CTA Science

  • Posted on: 19 January 2015
  • By: aalbert
The Cherenkov Telescope Array (CTA) is an international observatory that will study the cosmos using very-high-energy (VHE) gamma rays with energies above 30 billion electron volts (30 GeV).  A gamma ray is a type of light particle (photon) and is the most energetic form of light in the electromagnetic spectrum.  For reference, 1 trillion electron volts (1 TeV) is roughly the kinetic energy of a flying mosquito.  But a mosquito is made up of about ten trillion trillion atoms. Imagine packing that much energy into a single photon! These gamma rays are produced by the most extreme phenomena in the Universe, such as exploding stars and matter spiraling into black holes. By using the Universe as our laboratory, we are able to probe high-energy processes that are not accessible in terrestrial labs. As a key example of this capability, we can use gamma rays to search for evidence of the mysterious invisible material that comprises ~85% of the matter in the Universe -- dark matter.
A 1 trillion electron volt (1 TeV) gamma ray has roughly the same amount of energy as a flying mosquito, but packed into a single photon.
By studying VHE gamma rays, we can investigate the mechanisms of particle acceleration in astrophysical sources, the physics of black holes and astrophysical jets, the nature of the Universe between galaxies, and fundamental physics.  CTA will provide exploratory surveys of extremely energetic sources across much of the Milky Way and beyond. CTA will be able to stare deeper into space than ever before with gamma rays (out to redshift z=2), giving us access to regions where active galaxy and gamma-ray burst source populations are expected to be peaked. This gives CTA huge discovery potential to learn more about cosmic sources and high-energy processes that produce VHE gamma rays and cosmic rays.
CTA will provide crucial information regarding the nature of dark matter, complementing direct-detection searches, like LUX and searches at the Large Hadron Collider. It will help connect what we learn about dark matter in terrestrial labs to its cosmic setting.  Using our knowledge of particle creation and evolution just after the Big Bang, along with measurements of the cosmic microwave background radiation, there is a predicted natural cross section for dark matter annihilation. We have good reasons to think dark matter might be a particle that can annihilate with another dark matter particle when they run into each other. The cross section just tells us how likely it is for an annihilation to happen.

Cartoon of dark matter particles annihilating.  Dark matter particles are flying around the Universe and may occansionally run into each other and annihilate.  How often this annihilation happens depends on the cross section and amount of dark matter.  The more annihilation, the more gamma rays produced, the brighter the signal.  The high the cross section, the brighter the gamma-ray signal.
A lot of energy will be released during these annihilations, which would create many energetic particles, including gamma rays. Together with our knowledge of how much dark matter there is in a specific target region, we can predict how bright of a gamma-ray signal we would see from dark matter annihilation in that region. Though we have a good hypothesis for the interaction scale of dark matter in the Universe, we do not know the mass of these particles. A current gamma-ray experiment, the Fermi Large Area Telescope (Fermi LAT), has, for the first time, reached the sensitivity necessary to probe the natural dark matter annihilation cross section.  
Since the Fermi LAT hasn't see a definitive dark matter signal, they have set upper limits on how big the cross section can be.  If the cross section (or rate of annihilation) were higher than the limit, then the Fermi LAT would have seen it.  Though we haven't seen dark matter yet, we have just started to scratch the surface along the natural dark matter cross section.  The Fermi LAT is only sensitive to dark matter masses from 1 to 100 GeV, whereas CTA will be sensitive to dark matter masses from 100 GeV to 10 TeV.  We have just begun to reach the sensitivities necessary to search for cosmic hints of dark matter, and CTA will play a crucial role by probing higher theoretical dark matter masses at the natural interaction scale inaccessible to current gamma-ray, direct-detection, and collider experiments.
Current and expected dark matter cross section upper limits from gamma-ray experiments for dark matter annihilation into a pair of b quarks.  If the dark matter annihilation cross section and mass was above the blue curve, we would have been bright enough to have been seen by with the Fermi LAT. The natural dark matter cross section, 3*10-26 cm3s-1, would produce the observed amount of dark matter today if the dark matter is a thermal relic. [The Fermi-LAT Collaboration]
As alluded to above, CTA will have synergies with several existing detectors. The energy range available to CTA will provide a window into the highest energy emission from astrophysical sources. These sources glow at other energies (wavelengths) as well and studying them through different energy “lenses” gives us a deeper understanding of the processes accelerating these particles to such high energies. The expected sensitivity of CTA will nicely complement the current all-sky measurements taken at slightly lower energies with the Fermi Large Area Telescope. Additionally, a deeper, more focused observation could be obtained with CTA based on triggers of bright gamma-ray bursts from monitors like HAWC or Swift.
The differential sensitivity for CTA in comparison to current gamma-ray experiments.  The sensitivity is defined as the dimmest source radiation flux needed for a detection with a significance of 5 standard deviations.  The H.E.S.S. sensitivity (not shown) is similar to that of VERITAS. [The VERITAS Collaboration]