Enigmatic explosions from the distant universe
About once a day, something remarkable happens: the sky is lit up by a brilliant flash of energy. For a fleeting few seconds, this mysterious burst - coming from a seemingly random direction, different every time - ranks among the brightest objects in the sky.
Yet no one has ever witnessed such a flash directly: the energy comes almost entirely in the form of gamma rays, which human eyes cannot detect. Even if our eyes were sensitive to this extremely energetic form of radiation, gamma rays cannot penetrate the atmosphere. Only via orbiting satellites do we know of the presence of these mysterious blasts.
These events are known as gamma-ray bursts, or GRBs. They represent the most powerful explosions of energy in the cosmos since the Big Bang itself, corresponding to the equivalent of a thousand Earths vaporized into pure energy in a matter of seconds. One of the most enduring mysteries of the universe since their discovery in the 1960s, only recently have they begun to reveal their secrets.
We define a gamma-ray burst based on its observational properties: an intense flash of gamma rays, lasting anywhere from a fraction of a second to up to a few minutes.
Gamma-ray bursts have a few other common features. We believe them to be beamed - the energy does not escape from the explosion everywhere equally, but is focused into a narrow jet (or more likely, two oppositely-directed jets.) The burst itself is also normally followed by a much longer-lived (but also much fainter) signal, visible at optical and other wavelengths. This so-called "afterglow", discovered only in the 1990s, allows us to pinpoint the origin of the GRB - something not possible from the short-lived gamma-ray signal alone.
For a long time, it was believed that GRBs must come from within our own Galaxy. It seemed impossible that they could be much more distant: for a gamma-ray burst to have come from a distant galaxy, it would have to be incredibly powerful to explain its observed brightness.
And yet we now know that, except perhaps for a few rare exceptions, most GRBs do indeed come from other galaxies - often from among the most distant galaxies in the known universe! The closest GRB known to date is still over a hundred million light-years away, and most of them come from billions of light years. To outshine our own Galaxy's closest stars in our sky from distances that are literally billions of times further away, stupendous amounts of energy are required.
No one knows for sure! Our best theory to date is based upon several observed facts. First, the only way to generate huge quantities is via gravitational collapse, and black holes can be very efficient at turning this energy into explosive power. Second, some of the closest GRBs appear to occur simultaneously with supernovae: explosions of stars at the end of their lives. Finally, almost all GRBs happen in galaxies containing large numbers of very massive stars.
Our conclusion: GRBs happen when an extremely massive star, at the end of its life, runs out of fuel and can no longer support itself. It collapses onto its core, crushing it into a black hole. Matter from the star falls towards the black hole at its center, and before it falls in, some of its energy is focused into powerful jets that pummel out of the north and south poles of the star, making a gamma-ray burst. The rest of the star explodes as a supernova soon afterwards.
Other origins are also possible. For example, some GRBs may be due to two ultra-dense neutron stars smashing into each other; and a small fraction may be magnetic eruptions on neutron stars in very nearby galaxies.
Page authored by D. Perley (University of California, Berkeley)
All images were taken from the NASA public image archive and are in the public domain. (HST image is credit of S. R. Kulkarni and S. G. Djorgovski; the GRB animation is credit Aurore Simonnet SSU NASA E/PO.)