Abstract
Surveys open up unbiased discovery space and generate legacy datasets of long-lasting value. One of the goals of imaging arrays of Cherenkov telescopes like CTA is to survey areas of the sky for faint very high energy gamma-ray (VHE) sources, especially sources that would not have drawn attention were it not for their VHE emission (e.g. the Galactic “dark accelerators”). More than half the currently known VHE sources are to be found in the Galactic Plane. Using standard techniques, CTA can carry out a survey of the region |ℓ|⩽60°,|b|⩽2° in 250h (1/4th the available time per year at one location) down to a uniform sensitivity of 3mCrab (a “Galactic Plane survey”). CTA could also survey 1/4th of the sky down to a sensitivity of 20mCrab in 370h of observing time (an “all-sky survey”), which complements well the surveys by the Fermi/LAT at lower energies and extended air shower arrays at higher energies. Observations in (non-standard) divergent pointing mode may shorten the “all-sky survey” time to about 100h with no loss in survey sensitivity. We present the scientific rationale for these surveys, their place in the multi-wavelength context, their possible impact and their feasibility. We find that the Galactic Plane survey has the potential to detect hundreds of sources. Implementing such a survey should be a major goal of CTA. Additionally, about a dozen blazars, or counterparts to Fermi/LAT sources, are expected to be detected by the all-sky survey, whose prime motivation is the search for extragalactic “dark accelerators”.
Highlights
Surveys constitute an unbiased, systematic exploratory approach; they favor discoveries of unknown source classes; they allow for scheduling ease and homogeneous data reduction; they provide legacy datasets for future reference
We examined two survey strategies to cover a given area of the sky: a single row of pointings (e.g. Galactic Plane survey) and multiple evenly-spaced rows of pointings
We searched for the optimal longitude step in between pointings, for a single row of pointings aligned along the Galactic plane at b 1⁄4 0
Summary
Systematic exploratory approach; they favor discoveries of unknown source classes; they allow for scheduling ease and homogeneous data reduction; they provide legacy datasets for future reference. Surveys of different extents and depths are amongst the scientific goals of all major facilities that are planned or in operation. This is critical for observational domains that are opening up, such as very high energy (VHE P 30 GeV) gamma rays, with wide scope for surprises. The Galactic Plane survey carried out by HESS led to the detection of dozens of sources, many of which were unexpected; among these, the dark accelerators, have no obvious counterparts at other wavelengths [1,2,3]. In high energy gamma rays (HE P 30 MeV), the Fermi/LAT catalog [4] has a major impact on our knowledge of the HE sky with statistical studies rendered.
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