arXiv subjects

Takayuki J. Hayashi

Publications and source records attributed to Takayuki J. Hayashi.

8 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↗

arXiv2025

Origin of Radio Emission in Three Nearby Ultraluminous Infrared Galaxies with Signatures of Luminous Buried Active Galactic Nuclei

We report multifrequency Very Long Baseline Array (VLBA) observations at 2.3 and 8.4GHz of three nearby ultraluminous infrared galaxies, identified via mid-infrared spectroscopic analyses as hosting deeply embedded active galactic nuclei (AGNs). Milliarcsecond-scale observations at both frequencies reveal compact continuum emission in IRAS F00188$-$0856 and IRAS F01298$-$0744, accounting for $\sim10$% of the flux density measured on arcsecond scales. The non-detection in IRAS F00091$-$0738 and the lower limit on the intrinsic 8.4 GHz brightness temperature of $10^{6.1}$ K in IRAS F01298$-$0744 yield no conclusive evidence of AGN-driven radio emission, whereas the measurement of $10^{7.8}$ K in IRAS F00188$-$0856 confirms an AGN origin. Thus, the mid-infrared AGN classification remains robust, with at least one object exhibiting compact radio emission indicative of AGN activity. We further investigate the high-frequency spectral steepening observed in all three galaxies. In each case, this steepening arises from spectral aging in diffuse kpc-scale emission, which is resolved out by the VLBA observations. One possible explanation for the steepening of the sample is merger-induced particle acceleration. IRAS F00188$-$0856 exhibits a peaked radio spectrum, characteristic of a young radio source, with the high-frequency steepening attributable to this AGN activity. Consequently, the spectral steepening at high frequencies arises from particles accelerated by merger dynamics or AGN activity.

astro-ph.GA↗

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

VLBI Detection of an Active Radio Source Potentially Driving 100-kpc Scale Emission in the Ultraluminous Infrared Galaxy IRAS F01004$-$2237

The nearby ultraluminous infrared galaxy (ULIRG) IRAS F01004$-$2237 exhibits 100-kpc scale continuum emission at radio wavelengths. The absence of extended X-ray emission in IRAS F01004$-$2237 has suggested an active galactic nucleus (AGN) origin for the extended radio emission, whose properties and role in merging systems still need to be better understood. We present the results of multi-frequency observations of IRAS F01004$-$2237 conducted by the Very Long Baseline Array at 2.3 and 8.4 GHz. Compact 8.4-GHz continuum emission was detected on a 1-pc scale in the nuclear region with an intrinsic brightness temperature of $10^{8.1}$ K suggesting that the radio source is originated from an AGN, potentially driving the extended emission. In contrast, no significant emission was observed at 2.3 GHz, indicating the presence of low-frequency absorption. This absorption cannot be attributed solely to synchrotron self-absorption; alternatively, free-free absorption due to thermal plasma is mainly at work in the spectrum. From combined perspectives, including mid-infrared and X-ray data, the AGN is obscured in a dense environment. The kinetic power of the nonthermal jet, as inferred from the extended emission, can play a more important role in dispersing the surrounding medium than the thermal outflow in IRAS F01004$-$2237. These findings hint that jet activities in ULIRGs may contribute to AGN feedback during galaxy evolution induced by merger events.

astro-ph.GA↗

arXiv2024

A Giant Metrewave Radio Telescope Survey of Radio-loud Broad Absorption Line Quasars

A substantial fraction of quasars display broad absorption lines (BALs) in their rest-frame ultraviolet spectra. While the origin of BALs is thought to be related to the accretion disc wind, it remains unclear whether the observed ratio of BAL to non-BAL quasars is due to orientation. We conducted observations of 48 BAL quasars and the same number of non-BAL quasars at 322 MHz using the Giant Metrewave Radio Telescope. Combined with previous flux measurements ranging from MHz to GHz frequencies, we compared continuum radio spectra between the two quasar groups. These data offer insights into low-frequency radio properties that have been difficult to investigate with previous observations only at GHz frequencies. Our results present that $73\pm13$ per cent of the BAL quasars exhibit steep or peaked spectra, a higher proportion than $44 \pm 14$ per cent observed in the non-BAL quasars. In contrast, there are no discernible differences between the two quasar groups in the radio luminosity, peak frequency, and spectral index distributions of sources with steep or peaked spectra and sources with flat or inverted spectra. Generally, as the jet axis and line of sight become closer to parallel, quasars exhibit flat or inverted spectra rather than steep or peaked ones. Therefore, these results suggest that BAL quasars are more frequently observed farther from the jet axis than non-BAL quasars. However, given that a certain proportion of BAL quasars exhibit flat or inverted spectra, more than the simple orientation scenario is required to elucidate the radio properties of BAL quasars.

astro-ph.HE↗

arXiv2021

Radio properties of ten nearby ultraluminous infrared galaxies with signatures of luminous buried active galactic nuclei

We present the results of our multifrequency observations for 10 ultraluminous infrared galaxies (ULIRGs) made by the Karl G. Jansky Very Large Array at 1.4, 5.5, 9.0, and 14.0 GHz. Our sample is selected from ULIRGs whose active galactic nuclei (AGNs) are not found at optical wavelengths ($\sim$70% of the entire ULIRGs), but whose presence is suggested by mid-infrared or submillimetre observations ($>50$% of the non-AGN ULIRGs at optical wavelengths). The statistical properties of the targets are similar to those of the entire ULIRG sample, which implies that ULIRGs have common radiative processes regardless of the presence of optical AGNs, and thus AGNs might equally contribute to the radio emission of every ULIRG. Although their spectra are mainly explained by starbursts and/or merger activity, some individual sources suggest contributions from AGNs. IRAS 00188$-$0856, whose optical morphology is not disturbed, shows a large nonthermal fraction and a spectral break at high frequency, which can be explained by synchrotron ageing of nonthermal plasma emitted from AGNs. In addition, we find 100-kpc scale extended emission associated with IRAS 01004$-$2237. The two-sided morphology and absence of extended X-ray emission suggest that this system is not induced by a merger in a cluster but originates from AGN activity.

astro-ph.GA↗

arXiv2013

VLBA Multi-frequency Polarimetric imaging of Radio-loud Broad Absorption Line Quasars

We conducted the first multi-frequency polarimetric imaging of four broad absorption line (BAL) quasars using Very Long Baseline Array at milli-arcsecond resolutions to investigate the inclination of the non-thermal jet and test the hypothesis that radio sources in BAL quasars are still young. Among these four sources, J0928+446, J1018+0530, and J1405+4056 show one-sided structures in parsec scales, and polarized emission detected in the core. These characteristics are consistent with those of blazars. We set constraints on viewing angles to $<$66 deg for these jets, in the framework of a Doppler beaming effect. J1159+0112 exhibits an unpolarized gigahertz peaked spectrum component and several discrete blobs with steep spectra on both sides of the central component across $\sim$1 kpc. These properties are consistent with those of young radio sources. We discuss the structures of jets and AGN wind.

astro-ph.HE↗

arXiv2013

Multifrequency VLBI Observations of the Broad Absorption Line Quasar J1020+4320: Recently Restarted Jet Activity?

This paper reports very-long-baseline interferometry observations of the radio-loud broad absorption line (BAL) quasar J1020+4320 at 1.7, 2.3, 6.7, and 8.4 GHz using the Japanese VLBI network (JVN) and European VLBI network (EVN). The radio morphology is compact with a size of ~10 pc. The convex radio spectrum is stable over the last decade; an observed peak frequency of 3.2 GHz is equivalent to 9.5 GHz in the rest frame, suggesting an age of the order of ~100 years as a radio source, according to an observed correlation between linear size and peak frequency of compact steep spectrum (CSS) and giga-hertz peaked spectrum (GPS) radio sources. A low-frequency radio excess suggests relic of past jet activity. J1020+4320 may be one of the quasars with recurrent and short-lived jet activity during a BAL-outflowing phase.

astro-ph.HE↗