  (Abridged) We interpret gamma ray bursts as relativistic, electromagnetic
explosions. Specifically, we propose that they are created when a rotating,
relativistic, stellar-mass progenitor loses much of its rotational energy in
the form of a Poynting flux during an active period lasting $\sim 100$ s.
Initially, a non-spherically symmetric, electromagnetically-dominated bubble
expands non-relativistically inside the star, most rapidly along the rotational
axis of the progenitor. After the bubble breaks out from the stellar surface
and most of the electron-positron pairs annihilate, the bubble expansion
becomes highly relativistic. After the end of the source activity most of the
electromagnetic energy is concentrated in a thin shell inside the contact
discontinuity between the ejecta and the shocked circumstellar material. This
electromagnetic shell pushes a relativistic blast wave into the circumstellar
medium. Current-driven instabilities develop in this shell at a radius
$\sim3\times10^{16}$ cm and lead to dissipation of magnetic field and
acceleration of pairs which are responsible for the $\gamma$-ray burst. At
larger radii, the energy contained in the electromagnetic shell is mostly
transferred to the preceding blast wave. Particles accelerated at the forward
shock may combine with electromagnetic field from the electromagnetic shell to
produce the afterglow emission.
