arXiv subjects

Kotaro Niinuma

Publications and source records attributed to Kotaro Niinuma.

At least 19 recordsLinked to original sources

arXiv2026

Resolving Nearby Supermassive Black Holes with the Black Hole Explorer

Recent Event Horizon Telescope results have demonstrated unique and transformative science in gravitational physics and black hole astrophysics enabled by event-horizon-scale imaging of supermassive black holes (SMBHs). Nevertheless, the angular resolution of current ground-based very long baseline interferometry (VLBI) arrays limits such studies to only two sources, precluding systematic investigations of horizon-scale emission across a nearby SMBH population. The proposed Black Hole Explorer (BHEX), a millimeter/submillimeter space VLBI mission, would overcome this limitation by delivering substantially higher angular resolution. Here, we present a series of simulated observations to assess a population of nearby horizon-scale targets accessible with BHEX. Based on a recently developed SMBH number density model, we find that BHEX could infer black hole masses for ~70-90 sources from size measurements, constrain magnetic field structures through linear polarization imaging for ~20-30 sources, and resolve black hole shadows for ~20-25 sources. Targeted observations of ~50 nearby SMBHs are expected to yield measurements for ~30 source sizes and ~10 shadows and linear-polarization patterns. These projections are supported by detailed imaging simulations of general relativistic magnetohydrodynamic (GRMHD) models for eleven nearby SMBHs. Together, our results highlight BHEX as a powerful facility for revealing the demographics of SMBH properties across diverse accretion states, radio loudness, host galaxy environments, and viewing geometries.

astro-ph.HE↗

arXiv2026

Radio Constraints on the Circumstellar Environment of the Type IIb Supernova SN 2024iss

Type~IIb supernovae exhibit diverse progenitor properties, and radio observations offer a unique probe of their mass-loss histories shortly before the explosion. We present Japanese VLBI Network single-baseline monitoring of the nearby Type IIb SN 2024iss at 6.9 and 8.4 GHz, spanning approximately one year after its discovery. Our radio observations have detected its emission at 10 and 23 days after the explosion, with subsequent epochs yielding non-detections. Based on the peak radio luminosity and peak time, SN 2024iss exhibits radio properties highly comparable to those of compact-envelope events. Using a synchrotron self-absorption (SSA) modeling, we estimate a progenitor mass-loss rate of $\dot{M} \approx 2.5 \times 10^{-6}\>M_{\odot}\>{\rm yr^{-1}}$ for a compact progenitor wind velocity of $100 \>{\rm km\>s^{-1}}$. Furthermore, our SSA analysis yields a mean expansion velocity of $V_{\rm sh} \approx 3.3 \times 10^4\>{\rm km\>s^{-1}}$, which exceeds the theoretical shock velocity derived from the self-similar solution by a factor of $\sim 2.4$. Even for the conservative upper-bound peak time, the SSA-derived velocity remains larger than the theoretical expectation by a factor of $\gtrsim 1.7$. To explain this velocity excess, we propose the presence of a confined, dense circumstellar matter (CSM) surrounding the progenitor. The shock emergence from this confined CSM may have accelerated the forward shock, pointing to a highly complex and non-steady mass-loss history shortly before the explosion.

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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

VLBI Astrometry of Magnetars

The origin of the strongest magnetic fields in the Universe, i.e., the origin of magnetars, is a longstanding question. An enhanced dynamo effect in an irregular supernova explosion is a possible origin, which implies a stronger kick velocity and a higher proper motion of the magnetar compared to those of ordinary pulsars as well as an irregular morphology of the host supernova remnant (SNR). However, this hypothesis is not well studied yet, because there is a lack of precise measurement of the proper motion of magnetar and of identification of the host SNR. VLBI astrometry of magnetars is a unique tool to examine the hypothesis. In this chapter, we introduce the MONSTER (Monitoring Observations of the Neutron Stars That Evolve Rapidly) Project. SKA-VLBI's unprecedented sensitivity and the highest angular resolution will allow us to dramatically expand the survey volume in which we can measure the proper motion of magnetars within a radio outburst period of a few months.

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arXiv2026

VLBI-Enabled Localization of Continuous GW Sources

Pulsar timing arrays (PTAs) are opening the nanohertz gravitational-wave (GW) band by timing millisecond pulsars (MSPs) to target signals from supermassive black hole binaries (SMBHBs). Beyond evidence for a stochastic background, a central SKA-era objective is detecting individual continuous-wave (CW) sources. The scientific payoff hinges on localization: conventional PTA searches yield uncertainties of tens-hundreds of deg$^2$, too large to identify a unique host, obtain a redshift, infer intrinsic masses, or pursue electromagnetic counterparts. This limitation is chiefly geometric: the CW response includes Earth and pulsar terms, and poorly known pulsar distances make the pulsar-term phase a free parameter that degrades triangulation. If distances to a few MSPs are known to better than a GW wavelength ($\sim$ 1 pc), these phases are fixed and localization improves by orders of magnitude. Simulations indicate that with sub-parsec distances for a handful of nearby MSPs, the uncertainty can shrink to $\sim 10^{-3}$ deg$^2$ (arcminute scale), enabling unique host association and multi-messenger follow-up. Achieving such distances requires $\sim$ 10 $μ$arcsec parallaxes for MSPs within a few hundred parsecs, a precision now approached with Very Long Baseline Interferometry (VLBI) and expected to become practical with phased-array SKA1-Mid operating as a sensitive VLBI element. SKA1's multi-beam, multi-calibrator astrometry will provide the independent distance priors needed for PTAs to localize nanohertz GW sources and measure SMBHB parameters and environments. We assess VLBI's role in PTA CW searches and propose a concrete SKA1-Mid observing strategy for nearby MSPs to deliver the required sub-parsec distances.

astro-ph.IM↗

arXiv2026

Fast variability and circular polarization of the 6.7 GHz methanol maser in G33.641$-$0.228

The 6.7 GHz methanol maser in a high-mass star-forming region G33.641$-$0.228 is known to exhibit burst-like flux variability due to an unknown mechanism. To investigate the burst mechanism, we conducted high-cadence flux and circular polarization monitoring observations, simultaneously using left- and right-hand circular polarizations. We found that the flux density increased and decreased on a short timescale of 0.3 d during a burst. We also found strong circular polarization, reaching up to 20\% in the component exhibiting the bursts. Circular polarization of 0--20\% was continuously observed from 2009 to 2016, even in the quiescent period. The polarization also varied on timescales of less than one day. When a burst occurred and the flux density increased, the circular polarization decreased to zero. To explain the observational properties of the flux variability and circular polarization, we propose a model in which an explosive event similar to a solar radio burst occurs on the line of sight behind the maser cloud, producing circularly polarized continuum emission due to gyro-synchrotron or gyro-resonance radiation, which is then amplified by the maser.

astro-ph.SR↗

arXiv2025

A Possible Shutting-Down Event of Mass Accretion in An Active Galactic Nucleus at z~1.8

We present the discovery of a large gradual apparent fading event in optical and near-infrared wavelengths in a quasar at z=1.767 by a factor of 20-30 (in optical) over a period of ~20 years in the observed frame. This pronounced fading trend in brightness was first identified by comparing the magnitudes measured in the Subaru/Hyper Suprime-Cam (HSC) images with those in the Sloan Digital Sky Survey (SDSS) images for ~3x10^4 quasars spectroscopically identified by SDSS. We performed follow-up observations, including optical imaging and spectroscopy as well as near-infrared imaging, with >4m-class telescopes such as Subaru, GTC, Keck, and SOAR telescopes. We combine these new data with the archival data to examine the variability behavior over ~20 years in detail and even the longer-term trend of the variability over ~70 years in the observed frame. We find that (i) the AGN component likely faded by a factor of ~50 from the early 2000s to 2023 and (ii) the observed brightness decline is best explained by a substantial decrease in accretion rate rather than time-varying line-of-sight dust obscuration. These findings are derived from multi-component (time-varying AGN + constant galaxy) spectral energy distribution fitting over multi-epochs, which is well consistent with the optical spectra. The Eddington ratio decreases by a factor of ~50, from ~0.4 to ~0.008 if we use the black hole mass measured with the SDSS spectrum, which could be highly uncertain because of the very large variability. The total brightness is dominated by the host galaxy in the rest-frame optical wavelength rather than the AGN as of 2023.

astro-ph.GA↗

arXiv2024

Radio Follow-up Observations of SN 2023ixf by Japanese and Korean VLBIs

We report on radio follow-up observations of the nearby Type II supernova, SN 2023ixf, spanning from 1.7 to 269.9 days after the explosion, conducted using three very long baseline interferometers (VLBIs), which are the Japanese VLBI Network (JVN), the VLBI Exploration of Radio Astrometry (VERA), and the Korean VLBI Network (KVN). In three observation epochs (152.3, 206.1, and 269.9 days), we detected emission at the 6.9 and 8.4 GHz bands, with a flux density of $\sim 5$ mJy. The flux density reached a peak at around 206.1 days, which is longer than the timescale to reach the peak observed in typical Type II supernovae. Based on the analytical model of radio emission, our late-time detections were inferred to be due to the decreasing optical depth. In this case, the mass-loss rate of the progenitor is estimated to have increased from $\sim 10^{-6} - 10^{-5}\, M_{\odot}\,{\rm yr^{-1}}$ to $\sim 10^{-4}\, M_{\odot}\,{\rm yr^{-1}}$ between 28 and 6 years before the explosion. Our radio constraints are also consistent with the mass-loss rate to produce a confined circumstellar medium proposed by previous studies, which suggest that the mass-loss rate increased from $\sim 10^{-4}\, M_{\odot}\,{\rm yr^{-1}}$ to $\gtrsim 10^{-2}\, M_{\odot}\,{\rm yr^{-1}}$ in the last few years before the explosion.

astro-ph.HE↗

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.

astro-ph.HE↗

arXiv2024

The Black Hole Explorer: Motivation and Vision

We present the Black Hole Explorer (BHEX), a mission that will produce the sharpest images in the history of astronomy by extending submillimeter Very-Long-Baseline Interferometry (VLBI) to space. BHEX will discover and measure the bright and narrow "photon ring" that is predicted to exist in images of black holes, produced from light that has orbited the black hole before escaping. This discovery will expose universal features of a black hole's spacetime that are distinct from the complex astrophysics of the emitting plasma, allowing the first direct measurements of a supermassive black hole's spin. In addition to studying the properties of the nearby supermassive black holes M87* and Sgr A*, BHEX will measure the properties of dozens of additional supermassive black holes, providing crucial insights into the processes that drive their creation and growth. BHEX will also connect these supermassive black holes to their relativistic jets, elucidating the power source for the brightest and most efficient engines in the universe. BHEX will address fundamental open questions in the physics and astrophysics of black holes that cannot be answered without submillimeter space VLBI. The mission is enabled by recent technological breakthroughs, including the development of ultra-high-speed downlink using laser communications, and it leverages billions of dollars of existing ground infrastructure. We present the motivation for BHEX, its science goals and associated requirements, and the pathway to launch within the next decade.

astro-ph.IM↗

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↗

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.

astro-ph.HE↗

arXiv2023

Very Long Baseline Interferometry imaging of H2O maser emission in the nearby radio galaxy NGC 4261

We report dual-frequency very long baseline interferometry (VLBI) observations at 22 and 43 GHz toward the nucleus of a nearby radio galaxy NGC 4261. In particular, we present a VLBI image of the 22 GHz H2O maser line and its location in the circumnuclear region of NGC 4261. H2O maser emission is marginally detected above the three times the rms level within a velocity range of approximately 2250-2450 km/s, slightly red-shifted with respect to the systemic velocity. H2O maser emission is located approximately 1 milliarcsecond (mas) east of the brightest continuum component at 22 GHz, where the continuum spectrum is optically thick, that is at the free-free absorbed receding jet by ionized gas. A positional coincidence between H2O maser emission and an ionized gas disk implies that the H2O maser emission arises from the near side of the disk, amplifying continuum emission from the background receding jet. If the disk axis is oriented 64 degree relative to the line of sight, the H2O maser emission is expected to be at a mean radius of 0.3 pc in the disk. The broad line width of the H2O maser emission can be attributed to complex kinematics in the immediate vicinity of the supermassive black hole (SMBH), including ongoing gas infall onto the SMBH, turbulence, and outflow. This is analogous to the multi-phase circumnuclear torus model in the nearest radio-loud H2O megamaser source NGC 1052. An alternative explanation for H2O maser association is the shock region between the jet and the ambient molecular clouds. However, this explanation fails to describe the explicit association of H2O maser emission only with the free-free absorbed receding jet.

astro-ph.GA↗

arXiv2023

Spectral analysis of a parsec-scale jet in M87: Observational constraint on the magnetic field strengths in the jet

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 $α_{22-43 GHz}$ ~ -0.7 at ~ 2 mas to $α_{22-43 GHz}$ ~ -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.

astro-ph.HE↗

arXiv2022

Overview of the Observing System and Initial Scientific Accomplishments of the East Asian VLBI Network (EAVN)

The East Asian VLBI Network (EAVN) is an international VLBI facility in East Asia and is operated under mutual collaboration between East Asian countries, as well as part of Southeast Asian and European countries. EAVN currently consists of 16 radio telescopes and three correlators located in China, Japan, and Korea, and is operated mainly at three frequency bands, 6.7, 22, and 43 GHz with the longest baseline length of 5078 km, resulting in the highest angular resolution of 0.28 milliarcseconds at 43 GHz. One of distinct capabilities of EAVN is multi-frequency simultaneous data reception at nine telescopes, which enable us to employ the frequency phase transfer technique to obtain better sensitivity at higher observing frequencies. EAVN started its open-use program in the second half of 2018, providing a total observing time of more than 1100 hours in a year. EAVN fills geographical gap in global VLBI array, resulting in enabling us to conduct contiguous high-resolution VLBI observations. EAVN has produced various scientific accomplishments especially in observations toward active galactic nuclei, evolved stars, and star-forming regions. These activities motivate us to initiate launch of the 'Global VLBI Alliance' to provide an opportunity of VLBI observation with the longest baselines on the earth.

astro-ph.IM↗

arXiv2022

Detection of a bright burst from the repeating FRB 20201124A at 2 GHz

We present a detection of a bright burst from FRB 20201124A, which is one of the most active repeating FRBs, based on S-band observations with the 64-m radio telescope at the Usuda Deep Space Center/JAXA. This is the first FRB observed by using a Japanese facility. Our detection at 2 GHz in February 2022 is the highest frequency for this FRB and the fluence of $>$ 189 Jy ms is one of the brightest bursts from this FRB source. We place an upper limit on the spectral index $α$ = -2.14 from the detection of the S band and non-detection of the X band at the same time. We compare an event rate of the detected burst with ones of the previous research, and suggest that the power-law of the luminosity function might be broken at lower fluence, and the fluences of bright FRBs distribute up to over 2 GHz with the power-law against frequency. In addition, we show the energy density of the burst detected in this work was comparable to the bright population of one-off FRBs. We propose that repeating FRBs can be as bright as one-off FRBs, and only their brightest bursts could be detected so some of repeating FRBs intrinsically might have been classified as one-off FRBs.

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arXiv2021

The intrinsic structure of Sagittarius A* at 1.3 cm and 7 mm

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 $μ$as (axial ratio $\sim$1.19$^{+0.24}_{-0.19}$) and $\sim$300$\pm$25 $μ$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.

astro-ph.HE↗

arXiv2021

Morphological transition of the compact radio lobe in 3C84 via the strong jet-cloud collision

We report multi-epoch Very Long Baseline Interferometric (VLBI) observations of the compact radio lobe in the radio galaxy 3C84 (NGC1275) during 2016 - 2020. The image sequence of 3C84 reveals that the hotspot in the radio lobe showed the one-year long frustration in 2017 within a compact region of about 0.07 parsec, suggesting a strong collision between the jet and a compact dense cloud with the estimated average density about $(4\-- 6)\times 10^{5}~{\rm cm^{-3}}$. Although the hotspot and the radio lobe began to move south again after its breakout, the radio lobe showed a morphological transition from FR II- to FR I-class radio lobe and its radio flux became fainter. This is the first detection of the dynamical feedback from the cloud to the jet where the cloud located on the jet axis significantly interferes with the jet propagation and evolution at the central one-parsec region in active galactic nucleus.

astro-ph.HE↗