Gamma Ray Bursts
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.
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.
Further Reading
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 |
|
Photons from a gamma ray burst racing towards the Fermi satellite. (Image: NASA/Sonoma State University/Aurore Simonnet )
|