Supernova explosions, triggered when the fuel within a star reignites or its core collapses, launch a shock wave that sweeps through a few light-years of space in only a few hundred years. The remnants of these explosions are now recognized widely as one of nature’s major particle accelerators. The theory is that charged particles increase in energy through repeated encounters with magnetic “mirrors” or changing magnetic fields in the shocks. Now, a team of researchers has brought some of these processes down to Earth, in an experiment to investigate the turbulent amplification of magnetic fields in the supernova remnant, Cassiopeia A, which was first seen about 300 years ago in the constellation Cassiopeia.
Radio observations of Cassiopeia A have revealed regions within the expanding remnant that are consistent with synchrotron radiation emission from giga-electron-volt electrons spiralling in a magnetic field of a few milligauss – 100 times higher than expected from the standard shock compression of the interstellar medium. The origin of such high magnetic fields, which help to make Cassiopeia A a particularly effective particle accelerator and bright radio source, appears to lie with regions of turbulence that could amplify the magnetic field and that could be related to puzzling irregular “knots” seen in optical observations. One explanation for these knots is that the shock produces turbulence as it passes through a region of space that already contains dense clumps or clouds of gas.
To investigate these possibilities, an international team led by Gianluca Gregori at Oxford University used the Vulcan laser facility at the UK’s Rutherford Appleton Laboratory to focus three laser beams onto a carbon rod 0.5 mm thick in a chamber filled with low-density gas. The heat generated made the rod explode, creating a blast that expanded through the surrounding gas, mimicking a supernova shock wave. To simulate the clumps that might surround an exploding star, the team introduced a mesh of fine plastic wires 0.4 mm thick with cells 1.1 mm square at a distance of 1 cm from the rod. Using hydrodynamical scaling relations, the team can relate the experimental conditions 0.3 μs after the laser burst to Cassiopeia A as it is now, about 310 years after the supernova explosion. With the same scaling, the wire thickness corresponds to a distance of about one parsec in the remnant.
The researchers used various techniques to monitor the evolution of the shock wave, including an induction coil to measure the magnetic fields produced. The measurements show that the grid produces additional turbulent flow and gives rise to magnetic-field components that are 2–3 times larger than without it. The results are also in good agreement with the output from numerical simulation code, in particular, the magnetohydrodynamic code FLASH, developed by Don Lamb at Chicago University. The simulations reproduce well the position of the shock, the peak electron density and temperature – with and without the grid – and confirm that the magnetic field is indeed enhanced as a result of induced turbulence created as the shock moves through the grid.
These results demonstrate that the amplification of the magnetic field within the Cassiopeia A “particle accelerator” might indeed arise from the interaction of the shock with a clumpy interstellar medium. Importantly, the experiment also gives valuable confirmation of the simulations, providing for the first time an experimental means to validate the simulation codes used for many astrophysical phenomena.
Further reading
J Meinecke et al. 2014 Nature Physics 10 520.
