Dusty gamma-ray puzzles

Low proportion of star material surprises astronomers

Artistic representation of the environment of GRB 020819B, the origin of a gamma-ray burst, based on the ALMA-measured radio data. © NAOJ
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Enigmatic explosion: For the first time, astronomers have explored a dark gamma-ray burst - a gamma-ray burst that is obscured by dense dust clouds. The surprising thing: Actually, this explosion should not have taken place there. Because it lacked the basic material for massive stars - the main players of such flashes, as the researchers report in the journal "Nature".

Gamma Ray Bursts (GRBs) are the brightest explosions in the universe. In the few seconds that such a lightning lasts, it releases about as much energy as our sun in its entire lifetime of about 10 billion years. GRBs of more than a few seconds are likely to result from massive explosions at the end of life of massive stars. The shorter flashes lasting less than two seconds, on the other hand, probably come from fused neutron stars.

Dust clouds hide starburst

Most flashes are followed by a slowly fading afterglow that can last for several weeks. Astronomers suspect that this afterglow is caused by collisions between the material ejected during the explosion and surrounding gas. However, some GRBs, the so-called dark flashes, mysteriously show no afterglow. The common explanation for this are so far dust clouds that absorb the radiation and so they do not penetrate to the observers on Earth.

However, astronomers around Bunyo Hatsukade of the National Astronomical Observatory in Japan have now come to a surprise: with the ALMA radio telescope of the European Southern Observatory in Chile, they found only relatively in the original galaxy of such a dark lightning, the object GRB 020819B at 4.3 billion light-years distance little molecular gas, but a lot of dust. On the one hand this supports the assumption that the dust absorbs the afterglow.

Enigmatic gas shortage

The small amount of molecular gas, however, gives rise to puzzles: it is the raw material from which stars are formed - without gas, no such massive stars can form, which eventually explode with a GRB. For comparison: in our Milky Way, dust accounts for about one percent of the mass of the gas - in the observed galaxies, the dust content is over ten percent. display

"We did not expect GRBs to occur in such a dusty environment with a low molecular gas to dust ratio, " explains Hatsukade. "This suggests that the gamma-ray burst occurred in an environment that is very different from a typical star formation region." This, in turn, suggests that the environment in the star formation region of the massive stars, who died as GRB, before whose explosion changed a lot: from too much gas to too much dust, which eventually even obscured the afterglow of the eventual GRB.

UV radiation blasts off gas

This change, the astronomers suspect, is caused by strong ultraviolet radiation, which also emanates from very hot and massive stars. It breaks the bonds between the atoms in the gas, which is why no molecular gas can exist in the immediate vicinity of such a star. This also applies to the stars whose explosion causes a GRB. The portion of the gas that does not serve as the base material for a star is then blown away by it.

Generation of a gamma-ray burst by the death of a massive star NASA / JPL

Since these are so far only individual data, the astronomers want to carry out further research, also with the high-resolution ALMA telescope. "We need to continue to make observations for other original GRB galaxies to see if these are general conditions around a GRB, " Hatsukade said. "We look forward to future research with the improved performance of ALMA."

(ESO, 12.06.2014 - AKR)