Measurement of five flashes from the depths of the universe

They shine far in the distance, briefly and extremely brightly: Roughly once a day, researchers can observe so-called gamma-ray bursts coming from the depths of the universe. These flashes of light occur – hence their name – in the high energy regime of gamma radiation. They discharge, in just a few seconds, more energy than the sun emits over millions of years. So far, however, their origin is still to a large extent unclear.

To make headway on this question, an international research team has now measured a particular property of these flashes of light: their degree of polarisation. Light propagates as waves; the degree of polarisation indicates whether such waves from a source oscillate parallel to each other – that would be a high degree of polarisation – or randomly to each other, in which case this value would be small.

The results the researchers have now published show that gamma-ray bursts have a relatively low degree of polarisation. Moreover, the researchers observed indications of a change in the polarisation angle – that is, the orientation of this slight polarisation – over the course of individual gamma-ray bursts.

Sorting out theories on gamma-ray bursts

The various existing theories on the origin of gamma-ray bursts lead to predictions of different levels of polarisation. Phenomena under consideration as the possible source of a gamma-ray burst include the collapse of a massive star into a black hole, the fusion of two neutron stars, a special kind of supernova, and a series of other processes involving high energies and masses. Thanks to our data, some of these theories on the origin mechanisms can now arguably be ruled out, says Wojciech Hajdas of the Paul Scherer Institute PSI, who played a leading role in the development of POLAR, the instrument that delivered the new measurement data.

POLAR is a state-of-the-art detector that PSI researchers working with Hajdas developed in collaboration with scientists from the University of Geneva and IHEP-CAS, the Institute of High Energy Physics of the Chinese Academy of Sciences. In September 2016, the instrument was launched into orbit aboard the newest Chinese space station, Tiangong 2. From that vantage point, POLAR collected data by capturing, relaying, and analysing the light of gamma-ray bursts with the help of 1600 special plastic bars. The flight into space was crucial for the project, since a precise ground-based measurement of the degree of polarisation is hindered by Earth's atmosphere.

Both the concept and the electronics of the POLAR detector system were essentially developed at PSI. The signal readout module and the central computer of POLAR were then produced and tested at PSI. In addition, the researchers developed their own software to process the data. To calibrate POLAR before its mission, the researchers used X-rays from the Swiss Light Source SLS at PSI and the European Synchrotron Radiation Facility (ESRF) in Grenoble. The Zurich-based company Art of Technology fabricated the power supplies for POLAR. At the University of Geneva, the plastic bars, mechanics and housing were built, and then the components of POLAR were put together.

The virtual PSI POLAR Data Center

In all, POLAR was able to register several dozen of gamma-ray bursts; of these, five events with the greatest predictive power have now been evaluated by the international research team. In addition, the PSI researchers have made the data from all registered events available to the research community on the virtual platform PSI POLAR Data Center. Since a number of the gamma-ray bursts measured by POLAR were also observed by other detectors around the world, the platform also serves, among other things, to help researchers cross-calibrate their own respective data sets.

Text: Paul Scherrer Institute/Laura Hennemann

About PSI

The Paul Scherrer Institute PSI develops, builds and operates large, complex research facilities and makes them available to the national and international research community. The institute's own key research priorities are in the fields of matter and materials, energy and environment and human health. PSI is committed to the training of future generations. Therefore about one quarter of our staff are post-docs, post-graduates or apprentices. Altogether PSI employs 2100 people, thus being the largest research institute in Switzerland. The annual budget amounts to approximately CHF 390 million. PSI is part of the ETH Domain, with the other members being the two Swiss Federal Institutes of Technology, ETH Zurich and EPFL Lausanne, as well as Eawag (Swiss Federal Institute of Aquatic Science and Technology), Empa (Swiss Federal Laboratories for Materials Science and Technology) and WSL (Swiss Federal Institute for Forest, Snow and Landscape Research).

(Last updated in May 2018)