National Centre for Nuclear Research: The first results of the POLAR detector measurements

The POLAR detector is the largest device of this type designed to measure the polarization of gamma-ray bursts (GRB) - short-term X-ray signals from sources located at cosmological distances from the Earth. In just a few seconds, these sources emit more energy than the Sun during its entire life - but it is unclear how they do it!

Representatives of the National Centre for Nuclear Research (NCBJ) explain in the press release sent to PAP that the directions of gamma-ray bursts are not repeated - which means that emissions are probably accompanied by an irreversible cosmic disaster.

Currently, GRBs are observed on average once a day, by several satellite detectors. Since the 1960s and 1970s, the directions of bursts, intensities and energies of gamma photons and their variability over time have been measured. The POLAR detector launched in September 2016 aboard the Chinese space laboratory Tiangong-2, has opened a new "window": measuring the polarization of this radiation.

POLAR - as the NCBJ representatives emphasize - is large enough and sufficiently precise to perform measurements of many bursts and reliably determine the polarization of gamma quanta from GRB. The first data on the polarization of five gamma-ray bursts published in Nature Astronomy show that the determined degree of polarization of photons in bursts is very small in all cases. In the case of the brightest burst, it was possible to measure the polarization separately in subsequent moments. It turned out that in each moment of measurement a high polarization was found, but the direction of polarization was rotating over time.

The NCBJ representatives reveal that this phenomenon may be very interesting for scientists: the observed polarization requires a directional arrangement of the source of emission, and the fast variation of the direction of polarization suggests a new - unknown and unexplored - emitter property. But in order to better understand the GRB emission process, we need to build a much larger detector than POLAR. Currently, scientists are preparing a more efficient detector POLAR-2. They hope to launch it in 2022 on the next Chinese space station.

Polish scientists and engineers from NCBJ cooperated on key elements of the POLAR experiment - POLAR is the result of cooperation between Polish, Chinese and Swiss scientists. One of the achievements of this cooperation was the design and construction of the central case selection system and software. Due to the limited possibility of communication between the satellite device and the Earth, the transmitted data must be subject to selection in space. For example, events caused by ionising cosmic ray particles are rejected. The events of highest interest to scientists are those, during which the detector has at least a double dispersion of the gamma photon in a very short space of time. Such cases are used to determine the polarization of gamma-ray photons with a burst.

In the NCBJ laboratory in Łódź, a prototype of a high-voltage power supply for POLAR`s 25 photomultipliers was built according to the design of outstanding electronics engineer Jacek Karczmarczyk, who died in 2016. The NCBJ also prototyped plastic scintillation detectors for the detection of gamma radiation. The matrix of 1600 such scintillators is the heart of the POLAR detector.

Polish researchers and engineers also participated in all phases of detector testing during the qualifying and functional tests. According to the representatives of NCBJ, all components of the detector must withstand extreme conditions: vacuum, violent shocks, high load, high and low temperatures, as well as high doses of radiation.

PAP - Science in Poland

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