We investigate potential sources of UltraHigh-Energy Cosmic Rays (UHECRs) and their acceleration mechanisms, focusing on astrophysical phenomena associated with massive stellar deaths and supermassive black holes. These phenomena include gamma-ray bursts (GRBs), engine-driven supernovae/hypernovae, magnetars, newly born pulsars, binary neutron star mergers (BNS), tidal disruption events (TDEs), and active galactic nuclei (AGN). While high-luminosity GRBs (HL GRBs) are constrained as UHECR sources by high-energy neutrino observations, low-luminosity GRBs (LL GRBs) and engine-driven supernovae emerge as more promising candidates, with intermediate-mass nuclei as the dominant components. Compact binary mergers and r-process nucleosynthesis in neutron-rich environments may also contribute to ultraheavy UHECRs.

The composition models of TDEs depend on the properties of the disrupted stars. AGN, particularly radio galaxies, are significant sources, with acceleration occurring in their large-scale jets and lobes. Shear acceleration mechanisms have been proposed as a viable alternative for accelerating UHECRs, as they involve the acceleration of low-energy cosmic rays and are compatible with the observed spectrum and composition.

Future multi-messenger observations, especially from upcoming observatories, are expected to provide critical data to refine our understanding of UHECR origins, test existing models, and explore new acceleration mechanisms.