Light Shed on Radiation from Outer Space


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The origin of cosmic rays -- the radiation that provides intriguing insights into the nature of matter throughout the universe -- may have been clarified in a new Institute study, parts of which appeared in a recent issue of Astrophysical Journal Letters.

Profs. Mordehai Milgrom and Vladimir Usov of the Department of Condensed Matter Physics have found evidence that cosmic rays of particularly high energy originate from the same cosmological outbursts that give rise to gamma-ray bursts -- electromagnetic waves that are similar to X-rays, but with shorter wavelengths. If this new association between cosmic rays and gamma rays proves correct, it will greatly enhance our understanding of their source, and facilitate the effort to decipher the valuable clues they both evince about the physical conditions of our galaxy and beyond.

Milgrom and Usov analyzed the two highest-energy cosmic ray showers known, and traced the likely course of their journey to earth on a cosmic map in order to locate their probable source. They then went through the records of gamma-ray bursts from the year or two preceding the arrival of these showers to determine whether any of the bursts appeared to originate from the same location as the cosmic rays. In both cases, they found a strong correspondence between the regions where the cosmic rays and the gamma rays seem to have originated. This means that both types of rays were evidently produced by the same cataclysmic events.

Illustration of a cataclysmic event

Although the cosmic rays seem to begin their journey together with the gamma rays, they are detected a few months or even years later because they do not travel in a straight line. Being charged particles, they are affected by magnetic fields both within and between galaxies, which cause them to move in a snake-like fashion and delay their arrival on Earth.

"The two cases we examined do not in themselves establish the association, but they clearly suggest that these two intriguing phenomenan are related," Prof. Milgrom says. "It might be possible to corroborate our theory within the next few years, and if this happens, it will impose very important constraints on both systems and make our attempts to understand their nature far easier."
Prof. Usov joined the Weizmann Institute in 1990 from the Space Research Institute in Moscow.