arXiv subjects

Antonios Nathanail

Publications and source records attributed to Antonios Nathanail.

At least 19 recordsLinked to original sources

arXiv2026

Dynamics and Spectra-Polarimetric Signatures of GRMHD Simulations with Multiple Magnetic Loops

Relativistic jets are a common outcome of accretion onto black holes, yet their presence and variability depend strongly on the magnetic and dynamical state of the accretion flow. While some systems, such as Blazars and Quasars, launch powerful persistent jets, others, including the Galactic Centre black hole Sgr~A$^\star$, show only weak or transient outflows. The physical conditions leading to the onset or suppression of jet activity remain poorly understood. We investigate the accretion flow conditions that produce transient jets or inhibit jet formation, aiming to improve our understanding of black holes that accrete without strong, steady outflows. We further predict observational signatures in total and polarized emission for comparison with recent observations of Sgr~A$^\star$ in the quiescent state from the radio to the $\gamma$-ray regime. We perform three-dimensional GRMHD simulations of an accreting black hole surrounded by a torus threaded by a poloidal multi-loop magnetic field of alternating polarities. We follow the evolution of the accretion rate, magnetic flux, and jet power, and analyze angular momentum transport. In addition, radiative transfer calculations including Compton scattering are used to derive synthetic total and polarized emission. The simulations show strong variability in jet power while the initial magnetic polarity loops accrete, followed by weaker activity at later times as the system approaches a semi-MAD state. The emission from the disk is relatively stable, weakly polarized and consistent with the quiet state values reported for SgrA$^\star$. The resulting jet emission is strongly suppressed, depolarized by Faraday rotation and conversion in the surrounding cold plasma. Upscattering calculations yield near-infrared (NIR) high energy light curves that respect observational constraints of the quiescent NIR and X-ray fluxes in SgrA$^\star$

astro-ph.HE↗

arXiv2026

Probing radiation micro-physics in M 87 I. Total intensity and broad-band spectra

Next generation Very Long Baseline Interferometers (VLBI) will provide dense sampling of the Fourier space together with high signal to noise ratios allowing to reliably observe and image faint jet structure in M 87 at mm-wavelength. The proposed next generation Event Horizon Telescope (ngEHT) and next generation Very Large Array (ngVLA) offers the unique capability to simultaneously resolve and image the accretion flow around the supermassive black hole in M 87 together with the jet launching and acceleration zone. In order to explore these capabilities and to provide theoretical expectations we perform general relativistic magnetohydrodynamic simulations of accretion on to black holes and jet launching. M 87 has been the target for multiple observations across the entire electromagnetic spectrum. Among these VLBI observations provide unique capability to resolve the jet structure down to several gravitational radii. In this work we provide possible observable signatures which will allow us to distinguish between different electron heating models and particle distributions. We use general relativistic magnetohydrodynamics and simulate the accretion of the magnetised plasma onto Kerr-black holes in 3D. The multi-frequency radiative signatures of these simulations are computed taking different electron heating and distribution functions into account. The results of our simulations show that with a dynamical range of $1\times 10^4$ and a frequency range from 86 GHz to 345 GHz observations with future VLBI arrays have the potential to tell turbulent and magnetic reconnection electron heating and the electron distribution function apart.

astro-ph.HE↗

arXiv2026

An adaptive framework for the axisymmetric pulsar magnetosphere using physics-informed Kolmogorov-Arnold networks

The pulsar magnetosphere has only recently been addressed using Physics-Informed Neural Networks (PINNs), by deploying a domain-decomposition approach and treating the separatrix and equatorial current sheet as infinitesimally thin discontinuities. However, this baseline requires extensive manual hyperparameter tuning, achieves limited final accuracy and demands several hours of training. We refine this framework by introducing domain-specific neural architectures based on Kolmogorov-Arnold networks, an automated adaptive training pipeline and a physics-based convergence criterion that eliminate the need for manual calibration. The proposed methodology delivers self-consistent axisymmetric magnetosphere solutions with mean squared errors of the PDE residuals at O(1e-6) in double precision - an improvement of two orders of magnitude over the baseline - while achieving convergence in under 20 minutes in single precision. Importantly, the method reliably resolves stellar radii reduced by up to 80% compared to the baseline, overcoming the severe spatial scale disparities that also challenge traditional solvers. Furthermore, by varying the flux that opens to infinity, we provide a correction to the equation that connects it to the equatorial T-point's position. The complete framework is released as the open-source library PulsarX.

physics.comp-ph↗

arXiv2026

The azimuthal structure of magnetically arrested disks during flux eruption events

We analyze data from a standard 3D general-relativistic magnetohydrodynamics (GRMHD) simulation, focusing on equatorial slices in order to examine the details and the evolution of the azimuthal structure of the accreting matter. During flux eruption events, the non-axisymmetric features of the equatorial inner accretion disk are considerably enhanced, with this enhancement being more prominent close to the black hole. Our analysis of the azimuthal structure of the equatorial accretion disk finds that the matter distribution in the vicinity of the horizon is dominated by low azimuthal mode numbers, specifically by the $m = 2$, and $m = 1$ modes, indicating that the non-axisymmetry of the disk during flux eruption events is enhanced due to the emergence of features with a large angular size on the equatorial plane. Our results suggest that the morphology of the equatorial accretion flow close to the black hole is mainly determined by the formation and motion of vertical magnetic flux bundles. These bundles are formed when the initially horizontal magnetic field reconnects into a vertical configuration, effectively detaching from the black hole horizon. This reconnection occurs in a low-density, highly magnetized region on the equatorial plane that expands over time as more field lines undergo vertical reconfiguration. The resulting vertical flux tubes, filled with low-density plasma, are then transported outwards due to magnetic buoyancy. Our results present a detailed quantitative description of the morphology of MADs and of its evolution during flux eruptions, complemented by a description of the physical process by which excess magnetic flux is detached from the black hole, vertically reconfigured, and expelled.

astro-ph.HE↗

arXiv2026

Black hole Limits Redefined: Extreme Efficiency in Black Hole Jets

Relativistic jets from black holes can extract energy not only from accretion but also directly from the black hole's spin, as described by the Blandford-Znajek mechanism. A longstanding question is whether magnetic flux can accumulate near the event horizon to such an extent that it halts accretion entirely, enabling energy extraction purely from spin. Previous studies have shown that accretion persists through instabilities and that jet power only modestly exceeds the accretion energy budget, yet some observational results suggest much higher efficiencies. Here we present state-of-the-art general relativistic magnetohydrodynamic (GRMHD) simulations that sustain a quasi-steady magnetically arrested disk state for approximately 10,000 dynamical times, during which accretion is globally suppressed across the full azimuthal extent. In this regime, jet power exceeds the accretion energy input by more than two orders of magnitude, demonstrating a previously unachieved level of efficiency. These results challenge conventional assumptions about the limits of black hole energy extraction and suggest a new framework for interpreting powerful jet systems. Our findings raise important questions about the long-term stability of such states and the fundamental limits of the Blandford-Znajek process.

astro-ph.HE↗

arXiv2026

Radiative Cooling Effects on Plasmoid Formation in Black Hole Accretion Flows with Multiple Magnetic Loops

We investigate the influence of radiative cooling on plasmoid formation in black hole accretion flows using 2D and 3D two-temperature GRMHD simulations with multi-loop magnetic fields. Our results show that radiative cooling suppresses the transition to a MAD state by reducing magnetic flux accumulation near the horizon, modifies the disk structure via lower electron temperatures and increased equatorial density, and alters reconnection properties:compressing current sheets, shortening plasmoid lifetimes, and increasing their frequency. We also find enhanced negative energy-at-infinity density in plasmoids near the ergosphere. These findings indicate that radiative cooling critically shapes both large scale accretion dynamics and small-scale reconnection phenomena, potentially modulating black hole energy extraction through reconnection-driven Penrose processes.

astro-ph.HE↗

arXiv2025

Supermassive black-hole imaging with a self-consistent electron-temperature prescription

The recent 230 GHz observations by the Event Horizon Telescope have resolved the innermost structure of the M87 galaxy, revealing a ring-like feature consistent with thermal synchrotron emission from a magnetized torus surrounding a rotating supermassive black hole. Moreover, Global Millimeter VLBI Array observations at 86 GHz have revealed a larger-scale, edge-brightened jet with clear signatures of non-thermal emission. The theoretical modelling of these observations involves advanced general-relativistic magnetohydrodynamic simulations of magnetized accretion disks around rotating black holes, together with the associated synchrotron emission, which is normally treated with simplified expressions for the electron temperature and assuming a purely thermal distribution. However, an important non-thermal component is expected to be present, making the thermal-emission model not only an approximation, but also a source of degeneracy in the modelling. In view of this, we here present the first application of an ab-initio approach to the electron temperature derived from microscopic simulations of turbulent collisionless plasmas. The novel method, which has no tuneable coefficients and is fully specified by the thermodynamical and magnetic properties of the plasma, provides a better description of the jet morphology and width at 86 GHz, as well as of the broadband spectral emission. These findings highlight the importance of incorporating microscopic plasma physics in black-hole imaging and emphasise the crucial role of magnetic reconnection in electron heating and acceleration processes.

astro-ph.HE↗

arXiv2025

The Physical Origin and Time Lag of Multi-Frequency Flares from SgrA*

Sagittarius~A$^*$, the supermassive black hole at the center of our galaxy, exhibits flares across various wavelengths, yet their origins remain elusive. We performed 3D two-temperature General Relativistic Magnetohydrodynamic (GRMHD) simulations of magnetized accretion flows initialized from multi-loop magnetic field configuration onto a rotating black hole and conducted General Relativistic Radiative Transfer (GRRT) calculations considering contributions from both thermal and non-thermal synchrotron emission processes. Our results indicate that the polarity inversion events from the multi-loop magnetic field configurations can generate $138\,\rm THz$ flares consistent with observations with the help of non-thermal emission. By tracing the intensity evolution of light rays in GRRT calculations, we identify the precise location of the flaring region and confirm that it originates from a large-scale polarity inversion event. We observe time delays between different frequencies, with lower-frequency radio flares lagging behind higher frequencies due to plasma self-absorption in the disk. The time delay between near-infrared and 43 GHz flares can reach up to $\sim 50$ min, during which the flaring region gradually shifts outward, becoming visible at lower frequencies. Our study confirms that large-scale polarity inversion in a Standard And Normal Evolution (SANE) accretion flow with a multi-loop initial magnetic configuration can be a potential mechanism driving flares from Sgr~A$^*$.

astro-ph.HE↗

arXiv2025

Identifying Long Radio Transients with Accompanying X-Ray Emission as Disk-Jet Precessing Black Holes: The Case of ASKAP J1832-0911

Aims: In this work we investigate whether the 2 min bursts every 44 min from ASKAP J1832-0911 can be explained by Lense-Thirring precession of an intermediate-mass black hole (IMBH) accretion disk launching a Blandford-Znajek jet, as an alternative to magnetar or white-dwarf models. Methods: We derive the Lense-Thirring period $ P_{\rm LT}=\frac{\pi G M}{a c^3}r^3 $ and solve $P_{\rm LT}=44$ min for black-hole mass M and dimensionless radius $r=R/R_g$. We estimate the equipartition field B at r, compute the Blandford-Znajek power $P_{\rm BZ}$, and the power expected from a gap at the black hole magnetosphere, and compare the resulting jet luminosity to the observed radio and X-ray fluxes at $D\approx4.5$ kpc. We also evaluate expected high-frequency variability and the angular size for Very Long Baseline Interferometry (VLBI) observations. Results: For $a\sim0.3\!-\!0.9$, an IMBH with $M\sim10^3\!-\!10^5\,M_\odot$ yields $r\sim10\!-\!40\,R_g$ and $P_{\rm LT}=44$ min. Equipartition gives $B\sim10^5$ G at r, leading to $P_{\rm BZ}\sim10^{35\!-\!39}$ erg ${\rm s^{-1}}$. With radiative efficiency $\epsilon_j\sim10^{-2}\!-\!10^{-1}$, the predicted $L_{\rm jet}\sim10^{34\!-\!36}$ erg ${\rm s^{-1}}$ matches the observed $F_X\sim10^{-12}$ erg ${\rm cm^{-2}}$ ${\rm s^{-1}}$ and radio flux, variability on $\lesssim100$ s could be a smoking gun of this model. Conclusions:The IMBH precessing-jet model simultaneously explains the periodicity, energetics, and duty cycle of ASKAP J1832-0911. Only high-time-resolution X-ray timing (to check $\sim$s pulsations) and multi-frequency radio polarimetry can definitively distinguish it from magnetar or white-dwarf scenarios.

astro-ph.HE↗

arXiv2025

Magnetically arrested disk flux eruption events to describe SgrA* flares

Context. Magnetically arrested disks are among the most suitable candidates for describing the gas accretion and observed emission in the vicinity of supermassive black holes. Aims. This work aims to provide a direct correlation between the quasi-periodic flux eruption events, characteristic of MAD accretion disk simulations, and the observed flaring behavior in the Galactic center. Methods. We employ a MAD accretion disk with a distinct counter-clockwise rotation and investigate the evolution of magnetized flux tubes generated during a prominent flux eruption event. Although these flux tubes demonstrate a clockwise pattern, they experience significant dragging by the accretion disk's rotation. This study models the motion of hot spots, formed on the disk's equatorial plane due to magnetic reconnection, as they travel along the magnetized flux tubes at a fraction of the speed of light. Results. Hot spots with a relativistic ejection velocity are able to balance out the counter-clockwise dragging of the flux tube's foot-point on the disk and demonstrate a clockwise motion in the sky, that is in good agreement with the NIR flares in the Galactic center. In addition, our flare models favor face-on inclinations in the range $[0^\circ, 34^\circ]$ and $[163^\circ, 180^\circ]$ for SgrA*. Conclusions. The flux eruption events that arise naturally in the MAD accretion state provide a promising framework for reproducing the observed flaring behavior in the vicinity of SgrA*.

astro-ph.HE↗

arXiv2024

The impact of resistivity on the variability of black hole accretion flows

Context. The accretion of magnetized plasma onto black holes is a complex and dynamic process, where the magnetic field plays a crucial role. The amount of magnetic flux accumulated near the event horizon significantly impacts the accretion flow behavior. Resistivity, a measure of how easily magnetic fields can dissipate, is thought to be a key factor influencing this process. This work explores the influence of resistivity on accretion flow variability. We investigate simulations reaching the magnetically arrested disk (MAD) limit and those with an initial multi-loop magnetic field configuration. Methods. We employ 3D resistive general relativistic magnetohydrodynamic (GRMHD) simulations to model the accretion process under various regimes, where resistivity has a global uniform value. Results. Our findings reveal distinct flow behaviors depending on resistivity. High resistivity simulations never achieve the MAD state, indicating a disturbed magnetic flux accumulation process. Conversely, low resistivity simulations converge towards the ideal MHD limit. The key results are: i) For the standard MAD model, resistivity plays a minimal role in flow variability, suggesting that flux eruption events dominate the dynamics. ii) High resistivity simulations exhibit strong magnetic field diffusion into the disk, rearranging efficient magnetic flux accumulation from the accretion flow. iii) In multi-loop simulations, resistivity significantly reduces flow variability, which was not expected. However, magnetic flux accumulation becomes more variable due to frequent reconnection events at very low resistivity values. Conclusions. This study shows that resistivity affects how much the flow is distorted due to magnetic field dissipation. Our findings provide new insights into the interplay between magnetic field accumulation, resistivity, variability and the dynamics of black hole accretion.

astro-ph.HE↗

arXiv2024

Parameter study for hot spot trajectories around Sgr$A*$

Intense flaring events in the near-infrared and X-ray wavebands of our Galactic Center have been the subject of research for decades. In recent years, the GRAVITY instrument of the Very Large Telescope captured the motion and polarimetric signature of such a flare in close proximity to the supermassive black hole. This study aims to investigate a broad parameter space for hot spot motion in the vicinity of Sgr$A*$ and reproduce the observed flaring behavior. To this end, we have developed a General Relativistic Radiative Transfer code and conducted a parameter study including both planar and ejected hot spot configurations around supermassive black holes. Super-Keplerian orbital frequencies are favored by circular equatorial, cylindrical and parabolic models, whereas conical hot spot trajectories provide a better fit for orbital frequencies below the Keplerian value. Additionally, a distant observer cannot effectively differentiate between Schwarzschild and Kerr black holes, as well as face-on orbits at different observation angles.

astro-ph.HE↗

arXiv2024

Dynamics and Emission Properties of Flux Ropes from Two-Temperature GRMHD Simulations with Multiple Magnetic Loops

Flux ropes erupting from the vicinity of the black hole are thought to be a potential model for the flares observed in Sgr\,A$^*$. In this study, we examine the radiative properties of flux ropes that emerged from the vicinity of the black hole. We have performed three-dimensional two-temperature General Relativistic Magnetohydrodynamic (GRMHD) simulations of magnetized accretion flows with alternating multiple magnetic loops, and General Relativistic Radiation Transfer (GRRT) calculations. In GRMHD simulations, two different sizes of initial magnetic loops are implemented. In the small loop case, magnetic dissipation leads to a weaker excitement of magneto-rotational instability inside the torus which generates a lower accretion rate compared to the large loop case. However, it makes more generation of flux ropes due to frequent reconnection by magnetic loops with different polarities. By calculating the thermal synchrotron emission, we found that the variability of light curves and emitting region are tightly related. At $230\,\rm GHz$ and higher frequency, the emission from the flux ropes is relatively stronger compared with the background, which is responsible for the filamentary structure in the images. At lower frequencies, e.g. $43\,\rm GHz$, emission comes from more extended regions, which have a less filamentary structure in the image. Our study shows self-consistent electron temperature models are essential for the calculation of thermal synchrotron radiation and the morphology of the GRRT images. Flux ropes contribute considerable emission at frequencies $\gtrsim 230\,\rm GHz$.

astro-ph.HE↗

arXiv2023

Impact of anisotropic ejecta on jet dynamics and afterglow emission in binary neutron-star mergers

Binary neutron stars mergers widely accepted as potential progenitors of short gamma-ray bursts. After the remnant of the merger has collapsed to a black hole, a jet is powered and may breakout from the the matter expelled during the collision and the subsequent wind emission. The interaction of the jet with the ejecta may affect its dynamics and the resulting electromagnetic counterparts. We here examine how an inhomogeneous and anisotropic distribution of ejecta affects such dynamics, dictating the properties of the jet-ejecta cocoon and of the afterglow radiated by the jet upon deceleration. More specifically, we carry out general-relativistic hydrodynamical simulations of relativistic jets launched within a variety of geometrically inhomogeneous and anisotropic distributions of ejected matter. We find that different anisotropies impact the variance of the afterglow light-curves as a function of the jet luminosity and ejected mass. A considerable amount of the jet energy is deposited in the cocoon through the jet-ejecta interaction with a small but important dependence on the properties of the ejecta. Furthermore, all configurations show a two-component behaviour for the polar structure of the jet, with a narrow core at large energies and Lorentz factors and a shallow segment at high latitudes from the jet axis. Hence, afterglows measured on off-axis lines of sight could be used to deduce the properties of the ejected matter, but also that the latter need to be properly accounted for when modelling the afterglow signal and the jet-launching mechanisms.

gr-qc↗

arXiv2023

Future Prospects for Constraining Black-Hole Spacetime: Horizon-scale Variability of Astrophysical Jet

The Event Horizon Telescope (EHT) Collaboration has recently published the first horizon-scale images of the supermassive black holes M87* and Sgr A* and provided some first information on the physical conditions in their vicinity. The comparison between the observations and the three-dimensional general-relativistic magnetohydrodynamic (GRMHD) simulations has enabled the EHT to set initial constraints on the properties of these black-hole spacetimes. However, accurately distinguishing the properties of the accretion flow from those of the spacetime, most notably, the black-hole mass and spin, remains challenging because of the degeneracies the emitted radiation suffers when varying the properties of the plasma and those of the spacetime. The next-generation EHT (ngEHT) observations are expected to remove some of these degeneracies by exploring the complex interplay between the disk-jet dynamics, which represents one of the most promising tools for extracting information on the black-hole spin. By using GRMHD simulations of magnetically arrested disks (MADs) and general-relativistic radiative-transfer (GRRT) calculations of the emitted radiation, we have studied the properties of the jet and the accretion-disk dynamics on spatial scales that are comparable with the horizon. In this way, we are able to highlight that the radial and azimuthal dynamics of the jet are well correlated with the black-hole spin. Based on the resolution and image reconstruction capabilities of the ngEHT observations of M87*, we can assess the detectability and associated uncertainty of this correlation. Overall, our results serve to assess what are the prospects for constraining the black-hole spin with future EHT observations.

astro-ph.HE↗

arXiv2023

Two-Temperature GRMHD Simulations of Black Hole Accretion Flows with Multiple Magnetic Loops

We have performed a series of two-dimensional two-temperature general relativistic magnetohydrodynamic simulations of magnetized accretion flows initiated from tori with different sizes and poloidal magnetic loop polarities. In these two temperature simulations, we trace the process of heating electrons through turbulence and reconnection, most of the time these electrons are trapped in plasmoids. We found that the accretion process strongly depends on the size of the magnetic loops. The accretion flows never reach the magnetically arrested (MAD) regime in small loop cases. Interaction between magnetic field with different polarities dissipates and decreases the efficiency of magneto-rotational instability. The dependency on the wavelength of the loops places a lower limit on the loop size. In the large loop cases, after reaching a quasi-steady phase, a transition from Standard And Normal Evolution (SANE) flow to MAD flow is observed. The transition of the accretion state and the transition time depends on the initial loop wavelength. The formation of plasmoids strongly depends on the size of the magnetic loops. The frequent magnetic reconnection between the magnetic loops is responsible for the formation of most of the plasmoids. For some plasmoids, Kelvin-Helmholtz and tearing instabilities are coexisting, showing another channel of plasmoid formation. The simulations present that electrons in the plasmoids are well-heated up by turbulent and magnetic reconnection. Different properties of plasmoid formation in different magnetic field configurations provide new insights for the understanding of flaring activity and electron thermodynamics in Sgr A*.

astro-ph.HE↗

arXiv2022

The Photon Ring in M87*

We report measurements of the gravitationally lensed secondary image -- the first in an infinite series of so-called "photon rings" -- around the supermassive black hole M87* via simultaneous modeling and imaging of the 2017 Event Horizon Telescope (EHT) observations. The inferred ring size remains constant across the seven days of the 2017 EHT observing campaign and is consistent with theoretical expectations, providing clear evidence that such measurements probe spacetime and a striking confirmation of the models underlying the first set of EHT results. The residual diffuse emission evolves on timescales comparable to one week. We are able to detect with high significance a southwestern extension consistent with that expected from the base of a jet that is rapidly rotating in the clockwise direction. This result adds further support to the identification of the jet in M87* with a black hole spin-driven outflow, launched via the Blandford-Znajek process. We present three revised estimates for the mass of M87* based on identifying the modeled thin ring component with the bright ringlike features seen in simulated images, one of which is only weakly sensitive to the astrophysics of the emission region. All three estimates agree with each other and previously reported values. Our strongest mass constraint combines information from both the ring and the diffuse emission region, which together imply a mass-to-distance ratio of $4.20^{+0.12}_{-0.06}~μ{\rm as}$ and a corresponding black hole mass of $(7.13\pm0.39)\times10^9M_\odot$, where the error on the latter is now dominated by the systematic uncertainty arising from the uncertain distance to M87*.

astro-ph.HE↗

arXiv2022

Magnetic field structure in the vicinity of a super-massive black hole in low luminosity galaxies: the case of Sgr A*

Observations of $\rm SgrA^*$ have provided a lot of insight on low-luminosity accretion, with a handful of bright flares accompanied with orbital motion close to the horizon. It has been proposed that gas supply comes from stellar winds in the neighborhood of the supermassive black hole. We here argue that the flow at the vicinity of the black hole has a low magnetization and a structure of alternating polarity totally dictated by the well studied and long-ago proposed MRI turbulent process. This can be the case, provided that in larger distances from the black hole magnetic diffusivity is dominant and thus the magnetic field will never reach equipartition values. For $\rm SgrA^*$, we show the immediate consequences of this specific magnetic field geometry, which are: (i) an intermittent flow that passes from quiescent states to flaring activity, (ii) no quasi-steady-state jet, (iii) no possibility of a magnetically arrested configuration. Moreover a further distinctive feature of this geometry is the intense magnetic reconnection events, occurring as layers of opposite magnetic polarity are accreted, in the vicinity of the black hole. Finally, we argue that the absence of a jet structure in such case will be a smoking gun in 43 \& 86 GHz observations.

astro-ph.HE↗