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GRB 970228
Artist's conception of the Swift satellite observing a supermassive black hole emitting jets of x-rays and gamma rays.

(Credit: NASA) (Larger image)

Lingering Mysteries

Although astronomers feel they have a good grasp on what triggers gamma-ray bursts with the collapsar/hypernova model, they know that many questions remain. To begin with, as we discussed at the outset, this model only deals with long-duration GRBs -- those lasting more than 2 seconds and having an average duration of about 30 seconds -- and that have a clearly defined burst followed by a clearly defined afterglow of progressively less energetic light. In 2003, the High Energy Transient Explorer 2 (HETE-2) began to see evidence of afterglows from short duration GRBs. But the afterglows in these initial studies were too short to determine a distance to these bursts. Furthermore, the properties of these short-duration bursts indicate that they appear to be triggered by a fundamentally different physical process, perhaps involving the merger of neutron stars. No one really knows. In addition, some GRBs are insufficiently energetic and fall into a category called "X-ray flashes" (XRFs). The BATSE instrument could not "see" these XRFs.

One suggestion for the existence of such short-duration GRBs and XRFs -- is that our viewing angle from Earth is slightly off the blast axis, so we're really looking at the very edge of the radiation "cone" created by these bursts. Yet this idea would seem to have trouble explaining the observation that some GRBs seem to "turn off" only to briefly "turn back on" at full power. Such findings blur the tidy distinction between GRBs and their less energetic afterglow assumed in the collapsar model.

Other lingering mysteries concern the nature of the narrow jets of radiation and material shot out from the collapsing star. Are the jets of radiation uniform or do they vary in intensity? Astronomers now think they vary to some extent, with their energy rising and falling. Perhaps this would explain some of the variability seen in gamma-ray bursts in their intensities with time. Another question concerns how clumpy or uniform the stellar material in the jets -- is there a uniform density or does it vary?

Knowing the nature of the material in the jets is important in determining whether the model of gamma-ray bursts presented here is the definitive account of their origin. This model, which is called the collapsar/hypernova model, is also sometimes known as the "fireball" model because the gamma-rays are produced inside the star as the stellar core collapses and the jets of material explode outward. The interaction of the jets with the interstellar medium outside the star produces the less energetic afterglow. An alternate view held by some astronomers is known as the "cannon ball model." In this model, the gamma-rays are produced when blobs of stellar material (the "cannon balls") in the jets collide with the material in the interstellar medium, rather than inside the star.

In mid 2005, NASA is planning to launch into orbit the Swift Gamma-Ray Burst Mission. The Swift satellite will be NASA's most sophisticated GRB-detecting satellite ever with sensitivity five times better than the BATSE. It will also perform follow-through observations of the afterglow with an X-ray and a UV/optical telescopes. These will both automatically be pointed to a burst location within a minute a GRB is detected. The light from the afterglow will be analyzed to look for the characteristic "light curves" of a supernova explosion.

Once successfully in operation, the Swift mission will join HETE, RXTE, Integral, and the IPN array as the space-based side of the on-going, collaborative international effort by scientists on Earth to gain better understanding into gamma-ray bursts and what these titanic, distant explosions reveal about our awesome Universe.

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Nicholas White (Director), within the Exploration of the Universe Division (EUD) at NASA's Goddard Space Flight Center.

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Project Leader: Dr. Jim Lochner
Curator:Meredith Bene
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