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Many active galaxies harbor powerful relativistic jets, however, the detailed mechanisms of their formation and acceleration remain poorly understood. To investigate the area of jet acceleration and collimation with the highest available angular resolution, we study the innermost region of the bipolar jet in the nearby low-ionization nuclear emission-line region (LINER) galaxy NGC 1052. We combined observations of NGC 1052 taken with VLBA, GMVA, and EHT over one week in the spring of 2017. For the first time, NGC 1052 was detected with the EHT, providing a size of the central region in-between both jet bases of 250 RS (Schwarzschild radii) perpendicular to the jet axes. This size estimate supports previous studies of the jets expansion profile which suggest two breaks of the profile at around 300 RS and 10000 RS distances to the core. Furthermore, we estimated the magnetic field to be 1.25 Gauss at a distance of 22 {\mu}as from the central engine by fitting a synchrotron-self absorption spectrum to the innermost emission feature, which shows a spectral turn-over at about 130 GHz. Assuming a purely poloidal magnetic field, this implies an upper limit on the magnetic field strength at the event horizon of 26000 Gauss, which is consistent with previous measurements. The complex, low-brightness, double-sided jet structure in NGC 1052 makes it a challenge to detect the source at millimeter (mm) wavelengths. However, our first EHT observations have demonstrated that detection is possible up to at least 230 GHz. This study offers a glimpse through the dense surrounding torus and into the innermost central region, where the jets are formed. This has enabled us to finally resolve this region and provide improved constraints on its expansion and magnetic field strength., Comment: 22 pages, 10 figures, published in A&A

The 2017 observing campaign of the Event Horizon Telescope (EHT) delivered the first very long baseline interferometry (VLBI) images at the observing frequency of 230 GHz, leading to a number of unique studies on black holes and relativistic jets from active galactic nuclei (AGN). In total, eighteen sources were observed: the main science targets, Sgr A* and M87 along with various calibrators. We investigated the morphology of the sixteen AGN in the EHT 2017 data set, focusing on the properties of the VLBI cores: size, flux density, and brightness temperature. We studied their dependence on the observing frequency in order to compare it with the Blandford-K\"onigl (BK) jet model. We modeled the source structure of seven AGN in the EHT 2017 data set using linearly polarized circular Gaussian components and collected results for the other nine AGN from dedicated EHT publications, complemented by lower frequency data in the 2-86 GHz range. Then, we studied the dependences of the VLBI core flux density, size, and brightness temperature on the frequency measured in the AGN host frame. We compared the observations with the BK jet model and estimated the magnetic field strength dependence on the distance from the central black hole. Our results indicate a deviation from the standard BK model, particularly in the decrease of the brightness temperature with the observing frequency. Either bulk acceleration of the jet material, energy transfer from the magnetic field to the particles, or both are required to explain the observations.

Using the Event Horizon Telescope (EHT), the gravitationally lensed rings around the supermassive black holes (SMBHs) in Messier 87 (M87) and Sagittarius A* (Sgr A*) have now been successfully imaged at a resolution under 10 gravitational radii (R$_{\rm g}$ $ = \rm{GM/c^2}$). To expand studies beyond M87 and Sgr A*, we have constructed the Event Horizon and Environs (ETHER) sample, a comprehensive database encompassing approximately 3.15 million SMBH mass estimates, $\sim$ 20,000 Very-Long Baseline Interferometry (VLBI) radio flux densities, and $\sim$ 36,000 hard X-ray flux densities. This database is designed to identify and optimize target selection for the EHT and its upgrades on the ground and in space. We have identified a Gold Sample (GS) of nearby low-luminosity Active Galactic Nuclei (AGNs) within it that are ideal for studying jet bases and potentially imaging black hole shadows. We observed 27 of these AGNs using the EHT from 2022 to 2024, providing an opportunity to resolve and image accretion flows and jets at resolutions of $\leq$ 100 R$_{\rm g}$. Only a few SMBHs have sufficiently high enough flux density to be imaged at scales of $\leq$ 50 R$_{\rm g}$ with the present EHT. Among these are M87, Sgr A*, NGC4594 (Sombrero/M104), NGC4261, and NGC4374 (Messier 84/M84). Of these, NGC4261, Sombrero, and M84 have been observed and/or are scheduled for deep imaging with EHT+ALMA from 2023 to 2025. Sombrero, NGC4261, M84, NGC4278, and NGC5232 are clearly detected in our EHT+ALMA observations in 2022, indicating that the 230 GHz flux density from the accretion flows is significantly high. Ongoing imaging of the ETHER GS will enable measurements of black hole mass and spin, help constrain General Relativity, and enrich our understanding of jet launching and accretion inflows across a broad multi-parameter space, including black hole mass, spin, accretion rate, and orientation., Comment: 9 pages, 6 figures, 1 table, published in Proceedings of the 16th EVN Symposium, Ed. E. Ros, P. Benke, S.A. Dzib, I. Rottmann, & J.A. Zensus, Bonn: Max-Planck-Institut f\"ur Radioastronomie, 2024, pages 75-84, https://cloud.mpifr-bonn.mpg.de/index.php/s/BkX2CC2Xjn2aKR4

The first very long baseline interferometry (VLBI) detections at 870$\mu$m wavelength (345$\,$GHz frequency) are reported, achieving the highest diffraction-limited angular resolution yet obtained from the surface of the Earth, and the highest-frequency example of the VLBI technique to date. These include strong detections for multiple sources observed on inter-continental baselines between telescopes in Chile, Hawaii, and Spain, obtained during observations in October 2018. The longest-baseline detections approach 11$\,$G$\lambda$ corresponding to an angular resolution, or fringe spacing, of 19$\mu$as. The Allan deviation of the visibility phase at 870$\mu$m is comparable to that at 1.3$\,$mm on the relevant integration time scales between 2 and 100$\,$s. The detections confirm that the sensitivity and signal chain stability of stations in the Event Horizon Telescope (EHT) array are suitable for VLBI observations at 870$\mu$m. Operation at this short wavelength, combined with anticipated enhancements of the EHT, will lead to a unique high angular resolution instrument for black hole studies, capable of resolving the event horizons of supermassive black holes in both space and time., Comment: Corresponding author: S. Doeleman

Optically thick non-thermal synchrotron sources notably produce linear polarization vectors being parallel to projected magnetic field lines on the observer's screen, although they are perpendicular in well-known optically thin cases. To elucidate the complex relationship between the vectors and fields and to investigate the energy and spatial distribution of non-thermal electrons through the images, we perform polarization radiative transfer calculations at submillimeter wavelengths. Here the calculations are based on semi-analytic force-free jet models with non-thermal electrons with a power-law energy distribution. In calculated images, we find a $90^\circ$-flip of linear polarization (LP) vectors at the base of counter-side (receding) jet near a black hole, which occurs because of large optical depths for synchrotron self-absorption effect. The $90^\circ$-flip of LP vectors is also seen on the photon ring at a high frequency, since the optical depth along the rays is large there due to the light bending effect. In addition, we see the flip of the sign of circular polarization (CP) components on the counter jet and photon ring. Furthermore, we show that these polarization flips are synthesized with large values in the spectral index map, and also give rise to outstanding features in the Faraday Rotation Measure (RM) map. Since the conditions of flipping depend on the magnetic field strength and configuration and the energy distribution of electrons, we can expect that the polarization flips will provide us with an observational evidence for the presence of non-thermal electrons around the black hole, and a clue to the magnetically driving mechanism of plasma jets., Comment: 22 pages, 16 figures. Accepted for publication in PASJ

We study magnetic field strengths along the jet in NGC~315. First, we estimated the angular velocity of rotation in the jet magnetosphere by comparing the measured velocity profile of NGC~315 with the magneto-hydrodynamic jet model of proposed by Tomimatsu and Takahashi. Similar to the case of M87, we find that the model can reproduce the logarithmic feature of the velocity profile and suggest a slowly rotating black hole magnetosphere for NGC~315. By substituting the estimated $\Omega_{F}$ into the jet power predicted by the Blandford-Znajek mechanism, we estimate the magnetic field strength near the event horizon of the central black hole as $5\times 10^{3}~{\rm G}\lesssim B_{H}\lesssim 2\times 10^{4}~{\rm G}$. We then estimate magnetic-field strengths along the jet by comparing the spectral index distribution obtained from VLBI observations with a synchrotron-emitting jet model. Then we constrain the magnetic field strength at a de-projected distance $z$ from the black hole to be in the range $0.06~{\rm G}\lesssim B(z)\lesssim 0.9~{\rm G}$ for $5.2 \times 10^{3}~r_{g}\lesssim z \lesssim 4.9 \times 10^{4}~r_{g}$, where $r_{g}$ represents the gravitational radius. By combining the obtained field strengths at the event horizon and the downstream section of the jet, we find that the accretion flow at the jet base is consistent with a magnetically arrested disk (MAD). We discuss a comparison of the jet power and the magnetic flux anchored to the event horizon in NGC~315 and M87., Comment: Accepted for publication in The Astrophysical Journal

We perform axisymmetric two-dimensional radiation-hydrodynamic simulations of super-Eddington accretion flow and outflow around black holes to examine the properties of radiation and outflow as functions of the black hole mass and the accretion rate onto the black hole ($\dot M_{\rm BH}$). We find that the $\dot{m}_{\rm BH} (\equiv \dot{M}_{\rm BH}c^2 /L_{\rm Edd})$ dependence of $L_{\rm rad}/L_{\rm Edd}$ and $L_{\rm mech}/L_{\rm Edd}$ found for stellar-mass black hole can apply to the high mass cases, where $L_{\rm rad}$ is the radiation luminosity, $L_{\rm mech}$ is the mechanical luminosity, $c$ is the speed of light, and $L_{\rm Edd}$ is the Eddington luminosity. Such universalities can appear in the regime, in which electron scattering opacity dominates over absorption opacity. Further, the normalized isotropic mechanical luminosity $L_{\rm mech}^{\rm ISO}/L_{\rm Edd}$ (evaluated by normalized density and velocity at $\theta=10^\circ$) exhibits a broken power-law relationship with ${\dot m}_{\rm BH}$; $L_{\rm mech}^{\rm ISO}/ L_{\rm Edd} \propto{\dot m}_{\rm BH}^{2.7}$ (or $\propto {\dot m}_{\rm BH}^{0.7}$) below (above) ${\dot m}_{\rm BH}\sim 400$. This is because the radial velocity stays nearly constant (or even decreases) below (above) the break with increase of $\dot m_{\rm BH}$. We also find that the luminosity ratio is $L_{\rm mech}/L_{\rm rad}^{\rm ISO} \sim$ 0.05 at ${\dot m}_{\rm BH} \sim 100$, which is roughly consistent with the observations of NLS1, 1H 0323+103., Comment: 14 pages, 8 figures, 3 tables, accepted for publication in PASJ. arXiv admin note: text overlap with arXiv:2209.01427

Strong soft X-ray emission called soft X-ray excess is often observed in luminous active galactic nuclei (AGN). It has been suggested that the soft X-rays are emitted from a warm ($T=10^6\sim10^7\ \rm{K}$) region that is optically thick for the Thomson scattering (warm Comptonization region). Motivated by the recent observations that soft X-ray excess appears in changing look AGN (CLAGN) during the state transition from a dim state without broad emission lines to a bright state with broad emission lines, we performed global three-dimensional radiation magnetohydrodynamic simulations assuming that the mass accretion rate increases and becomes around $10$\% of the Eddington accretion rate. The simulation successfully reproduces a warm, Thomson-thick region outside the hot radiatively inefficient accretion flow near the black hole. The warm region is formed by efficient radiative cooling due to inverse Compton scattering. The calculated luminosity $0.01L_{{\rm Edd}}-0.08L_{{\rm Edd}}$ is consistent with the luminosity of CLAGN. We also found that the warm Comptonization region is well described by the steady model of magnetized disks supported by azimuthal magnetic fields. When the anti-parallel azimuthal magnetic fields supporting the radiatively cooled region reconnect around the equatorial plane of the disk, the temperature of the region becomes higher by releasing the magnetic energy transported to the region., Comment: Published in ApJ

Black Hole Explorer (BHEX) is a space VLBI mission concept, which can probe the black hole spacetime and the plasma properties including the magnetic fields of the accretion flows and relativistic jets. We propose science cases anticipated to be addressed by BHEX mainly via the imaging of the target objects, whose observational features appear in several microarcsecond scale. An appearance of a crescent-shaped shadow in a bright state of the M87 will be able to constrain the magnitude of the black hole spin. A possible appearance of the plasma injection region in the vicinity of the black hole results in the formation of the multiple ring structure and may enable us to understand the jet formation processes. In addition, The separation of linear and circular polarization fluxes and reversal of circular polarization will constrain the magnetic field structure and the thermal properties of the electrons, respectively. Other topics including the test of the gravitational theory are also being discussed., Comment: 14 pages, 4figures, Proceedings of SPIE 2024

The Black Hole Explorer (BHEX) is a next-generation space very long baseline interferometry (VLBI) mission concept that will extend the ground-based millimeter/submillimeter arrays into space. The mission, closely aligned with the science priorities of the Japanese VLBI community, involves an active engagement of this community in the development of the mission, resulting in the formation of the Black Hole Explorer Japan Consortium. Here we present the current Japanese vision for the mission, ranging from scientific objectives to instrumentation. The Consortium anticipates a wide range of scientific investigations, from diverse black hole physics and astrophysics studied through the primary VLBI mode, to the molecular universe explored via a potential single-dish observation mode in the previously unexplored 50-70\,GHz band that would make BHEX the highest-sensitivity explorer ever of molecular oxygen. A potential major contribution for the onboard instrument involves supplying essential elements for its high-sensitivity dual-band receiving system, which includes a broadband 300\,GHz SIS mixer and a space-certified multi-stage 4.5K cryocooler akin to those used in the Hitomi and XRISM satellites by the Japan Aerospace Exploration Agency. Additionally, the Consortium explores enhancing and supporting BHEX operations through the use of millimeter/submillimeter facilities developed by the National Astronomical Observatory of Japan, coupled with a network of laser communication stations operated by the National Institute of Information and Communication Technology., Comment: Submitted to SPIE Conference Series, 17 pages for the main text, 33 pages for the references, 4 figures, SPIE Astronomical Telescopes + Instrumentation 2024

By performing general relativistic radiative transfer calculations, we show the radio images of relativistic jets including highly magnetized regions inside jet funnels, based on steady, axisymmetric, and semi-analytic general relativistic magnetohydrodynamics models. It is found that multiple ring images appear at the photon frequency of 230 GHz for nearly pole-on observers, because of the strong light bending effect on photons generated at the separation surfaces which is the boundary between the inflow and outflow flows in the jet funnel. A bright teardrop-shaped component, which extends from the bright rings of the separation surface, also appears in the counter jet region. The diameter of the brightest outermost ring originated from the counter jet is $\sim 60 ~ \mu {\rm as}$, which is consistent with the ring-like images of M87 at 86 GHz observed with GMVA, ALMA and GLT, whose ring-diameter is $\sim 64^{+4}_{-8} ~\mu {\rm as}$. The thinner and smaller-diameter rings are exhibited when the black-hole spin magnitude is higher. These morphological features are expected to appear without being prominently affected by the detailed MHD-plasma parameters of our GRMHD jet model, since the location of the separation surfaces is mainly regulated by the black hole spin. Our GRMHD model and the emission features of the images in the horizon-scale, highly magnetized jet funnel may be tested by future observations, e.g., the next-generation Event Horizon Telescope and the Black Hole Explorer., Comment: 14 pages, 7 figures, , accepted for publication in ApJ

The nearby radio galaxy M87 offers a unique opportunity to explore the connections between the central supermassive black hole and relativistic jets. Previous studies of the inner region of M87 revealed a wide opening angle for the jet originating near the black hole. The Event Horizon Telescope resolved the central radio source and found an asymmetric ring structure consistent with expectations from General Relativity. With a baseline of 17 years of observations, there was a shift in the jet's transverse position, possibly arising from an eight to ten-year quasi-periodicity. However, the origin of this sideways shift remains unclear. Here we report an analysis of radio observations over 22 years that suggests a period of about 11 years in the position angle variation of the jet. We infer that we are seeing a spinning black hole that induces the Lense-Thirring precession of a misaligned accretion disk. Similar jet precession may commonly occur in other active galactic nuclei but has been challenging to detect owing to the small magnitude and long period of the variation., Comment: 41 pages, 7 figures, 7 tables

A phase-resolved analysis on the X-ray spectrum of Ultra-Luminous X-ray Pulsar (ULXP) NGC 300 ULX-1 is performed with data taken with XMM-Newton and NuSTAR on 2016 December 16th. In addition to the classical phase-restricting analysis, a method developed in active galactic nuclei studies is newly employed for ULXP. It has revealed that the pulsation cycle of the source can be divided into two intervals in terms of X-ray variability. This suggests the rotating flow consists of at least two representative emission regions. Furthermore, the new method successfully decomposed the spectrum into an independent pair in each interval. One is an unchanging-component spectrum that can be reproduced by a standard disk model with a $720^{+220}_{-120}$ km inner radius and a $0.25\pm0.03$ keV peak temperature. The other is the spectrum of the component that coincides with the pulsation. This was explained with a Comptonization of a $0.22^{+0.2}_{-0.1}$ keV blackbody and exhibited a harder photon index in the brighter phase interval of two. The results are consistent with a picture that the pulsating emission originates from a funnel-like flow formed within the magnetosphere, and the inner flow exhibiting a harder continuum is observed exclusively when the opening cone points to the observer., Comment: 27 pages, 15 figures, accepted for publication in ApJ

The Event Horizon Telescope (EHT) observed in 2017 the supermassive black hole at the center of the Milky Way, Sagittarius A* (Sgr A*), at a frequency of 228.1 GHz ($\lambda$=1.3 mm). The fundamental physics tests that even a single pulsar orbiting Sgr A* would enable motivate searching for pulsars in EHT datasets. The high observing frequency means that pulsars - which typically exhibit steep emission spectra - are expected to be very faint. However, it also negates pulse scattering, an effect that could hinder pulsar detections in the Galactic Center. Additionally, magnetars or a secondary inverse Compton emission could be stronger at millimeter wavelengths than at lower frequencies. We present a search for pulsars close to Sgr A* using the data from the three most-sensitive stations in the EHT 2017 campaign: the Atacama Large Millimeter/submillimeter Array, the Large Millimeter Telescope and the IRAM 30 m Telescope. We apply three detection methods based on Fourier-domain analysis, the Fast-Folding-Algorithm and single pulse search targeting both pulsars and burst-like transient emission; using the simultaneity of the observations to confirm potential candidates. No new pulsars or significant bursts were found. Being the first pulsar search ever carried out at such high radio frequencies, we detail our analysis methods and give a detailed estimation of the sensitivity of the search. We conclude that the EHT 2017 observations are only sensitive to a small fraction ($\lesssim$2.2%) of the pulsars that may exist close to Sgr A*, motivating further searches for fainter pulsars in the region., Comment: 33 pages, 7 figures, 6 Tables. Accepted for publication in ApJ

To demonstrate the magnetic energy dissipation via relativistic shocks, we carry out spherically symmetrical one-dimensional special relativistic magneto-hydrodynamic simulations of highly magnetised outflows with an adaptive mesh refinement method. We first investigate the detail of the dynamical energy dissipation via interaction between a single ejecta and an external medium. The energy dissipation timescales, which affect the early behaviour of the afterglow emission in gamma-ray bursts, are estimated for a wide range of magnetisation. In addition, we demonstrate the internal shock dissipation in multiple interactions between magnetically dominated relativistic ejecta and kinetically dominated non-relativistic winds. Our numerical results show that almost 10% of the magnetic energy in the ejecta can be converted into the thermal energy of the relativistic and low-magnetised outflows via shocks in the rarefaction waves or the winds. Such hot and less magnetised outflows are relevant for observed non-thermal emissions in blazars or gamma-ray bursts., Comment: 12 pages, 14 figures, Animations are available from here https://www.youtube.com/playlist?list=PLgnUM4yGp9oLxqXlcKcc8ftJne9kTpHEs

The nearby radio galaxy M87 is a prime target for studying black hole accretion and jet formation^{1,2}. Event Horizon Telescope observations of M87 in 2017, at a wavelength of 1.3 mm, revealed a ring-like structure, which was interpreted as gravitationally lensed emission around a central black hole^3. Here we report images of M87 obtained in 2018, at a wavelength of 3.5 mm, showing that the compact radio core is spatially resolved. High-resolution imaging shows a ring-like structure of 8.4_{-1.1}^{+0.5} Schwarzschild radii in diameter, approximately 50% larger than that seen at 1.3 mm. The outer edge at 3.5 mm is also larger than that at 1.3 mm. This larger and thicker ring indicates a substantial contribution from the accretion flow with absorption effects in addition to the gravitationally lensed ring-like emission. The images show that the edge-brightened jet connects to the accretion flow of the black hole. Close to the black hole, the emission profile of the jet-launching region is wider than the expected profile of a black-hole-driven jet, suggesting the possible presence of a wind associated with the accretion flow., Comment: 50 pages, 18 figures, 3 tables, author's version of the paper published in Nature

Interpretation of resolved polarized images of black holes by the Event Horizon Telescope (EHT) requires predictions of the polarized emission observable by an Earth-based instrument for a particular model of the black hole accretion system. Such predictions are generated by general relativistic radiative transfer (GRRT) codes, which integrate the equations of polarized radiative transfer in curved spacetime. A selection of ray-tracing GRRT codes used within the EHT collaboration is evaluated for accuracy and consistency in producing a selection of test images, demonstrating that the various methods and implementations of radiative transfer calculations are highly consistent. When imaging an analytic accretion model, we find that all codes produce images similar within a pixel-wise normalized mean squared error (NMSE) of 0.012 in the worst case. When imaging a snapshot from a cell-based magnetohydrodynamic simulation, we find all test images to be similar within NMSEs of 0.02, 0.04, 0.04, and 0.12 in Stokes I, Q, U , and V respectively. We additionally find the values of several image metrics relevant to published EHT results to be in agreement to much better precision than measurement uncertainties., Comment: Accepted for publication in ApJ

In this work, we studied the Galactic Center supermassive black hole (SMBH), Sagittarius A* (Sgr A*), with the KVN and VERA Array (KaVA)/East Asian VLBI Network (EAVN) monitoring observations. Especially on 13 May 2019, Sgr A* experienced an unprecedented bright near infra-red (NIR) flare; so, we find a possible counterpart at 43 GHz (7 mm). As a result, a large temporal variation of the flux density at the level 15.4%, with the highest flux density of 2.04 Jy, is found on 11 May 2019. Interestingly, the intrinsic sizes are also variable, and the area and major-axis size show a marginal correlation with flux density with >2{\sigma}. Thus, we interpret that the emission region at 43 GHz follows the larger-when-brighter relation in 2019. The possible origins are discussed with an emergence of a weak jet/outflow component and the position angle change of the rotation axis of the accretion disk in time., Comment: 15 pages, 7 figures, 3 tables, Published in the Galaxies Special Issue "Challenges in Understanding Black Hole Powered Jets with VLBI"

Recent VLBI monitoring has found transverse motions of the M87 jet. However, due to the limited cadence of previous observations, details of the transverse motion have not been fully revealed yet. We have regularly monitored the M87 jet at KVN and VERA Array (KaVA) 22 GHz from December 2013 to June 2016. The average time interval of the observation is ~ 0.1 year, which is suitable for tracking short-term structural changes. From these observations, the M87 jet is well represented by double ridge lines in the region 2 - 12 mas from the core. We found that the ridge lines exhibit transverse oscillations in all observed regions with an average period of $0.94\pm0.12$ years. When the sinusoidal fit is performed, we found that the amplitude of this oscillation is an order of $\sim0.1$ mas, and the oscillations in the northern and southern limbs are almost in phase. Considering the amplitude, it does not originate from Earth's parallax. We propose possible scenarios of the transverse oscillation, such as the propagation of jet instabilities or magneto-hydrodynamic (MHD) waves or perturbed mass injection around magnetically dominated accretion flows., Comment: 14 pages, 6 figures. Contribution to the Galaxies Special Issue "Challenges in Understanding Black Hole Powered Jets with VLBI"

Because of its proximity and the large size of its black hole, M87 is one of the best targets for studying the launching mechanism of active galactic nucleus jets. Currently, magnetic fields are considered to be an essential factor in the launching and accelerating of the jet. However, current observational estimates of the magnetic field strength of the M87 jet are limited to the innermost part of the jet or to HST-1. No attempt has yet been made to measure the magnetic field strength in between. We aim to infer the magnetic field strength of the M87 jet out to a distance of several thousand $r_s$ by tracking the distance-dependent changes in the synchrotron spectrum of the jet from high-resolution very long baseline interferometry observations. In order to obtain high-quality spectral index maps, quasi-simultaneous observations at 22 and 43 GHz were conducted using the KVN and VERA Array (KaVA) and the VLBA. We compared the spectral index distributions obtained from the observations with a model and placed limits on the magnetic field strengths as a function of distance. The overall spectral morphology is broadly consistent over the course of these observations. The observed synchrotron spectrum rapidly steepens from $\alpha_{22-43 GHz}$ ~ -0.7 at ~ 2 mas to $\alpha_{22-43 GHz}$ ~ -2.5 at ~ 6 mas. A spectral index model in which nonthermal electron injections inside the jet decrease with distance can adequately reproduce the observed trend. This suggests the magnetic field strength of the jet at a distance of 2 - 10 mas (~ 900 $r_s$ - ~ 4500 $r_s$ in the deprojected distance) has a range of $B=(0.3 - 1.0 G)(z/2 mas)^{-0.73}$. Extrapolating to the EHT scale yields consistent results, suggesting that the majority of the magnetic flux of the jet near the black hole is preserved out to ~ 4500 $r_s$ without significant dissipation., Comment: 16 pages, 9 figures. Accepted for publication in Astronomy & Astrophysics

Here, we demonstrate that polarization properties show a wide diversity depending on viewing angles. To simulate images of a supermassive black hole and surrounding plasma, we performed a full-polarimetric general relativistic radiative transfer based on three-dimensional general relativistic magnetohydrodynamics models with moderate magnetic strengths. Under an assumption of a hot-jet and cold-disk in the electron temperature prescription, we confirmed a typical scenario where polarized synchrotron emissions from the funnel jet experience Faraday rotation and conversion in the equatorial disk. Further, we found that linear polarization vectors are inevitably depolarized for edge-on-like observers, whereas a portion of vectors survive and reach the observers in face-on-like cases. We also found that circular polarization components have persistent signs in the face-on cases, and changing signs in the edge-on cases. It is confirmed that these features are smoothly connected via intermediate viewing-angle cases. These results are due to Faraday rotation/conversion for different viewing angles, and suggest that a combination of linear and circular polarimetry can give a constraint on the inclination between the observer and black hole's (and/or disk's) rotating-axis and plasma properties in the jet--disk structure. These can also lead to a more statistical and unified interpretation for a diversity of emissions from active galactic nuclei., Comment: 11 pages, 6 figures. Contribution to the Galaxies Special Issue "Challenges in Understanding Black Hole Powered Jets with VLBI". Movie available at https://youtu.be/065qAx6Tff0

Motivated by the measured velocity profile of the M87 jet using the KVN and VERA Array (KaVA) by Park et al. indicating that the starting position of the jet acceleration is farther from the central engine of the jet than predicted in general relativistic magnetohydrodynamic simulations, we explore how to mitigate the apparent discrepancy between the simulations and the KaVA observation. We use a semi-analytic jet model proposed by Tomimatsu and Takahashi. consistently solving the trans-magnetic field structure but neglecting any dissipation effects. By comparing the jet model with the observed M87 jet velocity profile, we find that the model can reproduce the logarithmic feature of the velocity profile, and can fit the observed data when choosing $c/(100r_{g}) \lesssim \Omega_{F} \lesssim c/(70r_{g})$ where $r_{g}$ is the gravitational radius. While a total specific energy (${\cal E}$) of the jet changes the terminal bulk Lorentz factor of the jet, a slower angular velocity of the black hole magnetosphere (funnel region) ($\Omega_{F}$) makes a light-cylinder radius ($r_{\rm lc}$) larger and it consequently pushes out a location of a starting point of the jet acceleration. Using the estimated $\Omega_{F}$ we further estimate the magnetic field strength on the event horizon scale in M87 by assuming Blandford-Znajek (BZ) process is in action. The corresponding magnetic flux threading the event horizon of M87 is in good agreement with a magnetically arrested disc (MAD) regime., Comment: 27 pages, 10 figures; accepted for publication in ApJ

In order to precisely evaluate the impacts by super-Eddington accretors to their environments, it is essential to assure a large enough simulation box and long computational time to avoid any artefacts from numerical settings as much as possible. In this paper, we carry out axisymmetric two-dimensional radiation hydrodynamic simulations around a $10~M_\odot$ black hole in large simulation boxes and study the large-scale outflow structure and radiation properties of super-Eddington accretion flow for a variety of black hole accretion rates, ${\dot M}_{\rm BH} = (110 - 380) ~L_{\rm Edd}/c^2$. The Keplerian radius of the inflow material, at which centrifugal force balances with gravitational force, is fixed to 2430 Schwarzschild radii.We find that the mechanical luminosity grows more rapidly than the radiation luminosity with an increase of ${\dot M}_{\rm BH}$. When seen from a nearly face-on direction, especially, the isotropic mechanical luminosity grows in proportion to ${\dot M}_{\rm BH}^{2.7}$, while the total mechanical luminosity is proportional to ${\dot M}_{\rm BH}^{1.7}$. The reason for the former is that the higher ${\dot M}_{\rm BH}$ is, the more vertically inflated becomes the disk surface, which makes radiation fields more confined in the region around the rotation axis, thereby strongly accelerating outflowing gas. The outflow is classified into pure outflow and failed outflow, depending whether outflowing gas can reach the outer boundary of the simulation box or not. The fraction of the failed outflow decreases with a decrease of ${\dot M}_{\rm BH}$. We analyze physical quantities along each outflow trajectory, finding that the Bernoulli parameter ($Be$) is not a good indicator to discriminate pure and failed outflows, since it is never constant because of continuous acceleration by radiation-pressure force.Pure outflow can arise, even if $Be < 0$ at the launching point., Comment: 18 pages, 13 figures, 3 tables, accepted for publication in PASJ

It is one of the biggest issues in black hole (BH) astrophysics how to precisely evaluate BH feedback to its environments. Aiming at studying the unique gas dynamics of super-Eddington flow around supermassive black hole (SMBH) seeds at high redshift, we carried out axisymmetric two dimensional radiation hydrodynamic simulations by a nested simulation-box method. Here we divide the simulation box into the inner zone at $(2 - 3 \times 10^3) r_{\rm{Sch}}$ (with $r_{\rm Sch}$ being the Schwarzschild radius) and the outer zone at $(2\times 10^{3} - 3\times 10^6) r_{\rm{Sch}}$, with smooth connection of the physical quantities, such as gas density, velocity, and radiation energy. We start the calculation by injecting mass through the outer boundary of the inner zone at a constant rate of $\dot{M}_{\rm{inj}}=10^3L_{\rm{Edd}}/c^2$, where $L_{\rm{Edd}}$ is the Eddington luminosity and $c$ is the speed of light. A powerful outflow is generated in the innermost region and it propagates from the inner zone to the outer zone. The outflows are characterized by a velocity of 0.02$c$ (0.7$c$) and density of $10^{-17}$ ($10^{-19}$) g cm$^{-3}$ for near the edge-on (face-on) direction. The outflow is gradually accelerated as it travels by accepting radiation-pressure force. The final mass outflow rate at the outermost boundary is $\dot{M}_{\rm{out}}\sim 0.3 \times \dot{M}_{\rm{inj}}$. By extrapolating the outflow structure to a further larger scale, we find that the momentum and energy fluxes at $r \sim 0.1$ pc are $\sim 10-100 L_{\rm{Edd}}/c $ and $\sim 0.1-10 L_{\rm{Edd}}$, respectively. Moreover, we find that the impacts are highly anisotropic in the sense that larger impacts are given towards the face-on direction than in the edge-on direction. These results indicate that the BH feedback will more efficiently work on the interstellar medium than that assumed in the cosmological simulations., Comment: 14 pages, 10 figures, published online in the Publications of the Astronomical Society of Japan

For testing different electron temperature ($T_{\rm e}$) prescriptions in general relativistic magnetohydrodynamics (GRMHD) simulations through observations, we propose to utilize linear polarization (LP) and circular polarization (CP) images. We calculate the polarization images based on a semi-Magnetically Arrested Disk (MAD) GRMHD model for various $T_{\rm e}$ parameters, bearing M87 in mind. We find an LP-CP separation in the images of the low-$T_{\rm e}$ disk cases at 230 GHz; namely, the LP flux mainly originates from downstream of the jet and the CP flux comes from the counter-side jet, while the total intensity is maximum at the jet base. This can be understood as follows: although the LP flux is generated through synchrotron emission widely around the black hole, most of the LP flux from the jet base does not reach the observer, since it undergoes Faraday rotation ($\propto T_{\rm e}^{-2}$) when passing through the outer cold disk and is thus depolarized. Hence, only the LP flux from the downstream (not passing the cold dense plasmas) can survive. Meanwhile, the CP flux is generated from the LP flux by Faraday conversion ($\propto T_{\rm e}$) in the inner hot region. Stronger CP flux is thus observed from the counter-side jet. Moreover, the LP-CP separation is more enhanced at a lower frequency such as 86 GHz but is rather weak at 43 GHz, since the media in the latter case is optically thick for synchrotron self-absorption so that all the fluxes should come from the photosphere. The same is true for cases with higher mass accretion rates and/or larger inclination angles., Comment: 30 pages, 31 figures, accepted for publication in ApJ

We study the kinetic plasma dynamics in collisionless relativistic jets with velocity shear, by carrying out particle-in-cell simulations in the transverse plane of a jet. It is discovered that intermittent magnetic reconnections (MRs) are driven by Mushroom instability (MI), which is an important kinetic-scale plasma instability in the plasma shear-flows with relativistic bulk speed. We refer to this sequence of kinetic plasma phenomena as "MI-driven MR". The MI-driven MRs intermittently occur with moving the location of the reconnection points from the vicinity of the initial velocity-shear surface towards the center of the jet. As a consequence, the number density of high energy electrons, which are accelerated by MI-driven MRs, increases with time in the region inside the initial velocity-shear surface with accompanying the generation and subsequent amplification of magnetic fields by MI. The maximum Lorentz factor of electrons increases with initial bulk Lorentz factor of the jet. A possible relation of MI-driven MR to the bright synchrotron emission in jet-spine of active galactic nucleus jets is also discussed., Comment: 13 pages, 11 figures, accepted for publication in ApJ

Sagittarius A* (Sgr A*), the Galactic Center supermassive black hole (SMBH), is one of the best targets to resolve the innermost region of SMBH with very long baseline interferometry (VLBI). In this study, we have carried out observations toward Sgr A* at 1.349 cm (22.223 GHz) and 6.950 mm (43.135 GHz) with the East Asian VLBI Network, as a part of the multi-wavelength campaign of the Event Horizon Telescope (EHT) in 2017 April. To mitigate scattering effects, the physically motivated scattering kernel model from Psaltis et al. (2018) and the scattering parameters from Johnson et al. (2018) have been applied. As a result, a single, symmetric Gaussian model well describes the intrinsic structure of Sgr A* at both wavelengths. From closure amplitudes, the major-axis sizes are ~704$\pm$102 $\mu$as (axial ratio $\sim$1.19$^{+0.24}_{-0.19}$) and $\sim$300$\pm$25 $\mu$as (axial ratio $\sim$1.28$\pm$0.2) at 1.349 cm and 6.95 mm respectively. Together with a quasi-simultaneous observation at 3.5 mm (86 GHz) by Issaoun et al. (2019), we show that the intrinsic size scales with observing wavelength as a power-law, with an index $\sim$1.2$\pm$0.2. Our results also provide estimates of the size and compact flux density at 1.3 mm, which can be incorporated into the analysis of the EHT observations. In terms of the origin of radio emission, we have compared the intrinsic structures with the accretion flow scenario, especially the radiatively inefficient accretion flow based on the Keplerian shell model. With this, we show that a nonthermal electron population is necessary to reproduce the source sizes., Comment: accepted to ApJ

Very-long-baseline interferometry (VLBI) observations of active galactic nuclei at millimeter wavelengths have the power to reveal the launching and initial collimation region of extragalactic radio jets, down to $10-100$ gravitational radii ($r_g=GM/c^2$) scales in nearby sources. Centaurus A is the closest radio-loud source to Earth. It bridges the gap in mass and accretion rate between the supermassive black holes (SMBHs) in Messier 87 and our galactic center. A large southern declination of $-43^{\circ}$ has however prevented VLBI imaging of Centaurus A below ${\lambda}1$cm thus far. Here, we show the millimeter VLBI image of the source, which we obtained with the Event Horizon Telescope at $228$GHz. Compared to previous observations, we image Centaurus A's jet at a tenfold higher frequency and sixteen times sharper resolution and thereby probe sub-lightday structures. We reveal a highly-collimated, asymmetrically edge-brightened jet as well as the fainter counterjet. We find that Centaurus A's source structure resembles the jet in Messier 87 on ${\sim}500r_g$ scales remarkably well. Furthermore, we identify the location of Centaurus A's SMBH with respect to its resolved jet core at ${\lambda}1.3$mm and conclude that the source's event horizon shadow should be visible at THz frequencies. This location further supports the universal scale invariance of black holes over a wide range of masses., Comment: 27 pages, 9 figures. This is a post-peer-review, pre-copyedit version of an article published in Nature Astronomy. The final authenticated version is available online at: https://doi.org/10.1038/s41550-021-01417-w

The black-hole images obtained with the Event Horizon Telescope (EHT) are expected to be variable at the dynamical timescale near their horizons. For the black hole at the center of the M87 galaxy, this timescale (5-61 days) is comparable to the 6-day extent of the 2017 EHT observations. Closure phases along baseline triangles are robust interferometric observables that are sensitive to the expected structural changes of the images but are free of station-based atmospheric and instrumental errors. We explored the day-to-day variability in closure phase measurements on all six linearly independent non-trivial baseline triangles that can be formed from the 2017 observations. We showed that three triangles exhibit very low day-to-day variability, with a dispersion of $\sim3-5^\circ$. The only triangles that exhibit substantially higher variability ($\sim90-180^\circ$) are the ones with baselines that cross visibility amplitude minima on the $u-v$ plane, as expected from theoretical modeling. We used two sets of General Relativistic magnetohydrodynamic simulations to explore the dependence of the predicted variability on various black-hole and accretion-flow parameters. We found that changing the magnetic field configuration, electron temperature model, or black-hole spin has a marginal effect on the model consistency with the observed level of variability. On the other hand, the most discriminating image characteristic of models is the fractional width of the bright ring of emission. Models that best reproduce the observed small level of variability are characterized by thin ring-like images with structures dominated by gravitational lensing effects and thus least affected by turbulence in the accreting plasmas., Comment: Accepted for Publication in ApJ

We present a general relativistic, ray-tracing radiative transfer code RAIKOU for multi-wavlength studies of spectra and images including the black hole shadows around Kerr black holes. Important radiative processes in hot plasmas around black holes, i.e., (cyclo-)synchrotron, bremsstrahlung emission/absorption and Compton/inverse-Compton scattering, are incorporated. The Maxwell-J\"uttner and single/broken power-law electron distribution functions are implemented to calculate the radiative transfer via both of the thermal and the nonthermal electrons. Two calculation algorithms are implemented for studies of both the images and broadband spectra. An observer-to-emitter algorithm, which inversely solve the radiative transfer equation from the observer screen to emitting plasmas, is suitable for efficient calculations of the images, e.g., the black hole shadows, and spectra without the Compton effects. On the other hand, an emitter-to-observer algorithm, by which photons are transported with a Monte-Carlo method including the effects of Compton/inverse-Compton scatterings, enables us to compute multi-wavelength spectra with their energy bands broadly ranging from radio to very-high-energy gamma-ray. The code is generally applicable to accretion flows around Kerr black holes with relativistic jets and winds/coronae with various mass accretion rate (i.e., radiatively inefficient accretion flows, super-Eddington accretion flows, and others). We demonstrate an application of the code to a radiatively innefficent accretion flow onto a supermassive black hole., Comment: 26 pages, 12 figures, submitted to ApJ

Our understanding of strong gravity near supermassive compact objects has recently improved thanks to the measurements made by the Event Horizon Telescope (EHT). We use here the M87* shadow size to infer constraints on the physical charges of a large variety of nonrotating or rotating black holes. For example, we show that the quality of the measurements is already sufficient to rule out that M87* is a highly charged dilaton black hole. Similarly, when considering black holes with two physical and independent charges, we are able to exclude considerable regions of the space of parameters for the doubly-charged dilaton and the Sen black holes., Comment: 15 pages, 4 figures, published in PRD on May 19

Synchrotron radiation from hot gas near a black hole results in a polarized image. The image polarization is determined by effects including the orientation of the magnetic field in the emitting region, relativistic motion of the gas, strong gravitational lensing by the black hole, and parallel transport in the curved spacetime. We explore these effects using a simple model of an axisymmetric, equatorial accretion disk around a Schwarzschild black hole. By using an approximate expression for the null geodesics derived by Beloborodov (2002) and conservation of the Walker-Penrose constant, we provide analytic estimates for the image polarization. We test this model using currently favored general relativistic magnetohydrodynamic simulations of M87*, using ring parameters given by the simulations. For a subset of these with modest Faraday effects, we show that the ring model broadly reproduces the polarimetric image morphology. Our model also predicts the polarization evolution for compact flaring regions, such as those observed from Sgr A* with GRAVITY. With suitably chosen parameters, our simple model can reproduce the EVPA pattern and relative polarized intensity in Event Horizon Telescope images of M87*. Under the physically motivated assumption that the magnetic field trails the fluid velocity, this comparison is consistent with the clockwise rotation inferred from total intensity images., Comment: 29 pages, 11 figures, published in ApJ on May 3

It is widely believed that super-Eddington accretion flow can produce powerful outflow, but where it originates from and how much mass and energy are carried away to which directions? To answer to these questions, we newly perform a large-box, two-dimensional radiation hydrodynamic simulation, paying special attention lest the results should depend on adopted initial and boundary conditions. We could achieve a quasi-steady state in an unprecedentedly large range, $r=2~r_{\rm S}$-$600~r_{\rm S}$ (with $r_{\rm S}$ being the Schwarzschild radius) from the black hole. The accretion rate onto the central $10 ~M_{\odot}$ black hole is $\dot{M}_{\rm BH} \sim 180 ~L_{\rm Edd}/c^{2}$, whereas the mass outflow rate is ${\dot M}_{\rm outflow} \sim 24 ~L_{\rm Edd}/c^2$ (where $L_{\rm Edd}$ and $c$ are the Eddington luminosity and the speed of light, respectively). The ratio (${\dot M}_{\rm outflow}/{\dot M}_{\rm BH} \sim 0.14$) is much less than those reported previously. By careful inspection we find that most of outflowing gas which reach the outer boundary originates from the region at $R\lesssim140~r_{\rm S}$, while gas at $140~r_{\rm S}$-$230 ~r_{\rm S}$ forms failed outflow. Therefore, significant outflow occurs inside the trapping radius $\sim 450 ~r_{\rm S}$. The mechanical energy flux (or mass flux) reaches its maximum in the direction of $\sim 15^\circ$ ($\sim 80^\circ$) from the rotation axis. The total mechanical luminosity is $L_{\rm mec}\sim 0.16~L_{\rm Edd}$, while the isotropic X-ray luminosity varies from $L_{\rm X}^{\rm ISO}\sim 2.9~L_{\rm Edd}$, (for a face-on observer) to $\sim 2.1~L_{\rm Edd}$ (for a nearly edge-on observer). The power ratio is $L_{\rm mec}/L_{\rm X}^{\rm ISO}\sim 0.05$-$0.08$, in good agreement with the observations of Ultra-Luminous X-ray sources surrounded by optical nebulae., Comment: 17pages, 15 figures, 3 tables, accepted for publication in PASJ on January 20, 2021

With unprecedented angular resolution, the Event Horizon Telescope (EHT) has opened a new era of black holes. We have previously calculated the expected polarization images of M87* with the EHT observations in mind. There, we demonstrated that circular polarization (CP) images, as well as the linear polarization (LP) maps, can convey quite useful information to us, such as the flow structure and magnetic field configuration around the black hole. In this paper, we make new predictions for the cases in which disk emission dominates over jet emission, bearing Sgr A* in mind. Here we set the proton-to-electron temperature ratio of the disk component to be Tp/Te $\sim$ 2 so as to suppress jet emission relative to emission from accretion flow. As a result, we obtain ring-like images and triple-forked images around the black hole for face-on and edge-on cases, respectively. We also find significant CP components in the images ($\gtrsim 10\%$ in fraction), both with positive and negative signs, amplified through the Faraday conversion, not sensitively depending on the inclination angles. Furthermore, we find a "separatrix" in the CP images, across which the sign of CP is reversed and on which the LP flux is brightest, that can be attributed to the helical magnetic field structure in the disk. These results indicate that future full polarization EHT images are a quite useful tracer of the magnetic field structure. We also discuss to what extent we will be able to extract information regarding the magnetic field configurations, under the scattering in the interstellar plasma, in future EHT polarimetric observations of Sgr A*., Comment: 17 pages, 13 figures, and 1 table. Accepted for publication in PASJ

The 2017 Event Horizon Telescope (EHT) observations of the central source in M87 have led to the first measurement of the size of a black-hole shadow. This observation offers a new and clean gravitational test of the black-hole metric in the strong-field regime. We show analytically that spacetimes that deviate from the Kerr metric but satisfy weak-field tests can lead to large deviations in the predicted black-hole shadows that are inconsistent with even the current EHT measurements. We use numerical calculations of regular, parametric, non-Kerr metrics to identify the common characteristic among these different parametrizations that control the predicted shadow size. We show that the shadow-size measurements place significant constraints on deviation parameters that control the second post-Newtonian and higher orders of each metric and are, therefore, inaccessible to weak-field tests. The new constraints are complementary to those imposed by observations of gravitational waves from stellar-mass sources., Comment: Physical Review Letters

We carry out general relativistic ray-tracing radiative-transfer calculations to study whether a localized emission from $e^{\pm}$ plasma rings created at the stagnation surface in the jet funnel, to which we refer as stagnation rings, can explain the ring image of M87 observed by Event Horizon Telescope (EHT) 2017. The resultant images consist of the direct image of the stagnation rings and the ring images formed via the strong deflection by the black-hole (BH) gravity, to which we refer as "quasi photon-ring". For the model with the BH spin $a_* = 0.99$, the direct image of the counter-jet and quasi photon-ring are almost coincident to the photon ring with diameter $\sim 40 \mu{\rm as}$, while the approaching jet shows the small ring-image inside them. The synthetic observation image assuming the EHT 2017 array is consistent with that observed in M87, because the array is a bit sparse to detect the inner ring image. This indicates that the ring image in M87 might contain the important feature of the jet bases in addition to the photon ring. We find that forthcoming EHT observations can resolve the stagnation-ring image and may enable us to explore the plasma-injection mechanism into the jet funnel., Comment: 8 pages, 3 figures, accepted for publication in ApJ

We calculate the pulsed fraction (PF) of the super-critical column accretion flows onto magnetized neutron stars (NSs), of which the magnetic axis is misaligned with the rotation axis, based on the simulation results by Kawashima et al.(2016, PASJ, 68, 83). Here, we solve the geodesic equation for light in the Schwarzschild spacetime in order to take into account the light bending effect. The gravitational redshift and the relativistic doppler effect from gas motions of the accretion columns are also incorporated. The pulsed emission appears since the observed luminosity, which exceeds the Eddington luminosity for the stellar-mass black holes, periodically changes via precession of the column caused by the rotation of the NS. The PF tends to increase as $\theta_{\rm obs}$ approaching to $\theta_{\rm B}$, where $\theta_{\rm obs}$ and $\theta_{\rm B}$ are the observer's viewing angle and the polar angle of the magnetic axis measured from the rotation axis. The maximum PF is around 50 %. Also, we find that the PF becomes less than 5 % for $\theta_{\rm obs} \lesssim 5^\circ$ or for $\theta_{\rm B} \lesssim 5^\circ$. Our results are consistent with observations of ultraluminous X-ray pulsars (ULXPs) with few exceptions, since the ULXPs mostly exhibit the PF of $\lesssim$ 50 %. Our present study supports the hypothesis that the ULXPs are powered by the super-critical column accretion onto NSs., Comment: 10 pages, 8 figures, accepted for publication in PASJ

The spectacular images of the M87 black hole taken by the Event Horizon Telescope (EHT) have opened a new era of black hole research. One of the next issues is to take polarization images around the central black hole (BH). Since radio emission is produced by synchrotron process, polarization properties should vividly reflect the magnetic field structure at the jet base and thus provide good information regarding the magnetic mechanism of jet formation. With this kept in mind we perform general relativistic (GR) radiative transfer calculations of polarized light based on the GR magnetohydrodynamic (MHD) simulation data of accretion flow and outflow in M87, to obtain their linear and circular polarization images in the horizon-scale. We found that the linear polarization components from the jet base and inner accretion flow should experience Faraday rotation when passing through magnetized plasmas around the BH, thus sensitively depending on the BH spin. Through the comparison with intensity image at 1.3mm by EHT and the polarized degree and the rotation measure (RM) at 1.3mm with the Submillimeter Array, the model with the spin parameter of a=0.9M_BH (with M_BH being the BH mass) is favored over other models with a=0.5M_BH or 0.99M_BH, though we need further systematic studies for confirmation. We also find in low-temperature models clear ring-like image in the circular polarization map, which arises because of Faraday conversion of the linearly polarized synchrotron emission and is thus indicative of magnetic field. This occurs only when the emission region is threaded with well-ordered magnetic fields and hence no clear images are expected in high-temperature disk models, in which disk emission is appreciable. We will be able to elucidate the field configuration through the comparison between the simulated polarization images and future polarimetry with EHT and other VLBI observations., Comment: Final version. 18 pages, 14 figures, 1 table. Accepted by PASJ

High-resolution imaging of supermassive black holes is now possible, with new applications to testing general relativity and horizon-scale accretion and relativistic jet formation processes. Over the coming decade, the EHT will propose to add new strategically placed VLBI elements operating at 1.3mm and 0.87mm wavelength. In parallel, development of next-generation backend instrumentation, coupled with high throughput correlation architectures, will boost sensitivity, allowing the new stations to be of modest collecting area while still improving imaging fidelity and angular resolution. The goal of these efforts is to move from imaging static horizon scale structure to dynamic reconstructions that capture the processes of accretion and jet launching in near real time., Comment: Astro2020 APC White Paper, 10 pages, 7 figures

The Event Horizon Telescope (EHT) recently produced the first horizon-scale image of a supermassive black hole. Expanding the array to include a 3-meter space telescope operating at >200 GHz enables mass measurements of many black holes, movies of black hole accretion flows, and new tests of general relativity that are impossible from the ground.

Imaging the immediate vicinity of supermassive black holes (SMBH) and extracting a BH-spin signature is one of the grand challenges in astrophysics. M87 is known as one of the best targets for imaging the BH shadow and it can be partially thick against synchrotron self-absorption (SSA), particularly in a flaring state with high mass accretion rate. However, little is known about influences of the SSA-thick region on BH shadow images. Here we investigate BH shadow images of M87 at 230 GHz properly taking into account the SSA-thick region. When the BH has a high spin value, the corresponding BH shadow image shows the positional offset between the center of the photon ring and that of the SSA-thick ring at the innermost stable circular orbit (ISCO) due to the frame-dragging effect in the Kerr spacetime. As a result, we find that a dark-crescent structure is generally produced between the photon ring and the SSA-thick ISCO ring in the BH shadow image. The scale size of the dark-crescent increase with BH spin: its width reaches up to $\sim 2$ gravitational radius when the BH spin is 99.8% of its maximum value. The dark crescent is regarded as a new signature of a highly spinning BH. This feature is expected to appear in flaring states with relatively high mass accretion rate rather than the quiescent states. We have simulated the image reconstruction of our theoretical image by assuming the current and future Event Horizon Telescope (EHT) array, and have found that the future EHT including space-very long baseline interferometry in 2020s can detect the dark crescent., Comment: 12 pages, 7 figures, accepted for publication in ApJ

To what extent can the one-dimensional slim disk model reproduce the multi-dimensional results of global radiation-hydrodynamic simulations of super-Eddington accretion? With this question in mind, we perform systematic simulation study of accretion flow onto a non-spinning black hole for a variety of black hole masses of $(10 - 10^7) M_{\odot}$ and mass accretion rates of $(1.4 \times 10^2 - 5.6 \times 10^3)L_{\rm Edd}/c^2$ (with $L_{\rm Edd}$ and $c$ being the Eddington luminosity and speed of light). In order to adequately resolve large-scale outflow structure, we extensively expand a simulation box to cover the space of $3000 r_{\rm S}$ (with $r_{\rm S}$ being the Schwarzschild radius), larger than those in most previous studies, so that we can put relatively large angular momentum to the gas injected from the outer simulation boundary. The adopted Keplerian radius, at which the centrifugal force balances with the gravitational force, is $r_{\rm K}=300 r_{\rm S}$. The injected mass first falls and is accumulated at around this radius and then slowly accretes towards the central black hole via viscosity. We simulate such accretion processes, taking inverse and bulk Compton scattering into account. The simulated accretion flow is in a quasi-steady state inside $r_{\rm qss}\sim 200 r_{\rm S}$. Within this radius the flow properties are, as a whole, in good agreement with those described by the slim disk model except that the radial density profile of the underlying disk is much flatter, $\rho\propto r^{-0.73}$ (cf. $\rho\propto r^{-3/2}$ in the slim disk model), due probably to efficient convection. We find very weak outflow from inside $r\sim 200 r_{\rm S}$ unlike the past studies., Comment: 15pages, 10figures, 2tables, accepted for publication in PASJ

We perform global three-dimensional (3D) radiation-hydrodynamic (RHD) simulations of out- flow from supercritical accretion flow around a 10 Msun black hole. We only solve the outflow part, starting from the axisymmetric 2D simulation data in a nearly steady state but with small perturbations in a sinusoidal form being added in the azimuthal direction. The mass accretion rate onto the black hole is ~10^2 L_E/c^2 in the underlying 2D simulation data and the outflow rate is ~10 L_E/c^2 (with LE and c being the Eddington luminosity and speed of light, respectively). We first confirm the emergence of clumpy outflow, which was discovered by the 2D RHD simulations, above the photosphere located at a few hundreds of Schwarzschild radii (r_S) from the central black hole. As prominent 3D features we find that the clumps have the shape of a torn sheet, rather than a cut string, and that they are rotating around the central black hole with a sub-Keplerian velocity at a distance of ~10^3 r_S from the center. The typical clump size is ~30 r_S or less in the radial direction, and is more elongated in the angular directions, ~hundreds of r_S at most. The sheet separation ranges from 50 to 150 r_S. We expect stochastic time variations when clumps pass across the line of the sight of a distant observer. Variation timescales are estimated to be several seconds for a black hole with mass of ten to several tens of Msun, in rough agreement with the observations of some ultra-luminous X-ray sources., Comment: 12 pages, 8 figures, accepted for publication in PASJ

X-ray continuum spectra of super-Eddington accretion flow are studied by means of Monte Carlo radiative transfer simulations based on the radiation hydrodynamic simulation data, in which both of thermal and bulk Compton scatterings are taken into account. We compare the calculated spectra of accretion flow around black holes with masses of $M_{\rm BH} = 10, 10^2, 10^3$, and $10^4 M_\odot$ for a fixed mass injection rate (from the computational boundary at $10^3 r_{\rm s}$) of $10^3 L_{\rm Edd}/c^2$ (with $r_{\rm s}$, $L_{\rm Edd}$, and $c$ being the Schwarzschild radius, the Eddington luminosity, and the speed of light, respectively). The soft X-ray spectra exhibit mass dependence in accordance with the standard-disk relation; the maximum surface temperature is scaled as $T \propto M_{\rm BH}^{-1/4}$. The spectra in the hard X-ray bands, by contrast, look quite similar among different models, if we normalize the radiation luminosity by $M_{\rm BH}$. This reflects that the hard component is created by thermal and bulk Compton scattering of soft photons originating from an accretion flow in the over-heated and/or funnel regions, the temperatures of which have no mass dependence. The hard X-ray spectra can be reproduced by a Wien spectrum with temperature of $T\sim 3$ keV accompanied by a hard excess at photon energy above several keV. The excess spectrum can be well fitted with a power law with a photon index of $\Gamma \sim 3$. This feature is in good agreement with that of the recent NuSTAR observations of ULX (Ultra-Luminous X-ray sources)., Comment: 10 pages, 9 figures, accepted for publication in PASJ

A possibility of time-delayed radio brightenings of Sgr A* triggered by the pericenter passage of the G2 cloud is studied by carrying out global three-dimensional magnetohydrodynamic simulations taking into account the radiative cooling of the tidal debris of the G2 cloud. Magnetic fields in the accretion flow are strongly perturbed and re-organized after the passage of G2. We have found that the magnetic energy in the accretion flow increases by a factor 3-4 in 5-10 years after the pericenter passage of G2 by a dynamo mechanism driven by the magneto-rotational instability. Since this B-field amplification enhances the synchrotron emission from the disk and the outflow, the radio and the infrared luminosity of Sgr A* is expected to increase around A.D. 2020. The time-delay of the radio brightening enables us to determine the rotation axis of the preexisting disk., Comment: accepted for publication in PASJ

The formation mechanism of CO clouds observed with NANTEN2 and Mopra telescope toward the stellar cluster Westerlund 2 is studied by three-dimensional magnetohydrodynamic (MHD) simulations taking into account the interstellar cooling. These molecular clouds show a peculiar shape composing of an arc-shaped cloud in one side of a TeV-ray source HESS J1023-575 and a linear distribution of clouds (jet clouds) in another side. We propose that these clouds are formed by the interaction of a jet with interstellar neutral hydrogen (HI) clumps. By studying the dependence of the shape of dense cold clouds formed by shock compression and cooling on the filling factor of HI clumps, we found that the density distribution of HI clumps determines the shape of molecular clouds formed by the jet-cloud interaction; arc-clouds are formed when the filling factor is large. On the other hand, when the filling factor is small, molecular clouds align with the jet. The jet propagates faster in models with small filling factors., Comment: 26 pages, 13 figures; published in ApJ

We present a new magnetohydrodynamic (MHD) simulation code with the aim of providing accurate numerical solutions to astrophysical phenomena where discontinuities, shock waves, and turbulence are inherently important. The code implements the HLLD approximate Riemann solver, the fifth-order-monotonicity-preserving interpolation (MP5) scheme, and the hyperbolic divergence cleaning method for a magnetic field. This choice of schemes significantly improved numerical accuracy and stability, and saved computational costs in multidimensional problems. Numerical tests of one- and two-dimensional problems showed the advantages of using the high-order scheme by comparing with results from a standard second-order TVD MUSCL scheme. The present code enabled us to explore long-term evolution of a three-dimensional accretion disk around a black hole, in which compressible MHD turbulence caused continuous mass accretion via nonlinear growth of the magneto-rotational instability (MRI). Numerical tests with various computational cell sizes exhibited a convergent picture of the early nonlinear growth of the MRI in a global model, and indicated that the MP5 scheme has more than twice the resolution of the MUSCL scheme in practical applications., Comment: 45 pages, 18 figures, published in Publ. Astron. Soc. Japan

Prompted by the recent discovery of pulsed emission from an ultra-luminous X-ray source, M82 X-2 ("ULX-pulsar"), we perform a two-dimensional radiation-hydrodynamic simulation of a super-critical accretion flow onto a neutron star through a narrow accretion column. We set an accretion column with a cone shape filled with tenuous gas with density of $10^{-4} {\rm g}~ {\rm cm}^{-3}$ above a neutron star and solve the two dimensional gas motion and radiative transfer within the column. The side boundaries are set such that radiation can freely escape, while gas cannot. Since the initial gas layer is not in a hydrostatic balance, the column gas falls onto the neutron-star surface, thereby a shock being generated. As a result, the accretion column is composed of two regions: an upper, nearly free-fall region and a lower settling region, as was noted by Basko \& Sunyaev (1976). The average accretion rate is very high; ${\dot M}\sim 10^{2-3} L_{\rm E}/c^2$ (with $L_{\rm E}$ being the Eddington luminosity), and so radiation energy dominates over gas internal energy entirely within the column. Despite the high accretion rate, the radiation flux in the laboratory frame is kept barely below $L_{\rm E}/(4\pi r^2)$ at a distance $r$ in the settling region so that matter can slowly accrete. This adjustment is made possible, since large amount of photons produced via dissipation of kinetic energy of matter can escape through the side boundaries. The total luminosity can greatly exceed $L_{\rm E}$ by several orders of magnitude, whereas the apparent luminosity observed from the top of the column is much less. Due to such highly anisotropic radiation fields, observed flux should exhibit periodic variations with the rotation period, provided that the rotation and magnetic axes are misaligned., Comment: accepted for publication in PASJ

Using three-dimensional general relativistic radiation magnetohydrodynamics simulations of accretion flows around stellar mass black holes, we report that the relatively cold disk ($\gtrsim 10^{7}$K) is truncated near the black hole. Hot and less-dense regions, of which the gas temperature is $ \gtrsim 10^9$K and more than ten times higher than the radiation temperature (overheated regions), appear within the truncation radius. The overheated regions also appear above as well as below the disk, and sandwich the cold disk, leading to the effective Compton upscattering. The truncation radius is $\sim 30 r_{\rm g}$ for $\dot{M} \sim L_{\rm Edd}/c^2$, where $r_{\rm g}, \dot M, L_\mathrm{Edd}, c$ are the gravitational radius, mass accretion rate, Eddington luminosity, and light speed. Our results are consistent with observations of very high state, whereby the truncated disk is thought to be embedded in the hot rarefied regions. The truncation radius shifts inward to $\sim 10 r_{\rm g}$ with increasing mass accretion rate $\dot{M} \sim 100 L_{\rm Edd}/c^2$, which is very close to an innermost stable circular orbit. This model corresponds to the slim disk state observed in ultra luminous X-ray sources. Although the overheated regions shrink if the Compton cooling effectively reduces the gas temperature, the sandwich-structure does not disappear at the range of $\dot{M} \lesssim 100L_{\rm Edd}/c^2$. Our simulations also reveal that the gas temperature in the overheated regions depends on black hole spin, which would be due to efficient energy transport from black hole to disks through the Poynting flux, resulting gas heating., Comment: 9 pages, 8 figures, 1 tables accepted for publication in ApJ

By performing two-dimensional radiation hydrodynamics simulations with large computational domain of 5000 Schwarzschild radius, we revealed that wide-angle outflow is launched via the radiation force from the super-critical accretion flows around black holes. The angular size of the outflow, of which the radial velocity (v_r) is over the escape velocity (v_esc), increases with an increase of the distance from the black hole. As a result, the mass is blown away with speed of v_r > v_esc in all direction except for the very vicinity of the equatorial plane, theta=0-85^circ, where theta is the polar angle. The mass ejected from the outer boundary per unit time by the outflow is larger than the mass accretion rate onto the black hole, ~150L_Edd/c^2, where L_Edd and c are the Eddington luminosity and the speed of light. Kinetic power of such wide-angle high-velocity outflow is comparable to the photon luminosity and is a few times larger than the Eddington luminosity. This corresponds to ~10^39-10^40 erg/s for the stellar mass black holes. Our model consistent with the observations of shock excited bubbles observed in some ultra-luminous X-ray sources (ULXs), supporting a hypothesis that ULXs are powered by the super-critical accretion onto stellar mass black holes., Comment: 9 pages, 8 figures, accepted for publication in PASJ