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
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.
On November 20th, 2004, NASA launched into orbit the Swift
Gamma-Ray Burst Mission. The Swift satellite is NASA's most
sophisticated GRB-detecting satellite ever with sensitivity five times
better than the BATSE. It also performs follow-through
observations of the afterglow with X-ray and UV/optical
telescopes. These are both automatically pointed to a burst location within a minute of a GRB being detected. The light from the afterglow is analyzed to look for the characteristic "light curves" of a supernova explosion.
The Swift mission joins 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.