Background for CSIRO media
release: Monster star blast ‘brighter than full moon’
until 0600 AEDT 19 February 2005 (1900 GMT 18 February)
Magnetars are a kind of neutron star—a city-sized ball of neutrons created
from a star’s core when the star explodes as a supernova at the end of
its life. Magnetars have magnetic fields a thousand million million times stronger
than Earth’s [about 1000 trillion gauss, compared to Earth’s 0.5
gauss]. Only about a dozen are known.
At the time of the flare, the gamma-ray detector on NASA’s Swift satellite
was pointing away from SGR 1806-20. But the flare was so powerful that the radiation
travelled through the back of the satellite and into the detector, saturating
Four of the known magnetars are also called ‘soft gamma repeaters’,
or SGRs, because they flare up randomly and release gamma rays. Such episodes
usually release about 1030 to 1035 watts for about a second, or thousands to
millions of times more energy than our Sun does.
But the flare from SGR 1806-20 is about 100 times greater than any seen before
from a soft gamma repeater.
The radio observations show that material expelled at high speed from the star
is now ploughing through the thin gas lying in space around the star. The radio
nebula is expanding at about a quarter of the speed of light.
The tremendous magnetic fields around a magnetar are responsible for SGR outbursts,
but scientists don't know exact mechanism. The flare could be analogous to solar
flares in which magnetic field lines become twisted and then suddenly ‘snap’,
reconnecting in new configurations. But the flare might also have resulted from
a quake on the surface of the neutron star.
SGR 1806-20 is a well-known object. Its rate of rotation had previously been
measured to be 7.56 seconds. Surprisingly, the giant flare does not seem to
have slowed its spin.
A scientific debate raged in the 1980s over whether gamma-ray bursts were star
explosions from beyond our Galaxy or eruptions on nearby neutron stars. By the
late 1990s it became clear that long-period gamma-ray bursts appear to be supernovae
that lead to the formation of black holes. The extraordinary flare from SGR
1806-20 suggests that such flares could account for some of the short-period
gamma-ray bursts (those lasting for less than two seconds).
More information on magnetars and soft gamma repeaters
Bryan Gaensler, Harvard-Smithsonian Center for Astrophysics
Dr Bob Sault, CSIRO Australia Telescope National Facility
+61-2-6790-4050 (observatory, direct)
+61-2-6790-4000 (observatory, main)
Dr Simon Johnston, CSIRO Australia Telescope National Facility
Dr Dick Hunstead, University of Sydney
+61-2- 9999 1847 (home)
Dr Phil Wilkinson, IPS Australia
+61-2-417 508 003 (mob)
Katherine Newton-McGee, University of Sydney
Dr Brian Schmidt, Research School of Astronomy and Astrophysics, ANU (for independent
Dr Andrew Melatos, University of Melbourne (for independent comment)
Helen Sim, CSIRO Australia Telescope National Facility