Astronomers identify the origin of photons in mysterious gamma rays

Scientists from the RIKEN Pioneering Research Cluster and collaborators have used simulations to show that photons emitted by long gamma-ray bursts – one of the most energetic events in the universe – originate from the photosphere – the visible part of the "relativistic jet" that is emitted by exploding stars.

Gamma-ray bursts are the most powerful electromagnetic phenomenon observed in the universe. They release as much energy in about a second as the sun will release during its lifetime. Although they were discovered in 1967, the mechanism behind this huge release of energy has long remained mysterious. Decades of studies have finally revealed that the long bursts – one of the types of bursts – came from relativistic jets of material ejected during the death of massive stars. However, the way gamma rays are produced from jets remains mysterious.

Current research, published in Nature Communications, began from a discovery called Yonetoku's relationship, which had originally been made by one of the authors of the paper. This relationship between spectral peak energy and the peak luminosity of GRBs is the closest correlation so far found in the properties of GRB emissions. It provides the best diagnostic to date to explain the emission mechanism and the most stringent test for any model of gamma ray bursts. Incidentally, this relationship also meant that long gamma-ray bursts could be used as a "standard candle" to measure distance, allowing us to look further into the past than type 1A supernovae, commonly used today but a lot weaker than bursts. . This would better understand the history of the universe and could shed light on mysteries such as dark matter and black energy.

Using computer simulations performed on several supercomputers, including Aterui of Japan's National Observatory of Astronomy, Hokusai of RIKEN and Cray xc40 of the Institute of Theoretical Physics of Yukawa, the Group's focus is on the so-called "photospheric emission" model, one of the reference models for the GRB emission mechanism. This model postulates that the photons visible on Earth are emitted by the photosphere of the relativistic jet. As the jet expands, it becomes easier for the photons to escape, as there are fewer objects available to disperse the light. Thus, the "critical density" – the place where it is possible for photons to escape – goes down through the jet to a material that was causing higher and higher densities.

To test the validity of the model, the team is committed to testing it taking into account the overall dynamics of relativistic jets and radiation transfer. Using a combination of three-dimensional relativistic hydrodynamic simulations and radiation transfer calculations to evaluate the photospheric emissions of a relativistic jet emerging from a massive star envelope, they were able to determine that, at least in the case of GRB long, the type associated with these massive star collapse – the model worked. Their simulations revealed that Yonetoku's relationship could be reproduced as a natural consequence of jet-stellar interactions. "For us," says Hirotaka Ito of the pioneering research group, "this strongly suggests that the photospheric emission is the transmission mechanism of the GRB."

He continues, "Although we have unraveled the origin of photons, there are still mysteries about how relativistic jets are generated by collapsing stars. Our calculations should provide valuable information to examine the fundamental mechanism of generating these extremely powerful events. "

Publication: Hirotaka Ito, et al., "The photospheric origin of Yonetoku's relationship in gamma-ray bursts," Nature Communications (2019), DOI: 10.1038 / s41467-019-09281-z.