Using simulations, researchers from the RIKEN Cluster for Pioneering Research and their colleagues have shown that the origin of photons emitted by long gamma-ray bursts—one of the most energetic events to occur in the universe—is the photosphere, which the visible portion of the “relativistic jet” that exploding stars emit.
Within a second, gamma-ray bursts, which are the most powerful electromagnetic phenomena to be viewed in the universe, emit so much energy as the sun releases over its entire lifetime. Despite being found out in the year 1967, the mechanism underlying this tremendous emission of energy has been mysterious for a long time. Following decades of research, it was finally discovered that the origin of long bursts—one type of burst—is the relativistic jets of matter released when massive stars die. Yet, it is still obscure exactly how the gamma-rays are generated from the jets.
The present study, reported in Nature Communications, started from a discovery known as the Yonetoku relation, originally made by one of the authors of the study. This relation between the peak luminosity and the spectral peak energy of GRBs is the most stringent correlation encountered so far in the properties of GRB emission. Thus, it offers the perfect diagnostic so far to explain the emission mechanism, and the most stringent test for any gamma-ray burst model.
Unexpectedly, the relationship is also suggestive of the fact that it could be possible to use long gamma-ray bursts as a “standard candle” for evaluating the distance, enabling researchers to look further into the past than type 1A supernovae, which are more common in use at present but are considerably dimmer when compared to the bursts. This would render it feasible to understand not just the history of the universe but also about mysteries like dark energy and dark matter.
The researchers used computer simulations conducted on various supercomputers, such as Aterui of the National Astronomical Observatory of Japan, Hokusai of RIKEN, and Cray xc40 of the Yukawa Institute for Theoretical Physics, to focus on what is known as the “photospheric emission” model, one of the pioneering models for the emission mechanism of GRBs.
According to this model, the photons visible on earth are released from the photosphere of the relativistic jet. The expansion of the jet makes it easier for the photons to escape from within the jet as there are fewer objects available to scatter the light. Therefore, the “critical density”—the point at which the photons are free to escape—moves downward through the jet, to material that was originally at higher and higher densities.
The researchers tested the validity of the model by testing it in a manner that accounted for the global dynamics of relativistic jets and radiation transfer. They used a combination of radiation transfer calculations and 3D relativistic hydrodynamical simulations to measure the photospheric emissions from a relativistic jet bursting out of a massive star envelope to determine that the model worked at least with respect to long GRBs—the type related to such collapsing massive stars.
As part of their simulations, it was demonstrated that it is possible to reproduce the Yonetoku relation as a natural outcome of the jet-stellar interactions.
To us, this strongly suggests that photospheric emission is the emission mechanism of GRBs. 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.
Hirotaka Ito, Cluster for Pioneering Research, RIKEN