arXiv subjects

Minyong Guo

Publications and source records attributed to Minyong Guo.

At least 37 records · Page 2Linked to original sources

arXiv2025

Semi-analytical Study on the Polarized Images of Black Hole due to Frame Dragging

An initially retrograde accretion flow is transformed into a prograde configuration before plunging into the black hole, as a result of the frame-dragging effect induced by the black hole's rotation. The polarized image of a black hole shaped by such an accretion flow manifests three distinctive critical locations: the turning point of the flow's primary image, the polarization-flip location on the image plane, and the position of the primary image corresponding to the flow's actual turning point in spacetime. Due to the influences of gravitational lensing and gravitational Faraday rotation, these three positions generally do not coincide. In this work, we examine a thin equatorial accretion disk composed of initially retrograde, geodesically moving flows, and conduct a systematic investigation into the interrelations and discrepancies among these critical locations. We elucidate the spatial hierarchy among the three, and for an on-axis observer, we derive approximate analytic expressions characterizing their positions.

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arXiv2025

Near-Horizon Polarization as a Diagnostic of Black Hole Spacetime

A key challenge in imaging supermassive black holes is disentangling gravitational effects from plasma physics in order to accurately determine spacetime properties, particularly black hole spin. In this Letter, we present a fully covariant and rigorous analysis of the synchrotron emission from accreting plasma in the equatorial plane in the stationary, axisymmetric, high-conductivity regime, and identify--for the first time--a distinctive near-horizon polarization pattern that remains robust across different flow structures. This pattern arises from strong frame dragging near the event horizon, which induces a degeneracy among plasma flow and magnetic field configurations, yielding a polarization signature determined solely by the spacetime geometry and the observer's inclination. The near-horizon polarization thus offers a clean and precise probe of black hole spin and other fundamental parameters. If future space-based millimeter VLBI observations can resolve synchrotron emission originating within approximately 1$\%$ of the event horizon radius in M87* or Sgr A*, this universal polarization pattern may become observable.

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arXiv2024

Magnetic reconnection under centrifugal and gravitational electromotive forces

We examine the physical implications of the centrifugal and gravitational electromotive forces on magnetic reconnection in a Kerr black hole background. We find that both forces increase the reconnection rate, though the underlying mechanisms differ substantially. The gravitational force leads to a separation of charge density, breaking the quasi-neutrality of the plasma. In contrast, the centrifugal electromotive force affects the electric current by reducing the effective length of the current sheet. This reduction arises from the non-Euclidean spatial geometry observed by a locally comoving observer with respect to the rotating sheet. This phenomenon amplifies both the transport of charged carriers and the thermal-inertia effect within the current sheet, irrespective of the presence of a black hole.

astro-ph.HE↗

arXiv2024

Coport: A New Public Code for Polarized Radiative Transfer in a Covariant Framework$^\spadesuit$

General relativistic radiative transfer calculations are essential for comparing theoretical models of black hole accretion flows and jets with observational data. In this work, we introduce Coport, a novel public code specifically designed for covariant polarized ray-tracing radiative transfer computations in any spacetime. Written in Julia, Coport includes an interface for visualizing numerical results obtained from HARM, a publicly available implementation of the general relativistic magnetohydrodynamics code. We validate the precision of our code by comparing its outputs with the results from a variety of established methodologies. This includes the verification against analytical solutions, the validation through thin-disk assessments, and the evaluation via thick-disk analyses. Notably, our code employs a methodology that eliminates the need for separating the computations of spacetime propagation and plasma propagation. Instead, it directly solves the coupled, covariant, polarized radiative transfer equation in curved spacetime, seamlessly integrating the effects of gravity with plasma influences. This approach sets our code apart from the existing alternatives and enhances its accuracy and efficiency.

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arXiv2024

Fast magnetic reconnection in Kerr spacetime

We develop a relativistic scenario of fast magnetic reconnection process, for general magnetohydrodynamical plasmas around Kerr black holes. Generalizing the Petschek model, we study various properties of the reconnection layer in distinct configurations. When current sheet forms in the zero-angular-momentum (ZAMO) frame which corotates with the black hole, the reconnection rate for both radial and azimuthal configurations is decreased by spacetime curvature. However, when the current sheet forms in a non-ZAMO frame, which rotates either faster or slower than the black hole, detail analysis establishes that for any given slow rotations (subrelativistic at most) and mildly relativistic inflow, the ZAMO observer will find asymmetric reconnection rates for radial configuration: it is decreased on one side of the current sheet and is increased on the other side in comparison to the unrotation limit. This is valid to both the Sweet-Parker and the Petschek scenario. The results clarify the effects of rotation on the reconnection layer in the laboratory frame in the flat spacetime limit.

astro-ph.HE↗

arXiv2024

Photon emissions from Kerr equatorial geodesic orbits

We consider the light emitters moving freely along the geodesics on the equatorial plane near a Kerr black hole and study the observability of these emitters. To do so, we assume these emitters emit the photons isotropically and monochromatically, and we compute the photon escaping probability (PEP) and the maximum observable blueshift (MOB) of the photons that reach infinity. We obtain numerical results of PEP and MOB for the emitters along various geodesic orbits, which exhibit distinct features for the trajectories of different classes. We exhaustively investigate the effects of the emitters' motion on the PEP and MOB. In particular, we find that the plunging emitters approaching the unstable circular orbits could have very good observability, before fading away suddenly. This interesting observational feature becomes more significant for the high-energy emitters near a high-spin black hole. As the radiatively-inefficient accretion flow may consist of such plunging emitters, the present work could be of great relevance to the astrophysical observations.

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arXiv2024

Images and flares of geodesic hotspots around a Kerr black hole

In this study, we develop a numerical method to generate images on an observer's screen, formed by radiation from hotspots on any timelike orbits outside a black hole. This method uses the calculation of fractional numbers, enabling us not only to produce the overall image but also to distinguish between primary, secondary, and higher-order images. Building upon this, we compute the images of hotspots from eight potential types of geodesic timelike orbits outside a Kerr black hole, summarizing the properties of both the overall and individual order images. Furthermore, we calculate the centroid motion and lightcurve. Notably, we observe flare phenomena across all orbit types and classify these flares into three categories based on the Doppler and gravitational redshift effects.

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arXiv2024

Is a photon ring invariably a closed structure?

In this study, we investigate the image of a rotating compact object (CO) illuminated by a geometrically thin, optically thin disk on the equatorial plane. As the radius of the CO's surface fluctuates, the CO may partially or entirely obscure the photon region. We observe that the perceived photon ring may exhibit discontinuities, deviating from a closed structure, and may even disappear entirely. We find that the disruption and disappearance of the photon ring are dependent on the observational angle$-$a novel phenomenon not previously observed in black hole imaging studies. Our study reveals that while the factors influencing this unique photon ring phenomenon are diverse and the outcomes complex, we can provide a clear and comprehensive explanation of the physical essence and variation trends of this phenomenon. We do this by introducing and analyzing the properties and interrelationships of three characteristic functions, $\tildeη$, $η_o$, and $η_s$ related to the photon impact parameters. Additionally, our analysis of the intensity cuts and inner shadows of the images uncovers patterns that differ significantly from the shadow curve.

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arXiv2024

Imaging thick accretion disks and jets surrounding black holes

Based on the horizon-scale magnetofluid model developed in [arXiv:2309.13304], we investigate the millimeter-wave images of a geometrically thick accretion disk or a funnel wall, i.e., the magnetofluid that encloses the base of the jet region, around a Kerr black hole. By employing the numerical method to solve the null geodesic and radiative transfer equations, we obtain the optical appearances at various observational angles and frequencies, generated by the thermal synchrotron radiation within the magnetofluid. For the thick disk, we specifically examine the impact of emission anisotropy on images, concluding that anisotropic synchrotron radiation could play an important role in the observability of the photon ring. For the funnel wall, we find that both the outflow and inflow funnel walls exhibit annular structures on the imaging plane. The outflow funnel wall yields a brighter primary image than the photon ring, whereas the inflow one does not. Based on our investigation, the inflow funnel wall model can not be ruled out by current observations of M87*.

astro-ph.HE↗

arXiv2024

Observational signatures of rotating black holes in the semiclassical gravity with trace anomaly

In a recent work by Fernandes [arXiv:2305.10382], an exact stationary and axisymmetric solution was discovered in semiclassical gravity with type-A trace anomaly, identified as a quantum-corrected version of the Kerr black hole. In this study, we explore the observational signatures of this black hole solution. Our investigation reveals that there exist prograde and retrograde light rings, whose radii increase monotonically with the coupling parameter $α$. We also observe that when $α$ is negative, the shadow area for the quantum-corrected black hole is smaller than that of the Kerr black hole, whereas when $α$ is positive, the area is larger. Furthermore, for a near-extremal black hole, its high-spin feature (the NHEKline) is found to be highly susceptible to disruption by $α$. Moreover, we discuss the images of the quantum-corrected black hole in the presence of a thin accretion disk and compare them to those of the Kerr black hole. Our study highlights the importance of near-horizon emission sources in detecting the effects of quantum corrections by black hole images.

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arXiv2024

A new analytical model of magnetofluids surrounding rotating black holes

In this study, we develop a simplified magnetofluid model in the framework of GRMHD. We consider an ideal, adiabatic fluid composed of two components, ions and electrons, having a constant ratio between their temperatures. The flows are assumed to be governed by gravity, enabling us to employ the ballistic approximation, treating the streamlines as timelike geodesics. We show that the model is analytically solvble around a rotating black hole if the angular velocity of the geodesic $u^θ$ is vanishing. In the corresponding solution, which is named the conical solution, we derive a comprehensive set of explicit expressions for the thermodynamics and the associated magnetic field. Furthermore, we explore the potential applications of our model to describe the thick disks and the jets at the horizon scale. Our model provides a direct pathway for the study of black hole imaging.

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arXiv2023

Imaging a Semi-Analytical Jet model Generated by 3D GRMHD Simulation

Employing 3D GRMHD simulation, we study the images of a geometrically thin jet, whose emissions concentrate on its surface, for accretion system surrounding a central spinning BH. By introducing a strong magnetic field, we observe three phases of BH accretion evolution: (a) initially, both the accretion rate and the magnetic flux on the horizon gradually increase; (b) at an intermediate stage, the magnetic flux approximately reaches saturation, and a jet forms via the Blandford-Znajek (BZ) mechanism; (c) ultimately, the entire system achieves a dynamic equilibrium, and a magnetically arrested disk (MAD) forms. We carefully study the jet images during the saturation and MAD regimes at various frequencies and from different observational angles. We reveal the presence of U-shaped brighter lines near the jet surface boundaries, which can be attributed to the photons whose trajectories skim over the jet surface. The existence of these brighter lines is a unique feature of a geometrically thin jet. Moreover, we notice that the jet images are relatively insensitive to the observed frequencies of interest. Additionally, we observe that the time-averaged images for the highly oscillating MAD regime show only slight differences from those of the saturation regime.

astro-ph.HE↗

arXiv2023

Polarized images of charged particles in vortical motions around a magnetized Kerr black hole

In this work, we study the images of a Kerr black hole (BH) immersed in uniform magnetic fields, illuminated by the synchrotron radiation of charged particles in the jet. We particularly focus on the spontaneously vortical motions (SVMs) of charged particles in the jet region and investigate the polarized images of electromagnetic radiations from the trajectories along SVMs. We notice that there is a critical value $ω_c$ for charged particle released at a given initial position and subjected an outward force, and once $|qB_0/m|=|ω_B|>|ω_c|$ charged particles can move along SVMs in the jet region. We obtain the polarized images of the electromagnetic radiations from the trajectories along SVMs. Our simplified model suggests that the SVM radiations can act as the light source to illuminate the BH and form a photon ring structure.

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arXiv2023

Circular orbits and polarized images of charged particles orbiting Kerr black hole with a weak magnetic field

In this work, we study circular motions of charged particles and their polarized images around the Kerr black hole immersed in a weak magnetic field. We pay special attention to the case that both the magnetic field and the charge-to-mass ratio are not big, thus the effective potential along the radial motion reduce to a cubic form approximately so that we can express the radius of the innermost stable circular orbit analytically in terms of the energy and angular momentum of charged particles. Moreover, we computed the polarized synchrotron radiations of these particles and obtained the polarized images semi-analytically for various spins, observational angles and prograde and retrograde orbits. In particular, We find that these parameters have significant impacts on the polarization rotation and the magnitude of the polarization flux.

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arXiv2023

Electromagnetic effects on charged particles in NHEK

We investigate the motions of charged particles in the near horizon region of an extreme Kerr black hole with weak electromagnetic fields. There is an enhanced symmetry in the NHEK geometry. We find that when the electromagnetic field respects this enhanced symmetry, which we refer to as the maximally symmetric electromagnetic (MSEM) field, the equations of motion of charged particles get simplified into a set of decoupled first-order differential equations. We discuss the motions of charged particles in two MSEM fields, one being the force-free field and the other being the vacuum fields. Even though the radial motions are similar to the geodesics in NHEK geometry, the angular motions could be affected by the electromagnetic field significantly. In particular, for the vacuum solution which is produced by a weakly charged black hole, there exist stable vortical motions if the electromagnetic parameter is above the critical value $\mB_c = \sqrt{3}$. These vortical motions do not cross the equatorial planes, and the charged particles in them radiate non-thermally. We discuss the corresponding astrophysical implications.

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arXiv2023

How different are shadows of compact objects with and without horizons?

In this work, we theoretically assume that a compact object (CO) can have a dark surface so that the CO is simplified to have no emissions and reflections. Considering that the radius of the surface can be located inside or outside the photon region, which is closely related to the shadow curve, we investigate if a CO without an event horizon could produce shadow structures similar to black holes and figure out how different of shadows of COs with and without horizons. In particular, by introducing the (possible) observational photon region, we analytically construct an exact correspondence between the shadow curves with the impact parameters of photons and find that there are indeed several differences for shadows of COs without horizons and black holes. More precisely, We found the shadow curve is still determined by the photon region when the radius of the surface is small enough to retain a whole photon region outside the shell. When only part of the photon region remains, the shadow curve is partially determined by the photon region, and the remaining portion of the shadow curve is partly controlled by the impact parameters of photons which has a turning point on the surface. When there's no photon region outside the surface, the shadow curve is totally controlled by the impact parameters of photons which has a turning point on the surface.

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arXiv2023

Shadows of Kerr black holes with a Gaussian-distributed plasma in the polar direction

In this work, we study the shadow of Kerr black hole surrounded by an axisymmetric plasma, whose density takes a Gaussian distribution in the polar direction. Along the radial direction, we consider two models: in model A the density of the plasma decays in a power law; in model B the density obeys a logarithmic Gaussian distribution. Using the numerical backward ray-tracing method, we find that the size of the shadow is sensitive to the inclination angle of the observer due to the polar distribution of the density of the plasma. In particular, we pay special attention to the model B and investigate the influence of the radial position of maximum density, the decay rate of the density towards the event horizon and the opening angle of the plasma on the shape and size of the Kerr black hole shadow. The effects of the plasmas studied in this work can be qualitatively explained by taking the plasmas as convex lenses with the refractive index being less than $1$.

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arXiv2022

Polarized Images of Synchrotron Radiations in Curved Spacetime

In this work, we derive two formulas encoding the polarization direction and luminosity of synchrotron radiations from the moving electrons in curved spacetime under the geometric optics approximation. As an application, we further study the polarized images of synchrotron radiations from electron sources in Schwarzschild black hole spacetime with a vertical and uniform magnetic field. In particular, by focusing on the circular orbits of electrons on the equatorial plane, we show the polarized images of the synchrotron radiations from these orbits for different observational angles and discuss the variations of the polarization directions concerning the angles.

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