  A new method of investigating ultra-high energy cosmic ray sources is
suggested. The method is based on analysis of gamma-ray emission that is
generated in extragalactic space when ultra-high energy cosmic particles
interact with cosmic background. We have found that intensity of the gamma-ray
emission depends on characteristics of cosmic ray sources, specifically on
their remoteness and initial particle energy spectra. In the Earth atmosphere
cosmic rays initiate air showers, therefore selecting quanta-initiated showers
(and excluding those from the galactic plane, gamma-ray sources, etc.) we can
obtain above mentioned source characteristics. We derive that the number of
quanta-initiated showers is 0 or ~3x1000 depending on source parameters,
typical statistics of showers registered at 10^14 eV being of ~10^8. The
difference is large enough to use this method for studying ultra-high energy
cosmic ray sources.
