  We report a major outburst from the Anomalous X-ray Pulsar 1E 2259+586, in
which over 80 X-ray bursts were detected in four hours using the Rossi X-ray
Timing Explorer. The bursts range in duration from 2 ms to 3 s and have
fluences in the 2-10 keV band that range from 3 x 10^{-11} to 5 x 10^{-9} erg
cm^{-2}. We simultaneously observed increases of the pulsed and persistent
X-ray emission by over an order of magnitude relative to quiescent levels. Both
decayed significantly during the course of our 14 ks observation. Correlated
spectral hardening was also observed, with the spectrum softening during the
observation. In addition, we observed a pulse profile change, in which the
amplitudes of the two peaks in the pulse profile were swapped. The profile
relaxed back to its pre-outburst morphology after ~6 days. The pulsar also
underwent a sudden spin-up (df/f = 4 x 10^{-6}), followed by a large (factor of
\~2) increase in spin-down rate which persisted for >18 days. We also observed,
using the Gemini-North telescope, an infrared enhancement, in which the K_s
(2.15 um) flux increased, relative to that measured in a observation made in
2000, by a factor of ~3, three days post-outburst. The IR counterpart then
faded by a factor of ~2 one week later. In addition, we report an upper limit
of 50 uJy on radio emission at 1.4 GHz two days post-outburst. The X-ray
properties of this outburst are like those seen only in Soft Gamma Repeaters.
This conclusively unifies Anomalous X-ray Pulsars and Soft Gamma Repeaters, as
predicted uniquely by the magnetar model.
