Discovering clues about the evolution of the universe – Measuring the hidden energy of gamma-ray bursts

Gamma Burst GRB191221B

Artist’s impression of the GRB191221B gamma-ray burst. Credit: Urata et al./Yu-Sin Huang/MITOS Science CO., LTD

Gamma-ray bursts are the brightest explosions in the cosmos, emitting intense gamma rays for brief periods. These bursts are classified as short or long, with long gamma-ray bursts being produced by the death of massive stars. That’s why they provide hidden clues about the evolution of the universe.

Gamma-ray bursts release not only gamma rays, but also radio waves, optical light, and X-rays. When the efficiency of converting energy from the explosion into emitted energy is high, the total energy of the explosion can be determined by adding up all the energy emitted. However, when the conversion efficiency is low or uncertain, measuring only the emitted energy is not sufficient to calculate the total explosion energy.

Today, a team of astrophysicists succeeded in measuring the hidden energy of a gamma-ray burst using the polarization of light. The team was led by Dr. Yuji Urata of National Central Taiwan University and MITOS Science CO., LTD, and Prof. Kenji Toma of Tohoku University Frontier Research Institute for Interdisciplinary Sciences (FRIS).

Details of their findings were recently published in the journal Natural astronomy.

When an electromagnetic wave is polarized, it means that the oscillation of this wave travels in one direction. While the light emitted by stars is not polarized, the reflection of this light is. Many everyday items such as sunglasses and light shields use polarization to block glare from lights traveling in a uniform direction.

The measurement of the degree of polarization is called polarimetry. In astrophysical observations, measuring the polarimetry of a celestial object is not as simple as measuring its luminosity. But it offers valuable information about the physical conditions of objects.

The team examined a gamma-ray burst that occurred on December 21, 2019 (GRB191221B). By using the[{” attribute=””>Very Large Telescope of the European Southern Observatory and Atacama Large Millimeter/submillimeter Array – some of the world’s most advanced optical and radio telescopes – they calculated the polarimetry of fast-fading emissions from GRB191221B. They then successfully measured the optical and radio polarizations simultaneously, finding the radio polarization degree to be significantly lower than the optical one.

“This difference in polarization at the two wavelengths reveals detailed physical conditions of the gamma-ray burst’s emission region,” said Toma. “In particular, it allowed us to measure the previously unmeasurable hidden energy.”

When accounting for the hidden energy, the team revealed that the total energy was about 3.5 times bigger than previous estimates.

With the explosion energy representing the gravitational energy of the progenitor star, being able to measure this figure has important ramifications for determining stars’ masses.

“Knowing the measurements of the progenitor star’s true masses will help in understanding the evolutionary history of the universe,” added Toma. “The first stars in the universe could be discovered if we can detect their long gamma-ray bursts.”

Reference: “Simultaneous radio and optical polarimetry of GRB 191221B afterglow” by Yuji Urata, Kenji Toma, Stefano Covino, Klaas Wiersema, Kuiyun Huang, Jiro Shimoda, Asuka Kuwata, Sota Nagao, Keiichi Asada, Hiroshi Nagai, Satoko Takahashi, Chao-En Chung, Glen Petitpas, Kazutaka Yamaoka, Luca Izzo, Johan Fynbo, Antonio de Ugarte Postigo, Maryam Arabsalmani, and Makoto Tashiro, 8 December 2022, Nature Astronomy.
DOI: 10.1038/s41550-022-01832-7

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