  The unified models of astrophysical sources to account for ultrahigh-energy
cosmic rays (UHECRs) and high-energy cosmic neutrinos with energies greater
than 100 TeV have been discussed. Based on model-independent arguments, we
argue that if the photomeson production is the dominant mechanism, the most
probable candidate sources are x-ray transient objects, allowing for the
semi-transparency for the photomeson production. We develop a generic model of
high-energy neutrino emitters accompanied by x-ray emission, and present how
multimessenger observations can place significant constraints on the source
parameters that characterize the common sources of neutrinos and UHECRs, such
as the cosmic-ray loading factor. The requirements of UHECR acceleration,
escape, and energetics further constrain the magnetic field and the bulk
Lorentz factor of the sources. The resulting bounds provide diagnoses of the
unified models, which demonstrates the importance of current and future x-ray
facilities such as MAXI and Einstein Probe.
