Gamma Ray Burst Afterglow Surprises Astronomers

NASA. Artist's impression of a Gamma Ray Burst at an unsafe distance
Theoretical models of the shockwaves from the most powerful explosions in the modern universe have been overturned, with a little help from an astrophysicist's three year old child.
 
Approximately once a day satellites pick up signals from gamma ray bursts (GRB)  – explosions so powerful we can sometimes detect them from distances of more than 13 billion light years away. The bursts are thought to come from either two neutron stars merging or massive stars collapsing to form black holes. Observations are thought to be so rare because the rays are only emitted along the axis of rotation of the newly formed black hole.
 
As the name suggests the original burst comes in the form of gamma rays, the most energetic waves in the electromagnetic spectrum. This is followed by an afterglow which, if the event is close enough, can be seen at all wavelengths. This can lead to a race to observe the glow with a variety of types of telescopes before it fades.
 
The University of Leicester's Dr Klaas Wiersema is notified when a GRB occurs that might suitable for viewing with the Very Large Telescope in Chile. He would normally have been asleep when notification for GRB 121024A came through on October 24, 2012 but his son was having trouble sleeping. He says, “I kept on running back and forth between my laptop, my phone to call the observatory in Chile, and my son's cot!"
 
The night presented a rare opportunity to study the afterglow in great detail, and Wiersema made full use of it, with his findings now published in Nature.
 
One of the ways in which GRB afterglows can be revealing is the polarization of the light emitted. Photons of light can be polarized either linearly or in a circular fashion. Linear waves are aligned in a particular direction, while circularly polarized light moves descends upon us like a corkscrew or someone climbing a spiral staircase. While sources like the sun produce a random mix photons with all polarizations, under certain circumstances light is emitted with a predominance in one sort of polarization or another. In the paper Wiersema and his fellow authors write, “Theoretical models predict low degrees of linear polarization and no circular polarization” from GRBs. 
 
While the models don't predict a lot of linear polarization they vary in just how little this will be, leading to a quest to measure this quantity. Last year a GRB from earlier in 2012 was found to be highly linearly polarized.
 
However, detecting circular polarization is not easy from something at such a great distance that fades as you watch it; no one had even tried to find it before. Wiersema's efforts that night were rewarded. 
 
“We know that the afterglow emission is formed by a shockwave, moving at very high velocities, in which electrons are being accelerated to tremendous energies. These fast moving electrons then produce the afterglow light that we detect,” Wiersema says. “Much to our surprise we clearly detected circular polarization.”
 
One of his coauthors, Dr Peter Curran of Curtin University says, the light “was about 1000 times more polarized than we expected.” Curran compared this to a 3D film, where contrasting circular polarizations are used to allow the glass in front of each eye to provide different images.
 
The paper states, “We show that the circular polarization is intrinsic to the afterglow and unlikely to be produced by dust scattering or plasma propagation effects.” The authors suggest “New models are required to produce the complex microphysics of realistic shocks in relativistic jets.”
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