RHESSI observes a magnetar

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Number: 3
1st Author: Hugh Hudson
2nd Author: Kevin Hurley
Published: 2005-07-05
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A fortunate spacecraft, RHESSI had the privilege of making decisive observations of the most powerful cosmic gamma-ray burst ever observed. Fortunate, because its source was only a few degrees from the Sun and so RHESSI (along with other Sun-pointed observatories) could observe it relatively well. What is a magnetar? See Robert Duncan's magnetar home page for virtually all knowledge of these amazing objects. Or see aquĆ­ for a Spanish-language version. In brief a magnetar is a kind of pulsar, therefore a neutron star (see here for a basic explanation) resulting from a gravitational collapse. This results in an extraordinarily dense stellar object, which may have an extraordinarily large magnetic field - some 1015 gauss (for comparison, the Earths magnetic field is only 0.5 gauss!).

The RHESSI observations

Here is a time history of the outburst of 2004-Dec-27 21:30:00:

Figure 1: The time history of the 27 December 2004 magnetar outburst.

The burst lasts for a few hundred seconds and exhibits a huge modulation, which produces the comb-shaped pattern. The inset shows the initial moment (the "giant flare" characteristic of these magnetar outbursts, of which only a few have ever been seen).

The wonderful thing about the RHESSI observations is that RHESSI is the first gamma-ray observatory in space to have a large volume of germanium detectors on board. These provide extremely high "non-dispersive" spectrometry, since they are a form of proportional counter; RHESSI can determine the energy of every photon detected individually. This is usually a good thing for gamma-ray astronomy, where there are few photons, but this burst was extraordinarily bright - brighter than the brightest solar flare, even though at a distance of 15 kpc beyond the far-off center of the Galaxy. The hard X-ray and gamma-ray spectrum turns out to be a blackbody spectrum at very high temperature, quite unlike anything RHESSI was designed for since the solar corona is entirely optically thin for these radiations.

Why magnetars are like solar flares

The amazing thing about this observation is that the basic theory of magnetar giant flare physics sounds very like the physics of solar flares, just scaled far, far up in energy (although far, far down in physical size) and in magnetic field. Here's a Robert Duncan cartoon:

Figure 2:Robert Duncan's cartoon of the basic physics of a magnetar burst.

One can see the basic ingredient of energy storage outside the star in the form of twisted magnetic loops - really, in the form of field-aligned electric currents.

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