  Variable X-ray and gamma-ray emission is characteristic of the most extreme
physical processes in the Universe, and studying the sources of these energetic
photons has been a major driver in astronomy for the past 50 years. Here we
present multiwavelength observations of a unique gamma-ray selected transient,
discovered by Swift, which was accompanied by bright emission across the
electromagnetic spectrum, and whose properties are unlike any previously
observed source. We pinpoint the event to the center of a small, star-forming
galaxy at redshift z=0.3534. Its high-energy emission has lasted much longer
than any gamma-ray burst, while its peak luminosity was about 100 times higher
than the brightest active galactic nuclei. The association of the outburst with
the center of its host galaxy suggests that this phenomenon has its origin in a
new, rare mechanism associated with a massive black hole in the nucleus of a
galaxy.
