  Detecting neutrinos and photons is crucial to identifying the sources of
ultrahigh-energy cosmic rays (UHECRs), especially for transient sources. We
focus on ultrahigh-energy gamma-ray emission from transient sources such as
gamma-ray bursts, since >EeV gamma rays can be more direct evidence of UHECRs
than PeV neutrinos and GeV-TeV gamma rays. We demonstrate that coincident
detections of about 1-100 events can be expected by current and future UHECR
detectors such as Auger and JEM-EUSO, and the detection probability can be
higher than that of neutrinos for nearby transient sources at <50-100 Mpc. They
may be useful for constraining the uncertain cosmic radio background as well as
knowing the source properties and maximum energy of UHECRs. They can also give
us more than 10^4 times stronger limits on the Lorentz-invariance violation
than current constraints.
