Gamma Ray Bursts
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.
Ghisellini, Gamma Ray Bursts: Basic Facts and Ideas; http://arxiv.org/abs/1010.3015
Xue at al., Very High Energy Gamma-Ray Afterglow Emission of Nearby Gamma Ray Bursts, The Astrophysical Journal (2009), 703, 1, p. 60-67; http://arxiv.org/abs/0907.4014