arXiv subjects

Lizhong Zhang

Publications and source records attributed to Lizhong Zhang.

At least 19 recordsLinked to original sources

arXiv2026

Geometrically Thin Sub-Eddington AGN Accretion Disks show a suppressed Lyman Edge

The observed ultraviolet continua of active galactic nuclei (AGN) generally lack a strong intrinsic HI Lyman edge predicted by classical optically thick accretion disk atmosphere models. We revisit this long-standing problem using our previous sub-Eddington ($L/L_{\rm Edd}\sim 0.03$) geometrically thin 3D accretion disk simulation evolved with gray angle-dependent radiation magnetohydrodynamics (MHD) around a $10^8M_{\odot}$ black hole. This disk is magnetic pressure dominated, and has surface densities more than two orders of magnitude below the corresponding radiation-pressure-supported $α$-disk prediction. By restarting the simulation with multiple frequency groups, we compute the emergent continuum directly from the frequency-dependent radiation fluxes and find no sharp HI Lyman edge at 13.6 eV. This suppressed Lyman edge is mainly caused by the low densities in a magnetic pressure supported disk where the Lyman opacity jump is far weaker than in standard disk models. These results indicate that magnetic pressure support may provide a possible solution to reconciling optically thick AGN disks with the smooth observed continuum emission around the Lyman edge. However, in this simulation, a substantial fraction of the emission near 13.6 eV is expected to originate at smaller radii closer to the central black hole, which is outside our simulation domain. More detailed predictions of the spectrum will require an extension to the inner disk, non-LTE radiation transfer, higher frequency resolution and a survey across a broader range of accretion rates.

astro-ph.HE↗

arXiv2026

Radiation GRMHD Models of Accretion onto Stellar-Mass Black Holes: III. Near-Eddington Accretion

We present a comprehensive analysis of four near-Eddington black hole accretion models from GRMHD simulations with full radiation transport. This study investigates the dynamical effects of magnetic field topology and black hole spin using two representative choices of each. Two stable near-Eddington solutions emerge in these models: a thin thermal disk embedded within a magnetic envelope when sufficient net vertical magnetic flux is present (e.g., vertical field $\gtrsim 5\times10^5$ G at $20r_g$), and a magnetically elevated disk when the net vertical flux is weak or absent. One model initialized without net vertical flux evolves into the thin disk solution, as strong, anisotropic radiation feedback at high accretion rates promotes the accumulation of vertical magnetic flux in the inner disk. In the thin thermal disk, accretion is driven primarily by mean-field Maxwell stress and proceeds largely within the magnetic envelope, while heat dissipation is spatially decoupled and concentrated near the midplane. However, in the magnetically elevated disk, accretion occurs throughout the disk body and is comparably driven by mean-field and turbulent stresses; heat dissipation therefore occurs locally through turbulence. Radiation transport is diffusion-dominated, enabling efficient radiative cooling ($\sim$4-10%). An optically thin wind is launched from the disk surface by combined radiative and magnetic forces, with stronger winds found in models with larger vertical magnetic flux and higher spin. Both strong and weak jets are produced: strong jets are persistent, highly relativistic, and magnetically driven, while weak jets are intermittent, mildly relativistic, and powered by a combination of magnetic and radiative forces.

astro-ph.HE↗

arXiv2026

Finite-Solid-Angle Boltzmann Radiation Transport on Dynamical Spacetimes in AthenaK

We extend the finite-solid-angle general relativistic radiation transport method of C. J. White et al. (2023) to time-dependent spacetimes represented in ADM form. This Valencia-type solver retains the angular transport and local implicit matter coupling of the original HARM-type solver, but replaces its time-independent Kerr--Schild tetrad and conserved-variable normalization with an Eulerian formulation. A Cholesky-gauge spatial tetrad makes the frame and its derivatives algebraic functions of the ADM variables, providing smooth, metric-compatible angular transport. The resulting transport system couples directly to an evolving spacetime, agnostic to the gauge evolution equations. Our test suite, including flat and curved beams, radiation--fluid coupling, and time-dependent geometries, establishes the accuracy and robustness of this approach. We further apply the Valencia-type solver to a radiative circumbinary disk, illustrating its potential for multi-messenger studies of dynamical strongly gravitating systems.

astro-ph.HE↗

arXiv2026

PACHA: Probing AGN Coronae with High-redshift AGN

The X-ray emission of active galactic nuclei (AGN) is generally attributed to inverse Compton scattering of accretion-disk photons by hot electrons in a compact corona. In local AGN, directly constraining coronal properties is challenging because the high-energy cutoff often lies beyond the NuSTAR bandpass. High-redshift, luminous quasars enable systematic constraints on the high-energy cutoff, as cosmological redshift shifts the spectal cutoff into the observable hard X-ray band. We present first results from the ``Probing the AGN Coronae with High-redshift AGN'' (PACHA) project, based on quasi-simultaneous NuSTAR and XMM-Newton observations of 13 radio-quiet AGN at $z>1$. We constrain the high-energy cutoff and coronal temperature at 90\% confidence level for 10 and 9 sources, respectively. The sample exhibits a mean cutoff energy of $E_{\rm cut}=80.8\pm8.1$ keV and a mean coronal temperature of $kT_{\rm e}=18.4\pm1.6$ keV, both significantly lower than those measured in local {\it Swift}-BAT AGN, while the mean optical depth ($τ=4.8\pm0.3$) is significantly higher. The uncertainties are at 1~$σ$. Combining our high-redshift sample with local AGN, we find a potential anti-correlation between cutoff energy and both X-ray luminosity and black hole mass, with no significant dependence on Eddington ratio. Within a hybrid coronal framework, the inferred temperatures lie well below the pair-production limits for purely thermal coronae, indicating a substantial efficient Compton cooling and/or non-thermal electron component. The detection of low coronal temperatures in high-luminosity AGN is broadly consistent with predictions from recent radiation MHD simulations that consider purely thermal electron populations, implying that non-thermal electrons may not be the primary drivers of the observed coronal properties in these systems.

astro-ph.HE↗

arXiv2026

The plunging region of thin accretion discs across the black hole spin range

We compute and test analytic models for the plunging region dynamics, thermodynamics, and magnetic fields against dedicated 3D global general relativistic magnetohydrodynamics (MHD) simulations of thin accretion discs around black holes across the spin range, using the code {\tt ATHENAK}. We find that the dynamics of the plunging fluid closely resembles that of a gravity-dominated geodesic plunge, with the best agreement at low spins. Additionally, we find good agreement between the thermodynamic framework and the simulated quantities across the spin range. Finally, we develop a new model for the magnetic fields in the plunging region that assumes a fixed geodesic inflow, into which the magnetic fields are frozen. Overall, our simulations are in good concordance with this model, albeit with some discrepancies that suggest a degree of non-ideal MHD dissipation. In addition, we investigate how the MHD stresses in the plunging region depend on the black hole spin, interpreting our results through the lens of our flux-freezing model. We find that the magnitude of the stress increases as the black hole spin is increased in the prograde direction. This question is of particular importance for observers who wish to determine the black hole spin from X-ray measurements of the inner accretion disc, since a low-stress, high-spin solution is degenerate with a high-stress, low-spin solution. The spin-stress relationship that we report is approximately orthogonal to the contour of degenerate spin-stress pairings, indicating that the degeneracy is not fundamental. We show this explicitly for the case of M33 X-7.

astro-ph.HE↗

arXiv2026

Profit-Oriented Planning and Multi-Market Operation Model for Hybrid Energy Storage Systems

The increasing penetration of renewable energy necessitates improved power system flexibility, driving the deployment of independent energy storage operators (ESOs). Existing research extensively investigates capacity sizing for price-taker storage systems or the operational coordination of aggregated distributed resources, lacking the joint optimization of capacity planning and multi-market bidding for a price-maker ESO with hybrid energy storage system (HESS) that preserves the technological heterogeneity of the integrated components. We propose a bi-level optimization framework to jointly optimize profit-oriented decisions on capacity and multi-market operation. The upper-level problem determines the optimal capacities of two heterogeneous storage systems while coordinating their bidding across day-ahead joint energy-reserve and real-time balancing markets. The lower-level problems represent market clearing of the system operator (SO). The model is reformulated into a mixed-integer linear program and solved with a Benders' decomposition algorithm. Results demonstrate that the ESO can allocate capacity between energy arbitrage and reserve provision strategically. The system with the high power-to-capacity ratio is used to capture arbitrage profits while the system with low power-to-capacity ratio is used to specialize in reserve markets. There can be internal power transfer between storage systems if there exist grid access constraints. The framework provides differentiated bidding strategies and market participation flexibility for HESS to enhance overall profitability.

econ.TH↗

arXiv2026

Radiation GRMHD Models of Accretion onto Stellar-Mass Black Holes: II. Super-Eddington Accretion

We present a comprehensive analysis of super-Eddington black hole accretion simulations that solve the GRMHD equations coupled with angle-discretized radiation transport. The simulations span a range of accretion rates, two black hole spins, and two magnetic field topologies, and include resolution studies as well as comparisons with non-radiative models. Super-Eddington accretion flows consistently develop geometrically thick disks supported by radiation pressure, regardless of magnetic field configuration. Radiation generated in the inner disk drives substantial outflows, forming conical funnel regions that limit photon escape and result in very low radiation efficiency. The accretion flows are highly turbulent with thermal energy transport dominated by radiation advection rather than diffusion. Angular momentum is primarily carried outward by Maxwell stress, with turbulent Reynolds stress playing a subdominant role. Both strong and weak jets are produced. Strong jets arise from sufficient net vertical magnetic flux and rapid black hole spin and can effectively evacuate the funnel, enabling radiation to escape through strong geometric beaming. In contrast, weak jets fail to clear the funnel, which becomes obscured by radiation-driven outflows and leads to distinct observational signatures. Spiral structures are observed in the plunging region, behaving like density waves. These super-Eddington models are applicable to a variety of astronomical systems, including ultraluminous X-ray sources, little red dots, and black hole transients.

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

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

The plunging region of a thin accretion disc around a Schwarzschild black hole

A set of analytic solutions for the plunging region thermodynamics have been developed recently under the assumption that the fluid undergoes a gravity-dominated geodesic plunge into the black hole. We test this model against a dedicated 3D global GRMHD simulation of a thin accretion disc around a Schwarzschild black hole using the code AthenaK. Provided that we account for non-adiabatic heating in the energetics, plausibly from grid-scale magnetic dissipation, we find an excellent agreement between the analytic model and the simulated quantities. These results are particularly important for existing and future electromagnetic black hole spin measurements, many of which do not to include the plunging fluid in their emission modelling. This exclusion typically stems from the assumption of a zero-stress boundary condition at the ISCO, forcing all thermodynamic quantities to vanish. Instead, we find a non-zero $δ_\mathcal{J}\approx 5.3 \%$ drop in the angular momentum over the plunging region, which is consistent with both prior simulations and observations. We demonstrate that this stress is small enough for the dynamics of the fluid in the plunging region to be well-described by geodesic trajectories, yet large enough to cause measurable dissipation near to the ISCO - keeping thermodynamic quantities from vanishing. In the plunging region, constant $α$-disc models are a physically inappropriate framework.

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↗

arXiv2024

Neural-Symbolic VideoQA: Learning Compositional Spatio-Temporal Reasoning for Real-world Video Question Answering

Compositional spatio-temporal reasoning poses a significant challenge in the field of video question answering (VideoQA). Existing approaches struggle to establish effective symbolic reasoning structures, which are crucial for answering compositional spatio-temporal questions. To address this challenge, we propose a neural-symbolic framework called Neural-Symbolic VideoQA (NS-VideoQA), specifically designed for real-world VideoQA tasks. The uniqueness and superiority of NS-VideoQA are two-fold: 1) It proposes a Scene Parser Network (SPN) to transform static-dynamic video scenes into Symbolic Representation (SR), structuralizing persons, objects, relations, and action chronologies. 2) A Symbolic Reasoning Machine (SRM) is designed for top-down question decompositions and bottom-up compositional reasonings. Specifically, a polymorphic program executor is constructed for internally consistent reasoning from SR to the final answer. As a result, Our NS-VideoQA not only improves the compositional spatio-temporal reasoning in real-world VideoQA task, but also enables step-by-step error analysis by tracing the intermediate results. Experimental evaluations on the AGQA Decomp benchmark demonstrate the effectiveness of the proposed NS-VideoQA framework. Empirical studies further confirm that NS-VideoQA exhibits internal consistency in answering compositional questions and significantly improves the capability of spatio-temporal and logical inference for VideoQA tasks.

cs.CV↗

arXiv2024

RetMIL: Retentive Multiple Instance Learning for Histopathological Whole Slide Image Classification

Histopathological whole slide image (WSI) analysis with deep learning has become a research focus in computational pathology. The current paradigm is mainly based on multiple instance learning (MIL), in which approaches with Transformer as the backbone are well discussed. These methods convert WSI tasks into sequence tasks by representing patches as tokens in the WSI sequence. However, the feature complexity brought by high heterogeneity and the ultra-long sequences brought by gigapixel size makes Transformer-based MIL suffer from the challenges of high memory consumption, slow inference speed, and lack of performance. To this end, we propose a retentive MIL method called RetMIL, which processes WSI sequences through hierarchical feature propagation structure. At the local level, the WSI sequence is divided into multiple subsequences. Tokens of each subsequence are updated through a parallel linear retention mechanism and aggregated utilizing an attention layer. At the global level, subsequences are fused into a global sequence, then updated through a serial retention mechanism, and finally the slide-level representation is obtained through a global attention pooling. We conduct experiments on two public CAMELYON and BRACS datasets and an public-internal LUNG dataset, confirming that RetMIL not only achieves state-of-the-art performance but also significantly reduces computational overhead. Our code will be accessed shortly.

cs.CV↗

arXiv2023

Dynamical Effects of Magnetic Opacity in Neutron Star Accretion Columns

We present relativistic, radiation magnetohydrodynamic simulations of supercritical neutron star accretion columns in Cartesian geometry, including temperature-dependent, polarization-averaged Rosseland mean opacities accounting for classical electron scattering in a magnetic field. Just as in our previous pure Thomson scattering simulations, vertical oscillations of the accretion shock and horizontally propagating entropy waves (photon bubbles) are present in all our simulations. However, at high magnetic fields $\gtrsim10^{12}$~G, the magnetic opacities produce significant differences in the overall structure and dynamics of the column. At fixed accretion rate, increasing the magnetic field strength results in a shorter accretion column, despite the fact that the overall opacity within the column is larger. Moreover, the vertical oscillation amplitude of the column is reduced. Increasing the accretion rate at high magnetic fields restores the height of the column. However, a new, slower instability takes place at these field strengths because they are in a regime where the opacity increases with temperature. This instability causes both the average height of the column and the oscillation amplitude to substantially increase on a time scale of $\sim10$~ms. We provide physical explanations for these results, and discuss their implications for the observed properties of these columns, including mixed fan-beam/pencil-beam emission patterns caused by the oscillations.

astro-ph.HE↗

arXiv2023

An Extension of the Athena++ Code Framework for Radiation-Magnetohydrodynamics in General Relativity Using a Finite-Solid-Angle Discretization

We extend the general-relativistic magnetohydrodynamics (GRMHD) capabilities of Athena++ to incorporate radiation. The intensity field in each finite-volume cell is discretized in angle, with explicit transport in both space and angle properly accounting for the effects of gravity on null geodesics, and with matter and radiation coupled in a locally implicit fashion. Here we describe the numerical procedure in detail, verifying its correctness with a suite of tests. Motivated in particular by black hole accretion in the high-accretion-rate, thin-disk regime, we demonstrate the application of the method to this problem. With excellent scaling on flagship computing clusters, the port of the algorithm to the GPU-enabled AthenaK code now allows the simulation of many previously intractable radiation-GRMHD systems.

astro-ph.HE↗

arXiv2023

Dynamics of Neutron Star Accretion Columns in Split-Monopole Magnetic Fields

We perform 2D axisymmetric radiative relativistic MHD simulations of radiation pressure supported neutron star accretion columns in split-monopole magnetic fields. The accretion columns exhibit quasi-periodic oscillations, which manifest in the luminosity power spectrum as 2-10 kHz peaks, together with broader extensions to somewhat higher frequencies. The peak frequency decreases for wider columns or higher mass accretion rates. In contrast to the case of shorter columns in uniform magnetic fields, pdV work contributes substantially to maintaining the radiation pressure inside the column against sideways radiative cooling. This is in part due to the compression associated with accretion along the converging magnetic field lines towards the stellar surface. Propagating entropy waves which are associated with the slow-diffusion photon bubble instability form in all our simulations. Radial advection of radiation from the oscillation itself as well as the entropy waves is also important in maintaining radiation pressure inside the column. The time-averaged profile of our fiducial simulation accretion is approximately consistent with the classical 1D stationary model provided one incorporates the correct column shape. We also quantify the porosity in all our accretion column simulations so that this may also in principle be used to improve 1D models.

astro-ph.HE↗

arXiv2023

New Identifications and Multi-wavelength Properties of Extragalactic Fermi Gamma-Ray Sources in the SPT-SZ Survey Field

The fourth Fermi Large Area Telescope (LAT) catalog (4FGL) contains 5064 $γ$-ray sources detected at high significance, but 26% of them still lack associations at other wavelengths. The SPT-SZ survey, conducted between 2008 and 2011 with the South Pole Telescope (SPT), covers 2500 $\mathrm{deg^2}$ of the Southern sky in three millimeter-wavelength (mm) bands and was used to construct a catalog of nearly 5000 emissive sources. In this study, we introduce a new cross-matching scheme to search for multi-wavelength counterparts of extragalactic $γ$-ray sources using a mm catalog. We apply a Poissonian probability to evaluate the rate of spurious false associations and compare the multi-wavelength associations from the radio, mm, near-infrared, and X-ray with 4FGL $γ$-ray sources. In the SPT-SZ survey field, 85% of 4FGL sources are associated with mm counterparts. These mm sources include 94% of previously associated 4FGL sources and 56% of previously unassociated 4FGL sources. The latter group contains 40 4FGL sources for which SPT has provided the first identified counterparts. Nearly all of the SPT-associated 4FGL sources can be described as flat-spectrum radio quasars or blazars. We find that the mm band is the most efficient wavelength for detecting $γ$-ray blazars when considering both completeness and purity. We also demonstrate that the mm band correlates better to the $γ$-ray band than the radio or X-ray bands. With the next generation of CMB experiments, this technique can be extended to greater sensitivities and more sky area to further complete the identifications of the remaining unknown $γ$-ray blazars.

astro-ph.HE↗