High-energy neutrino astronomy has blossomed. We have now found that the high-energy
cosmic background neutrino flux is extended beyond PeV. The energy flux above 100 TeV to
PeV is comparable to that of ultra-high energy cosmic rays (UHECRs), suggesting that the
neutrino background radiation may originate from the same class of objects to emit
UHECRs. However, most sources responsible for the 100-TeV energy neutrino emissions are
still unknown. We have identified two neutrino sources, a peculiar blazar galaxy TXS
0506+056 and a starburst galaxy NGC1068 but their flux and spectra have indicated that
these classes of galaxies cannot account for the major fraction of the high-energy
neutrino emissions, and thereby they are not UHECR sources. In this talk, we present
that the most probable candidate sources are x-ray transient objects such as
low-luminosity gamma-ray bursts or tidal disruption events, based on model-independent
arguments. We demonstrate how current and future multimessenger observations involving
X-rays pin down the unified origin of neutrinos and UHECRs. Followup observations to
search for X-ray transients responding to the IceCube neutrino signal detections are
especially powerful for accessing the parameter space to characterize such sources. We
argue that the Xtend instrument mounted in the XRISM mission has great potential to
probe such x-ray transient emissions associated with neutrinos, as its great flux
sensitivity and the 900 arcmin squared field of view exactly benefit the detection from
the majority of neutrino sources, which are most likely located in the deep space.