The purpose of this study is to investigate the association between high-energy neutrino events detected by IceCube and soft X-ray transients powered by failed or choked relativistic jets. Assuming photomeson interactions as the primary mechanism for neutrino production, such transients are promising candidates for high-energy neutrino emission. The photomeson resonance condition implies that...
IceCube has been detecting cosmic high-energy neutrinos for more than 10 years, but their sources are still unknown. Transient objects, such as peculiar supernovae and tidal disruption events are proposed as efficient neutrino emitters. To identify these transients as neutrino sources, IceCube is issuing neutrino alerts, which enable us to perform electromagnetic follow-up observations. In...
The Subaru telescope is one of the most powerful telescope for the multi-messenger astronomy. It has a wide-field imager, Hyper Suprime-Cam (HSC), with a field of view of 1.77 deg2, and multi-object spectrograph, Prime Focus Spectrograph (PFS), with 2,400 fibers in 1.25 deg2. In this presentation, we introduce the follow-up observations of gravitational waves with these instruments and future...
Since the start of operations, the IceCube Observatory has been characterizing the background flux of high-energy astrophysical neutrinos. However, only a fraction of this flux has been associated with known source populations. One of the most plausible production mechanisms for these neutrinos is photohadronic (pγ) interaction. In this scenario, the energies of PeV cosmic rays and sub-PeV...
The astrophysical origin of high-energy neutrinos detected by IceCube remains an open question. Explosive transients, such as supernovae and tidal disruption events, have been proposed as source candidates, but their contribution has not yet been firmly established. Identifying such transients as neutrino sources requires rapid and sensitive electromagnetic follow-up observations. However,...
HiZ-GUNDAM is a proposed JAXA-led mission for the 2030s designed to explore the early Universe through observations of high-redshift gamma-ray bursts (GRBs). Rapid discovery and characterization of high-energy transient phenomena are essential for identifying electromagnetic counterparts to gravitational-wave events, hidden neutrino sources, and distant GRBs. By combining wide-field soft...
During its fourth observing run (O4), the LIGO–Virgo–KAGRA Collaboration reported hundreds of gravitational-wave events. In this talk, I will summarize the gravitational-wave alerts issued during O4 and the multi-messenger follow-up observations they triggered. Despite extensive follow-up observations, no electromagnetic counterpart has so far been identified. I will then discuss what can...
Optical follow-up observations for IceCube high-energy neutrinos provide valuable insights into transient neutrino sources. However, their scientific interpretation depends critically on the analysis framework. Identifying a convincing counterpart or placing meaningful constraints from a non-detection requires a quantitative understanding of background contamination and detection efficiency....
Multi-messenger astrophysics has been a transformative approach to exploring the Universe, combining information from not only photons but also cosmic rays, neutrinos, and gravitational waves to probe the most energetic phenomena in Nature. Major discoveries have opened unprecedented opportunities to study the origins of cosmic particles and the physics of extreme environments. Japan has...
SS 433 is a unique Galactic microquasar that exhibits persistent supercritical accretion and relativistic baryonic jets. It is also one of the brightest microquasars in the TeV gamma-ray band and is considered a promising PeVatron candidate. We conducted SS 433 observations with the X-Ray Imaging Spectroscopy Mission XRISM, complemented by simultaneous optical spectroscopic monitoring. The...
