Giant Space Blasts a Two-step Process
An artist's impression of merging neutron stars, one of the theoretical progenitors of gamma-ray bursts.
Credit: Sonoma State University, NASA E/PO, Aurore Simonnet.

Scientists have taken another step closer to explaining mysterious gamma ray bursts, some of the most energetic and bright explosions in the sky.

Gamma ray bursts (GRBs) - the harbingers of death for some massive stars - are intense blasts of energy and radiation that eject from massive dying stars.

Now, scientists at the Istituto Nazionale di Astrofisica (INAF) in Italy have used NASA's Swift satellite - specially designed to automatically detect GRBs - to determine that GRBs are a two-step explosion.

"The first burst of energy, lasting less than a few minutes, is produced by shockwaves within the collapsing star," study author Sergio Campana told in an email. "Whereas the longer, less energetic afterglow is produced by collisions between ejected matter and the material around the star, witnessing the X-ray light curves in the transition period from prompt emission to afterglow."

This research is detailed in the Aug. 18 issue of the journal Nature.

The soft X-ray afterglow following the initial GRB can last for periods ranging from hours to weeks. This afterglow was initially thought to be the slow fading of the initial burst.

But after closely observing five GRBs and measuring the patterns of X-ray emission, the INAF scientists determined that the afterglow was instead caused by violent shock interactions caused by the initial high-intensity blast.

"The duration of GRBs, together with their spectral properties, suggest a classification into short and long bursts," Clemson University physicist Dieter Hartmann wrote in an accompanying 'News and Views' piece to the Nature article.

Short GRBs are believed to be caused by the merging of compact binary stars, such as two neutron stars. However, scientists are not yet positive this explanation is definite and plan to continue to use the Swift satellite to investigate the nature of these bursts.

When explaining long GRBs, scientists favor the "collapsar" model. In this model, a rapidly rotating massive star has undergone extreme gravitational collapse and created a black hole. A disk of material from the collapsed star forms around the outer rim of the black hole. Through complex processes, a jet of high-energy radiation shoots from this disk, bursting through the surface of the star at almost the speed of light.

Intense bursts of high-frequency gamma ray and X-ray radiation from exploding stars were discovered nearly four decades ago. Since NASA launched the Swift satellite in Nov. 2004, its detectors have picked up a burst every couple of days.

The Swift satellite is the most autonomous spacecraft NASA has designed, using its three telescopes to detect and observe GRBs all on its own, automatically detecting the first stage of the explosion and targeting itself on the event in about one minute.