One of the greatest mysteries of observational astronomy during the past 3 decades has been the nature of gamma-ray bursts (GRBs). The most powerful blasts in the cosmos, these flashes of light randomly appear throughout the sky every day, giving astronomers few clues about the origins of these elusive bursts.
On January 23, 1999, an intense gamma-ray burst exploded with the energy of 100 million billion stars. Hubble’s camera caught the interloper in a galaxy two-thirds of the way to the edge of the visible universe. One of the strongest Gamma-Ray Burst was observed by NASA on April 27th, 2013 named GRB 130427A. NASA released the data on November 21st, 2013. According to the Fermi space observatory, the Gamma-Ray had an energy of 94 billion electron volts.
Some GRBs last for only a fraction of a second — some as long as a minute — and beam so much energy in a focused searchlight that they make even supernovae appear weak in comparison. The long-standing mystery of gamma-ray bursts — are they powerful events in our Milky Way Galaxy or super-powerful events beyond it? — is now heading toward resolution.
The key to unravelling the nature of gamma-ray bursts comes in part from the discovery that they are narrowly focused beams. This realization allowed astronomers to estimate energies for individual bursts and hypothesize the number of total bursts occurring over a given time interval. “If you didn’t know the geometry,” says Shri Kulkarni of the California Institute of Technology, “and assumed spherical emission when it’s really conical, you could infer an energy release 1,000 times bigger than it really is. And if the beams are as narrow as we think they are, for every GRB I see, there are 1,000 I don’t see.”
A key moment in researching gamma- ray bursts occurred suddenly March 29, 2003, when a brilliant burst in the constellation Leo appeared in the data collectors of NASA’s High Energy and Transient Explorer (HETE-2) satellite. By immediately capturing the burst and its afterglow, HETE-2 showed GRB 030329 to be 2.6 billion light-years away and revealed its association with a bright supernova that exploded at the same time. This led researchers to link the most common type of GRB, those lasting 20 seconds or longer, with the collapse of massive stars about 30 or more times larger than the Sun. The stars go supernova and create powerful black holes in the process. The next big step came in November 2004, when NASA launched the Swift Gamma-Ray-Burst Mission.
The Swift satellite has orbited Earth since, observing gamma-ray bursts. Almost a year later, in October 2005, astronomers using Swift solved the 35-year-old mystery of one class of gamma-ray bursts known as short bursts, those lasting just a few milliseconds. What could produce enough radiation to equal that of a billion Suns in such a short period? On May 9, 2005, Swift detected a short burst, marking the first time for a short burst that astronomers detected an after- glow — something more common with longer bursts.
Are gamma-ray bursts common in normal galaxies? In 1997, Hubble’s Wide Field Planetary Camera 2 captured GRB 970228’s visible glow, the first that linked a gamma-ray burst with a specific host galaxy. Astronomers estimate the GRB’s host galaxy’s redshift is 0.835, which corresponds to a distance of hundreds of millions of light-years.
In 1997, astronomers using the Hubble Space Telescope imaged a gamma-ray burst that was briefly as bright as the rest of the universe. GRB 971214, which lies about 12 billion light-years away, released 100 times more energy than astronomers previously thought possible. “We had a hunch that short gamma-ray bursts came from a neutron star crashing into a black hole or another neutron star, but these new detections leave no doubt,” says Derek Fox, an astronomer at Pennsylvania State University. Fox’s team discovered the afterglow with NASA’s Chandra X-ray telescope.
The afterglow was also observed by a team led by Jens Hjorth of the University of Copenhagen, using the Danish 1.5m telescope at La Silla Observatory in Chile. Another gamma-ray burst was spotted July 9 with HETE-2. According to George Ricker of MIT, “The July 9 burst was like the dog that didn’t bark. Powerful telescopes detected no supernova as the gamma-ray burst faded, arguing against the explosion of a massive star.
Also, the July 9 burst, and probably the May 9 burst, are located in the outskirts of their host galaxies, just where old merging binaries are expected.” So after 35 years, a key piece of the puzzle about gamma-ray bursts appears solved. Astronomers do not yet know the details of how these incredibly energetic objects work, but they know the causes of many such bursts.
Over the coming months and years, Swift, HETE-2, and other instruments will further refine the picture until it becomes crystal clear — an exciting moment in science.
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