Cosmic explosions bursting with charged energy

Until recently, it has been impossible to collect usable multi-wavelength data from a GRB. In a huge collaborative project, an international team of researchers gathered comprehensive data from two GRBs that occurred in 2018 and 2019, using multiple ground-based telescopes and satellites. Their analyses, published in three papers in Nature, reveal the highest energy photons known to have been released by GRBs to date. The data also enabled the researchers to determine how these photons are generated, and provide new insights into how emissions from GRBs evolve over time. 

Razmik Mirzoyan at the Max Planck Institute of Physics in Munich, Germany, and his team, including Aquib Moin at the United Arab Emirates University, analysed and modelled multi-wavelength data from a GRB identified in January 2019. 

“In the first 30 seconds, the intensity of gamma rays from this GRB turned out to be 130 times stronger than from the Crab Nebula, the strongest steady source of very-high-energy gamma rays in our galaxy,” says Mirzoyan. “This is the strongest ever measured signal in ground-based very-high-energy gamma ray astronomy. We followed the development of the GRB spectrum from one minute after its explosion to a few hours later.”

After an initial bright GRB blast that lasts milliseconds to several tens of seconds, an afterglow occurs that can continue for days, in which the jet interacts with its surroundings, creating shockwaves that release emissions over a wide frequency range from radio waves to gamma rays. The team found that the GRB emitted photons at unprecedented energy levels, ranging from 0.2 to 1 teraelectronvolts, from around one minute after the initial blast. The researchers believe that electrons present during the blast transferred energy to these photons through a process called inverse Compton scattering. This dramatically increased the energy of the photons in the afterglow. 

Their results chimed with data from another GRB, analysed by Edna Ruiz-Velasco at the Max-Planck Institute for Nuclear Physics in Heidelberg, Germany and co-workers. In a later analysis, they found photons of 100 to 400 gigaelectronvolts in the afterglow ten hours after the initial blast. Crucially, the multi-wavelength spectrum datasets fit well with prediction models that incorporate inverse Compton scattering into GRB behaviour. 

“This marks a very significant step towards understanding the high energy phenomena at work in these most extreme cosmic explosions,” notes Werner Hofmann, Director Emeritus at Max Planck Institute for Nuclear Physics, who was not directly involved in the study. “It also proves that ground-based gamma ray telescopes can catch GRBs in the early afterglow phase, which is very exciting for future studies.”

“All our data are open access. I hope that the global scientific community will use them to make rapid progress in understanding these monster explosions,” says Mirzoyan.

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