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Dr. Alicja Wierzcholska’s blazars NASA

Dr. Alicja Wierzcholska’s blazars


Blazars are active galaxies centered around supermassive black holes. They generate strong electromagnetic radiation in a very wide spectrum – from visible light to radio, infrared, ultraviolet, X-ray, and gamma radiation. All those kinds of radiation are studied by Dr. Alicja Wierzcholska (PAS Institute of Nuclear Physics), winner of START competition organized by the Foundation for Polish Science.


Blazars emit plazma in relativistic jets pointing to the Earth at a small angle to the point of observation. Due to high velocities characterizing jet particles, the processes observed are altered by relativistic effects causing, among others, an increase of blazars’ observable luminosity. The stream of radiation observed at different frequencies varies, but the variations are not usually of periodic nature. This fact attracts the attention of scientists.

“My goal is to find physical processes defining what can be observed at a given frequency. I am especially interested in high-energy radiation and gamma radiation, which is observed in tera-electronvolt range”, says Dr. Alicja Wierzcholska.

Observing such particles is not an easy task since high-energy gamma photons collide with other particles in the atmosphere and do not reach the surface of the Earth. This is why a huge-scale instrument situated beyond the atmosphere would be needed to observe them. It is not possible to locate such a large instrument in space, so scientists use Cherenkov method of observation to detect those particles. “We use the atmosphere as a large microscope. High-energy photons are dispersed by particles in the atmosphere, thus creating cascades of secondary particles, which travel at high velocities (higher than light speed in corresponding milieu) and emit Cherenkov radiation. It consists of blue light registered in optical spectrum”, explains the astronomer.

Dr. Wierzcholska is member of H.E.S.S. (High Energy Stereoscopic System) cooperation for observing sources of gamma radiation. H.E.S.S. observatory consists of five telescopes located in Namibia. Why Namibia? “Observing short flashes of blue light requires special conditions, including complete darkness. Additionally, Africa offers a view of a particularly interesting part of the sky, including the center of our galaxy. Also, clean air is important, because it guarantees more precise results”, answers Dr. Wierzcholska. Apart from high-energy particles, the scientists also studies gamma, X-Ray and ultraviolet radiation generated by blazars. From investigating relations between emission at different frequencies, conclusions may be drawn as to which area of the blazar generates radiation.

Researchers have discovered and named many blazars, including over 50 high-energy objects. Dr. Wierzcholska is especially fond of the blazar called PKS 2155-304. That number refers to the location of the blazar, which generated a very strong flash of blue light observed in 2006 with H.E.S.S. telescope. Observations in Namibia are conducted in one-month turns. The Polish astronomer has worked there in 2010 and 2012; she is now planning another cycle of observation in Africa.

Blazar light, arriving to Earth with a significant delay, helps to explain the history of the universe. The radiation carries information on light encountered by the jet on its way to Earth, allowing scientists to track changes in radiation since the Big Bang.



© PAP – Nauka w Polsce, Karolina Duszczyk/ agt




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