Shigeo S. Kimura

Isolated stellar-mass black holes (IBHs) are expected to be abundant in the Milky Way, yet their electromagnetic signatures remain largely undetected. We investigate the detectability of IBHs in molecular clouds using a 1D, multiwavelength emission model that incorporates magnetic flux transport controlled by the magnetic Prandtl number P _m . We find that magnetically arrested disks (MADs) form for P _m ≳ 1, where the magnetic flux threading the black hole reaches a saturation value. On the other hand, MAD formation is restricted to a limited parameter range for P _m < 1, In our model, the outer parts of accretion disks, around 100 gravitational radii, efficiently emit infrared photons detectable by the Wide-field Infrared Survey Explorer (WISE). This feature is not captured by the conventional one-zone model. X-ray emission depends strongly on P _m . For P _m = 1, where a MAD is formed, X-ray emission is dominated by nonthermal radiation, whereas inverse Compton emission becomes dominant for P _m = 0.5, where the magnetic field is weaker than the saturation value. X-ray detection is plausible if IBHs are in dense molecular-cloud filaments within 1 kpc for P _m ≥ 1, although it is challenging for P _m < 1. These results demonstrate that magnetic flux transport plays a key role in shaping the multiwavelength observational signatures of IBHs.

Active galactic nuclei often exhibit highly collimated relativistic plasma outflows launched from the vicinity of their central black holes. One of the key theoretical challenges in understanding black hole jet formation is the origin of the plasma that feeds the jet, which remains poorly understood, particularly in explaining the observed jet emission. In this study, we focus on electron–positron pair production generated by high-energy photons from nonaxisymmetric magnetic reconnection near the black hole, as suggested by recent three-dimensional general-relativistic magnetohydrodynamics simulations. By employing general relativistic ray tracing, we calculate the spatial distribution of the pair production rate in the jet, taking into account photon propagation and collision angles in curved spacetime. We find that our scenario can supply a sufficient amount of plasma to explain the observed radio emission from the M87 jet. Furthermore, we show that a spinning black hole plays a crucial role in shaping the spatial distribution of the pairs, which in turn affects jet acceleration and the formation of magnetospheric spark gaps responsible for TeV emission.

Relativistic jets in active galactic nuclei (AGNs) are known to accelerate particles to extreme energies, yet the physical origin of very-high-energy (VHE) emission from low-luminosity (LL) AGNs remains unclear. NGC 4278, a nearby LL AGN, has recently been identified as a VHE source following detections by LHAASO. In this work, we present a multiwavelength and multimessenger analysis to investigate the origin of this emission. Swift X-Ray Telescope monitoring reveals a quasi-quiescent state characterized by the low X-ray flux. Modeling the broadband spectral energy distribution with the leptohadronic code AMES , we find that a standard one-zone synchrotron self-Compton model underpredicts the VHE flux, unless a relatively high Doppler factor ( δ ≳ 6) is invoked. Alternatively, an external inverse-Compton (EIC) scenario—scattering seed photons from a radiatively inefficient accretion flow—provides a good description of the broadband emission with modest jet power and Doppler factor. We further explore neutrino production within a leptohadronic framework. The EIC model in the quasi-quiescent state yields the largest predicted number of muon neutrinos, reaching N ν μ ∼ 0.001 over 15 yr of IceCube observations (assuming that 0.1% of the Eddington luminosity is converted into high-energy protons). Future multimessenger observations are essential to unveil the details of the high-energy processes of NGC 4278.

We report the first NuSTAR hard X-ray observations of the low-luminosity active galactic nucleus NGC 4278. The source is clearly detected beyond 10 keV with a hard X-ray spectrum consistent with a power law of photon index between 2.2 and 2.5 without evidence for a high-energy cutoff. The X-ray flux is low compared to the active state in 2021, but exhibits variability by a factor of ∼2 on a timescale of a month. We discuss the origin of the hard X-ray emission and explore its connection to gamma rays and high-energy neutrinos. We explain the X-ray data, including both quiescent and active states, using a radiatively inefficient accretion flow (RIAF) model with a variable accretion rate. We also show that TeV gamma rays cannot escape from the RIAF disk, and very high-energy gamma rays observed by LHAASO are likely to originate from outer regions such as jets and winds, which is consistent with our results favoring a magnetically arrested disk state of the RIAF disk. We also discuss hidden neutrino emission from RIAFs together with possible connections to coronae of active galactic nuclei with standard, radiatively efficient disks.

Accretion disks in active galactic nuclei (AGN) are promising sites for mergers of stellar-mass black holes (BHs) detectable via gravitational waves (GWs). These environments facilitate both in situ formation and dynamical capture of compact objects and their subsequent mergers. The uncertain origin of GW events detected by LIGO, Virgo, and KAGRA motivates searching for accompanying electromagnetic (EM) signatures. Here, we investigate postmerger EM flares associated with jets launched from merger remnants, as well as from the shocked ambient gas as the jet breaks out of the disk. We find that jet breakout produces luminous gamma-ray emission, detectable with MeV-band telescopes. Cooling emission from a shocked circum-BH minidisk, winds, and background AGN disk peaks in the UV and optical, with durations ranging from about an hour to a month, and can be identified through year-long monitoring of ∼10 ³ AGNs with luminosities ranging from ∼10 ⁴⁴ to ∼10 ⁴⁵ erg s ⁻¹ . With a single set of parameters, this postmerger jet model produces gamma-ray, hard X-ray, and optical flares similar to those claimed to be associated with GW events. Furthermore, by incorporating a transition from a high- to low-angular-momentum accretion state after the merger, the model avoids excessive BH growth, alleviating tensions with hyper-Eddington accretion scenarios.

Superluminous supernovae (SLSNe) are rare transients that are ∼10–100 times more luminous than ordinary stellar explosions, reaching peak optical luminosities ∼10 ⁴⁴ –10 ⁴⁵ erg s ⁻¹ . The energy source powering SLSNe remains uncertain. In this work, we explore the multiwavelength and multimessenger signatures of the scenario in which SLSNe are powered by a newly born millisecond magnetar. We model the dynamical evolution and emission from the coupled system comprised of the magnetar, wind, nebula, and supernova ejecta, consistently evaluating the pair multiplicity of the wind and nebula regions, and the bulk wind Lorentz factor governing the e ⁺ − e ⁻ injection spectra in the nebula. We compute the thermal and nonthermal electromagnetic signatures, neutrino signatures, and investigate their detection prospects. For SN 2017egm, the nearest observed SLSNe, our prediction for high-energy gamma rays matches the recent detection by the Fermi Large Area Telescope. For neutrinos, using SN 2017egm a canonical SLSNe, we find that in the era of the Vera C. Rubin Observatory, a stacking analysis with upcoming neutrino observatories can lead to 3 σ detection significance of neutrino events from a population of SLSNe within a decade of operation.

Gamma-ray bursts (GRBs) are among the most extreme transients in the Universe, but their explosion and emission mechanisms remain unclear. To investigate the nature of GRB jets, we focus here on X-ray flares (XFs) and extended emissions (EEs), which are X-ray emissions that occur 100–1000 s after the main burst. These emissions can be observed by recently developed multiwavelength facilities. In this paper, we calculate the associated multiwavelength emission from XFs and EEs under the hypothesis that they are optically thin synchrotron emissions produced by nonthermal electrons in relativistic jets. Considering a range of dissipation radii r _diss and Lorentz factors Γ of the jet, we determine the parameter space in which detectable emission can be produced at each wavelength. We find that simultaneous ultraviolet and very-high-energy gamma-ray emission associated with XFs or EEs can be detected by Swift/The Ultra-violet Optical Telescope (UVOT), SVOM/Visible Telescope (VT), and Cherenkov Telescope Array Observatory (CTAO) approximately every 3 yr. The detection and nondetection rates for each instrument are key to determining the uncertain yet essential parameters required to understand the physics of GRB jets.

We report the first identification of molecular clumps directly associated with the rebrightening regions of the large-scale X-ray jets of SS 433, based on ¹² CO ( J = 1–0) observations with the Nobeyama 45 m Radio Telescope. Multiple clumps are detected toward the eastern and western jet heads, showing clear spatial correlation with the X-ray emission. The X-ray emission peaks immediately downstream of the molecular clumps, while the hardness ratio is enhanced at their surfaces, indicating that the observed structures cannot be explained by absorption effects. These results provide direct evidence for shock–cloud interactions between the jets and the surrounding interstellar medium. We suggest that turbulence generated at the jet–cloud interface amplifies magnetic fields, producing the observed nonthermal X-ray emission. Our findings highlight the importance of jet–interstellar medium interactions in shaping the X-ray properties of microquasar jets.