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Relativistic jets are collimated plasma outflows with relativistic speeds. Astrophysical objects involving relativistic jets are a system comprising a compact object such as a black hole, surrounded by rotating accretion flows, with the relativistic jets produced near the central compact object. The most accepted models explaining the origin of relativistic jets involve magnetohydrodynamic (MHD) processes. Over the past few decades, many general relativistic MHD (GRMHD) codes have been developed and applied to model relativistic jet formation in various conditions. This short review provides an overview of the recent progress of GRMHD simulations in generating relativistic jets and their modeling for observations.

Reliable and efficient evaluation of the modulation amplitude is essential for achieving accurate spatial resolution in optical correlation-domain reflectometry (OCDR). Conventional measurement methods, such as those using optical or electrical spectrum analyzers, require additional equipment or independent setups, which increase system complexity and cost. In this study, we propose a simple and accurate method to estimate the modulation amplitude directly within the OCDR configuration by exploiting ghost peaks that appear when a small frequency shift is introduced in the optical path. The ghost-peak positions are analytically related to the modulation amplitude through a derived expression, enabling quantitative estimation without auxiliary instruments or analyzer setting changes. Experiments confirm that the estimated modulation amplitudes agree well with the reference values obtained by heterodyne detection, with errors below 1.4% across all tested modulation frequencies. Moreover, the approximate analytical formula achieves a deviation of less than 0.015%. These results demonstrate that the proposed method provides a compact and self-contained approach for evaluating the modulation amplitude in OCDR and may serve as a foundation for more stable and easily calibrated implementations in future correlation-domain reflectometry.

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 very long baseline interferometry 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.

An essential factor for determining the characteristics of an accretion flow is its angular momentum. According to the angular momentum of the flow, semi-analytical analysis suggests various types of accretion solutions. It is critical to test these with numerical simulations, using the most advanced framework available (general relativistic magnetohydrodynamics), to understand how the flow changes with different angular momentum. By changing the initial condition of the accretion torus minimally, we can simulate a steady, low-angular-momentum accretion flow around a Kerr black hole. We focus primarily on the lower limits of angular momentum and find that an accretion flow with an intermediate range of angular momentum differs significantly from high- or very-low-angular-momentum flows. The intermediate-angular-momentum accretion flow has the highest density, pressure, and temperature near the black hole, making it easier to observe. We find that the density and pressure have power-law scalings ρ ∝ r n−3/2 and p g ∝ r n−5/2, which only hold for very-low-angular-momentum cases. With the increase in flow angular momentum, it develops a nonaxisymmetric nature. In this case, simple self-similarity does not hold. We also find that the sonic surface moves away from the innermost stable circular orbit as the angular momentum decreases. Finally, we emphasize that an intermediate-angular-momentum flow could provide a possible solution to explaining the complex observation features of the supermassive black hole Sgr A* at our galactic center.

Large-scale magnetic fields are relevant for a number of dynamical processes in accretion disks, including driving turbulence, reconnection events, and launching outflows. Numerical simulations have indicated that the initial strengths and configurations of the large-scale magnetic fields have a direct imprint on the outcome of an accretion disk evolution. To facilitate future self-consistent simulations that include intrinsic dynamo processes, we derive and implement a subgrid model of a helical large-scale dynamo with dynamical quenching in general-relativistic resistive magnetohydrodynamical simulations of geometrically thin accretion disks. By incorporating previous numerical and analytical results of helical dynamos, our model features only one input parameter, the viscosity parameter αSS. We demonstrate that our model can reproduce butterfly diagrams seen in previous local and global simulations. With a rather aggressive parameter choice of αSS = 0.02 and a black hole spin aBH = 0.9375, our thin-disk model launches weak collimated polar outflows with a Lorentz factor ≃1.2, but no polar outflow is present with less vigorous turbulence or less positive aBH. With negative aBH, we find the field configurations to appear more similar to Newtonian cases, whereas for positive aBH, the poloidal field loops become distorted and the cycle period becomes sporadic or even disappears. Moreover, we demonstrate how αSS can avoid being prescribed and instead be determined by the local plasma beta. Such a fully dynamical subgrid dynamo allows for self-consistent amplification of the large-scale magnetic fields.

Quasiperiodic oscillations (QPOs) are very common in black hole accretion systems that are seen from the modulations in luminosity. Many supermassive black hole sources (e.g., RE J1034+396, 1H 0707-495, MCG-6-30-15, 1ES 1927+654, Sgr A*) have been observed to exhibit QPO-like variability in the range of mHz in different energy bands (e.g., radio, near-IR, X-rays). Due to the shorter infalling time, low-angular-momentum accretion flows can have resonance close to the black hole, which will raise variability cHz or beyond QPOs for supermassive black holes. In this study, for the first time, we show that such resonance conditions can be achieved in simulations of low-angular-momentum accretion flows onto a black hole. The QPOs could have values beyond νQPO ≳ 0.1–1 × 107M⊙/MBH cHz and the harmonics have a ratio of 2:1. Hunting these cHz QPOs down will provide a smoking gun signature for the presence of low-angular-momentum accretion flows around black holes (e.g., Sgr A*, 1ES 1927+654).

Intestinal stem cells (ISCs) of the fruit fly, Drosophila melanogaster , offer an excellent genetic model to explore homeostatic roles of ISCs in animal physiology. Among available genetic tools, the escargot ( esg ) -GAL4 driver, expressing the yeast transcription factor gene, GAL4 , under control of the esg gene promoter, has contributed significantly to ISC studies. This driver facilitates activation of genes of interest in proximity to a GAL4-binding element, Upstream Activating Sequence, in ISCs and progenitor enteroblasts (EBs). While esg-GAL4 has been considered an ISC/EB-specific driver, recent studies have shown that esg-GAL4 is also active in other tissues, such as neurons and ovaries. Therefore, the ISC/EB specificity of esg-GAL4 is questionable. In this study, we reveal esg-GAL4 expression in the corpus allatum (CA), responsible for juvenile hormone (JH) production. When driving the oncogenic gene, Ras V12 , esg-GAL4 induces overgrowth in ISCs/EBs as reported, but also increases CA cell number and size. Consistent with this observation, animals alter expression of JH-response genes. Our data show that esg-GAL4 -driven gene manipulation can systemically influence JH-mediated animal physiology, arguing for cautious use of esg-GAL4 as a “specific” ISC/EB driver to examine ISC/EB-mediated animal physiology.

Can a naked singularity (NkS) be distinguished from a black hole (BH)? We have investigated it with cutting-edge general relativistic magnetohydrodynamic simulations, followed by general relativistic radiation transfer calculation for magnetized accretion flow around NkS and BHs. Based on our simulations, the accreting matter close enough to the singularity repels due to effective potential. This prevents matter from reaching an NkS and forms a quasi-spherical symmetric density distribution around it, unlike the accretion flows around a BH. We observe 1 order of magnitude higher mass flux through the jet and much stronger wind from an NkS than a BH. We found that the jet launching mechanism in an NkS differs significantly from that in a BH. In the horizon-scale images, an NkS shows a photon arc instead of a photon ring that is shown around a BH. In summary, the flow dynamics and radiative properties around an NkS are distinctly different from a BH. These properties would be useful to either confirm or rule out such exotic compact objects through future observations.

Accretion physics has become more important recently due to the detection of the first horizon-scale images of the supermassive black holes of M 87* and Sgr A* by the Event Horizon Telescope. General relativistic magnetohydrodynamic (GRMHD) simulations of magnetized accretion flows onto a Kerr black hole have been used to interpret them. However, further testing the theory of gravity by using horizon-scale images requires performing consistent GRMHD simulations in non-Kerr spacetime. In this paper, we revisited the hydrodynamical equilibrium solution of the Fishbone and Moncrief (FM) torus that can be used to study any stationary, axisymmetric, vacuum, or nonvacuum spacetime. Further, we check the stability of the FM torus in non-Kerr spacetime by general relativistic hydrodynamic simulations. We find that FM torus in non-Kerr spacetime is indeed stable under long-term evolution. We conclude that the generalized FM torus solution would be very useful for creating new GRMHD libraries in extended Kerr black holes.

Theoretical models have long predicted the existence of shocks in multitransonic accretion flows onto a black hole, yet their fate under general relativistic simulations has not been fully tested. In this study, we present results from high-resolution two-dimensional general relativistic hydrodynamic and general relativistic magnetohydrodynamic simulations of low angular momentum accretion flows onto Kerr black holes, focusing on the formation of shocks in transonic accretion flow. We demonstrate that for specific combinations of energy and angular momentum, global shock solutions naturally emerge between multiple sonic points. These shocks are sustained in both corotating and counterrotating cases, and their locations depend on specific energy, angular momentum, and the spin of the black hole, which is in good agreement with analytical solutions. In magnetized flows, weak magnetic fields preserve the shock structure, whereas strong fields suppress it, enhancing turbulence and driving powerful, magnetically dominated jets/outflows. The strength and structure of the outflow also depend on a black hole spin and magnetization, with higher black hole spin parameters leading to faster jets. Shock solutions are found only in super-Alfvénic regions, where kinetic forces dominate. Our findings provide important insights into the physics of hot corona formation and jet launching in low angular momentum accretion systems such as Sgr A* (weak jet/outflow) and X-ray binaries.