Enigmatic Explosions from the distant universe

Photons from a gamma ray burst racing towards the Fermi satellite. (Image: NASA/Sonoma State University/Aurore Simonnet )

The Most Luminous Objects in the Universe
Gamma-Ray Bursts (GRBs) are the most powerful explosions in the Universe, and are by far the most luminous sources of light known apart from the Big Bang.
The peak luminosity of GRBs, equivalent to the light from millions of galaxies, means they can be detected across the entire Universe, allowing us to probe the evolution of stars back to before galaxies as we know them had formed. GRBs occur at random on the sky and then fade from view. Their rapid gamma- and X-ray variability indicates a small source size which, together with their huge luminosities and clearly non-thermal spectrum, requires the emitting region to be moving towards us at practically the speed of light. Indeed, GRBs are thought to be powered by ultra-relativistic jets produced by rapid accretion onto a newly formed stellar-mass black hole or a rapidly rotating highly-magnetised neutron star. The prompt gamma-ray emission is thought to originate from dissipation within the original outflow by shocks as shells of material collide, or by magnetic reconnection events.

Illustration of the production of a GRB by a massive collapsing star. http://arxiv.org/abs/astro-ph/0102255

The Highest Energy Emission from GRBs
Recent observations by the Swift and Fermi missions have revealed an even more complex behaviour than previously thought, featuring significant spectral and temporal evolution. As yet, no GRB has been detected at energies >100 GeV due to the limited sensitivity of current instruments and the large distance of these events. However, data from Fermi imply that a good fraction of the brightest GRBs could be detected by CTA in just a few minutes by quickly re-pointing some of the array.
Detecting GRBs in the CTA energy range would greatly enhance our knowledge of the intrinsic spectrum and the particle acceleration mechanism of GRBs. CTA could address the relative importance of of the various proposed emission processes, which divide mainly into leptonic and hadronic processes, and help determine if they are the origin of ultra-high-energy cosmic rays, the highest energy particles known to exist in the present Universe . The speed and the composition of the outflows can also be probed by CTA. They can even serve as beacons for probing cosmic radiation fields in the distant Universe, as well as in testing the accuracy of Einstein’s theory of relativity.
Overall, a large discovery space at high energies is readily accessible to CTA. The combination of GRBs being extreme astrophysical sources and cosmological probes make them prime targets for future high-energy experiments. With its large collecting area, energy range and rapid response, CTA is by far the most powerful and suitable VHE facility for GRB research and will open up a new energy range for their study.
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.

The faint, distant galaxy in which a gamma-ray burst exploded in 1997 appears as a faint smudge in the center of this image from the Hubble Space Telescope. Even in images from the Hubble, most gamma-ray bursts originate from so far away that the galaxies in which they occur appear blurry and faint.

The narrow twin beams of a gamma-ray burst blasts out of a dying star in this artist's conception, just as the star itself begins to explode in a supernova. Both events were triggered by the collapse of the star's core under its own gravity.

What is a gamma-ray burst?

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.

Where do gamma-ray bursts come from?

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.

What makes a gamma-ray burst?

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.