4 Conclusions

It is shown, that counterparts of cosmological GRB due to interaction of gamma-radiation with dense interstellar media are "longliving" objects, existing for years after GRB. To distinguish GRB counterpart from a supernova event, having similar energy output, it is necessary to take into account its unusual light curve and spectrum. In the optical region of the spectrum the strongest emission lines are H$_{\alpha}$ and H$_{\beta}$.

The volume occupied by dense interstellar medium in the parent galaxy is relatively small, so the probability of getting a bright counterpart of this kind is also small. Nevertheless, discovery of even one optical counterpart of GRB with properties described above would give an opportunity to probe the density of the interstellar medium around the burst, and therefore would give an indication of the burst progenitor.

Acknowledgements

Part of this work was supported by Russian Foundation of Basic Research (RFFI) under grant 96-02-16553.