A giant outburst of the soft gamma-ray repeater SGR 1806-20
SGR 1806-20 which belongs to the class of soft gamma-ray repeaters (SGR)
is believed to be a rotating neutron star with a super-strong magnetic
field (10^15 Gauss); a so-called magnetar. The tremendous outburst of
SGR 1806-20 on December 27, 2004 as seen by the large anticoincidence
shield (ACS) of the INTEGRAL-spectrometer SPI is shown in the figure.
The mean veto count rate of the ACS (~88000 counts/s) is interrupted at
21:30:26.539 UTC (T=0) by a steep count-rate increase (about a factor 25)
for about 0.7 s. This outburst is thought to be caused by a large-scale
rearrangement to a state of lower energy of a magnetar's super-strong
magnetic field (10^15 Gauss), which is the current model for soft
gamma-ray repeaters. The ~300 s long pulsating tail with a period
of 7.56 s is clearly seen and can be explained by a trapped fireball
which is co-rotating with the neutron star (magnetar). The initial peak
is proceeded at T= -143 s by a small precursor, which could be shown via
triangulation to originate from the position of SGR 1806-20.
