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Short-long or Long-short Gamma-ray Outburst May Signal New Way to Make Black Holes

(From the March/April 2007 issue of StarDate magazine)

Black holes are the darkest objects in the universe because their gravity is so strong that not even light can escape from them. Yet the birth of a black hole may create one of the brightest objects in the universe, known as a gamma-ray burst.

An example is a burst that appeared last June 14. The flash of gamma rays, which are the most powerful form of electromagnetic energy, lasted just 1 minute, 42 seconds. In that short span, the gamma-ray burst emitted more energy than our Sun will produce in its 10-billion-year lifetime. This burst was an oddball, though, which has astronomers wondering if they witnessed a new mechanism for producing black holes.

Before the June burst, astronomers had identified two basic types of gamma-ray bursts. The first lasts less than two seconds and may occur when two neutron stars merge to form a black hole, or a neutron star and a black hole merge to form a bigger black hole. The other type of burst lasts from two seconds up to a few minutes, and probably occurs when the core of a massive star collapses, also forming a black hole.

The June outburst had some traits of each type. It lasted long enough to be a collapsing star, but it did not produce a supernova explosion, which accompanies other long-duration bursts. On the other hand, it resembled a merger event in some ways, although it lasted far too long. No matter the details, though, the outburst probably produced a black hole.

So astronomers are trying to decide what happened. Perhaps this was a collapsing star, but the star was different from others. Or perhaps the black hole swallowed the star’s outer layers, preventing them from blasting into space. -- Damond Benningfield