  We study about ultra-high-energy cosmic rays (UHECRs) from transient sources,
propagating in the Galactic and intergalactic space. Based on the recent
observational results, we also estimate upper and lower bounds on the rate of
transient UHECR sources and required isotropic cosmic-ray energy input per
burst as 0.1 Gpc^-3 yr^-1 < rho_0 < 10^3.5 Gpc^-3 yr^-1 and 10^49.5 ergs <
E_HECR^iso < 10^54 ergs, through constraining the apparent burst duration,
i.e., dispersion in arrival times of UHECRs. Based on these bounds, we discuss
implications for proposed candidates such as gamma-ray bursts and active
galactic nuclei.
