arXiv subjects

Tomohisa Kawashima

Publications and source records attributed to Tomohisa Kawashima.

At least 19 recordsLinked to original sources

arXiv2026

Black Hole Explorer Mission Development in Japan

The Black Hole Explorer (BHEX) is a next-generation space very-long-baseline interferometry (VLBI) mission concept that will extend existing ground-based millimeter/submillimeter VLBI arrays to space. The Japanese astronomical community has contributed to BHEX mission development through the BHEX Japan Consortium, established in 2023. This paper provides a high-level summary of progress in Japan since 2024, including the establishment of the Black Hole Explorer Working Group (BHEX WG) at the Institute of Space and Astronautical Science (ISAS), JAXA, to conduct the Japanese side of the Pre-Phase~A mission studies. We outline recent advances in key instrument technologies, including concept design studies of a 4.5\,K closed-cycle mechanical cryocooler and prototype development of an ultra-wideband 300\,GHz Superconductor--Insulator--Superconductor (SIS) mixer for BHEX. We also describe ongoing upgrades to Japan's ground infrastructure to support 86\,GHz observations with VERA and simultaneous 86+230\,GHz observations with the Nobeyama 45\,m Telescope.

astro-ph.IM↗

arXiv2026

Transverse Oscillations and Wave Propagation in the Magnetically Dominated M87 Jet

We present an in-depth analysis of transverse oscillations in the M87 jet, as identified in our previous study (Ro et al. 2023a), which reported oscillatory patterns with a characteristic period of $\sim$1 year in the edge-brightened jet structure extending up to 12\,mas from the core. This work is based on high-cadence KaVA 22\,GHz observations conducted from December 2013 to June 2016. By analyzing the transverse velocity profiles and the spatial evolution of the oscillations, we find that the oscillations propagate downstream along the jet, with a wavelength of $\sim9-10$\,mas. A single-mode sinusoidal wave model applied to the ridge lines successfully reproduces the observed transverse oscillations and yields superluminal wave speeds of $\sim2.7-2.9\,c$, consistent with the bulk jet velocity in this region. These findings suggest that the transverse oscillations may be interpreted either as transverse MHD waves -- possibly excited by jet precession, nutation, or quasi-periodic magnetic flux eruptions near the central engine -- or as manifestations of jet instabilities, such as current-driven instabilities (CDIs). Further investigation is required to distinguish between these scenarios and to clarify the dominant physical mechanism.

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arXiv2025

Black Hole Spin Estimation with Time-variable Image of M87 During the Flaring State

By investigating the time-variable 230 GHz images using ray-tracing general relativistic radiative transfer calculation, we propose a novel method for estimating the spin parameter of the supermassive black hole at the M87 center by utilizing the sudden and short-term increase in emissivity in the innermost region of the accretion disk. It is found that the flux of the photon ring increases simultaneously as the flux of the direct ring, which brightens first, decreases, and then gradually diminishes, when the increase in emissivity persists for $15 t_{\rm g}$ with $t_{\rm g}$ being the light crossing time of the gravitational radius. The direct ring is formed by photons emitted from the vicinity of the innermost region of the disk and traveling directly to the observer without orbiting around the black hole, while the photon ring is formed by photons passing near the spherical photon orbit. The time-averaged width of the dark region between the direct ring and the photon ring (dark crescent) becomes thinner when the spin parameter is higher and the increase in the emissivity of the accretion disk is greater. The time variation of two rings also causes the intensity-weighted center to oscillate both in the direction of the black hole's angular momentum vector projected onto the screen ($Y$-direction) and in the perpendicular direction ($X$-direction). The amplitude of oscillatory time variation in the $X$-direction becomes large when the spin parameter is higher, and that in the $Y$-direction becomes large when the increase in the emissivity of the disk is large. The spin parameter can be estimated by combining the time-averaged dark crescent width and the ratio of the amplitudes in the $X$- and $Y$-directions. This method is applicable when the duration of the increase in emissivity of the accretion disk ranges at least from approximately 10-20 $t_{\rm g}$.

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arXiv2025

Constraining the Corona Geometry of Cyg X-1 with Broad Band Spectrum and Polarimetric Analysis Based on Observations in May 2022

Cygnus X-1 (Cyg X-1) exhibited a low hard state in 2022, observed by several missions. The IXPE reported that the polarization angle is aligned with the radio jet and gave a polarization degree approximately 4 times higher than the general expectations of $1\%$ through the analysis of the time-integrated data with a simple spectral model, indicating that the disk inclination is higher than a canonical value of about $30^{\circ}$. Many subsequent theoretical studies employed a non-standard model to explain this high PD. Here, we revisit the disk/corona spectrum through a detailed joint analysis using IXPE, NuSTAR, and NICER data. By investigating the time variability of the spectrum, we find that the two-Comptonization components model can better reproduce the data than the one-Comptonization component model originally adopted. We observed a lower disk photon temperature of about 0.15 keV. Detailed simulation suggests that lowering the disk temperature by a factor of 2 increases the PD by roughly 2 percentage points in the IXPE 2--8 keV band for a slab-like corona geometry, helping to reconcile the observed high PD with theoretical predictions. However, The simulated PDs are still significantly lower than the observed ones - even for a rather high $60^{\circ}$ inclination. We also investigated the polarization properties of a simple wedge-shaped corona with a truncated disk and a sandwiching slab corona. We find that the slab corona predicts an apparent energy dependence in PD while PA remains constant in the IXPE band, in agreement with the observed polarization. Therefore, we suggest that Cyg X-1 in 2022 May exhibits a two-Comptonization coronal emission with different optical depths, and the hard one is in a sandwiching slab geometry. We also discuss how the polarization is affected by other parameters of the black hole and the corona.

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arXiv2025

Helical Magnetic Field in the Acceleration--Collimation Zone of the M87 Jet

Relativistic jets from supermassive black holes are expected to be magnetically launched and guided, with magnetic energy systematically converted to bulk kinetic energy throughout an extended acceleration-collimation zone (ACZ). A key prediction of magnetohydrodynamic (MHD) models is a transition from poloidally dominated fields near the engine to toroidally dominated fields downstream, yet direct tests within the ACZ are hampered by weak polarization and strong Faraday rotation. We report quasi-simultaneous, high-sensitivity, multifrequency very long baseline interferometric polarimetry of M87 spanning 1.4-24.4GHz. We present high-fidelity, Faraday rotation-corrected maps of intrinsic linear polarization that continuously resolve the ACZ in the de-projected distance range of ~9e3 to ~3.6e5 gravitational radii from the black hole. The maps reveal pronounced north-south asymmetries in fractional linear polarization and electric vector position angle (EVPA), peaking in the inner ACZ at a projected distance of ~20mas along the jet and remaining prominent out to ~100mas. These signatures are best reproduced by models with a large-scale, ordered helical field that retains a substantial poloidal component-contrary to the rapid toroidal dominance expected under steady, ideal MHD. This tension implies ongoing magnetic dissipation that limits toroidal buildup over the ACZ. The handedness of the helix provides an independent constraint on the black hole's spin direction, supporting a spin vector oriented away from the observer, consistent with the orientation inferred from horizon-scale imaging. Farther downstream, the asymmetries diminish, and the EVPA and fractional polarization distributions become more symmetric; we tentatively interpret this as evolution toward a more poloidally dominated configuration, while noting current sensitivity and dynamic-range limits.

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arXiv2025

High-Energy Neutrino Emission from a Radiatively Inefficient Accretion Flow with a three-dimensional GRMHD Simulation

The high-energy particle production in the accretion flow onto black holes can be a key physics to explain the high-energy neutrino background. While the single-zone approximation has been commonly adopted in studies of the high-energy neutrino emission around black holes, the effects of the global plasma structure may be non-negligible. We carry out the first computations of cosmic-ray acceleration and high-energy neutrino emission via the hadronuclear ($pp$) interaction in global radiatively inefficient accretion flows and outflows around a supermassive black hole, using three-dimensional general relativistic magnetohydrodynamic simulation data. The Fokker-Planck equation for cosmic-ray protons is solved with a phenomenological model for the energy diffusion coefficient to express the turbulent acceleration in the sub-grid scale. The inhomogeneous and time variable structure of the accretion flow leads to a variety of particle energy distribution. The spectral energy distributions (SEDs) of neutrinos emitted from the entire region are flatter than those calculated under the single-zone approximation. In our model, the neutrino emission originating from cosmic rays advected with the outflow rather than the inflow predominates the SEDs. Such galactic nuclei can be significant sources of cosmic rays in those galaxies.

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arXiv2024

Demographics of black holes at $<$100 R$_{\rm g}$ scales: accretion flows, jets, and shadows

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.

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arXiv2024

Survey of Non-thermal Electron around Supermassive Black Holes through Polarization Flips

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.

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arXiv2024

Mapping the distribution of the magnetic field strength along the NGC 315 jet

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 $Ω_{F}$ 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.

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arXiv2024

Radiation and outflow properties of super-Eddington accretion flows around various mass classes of black holes: Dependence on the accretion rates

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 $θ=10^\circ$) 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.

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arXiv2024

Radiation MHD Simulations of Soft X-ray Emitting Regions in Changing Look AGN

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.

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arXiv2024

Black Hole Spacetime and Properties of Accretion Flows and Jets Probed by Black Hole Explorer: Science Cases Proposed by BHEX Japan Team

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.

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arXiv2024

The Japanese Vision for the Black Hole Explorer Mission

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.

astro-ph.IM↗

arXiv2024

Radio Images inside Highly Magnetized Jet Funnels Based on Semi-Analytic GRMHD Models

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 $\sim 60 ~ μ{\rm as}$, which is consistent with the ring-like images of M87 at 86 GHz observed with GMVA, ALMA and GLT, whose ring-diameter is $\sim 64^{+4}_{-8} ~μ{\rm as}$. 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.

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arXiv2023

Precessing jet nozzle connecting to a spinning black hole in M87

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.

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arXiv2023

Decomposing the Spectrum of Ultra-Luminous X-ray Pulsar NGC 300 ULX-1

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.

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arXiv2023

Dynamical Energy Dissipation of Relativistic Magnetic Bullets

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.

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arXiv2023

A ring-like accretion structure in M87 connecting its black hole and jet

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.

astro-ph.HE↗