Gamma-ray bursts are the most luminous explosions in the universe. They are classified as either short or long, with long gamma-ray bursts resulting from the extinction of massive stars. In addition to radio waves, optical lights, and X-rays, gamma-ray bursts also release gamma rays. When the conversion of explosion energy to radiated energy, i.e. conversion efficiency, is high, the total explosion energy can be calculated by simply adding all the radiated energy together. But when the conversion efficiency is low or unknown, more is needed than just measuring the energy emitted.
A team of astrophysicists has now succeeded in measuring the hidden energy of a gamma-ray burst using light polarization. When an electromagnetic wave oscillates in only one direction, it is said to be polarized. Although stars do not emit polarized light, they do reflect polarized light. Polarization is a common technique to reduce glare from light sources moving in one direction. Examples of these products are sunglasses and light curtains.
In astrophysical observations, measuring the polarimetry of a celestial body is not as simple as measuring its brightness. But it provides valuable information about the physical state of objects.
The group investigated a gamma-ray burst that occurred on Dec. 21, 2019. (GRB191221B). They calculated the polarimetry of fast fading emissions from GRB191221B using the European Southern Observatory’s Very Large Telescope and the Atacama Large Millimeter/submillimeter Array. The optical and radio polarizations were then successfully measured simultaneously and it was found that the degree of radio polarization was much lower than the optical one.
Professor Kenji Toma of Tohoku University’s Frontier Research Institute for Interdisciplinary Sciences (FRIS) said: “This difference in polarization at the two wavelengths reveals detailed physical conditions of the gamma-ray burst emission region. In particular, it allowed us to measure the previously unmeasurable hidden energy.”
The scientists found that the total energy was about 3.5 times what was expected, taking into account the hidden energy.
Being able to measure this quantity has important implications for calculating the masses of stars, as the explosion energy represents the gravitational energy of the precursor star.
toma said, “Knowing the dimensions of the true mass of the progenitor star will help understand the evolutionary history of the universe. The first stars in the universe could be discovered if we can detect their long gamma-ray bursts.”
- Yuji Urata, Kenji Toma, Stefano Covino, et al. Simultaneous Radio and Optical Polarimetry of GRB 191221B Afterglow. Nature Astronomy. DOI: 10.1038/s41550-022-01832-7