Highly luminous rapid flares are characteristic of processes around compact
objects like white dwarfs, neutron stars or black holes. In the high energy
regime of X- and gamma-rays, outbursts with variability time-scales of seconds
and faster are routinely observed, e.g. in gamma-ray bursts or Soft Gamma
Repeaters. In the optical, flaring activity on such time-scales has never been
observed outside the prompt phase of GRBs. This is mostly due to the fact that
outbursts with strong, fast flaring usually are discovered in the high-energy
regime. Most optical follow-up observations of such transients employ
instruments with integration times exceeding tens of seconds, which are
therefore unable to resolve fast variability. Here we show the observation of
extremely bright and rapid optical flaring in the galactic transient SWIFT
J195509.6+261406. Flaring of this kind has never previously been reported. Our
optical light-curves are phenomenologically similar to high energy light-curves
of Soft Gamma Repeaters and Anomalous X-ray Pulsars, which are thought to be
neutron stars with extremely high magnetic fields (magnetars). This suggests
similar emission processes may be at work, but in contrast to the other known
magnetars with strong emission in the optical.Comment: 8 pages, 3 figures. A substantially revised version of this
manuscript was published in Nature. Due to license issues, the accepted
manuscript will only be put on astro-ph as v2 6 months after this versio