Gamma-ray bursts reveal exciting information about the Universe’s evolution

A team of astrophysicists has succeeded in measuring a gamma-ray burst’s hidden energy by utilising light polarisation.

Gamma-ray bursts are the most luminous explosions in the Universe, allowing astrologists to observe intense gamma rays in short durations. They are classified as either short or long, with long gamma-ray bursts being the result of massive stars dying out. This means that they can provide hidden clues about the evolution of the Universe.

Gamma-ray bursts emit gamma rays, as well as radio waves, optical lights, and X-rays. When the conversion of explosion energy to emitted energy, i.e., the conversion efficiency, is high, the total explosion energy can be calculated by simply adding all the emitted energy. When the conversion efficiency is low or unknown, measuring the emitted energy alone is not enough.

The team was led by Dr Yuji Urata from the National Central University in Taiwan and MITOS Science CO., LTD and Professor Kenji Toma from Tohoku University’s Frontier Research Institute for Interdisciplinary Sciences (FRIS). Their work was published in the journal Nature Astronomy.

How polarisation measured previously hidden energy

When an electromagnetic wave is polarised, it means that the oscillation of the wave flows in one direction. While light emitted from stars is not polarised, the reflection of that light is. Many everyday items, such as sunglasses and light shields, utilise polarisation to block out the glare of light travelling in a uniform direction.

Measuring the degree of polarisation is referred to as polarimetry. In astrophysical observations, when measuring celestial objects like gamma-ray bursts, polarimetry is not as easy as measuring its brightness. But it offers valuable information on the physical conditions of objects.

The team looked at a gamma-ray burst which occurred on December 21, 2019 (GRB191221B). Using the Very Large Telescope of the European Southern Observatory, they calculated the polarimetry of fast-fading emissions from GRB191221B. Then, the team successfully measured the optical and radio polarisations simultaneously, finding the radio polarisation degree to be significantly lower than the optical one.

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

How can we use this information to understand the formation of the Universe?

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 the mass of stars.

Toma concluded: “Knowing the measurements of the progenitor star’s true mass will help in understanding the evolutionary history of the Universe.

“The first stars in the Universe could be discovered if we can detect their long gamma-ray bursts.”

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