Simulating Stimulating Bursts of Gamma Rays

May 5, 2015

Using powerful computer simulations, a KIPAC scientist explores the possible mechanisms behind the gamma-ray emission in the super explosions known as gamma-ray bursts.

Artist's conception of a GRB. We see the burst of gamma rays if the jets are oriented so that one points toward us. (Image courtesy of NASA)

Gamma-ray bursts (GRBs) are the most energetic events in the Universe. As such, they involve physics under conditions that can't be achieved anywhere else. As their name implies, the defining characteristic of GRBs is a temporary flash of gamma-rays, the most energetic photons of light. Typically a GRB has a so-called 'prompt' emission lasting few seconds where we see the flash of gamma rays, followed by a much longer 'afterglow' emission where we see visible, radio, and x-ray light. One of the most important goals of high energy astrophysics is to achieve a coherent picture of the astrophysics of gamma-ray burst emission.
 
GRBs are likely to originate from both the collapse of individual giant stars and the mergers of two neutron stars, when during the formation of a black hole intense beams of particles are shot out. Thus GRBs begin as extreme particle accelerators! The details of what actually happens to go from highly accelerated particles to the GRB emission we see can only be studied in detail with computer simulations, since the energy scales and huge sizes are beyond the reach of any laboratory experiment.
 
KIPAC postdoc Jonathan McKinney uses such simulations to explore how the prompt emission in GRBs comes to be. The simulations require as input the laws governing 'magnetohydrodyamics', the physics of very high energy charged particles and very strong magnetic fields, of the type that must be present in GRBs. The programs then follow particles, fields, and radiation as they all interact according to the rules laid down.
 
There are two primary mechanisms that are thought to be candidates for the prompt emission: 'internal shocks', which happen when bunches of the ultra-fast moving particles run into each other, and 'electromagnetic dissipation', where vast quantities of energy are released when intense electromagnetic fields twist up and spring free. The later would be a much higher energy analog of the flares and bubbles on the surface of the Sun. In a recent paper, McKinney and colleague Dimirty Uzdensky of the University of Colorado have described the implications from their detailed simulations on electromagnetic dissipation in GRBs. They explore in detail the potential for dissipation to give rise to observed characteristics of the prompt emission, such as its duration, intensity, and spectrum. They show that under certain constraints electromagnetic dissipation could be an important factor in prompt emission.
 
MHD simulations are a powerful and necessary tool for exploring not just the astrophysics of GRBs, but other high energy physics environments, such as the plasmas where fusion can occur. Developing the MHD capabilities to understand the physics in GRBs is an important piece of a larger effort to predict the behavior of plasmas in general.  Since the particles accelerated by the processes in GRBs often reach comparable energies to those achieved in our manmade particle accelerators here on Earth - although of course on much larger scales in the case of GRBs - it is interesting to contrast how nature achieves this acceleration with our methods.
 
This work is based on a paper submitted to Monthly Notices of the Royal Astronomical Society and available from astro-ph at arXiv:1101.1904
 
Science Contact:
Jonathan Mckinney
KIPAC
jmckinne@stanford.edu