the three local LRDs, in contrast to normal AGN variability. Using available multi-epoch spectra, we constrain the broad H$α$ line flux variability to be less than a few percent, without significant profile changes, over a rest-frame baseline of $\sim 15$ yrs in J1025 and J1047, respectively. The three LRDs stand out in the Balmer line properties compared with normal broad-line AGNs, with exceptionally large H$α$ equivalent widths and H$α$/H$β$ ratios far exceeding the Case B recombination value. In the context of recent theoretical models of LRDs as dense gas-enshrouded massive black holes with super-Eddington accretion, our results suggest that the photosphere emission is long-term stable and the broad Balmer lines are primarily collisonally excited. This scenario is consistent with the lack of variability, large H$α$/H$β$ ratios and little dust extinction, as well as the expected high gas density. Virial black hole mass estimates using broad H$α$ assuming photoionization are therefore highly questionable for LRDs.

astro-ph.GA↗

arXiv2025

The Denario project: Deep knowledge AI agents for scientific discovery

We present Denario, an AI multi-agent system designed to serve as a scientific research assistant. Denario can perform many different tasks, such as generating ideas, checking the literature, developing research plans, writing and executing code, making plots, and drafting and reviewing a scientific paper. The system has a modular architecture, allowing it to handle specific tasks, such as generating an idea, or carrying out end-to-end scientific analysis using Cmbagent as a deep-research backend. In this work, we describe in detail Denario and its modules, and illustrate its capabilities by presenting multiple AI-generated papers generated by it in many different scientific disciplines such as astrophysics, biology, biophysics, biomedical informatics, chemistry, material science, mathematical physics, medicine, neuroscience and planetary science. Denario also excels at combining ideas from different disciplines, and we illustrate this by showing a paper that applies methods from quantum physics and machine learning to astrophysical data. We report the evaluations performed on these papers by domain experts, who provided both numerical scores and review-like feedback. We then highlight the strengths, weaknesses, and limitations of the current system. Finally, we discuss the ethical implications of AI-driven research and reflect on how such technology relates to the philosophy of science. We publicly release the code at https://github.com/AstroPilot-AI/Denario. A Denario demo can also be run directly on the web at https://huggingface.co/spaces/astropilot-ai/Denario, and the full app will be deployed on the cloud.

cs.AI↗

arXiv2025

The Balmer Break and Optical Continuum of Little Red Dots from Super-Eddington Accretion

The physical origin of Little Red Dots (LRDs)--compact extragalactic sources with red rest-optical continua and broad Balmer lines--remains elusive. The redness of LRDs is likely intrinsic, suggesting optically thick gas emitting at a characteristic effective temperature of $\sim5000{\rm~K}$. Meanwhile, many LRD spectra exhibit a Balmer break, often attributed to absorption by a dense gas shell surrounding an AGN. Using semi-analytical atmosphere models and radiation transport calculations, we show that a super-Eddington accretion system can give rise to a Balmer break and a red optical color simultaneously, without invoking external gas absorption for the break or dust reddening. The break originates from a discontinuity in opacity across the Balmer limit, similar to that of early-type stars, but the lower photosphere density of super-Eddington systems, $ρ<10^{-9}{\rm~g~cm^{-3}}$, implies a significant opacity contrast even at a cool photosphere temperature of $\sim5000{\rm~K}$. Furthermore, while accretion in the form of a standard thin disk requires fine tuning to match the optical color of LRDs, an alternative scenario of a geometrically thick, roughly spherical accretion flow implies an effective temperature $4000{\rm~K}\lesssim T_{\rm eff}\lesssim6000{\rm~K}$ that is very insensitive to the accretion rate (analogous to the Hayashi line in stellar models). The continuum spectra from the latter scenario align with the Balmer break and optical color of currently known LRDs. We discuss predictions of our model and the prospects for more realistic spectra based on super-Eddington accretion simulations.

astro-ph.GA↗

arXiv2025

Radiation-magnetohydrodynamic Simulations of Accretion Flow Formation After a Tidal Disruption Event

We perform 3D radiation-magnetohydrodynamic simulations of the evolution of the fallback debris after a tidal disruption event. We focus on studying the effects of magnetic fields on the formation and early evolution of the accretion flow. We find that large magnetic fields can increase the debris stream thickness, moderately reducing the efficiency of the radiative acceleration of outflows during the first self-intersecting collisions. As gas accumulates and the collisions happen instead between the infalling stream and the accretion flow, magnetized and nonmagnetized systems evolve similarly at these early times: radiation-driven outflows dominate early after the initial stream-stream collision and a few days later, the accretion rate exceeds the mass outflow rate. We find that the MRI does not play a significant role in angular momentum transport and dissipation. Nor do we find evidence of a magnetocentrifugal driven outflow. Instead, collisions continue to dissipate kinetic energy into radiation that launches outflows and powers TDE luminosities reaching $L\sim4-6\times10^{44}$ erg s$^{-1}$. Shock-driven outflows and inflows redistribute angular momentum throughout the extent ($\sim50 r_s$) of the forming eccentric disk. Even in the presence of magnetic stresses, the accretion flow remains mostly eccentric with $e\sim0.2-0.3$ for $r\lesssim8r_s$ and $e\sim0.4-0.5$ for $10\lesssim r\,(r_s)\lesssim50$. Lastly, we find a polar angle-dependent density structure compatible with the viewing-angle effect, along with an additional azimuthal angle dependence established by the collisions.

astro-ph.HE↗

arXiv2025

Radiation Magnetohydrodynamic Simulation of sub-Eddington Circumbinary Disk in a 10:1 Massive Black Hole Binary

We present a global three-dimensional radiation magnetohydrodynamic (RMHD) simulation of a circumbinary disk (CBD) around a massive black hole binary (MBHB) with a total mass $2 \times 10^7\,M_{\odot}$ and mass ratio $0.1$, separated by $100\, GM_{\rm tot}/c^2$. The inclusion of radiation makes the disk thinner, denser, less eccentric at the inner edge, and more filamentary when compared to an otherwise identical locally isothermal MHD disk. The RMHD disk has accretion rate $\sim 0.23\,\dot{M}_{\mathrm{Edd}}$ and produces thermal emission peaking in the near-UV/optical with a luminosity of $\sim 1\, \% L_{\rm {Edd }}$. Compared with an equal-mass binary with the same total mass, the thermal emission of the CBD around the unequal-mass binary is several orders of magnitude brighter and much more variable at far-UV/soft X-rays frequencies. Similarly, we find that the light curve associated with the $0.1$ mass ratio binary exhibits dominant periodicity corresponding to 2 binary orbits, compared to the equal-mass binary that shows periodicity at 2.5-5 binary orbits. Our results highlight the importance of radiation for the structure and observational properties of MBHB circumbinary disks and have implications for detecting electromagnetic counterparts to LISA gravitational wave precursors and for the heavier binaries targeted by the Pulsar Timing Arrays.

astro-ph.HE↗

arXiv2025

Fragmentation-limited dust filtration in 2D simulations of planet-disk systems with dust coagulation. Parameter study and implications for the inner disk's dust mass budget and composition

Super-thermal gas giant planets or their progenitor cores are known to open deep gaps in protoplanetary disks, which stop large, drifting dust particles on their way to the inner disk. The possible separation of the disk into distinct reservoirs and the resulting dust depletion interior to the gap have important implications for planetesimal formation and the chemical and isotopic composition of the inner regions of protoplanetary disks. Dust fragmentation, however, maintains a reservoir of small grains which can traverse the gap. Dust evolution models are thus instrumental for studies of a gap's filtration efficiency. We present 2D multifluid hydrodynamic simulations of planet-disk systems with dust coagulation and fragmentation. For the first time, we evolve a series of 2D simulation with dust coagulation over 45000 planetary orbits and track the dust's size evolution and origin by using the TriPoD dust coagulation method. We investigate the effects of different planetary masses, fragmentation velocities, and viscosities on the inner disk's dust mass budget and composition, and highlight the advantages of multi-dimensional simulations over 1D models. Filtering can only be efficient for high planetary masses, high fragmentation velocities, and low diffusivities. Clear compositional distinctions between the inner and outer disk could not have been maintained by Jupiter's core if the fragmentation velocity was low, even if $α\lesssim 5 \times 10^{-4}$. Significant "contamination" of the inner disk by outer-disk dust occurs in much less than $2 \times 10^5$ yr in this case and even for more massive objects. This either places tight constraints on the physical conditions in the Solar nebula or mandates consideration of alternative explanations for the NC-CC dichotomy. Astrophysical constraints on the parameters could discriminate between these possibilities.

astro-ph.EP↗

arXiv2025

Radiation Hydrodynamic Simulations of Low-Energy Explosions of Red and Yellow Supergiants

A variety of physical processes leads to the low-energy ejection of material from the hydrogen-rich envelopes of red and yellow supergiants. These include common envelope events, stellar mergers, eruptive mass loss, and failed supernovae. These events may appear as luminous red novae, intermediate luminosity red transients, supernova imposters, or other transients with similar lightcurves and colors that are followed by the disappearance of the progenitor star (e.g. failed supernovae). The Vera C. Rubin Observatory will find these events in large numbers; detailed modeling of their lightcurves is essential for photometrically differentiating between these important physical processes in the lives of massive stars. We use one-dimensional, radiation hydrodynamic simulations to model the lightcurves of low-energy explosions of red and yellow supergiants. Red supergiant explosions have durations of 100-400 days, longer than Type IIp supernovae, while stripped, yellow supergiant explosions have durations of 10s of days. Our models probe the boundary between the radiation-pressure dominated and gas-pressure dominated regimes. We provide fitting formulae for the plateau luminosity and duration of the events. Finally, we show that the failed supernovae candidates in NGC 6946 and M31 are consistent with failed supernovae models for explosion energies of $\sim10^{47}-10^{49}$ erg.

astro-ph.SR↗

arXiv2025

Multi-band Emission from Star-Disk Collision and Implications for Quasi-Periodic Eruptions

We perform two-dimensional, multi-group radiation hydrodynamic simulations to explore the observational properties of a solar-like star colliding with an accretion disk around a supermassive black hole at separation of $\sim 100$ gravitational radii. We find that the star-disk collision produces ejecta on both sides of the disk. As the ejecta expand and cool, transient flares arise, reaching peak bolometric luminosity of up to $L\gtrsim10^{43}\rm erg~s^{-1}$. We estimate that the typical light curve rises and decays on an hour timescale. The spectral energy distribution (SED) peaks in $20-50$eV. The optical depth in soft X-rays is lower than the frequency-integrated optical depth, yielding $100$eV-$1$KeV luminosity $νL_ν\gtrsim10^{42}\rm erg~s^{-1}$. The ejecta aligned with the star's incident direction shows breakout emission, leading to asymmetric SED evolution of the two ejecta. The SED evolution is roughly consistent with those seen in short-period quasi-periodic eruptions (QPEs), which have eruption duration ranging from sub-hour to hours, but the ejecta cooling emission alone may not be sufficient to explain the longer duration flares. Increasing incident velocity generally produces a brighter and harder flare. A larger disk scale height prolongs the breakout emission but leads to a somewhat softer SED. A higher disk surface density can lead to higher ejecta temperature, reducing bound-free opacity and increasing luminosity. When lowering the disk surface density, we find that the ejecta becomes optically thin when the scattering optical depth across disk is at the order of $τ_{\rm disk}\sim200$, and the ejecta disappear when $τ_{\rm disk}\sim10$.

astro-ph.HE↗

arXiv2025

Radiation GRMHD Models of Accretion onto Stellar-Mass Black Holes: I. Survey of Eddington Ratios

We summarize results from a survey of radiation-dominated black hole accretion flows across a wide range of mass accretion rates, as well as two values of black hole spin and initial magnetic field geometry. These models apply an algorithm targeting direct solutions to the radiation transport equation in full general relativity and have been enabled by access to modern exascale computing systems. Super-Eddington accretion flows form geometrically thick radiation pressure supported disks that drive powerful equatorial outflows. A narrow funnel-shaped photosphere in the inner region results in very low radiative efficiencies in this regime. The structure of near- and sub-Eddington accretion depends on whether there is net vertical magnetic flux at the midplane of the disk. With net flux, the disk forms a thin, dense layer at the midplane surrounded by a magnetically-dominated corona, whereas without net flux the disk remains magnetically dominated everywhere. Although none of our models achieve the magnetically arrested disk (MAD) regime, those with net vertical flux and a rapidly spinning black hole still produce powerful relativistic jets. Our calculations adopt simple opacity models (with scalings appropriate to stellar-mass black hole accretion). We discuss the application of our results to observations of X-ray binaries and ultraluminous X-ray sources such as Cyg X-3 and SS433. We also speculate on the application of our super-Eddington models to the interpretation of little red dots (LRDs) recently discovered by JWST.

astro-ph.HE↗

arXiv2025

Do Little Red Dots Vary?

Little red dots (LRDs), high-redshift, compact, red objects with V-shaped spectra, are one of the most exciting and perplexing discoveries made by the James Webb Space Telescope (JWST). While the simplest explanation for LRDs is that they are high redshift active galactic nuclei (AGN), due to their compactness and frequent association with broad line emission, the lack of corresponding X-ray emission and observed variability cast doubt on this picture. Here, we simulate LRD light curves using both traditional models for sub-Eddington AGN variability derived empirically from lower-redshift AGN observations and moderately super-Eddington AGN disk models from radiation magnetohydrodynamic simulations to examine the reason for the lack of variability. We find that even though most LRDs have only been observed 2--4 times in a given waveband, we should still be detecting significantly more variability if traditional sub-Eddington AGN variability models can be applied to LRDs. Instead, our super-Eddington model light curves are consistent with the lack of observed LRD variability. In addition, the ongoing high-cadence {\sc nexus} campaign will detect changes in magnitude, $Δm>1$, for traditional sub-Eddington models, but will only observe significant continuum variability for the lowest mass LRDs for our super-Eddington AGN models. Even if LRDs lack continuum variability, we find that the ongoing spectroscopic JWST campaign {\sc twinkle} should observe broad emission line variability as long as soft X-ray irradiation manages to reach the broad line region from the inner disk. Our models show that super-Eddington accretion can easily explain the lack of continuum variability in LRDs.

astro-ph.GA↗

arXiv2025

Type IIb Supernova Progenitors in 3D: Variability and Episodic Mass Loss revealed by Radiation-Hydrodynamics Simulations

We present the first 3D Radiation-Hydrodynamics simulations of partially-stripped ($M_\mathrm{core}\sim10M_\odot$, $M_\mathrm{env}\sim0.1-1M_\odot$) Yellow Supergiant ($L\sim10^5$, $T_\mathrm{eff}\approx5000-8000$K) envelopes, constructed with Athena++. These envelope models represent the progenitors of Type IIb supernovae (SNe-IIb), which have lost a substantial fraction of their H-rich envelope before undergoing core-collapse. The luminosity-to-mass ratio is high in these extended envelopes, and convection is strongly driven by Hydrogen and Helium opacity peaks. This surface convection, coupled with changes in the opacity, sustains large-amplitude low-azimuthal-order radial pulsations, creating order-of-magnitude variability in the stellar luminosity on a timescale of tens of days. If persistent prior to a SN-IIb, these variations could herald the upcoming explosion. Supersonic fluid motions across the outer layers of the star lead to both successful and failed mass ejection events, which shape the circumstellar environment and drive episodic mass loss ($\sim10^{-6}-10^{-5}M_\odot/$yr, in outbursts). The resulting 3D gas distribution in the outer atmosphere, responsible for early-time supernova shock-breakout and shock-cooling emission, shows orders-of-magnitude fluctuations in both space and time at any given radial location. This intrinsically complex halo of bound and unbound material complicates predictions for early SN-IIb lightcurves relative to spherically-symmetric models. However, it does provide a natural, self-consistent explanation for the presence and diversity of dense circumstellar material observed or inferred around pulsating evolved stars.

astro-ph.SR↗

arXiv2025

Does magnetic field promote or suppress fragmentation in AGN disks? Results from local shearing box simulations with simple cooling

Accretion disks in Active Galactic Nuclei (AGN) are predicted to become gravitationally unstable substantially interior to the black hole's sphere of influence, at radii where the disk is simultaneously unstable to the magnetorotational instability (MRI). Using local shearing box simulations with net vertical flux and a simple cooling prescription, we investigate the effect of magnetic fields on fragmentation in the limit of ideal magnetohydrodyamics. Different levels of in-disk magnetic field from the magnetorotational instability are generated by varying the initial vertical-field plasma beta $β_0$. We find that the disk becomes magnetically dominated when $β_0 < 10^3$, and that this transition is accompanied by a drastic drop in fragmentation (as measured by the bound mass fraction) and gravitational stress. The destabilizing influence of radial magnetic fields, which are present locally and which may promote fragmentation via magnetic tension effects, is overwhelmed by magnetic elevation, which significantly reduces the mid-plane density. The magnetic suppression of fragmentation in magnetically elevated disks has implications for the radial extent of the accretion flow in AGN disks, and for the efficiency of in situ formation of disk-embedded stars that are progenitors for single and binary compact objects.

astro-ph.HE↗

arXiv2025

$\textit{Eppur Si Muove}$: Self-Sustained Streaming Motions in Multi-Phase MHD

Radiative cooling can drive dynamics in multi-phase gas. A dramatic example is hydrodynamic `shattering', the violent, pressure-driven fragmentation of a cooling cloud which falls drastically out of pressure balance with its surroundings. We run MHD simulations to understand how shattering is influenced by magnetic fields. In MHD, clouds do not `shatter' chaotically. Instead, after initial fragmentation, both hot and cold phases coherently `stream' in long-lived, field-aligned, self-sustaining gas flows, at high speed ($\sim 100 \, {\rm km \, s^{-1}}$). MHD thermal instability also produces such flows. They are due to the anisotropic nature of MHD pressure support, which only operates perpendicular to B-fields. Thus, even when $P_{\rm B} + P_{\rm gas} \approx$const, pressure balance only holds perpendicular to B-fields. Field-aligned gas pressure variations are unopposed, and results in gas velocities $v \sim (2 ΔP/ρ)^{1/2}$ from Bernoulli's principle. Strikingly, gas in adjacent flux tubes $\textit{counter-stream}$ in opposite directions. We show this arises from a cooling-induced, MHD version of the thin shell instability. Magnetic tension is important both in enabling corrugational instability and modifying its non-linear evolution. Even in high $β$ hot gas, streaming can arise, since magnetic pressure support grows as gas cools and compresses. Thermal conduction increases the sizes and velocities of streaming cloudlets, but does not qualitatively modify dynamics. These results are relevant to the counter-streaming gas flows observed in solar coronal rain, as well as multi-phase gas cooling and condensation in the ISM, CGM and ICM.

astro-ph.GA↗

arXiv2025

A Parameter Survey of Neutron Star Accretion Column Simulations

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.

astro-ph.HE↗

arXiv2025

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

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.

astro-ph.HE↗

arXiv2025

Accretion of AGN Stars under Influence of Disk Geometry

Massive stars can form within or be captured by AGN disks, influencing both the thermal structure and metallicity of the disk environment. In a previous work, we investigated isotropic accretion onto massive stars from a gas-rich, high-entropy background. Here, we consider a more realistic scenario by incorporating the stratified geometry of the background disk in our 3D radiation hydrodynamic simulatons. We find that accretion remains relatively isotropic when the disk is hot enough and the scale height is thicker than the accretion flow's nominal supersonic critical radius $R{crit}$ (sub-thermal). However, when the disk becomes cold, the accretion flow becomes significantly anisotropic (super-thermal). Escaping stellar and accretion luminosity can drive super-Eddington outflows in the polar region, while rapid accretion is sustained along the midplane. Eventually, the effective cross-section is constrained by the Hill radius and the disk scale height rather than the critical radius when the disk is cold enough. For our setup (stellar mass $\sim 50 M\odot$ and background density $ρ\sim 10^{-10}$ g/cm$^3$) the accretion rates is capped below $\sim 0.02M\odot$/year and the effective accretion parameter $α\sim 10^{-1}$ over disk temperature range $3 - 7 \times 10^4$ K. Spiral arms facilitate inward mass flux by driving outward angular momentum transport. Gap-opening effects may further reduce the long-term accretion rate, albeit to confirm which requires global simulations evolved over much longer viscous timescales.

astro-ph.HE↗

arXiv2025

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

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.

astro-ph.HE↗