SGR 1806-20
SGR (Soft Gamma ray Repeater) 1806-20 is the most powerful known magnetar. A magnetar is a neutron star with an extra-powerful magnetic field. On Dec 27, 2004, the Earth experienced a gamma ray burst originating from this magnetar. This was the brightest known extra-solar event so far recorded, with a brightness that (if it had been visible light instead of gamma rays) approached twice that of the moon. This is remarkable, considering that SGR 1806-20 is about 50,000 light years away from us.
Let's do some calculations! First, define constants:
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So, the first question is, what is the energy density of the magnetic field at the surface of the magnetar?
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That's some massive energy density, 1.59 × 
Joules per cubic meter. To get an idea of how much, let's convert this energy to mass, using Einstein's formula E = m
and see what we get.
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So we get a mass equivalent of 1.77×
kilograms, or 177 million tons per cubic meter. Or 177 tons per cubic centimeter, since there are a million cubic centimeters in a cubic meter. That's a lot of mass. Let's relate it to something more familiar, say nuclear explosions. The Hiroshima bomb had an energy equivalent of 15 kilotons of TNT (upper estimate), or about 63×
Joules. Converting this to mass equivalent:
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As we can see, the mass equivalent of the Hiroshima bomb is about 0.7 grams. So each cubic centimeter of space near the surface of this magnetar contains energy equivalent to 252 million Hiroshima bombs. The most powerful nuclear test by the United States was Castle Bravo, which was 1000 times stronger than Hiroshima. The magnetic field energy density would therefore be about a quarter of a million Castle Bravos per cubic centimeter.
