Might some gamma ray bursts be an observable signature of natural wormholes?

The extragalactic microlensing scenario for natural wormholes is examined. It is shown that the main features of wormhole lensing events upon the light of distant Active Galactic Nuclei (AGNs) are similar to some types of already observed Gamma Ray Bursts (GRBs). Using recent satellite data on GRBs, an upper limit to the negative mass density -- ${\cal O} (10^{-36})$ g cm$^{-3}$ -- under the form of wormhole-like objects is presented.Comment: extended version, additions on GRB physics, background sources and cosmological consequences. Two ps figures. Accpeted for publication in Phys. Rev.