University of Geneva | 2019 Jan 14
During the formation of a black hole a bright burst of very energetic light in the form of gamma-rays is produced, these events are called gamma-ray bursts. The physics behind this phenomenon includes many of the least understood fields within physics today: general gravity, extreme temperatures and acceleration of particles far beyond the energy of the most powerful particle accelerators on Earth. In order to analyse these gamma-ray bursts, researchers from the University of Geneva (UNIGE), in collaboration with the Paul Scherrer Institute (PSI) of Villigen, Switzerland, the Institute of High Energy Physics in Beijing and the National Center for Nuclear Research of Swierk in Poland, have built the POLAR instrument, sent in 2016 to the Chinese Tiangong-2 space laboratory, to analyze gamma-ray bursts. Contrary to the theories developed, the first results of POLAR reveal that the high energy photons coming from gamma-ray bursts are neither completely chaotic, nor completely organized, but a mixture of the two: within short time slices, the photons are found to oscillate in the same direction, but the oscillation direction changes with time. ...
When two neutron stars collide or a super massive star collapses into itself, a black hole is created. This birth is accompanied by a bright burst of gamma-rays – very energetic light such as that emitted by radioactive sources – called a gamma-ray burst (GRB).
“Our international teams have built together the first powerful and dedicated detector, called POLAR, capable of measuring the polarization of gamma-rays from GRBs. This instrument allows us to learn more about their source,” said Xin Wu ... Its operating system is rather simple. It is a square of 50 x 50 cm^2 consisting of 1,600 scintillator bars in which the gamma rays collide with the atoms that make up these bars. When a photon collides in a bar we can measure it, afterwards it can produce a second photon which can cause a second visible collision. “If the photons are polarized, we observe a directional dependency between the impact positions of the photons, continues Nicolas Produit ... On the contrary, if there is no polarization, the second photon resulting from the first collision will leave in a fully random direction.” ...
Detailed polarization measurements of the prompt emission of five gamma-ray bursts ~ Shuang-Nan Zhang et all
- Nature Astronomy (online 14 Jan 2019) DOI: 10.1038/s41550-018-0664-0