Scientists from the RIKEN Cluster for Pioneering Research and their collaborators have shown that the photons emitted by long gamma-ray bursts (GRBs) originate in the photosphere, that is, in the visible portion of the relativistic jet that is emitted by exploding stars.

Using computer simulations performed on supercomputers, the group focused on the so-called photospheric emission model, a leading model for the emission mechanism of GRBs. This model postulates that the photons visible on Earth are emitted from the photosphere of the relativistic jet. As the jet expands, it becomes easier for photons to escape from it, since there are fewer objects available to scatter the light. Thus, the “critical density” — that is, the place where it becomes possible for the photons to escape — moves downward through the jet to material that was originally at higher densities.

The researchers set out to test the validity of the model in a way that took into account the global dynamics of relativistic jets and radiation transfer. They used a combination of 3D relativistic hydrodynamical simulations and radiation transfer calculations to evaluate photospheric emissions from a relativistic jet breaking out of a massive star envelope, followed by a post-process radiation transfer calculation in 3D. They performed three sets of simulations to cover a wide range of model parameters. In each simulation, a jet with a different kinetic power was considered. The researchers were able to determine that at least in the case of long GRBs — the GRB type associated with collapsing massive stars — the model worked.

The team’s simulations revealed that the Yonetoku relation — the relation between the spectral peak energy and peak luminosity of GRBs, and the tightest correlation found so far in the properties of GRB emission — could be reproduced as a natural consequence of the jet-stellar interactions. “To us, this strongly suggests that photospheric emission is the emission mechanism of GRBs,” said researcher Hirotaka Ito.

“While we have elucidated the origin of the photons, there are still mysteries concerning how the relativistic jets themselves are generated by the collapsing stars. Our calculations should provide valuable insights for looking into the fundamental mechanism behind the generation of these tremendously powerful events,” Ito said.

The research was published in Nature Communications (https://doi.org/10.1038/s41467-019-09281-z).