Gamma-ray bursts (GRB) have left astronomers scratching their heads since the late 1960s when they were discovered by U.S. military satellites. Part of the mystery began to unlock when astronomers at Northwestern University detected the first observational evidence for the remnants of hypernovae, explosions hundreds of times more powerful than supernovae, last year. Hypernovae may be the possible source of GRBs, making them the most energetic events known in the Universe besides the Big Bang.
Northwestern astronomer Daniel Wang identified two hypernova remnants in galaxy M101, also known as the Pinwheel galaxy some 25 million light years away, in April 1999. The remnants were previously thought to be supernovae remnants, but Wang detected strong X-ray emission from them which led him to believe it was an explosion much more powerful than a supernova. One nebula, MF83, has a radius of over 430 light years and is one of the largest remnants known. The other nebula, NGC5471B, is expanding very fast at a velocity of 100 miles per second. The X-ray light from these nebulae is brighter than the brightest supernova remnants known. After Wang calculated the energy needed to produce these remnants, he concluded they were most likely a result from a hypernova.
These are two of the most unusual remnants known, Wang said. We see that they are bright in X-ray even at a distance of 25 million light years away. They must be from spectacular explosions.
Bohdan Paczynski, of Princeton University, first introduced the concept of a hypernova in 1998 as a why to explain GRBs. Gamma-ray bursts are brief but intense blasts of high-energy radiation. They only last for about 3 seconds, but in that brief time they can release enough energy to be more luminous than the rest of the universe. Paczynski theorized that a hypernova is most likely related to the formation of black holes. The collapse of a massive star and/or its merger with a neutron star could generate more energy than an average star explosion. This is a very possible theory because they have found evidence that GRBs appear close to massive star-forming regions.
The Burst and Transient Source Experiment (BATSE) instrument aboard NASA s Compton Gamma Ray Observatory satellite has recorded more than 2,000 bursts, about one a day, since its launch in 1991. It also plotted their positions all around the sky and found that GRBs don t concentrate in the plane of the Milky Way, where most of the Milky Way stars are located. Until then, astronomers didn t know if the GRBs are coming from objects on the outskirts of our galaxy or from distant galaxies. These findings pointed to an origin from distant galaxies. In 1997, the Italian-Dutch BeppoSAX satellite pinpointed bursts with enough accuracy to allow optical and radio telescopes to view the burst s sources for the first time.
On April 25, 1998, The galaxy ESO 184-G82 was host to a combined GRB and
supernova explosion. Using Hubble s Space Telescope Imaging Spectrograph,
astronomers obtained the first detailed images of a galaxy in which a GRB occurred. This burst, GRB 980425, is the closest ever detected at 125 million light years away and thus must have been 1000 to 1,000,000 times fainter than normal bursts. Also, an unusually bright supernova, SN 1998bw, was seen in exactly the same location merely a day after the burst. Several groups of astronomers have followed the development of this event closely over the last two years. On June 12, 2000, a group of European astronomers obtained very detailed observations of the galaxy ESO 184-G82, the first time such a galaxy has been observed in such fine detail. The new observations allow astronomers to investigate the GRB phenomenon in much greater detail. Most astronomers today believe that GRB 980425 and SN 1998bw (less than 24 hours apart) did come from the same source and has made connection between the two most energetic events in the Universe highly possible.
Astronomers, with these new discoveries, are coming closer to unlocking the mystery of the GRB phenomenon. They still know very little about the true nature of GRBs and hypernovae. I suspect GRBs may well be just a tip of an iceberg, as we have no clue why some explosions generate so much gamma-ray emission, Paczynski said. These discoveries will help our understanding of the history of the universe, how it all started. To work out the actual mechanisms involve, astronomers need more cases and more accurate positions of such events, allowing them to look at them from ground-based telescopes.