Rethinking the fireball medium: Investigating gamma-ray bursts to probe the environment between the stars

Artist’s impression of a gamma-ray burst. Credit: ESO/A. Roquette

Gamma-ray bursts (GRBs) are extremely energetic explosions that have been observed in distant galaxies; they are the most luminous explosions in the Universe. A team of scientists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) from the University of Western Australia recently studied a high redshift long GRB (a more common explosion lasting between 2 seconds to several minutes) called GRB160203A. After four hours, the afterglow of this specific GRB begins rebrightening, spiking in luminosity at different times. Using data from every telescope that observed the cosmic event with the ‘fireball’ simulation model, the OzGrav team concluded that these bright features are best explained by the jet of intense light, electrons, and swept-up debris (known as the fireball) crashing into irregularities of the environment, like a fast car hitting a speed bump.

The afterglow of GRB160203A has two main parts—the ‘well-behaved’ period, lasting four hours, and the ‘unusual’ period afterwards. The fireball model suggested that, in the well-behaved period, the fireball was in the interstellar medium—the space between the stars, which consists of gas with at least ten trillion times fewer particles than air in the same volume. In the unusual period, the fireball model failed to make any predictions about the environment of the burst due to the rebrightening events. This was our clue to investigate popular modifications to the model and explain the odd behaviour.
 

A gamma-ray burst normally decreases in brightness as time passes. As the fireball travels through space, it smashes into dense pockets of the interstellar medium and increases in brightness. We can see when these interactions happened by looking for the upwards moving patterns in the flux density.

The two most popular modifications to the fireball model are the magnetar collapse model and the termination shock model. The magnetar collapse model predicts that a magnetic neutron star (a small celestial object densely packed with neutrons) collapses, injecting energy into the fireball with the quickly changing magnetic field which is then converted to light.
 
The termination shock model proposes that the fireball is passing through the boundary between the stellar wind and the interstellar medium. As the front of the fireball crashes into the boundary and slows down, the back of the fireball catches up and smashes into the front. This ‘reverse shock’ releases energy in the form of light; however, neither model can explain why there are at least two rebrightening events. The magnetar cannot collapse twice and the fireball cannot cross the stellar/interstellar medium more than once.

The other models considered involved a non-uniform medium for the fireball—a turbulence model. As the fireball enters a region of higher density, it causes a reverse shock and releases light. The source of this spike in density could come from the wind surrounding a rare Wolf-Rayet star, or the natural turbulence of the interstellar medium. Wolf-Rayet stars constantly shed material in shells around themselves due to their unstable nature. The interstellar medium, like all turbulent fluids, has pockets of high and low density scattered throughout, like the chips in a chocolate chip biscuit.
OzGrav researcher and lead of the study Hayden Crisp says: ‘We created a model of the brightness of the GRB as if it was well-behaved throughout the observations. By comparing the modelled brightness to the actual brightness, the relationship between brightness and medium density showed the fireball medium’s density over time. We saw that the rebrightening events correspond to a 5 to 50x increase in the density of the fireball medium’.
 
Crisp adds: ‘Of the two plausible models, we prefer the turbulence model as the fireball model implies the well-behaved period is in the interstellar medium. Our main conclusion from this research is that the assumption of a uniform fireball medium is inappropriate in this case. A non-uniform environment may provide a new lens to examine the growing number of unusual bursts and provides a competitive model for explaining their features’.