Surveying the VHE Gamma-ray Sky
The CTA legacy will likely not be limited to individual observations, but will also comprise a survey of the inner Galactic plane and/or, depending on the final array capabilities, a deep survey of all or part of the extragalactic sky. Surveys provide coverage of large parts of the sky, maximise serendipitous detections, allow for optimal use of telescope time, and ensure the legacy of the project for the future scientific community.
Surveys of different extents and depths are among the scientific goals of all major facilities planned or in operation at any wavelengths. In view of both the H.E.S.S. and Fermi survey results, the usefulness of surveys is unquestioned, and many of the scientific cases goals of CTA can in fact be encompassed within such an observational strategy.
Two main possible alternatives for a CTA survey have been under study so far:
All-sky survey: With a possible field-of-view of 5 degrees, 500 pointings of 0.5 hours would cover a survey area of a quarter of the sky at the target sensitivity of 0.01 Crab. Hence, using about a quarter of the observing time in a year, a quarter of the sky can be surveyed down to a level of <0.01 Crab, which is equivalent to the flux level of the faintest active galactic nuclei currently detected at VHE energies.
Galactic plane survey: The H.E.S.S. Galactic plane survey covered 1.5% of the sky, at a sensitivity of 0.02 Crab above 200 GeV using about 250 hours of observing time. The increase in CTA sensitivity means that a similar investment in time can be expected to result in a sensitivity of 2-3 mCrab over the accessible region of the Galactic plane.
The high-energy phenomena which can be studied with CTA span a wide range of galactic and extragalactic astrophysics, of plasma physics, particle physics, dark matter and fundamental physics of space-time. They carry information on the birth and death of stars, on the matter circulation in the Galaxy, and on the history of the Universe. Optimisation of the layout of CTA with regards to these different science goals is a difficult task and detailed studies of the response of different array configurations to these scientific problems are conducted during the Design Study and the Preparatory Phase.
Aharonian et al., The H.E.S.S. Survey of the Inner Galaxy in Very High Energy Gamma Rays,
The Astrophysical Journal (2006), 636, 2, p. 777-797; http://arxiv.org/abs/astro-ph/0510397