Introducing the CTA concept

TLDR: The Cherenkov Telescope Array (CTA) as discussed by the authors is a very high-energy (VHE) gamma ray observatory with an international collaboration with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America.

About: This article is published in Astroparticle Physics.The article was published on 2013-03-01 and is currently open access. It has received 701 citations till now. The article focuses on the topics: Cherenkov Telescope Array.

Figures

Table 1: Properties of selected air-Cherenkov instruments, including three of historical interest (Whipple, HEGRA and CAT). Adapted from ref. [15]. Significances relate to a point-like source detectable at the 5σ significance level in a 50 h observation.

Figure 8: Integral sensitivity for CTA from MC simulations, together with the sensitivities in comparable conditions (50 h for IACTs, 1 year for Fermi-LAT and HAWC) for some gamma-ray observatories.

Figure 9: Angular resolution for CTA, compared with some existing and future VHE gamma-ray observatories. The solid line provides the angular resolution of CTA obtained from events with ten or more images, the dashed line shows the angular resolution for events with only two images.

Table 2: Performance goals for the CTA observatories. The sensitivity is given for 5 bins per decade. For the sensitivity and the collection area at high energies, separate values for the southern (S) and northern (N) arrays are given.

Figure 12: Approximate location of candidate sites in the range of ±20-30◦ latitude.

Figure 11: Current timeline of the CTA project.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Introducing the cta concept" ?

In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper the authors introduce the science goals and the concept of CTA, and provide an overview of the project. 

Q2. Why do ground-based gamma-ray detectors require a large detection area?

Due to the low flux of VHE photons (even at some tens of GeV), detectors for these energies require a large detection area, effectively ruling out space-borne instruments that directly detect the incident gamma rays. 

Q3. How much sensitivity does CTA have over the entire energy range?

In its core energy range, from about 100 GeV to several TeV, CTA will have milli-Crab (mCrab) sensitivity, i.e. a factor of 103 below the flux of the strongest steady source of VHE gamma rays (the Crab nebula), and a factor of 104 below the highest fluxes measured so far in bursts from transient sources. 

Q4. What are the main reasons why CTA is an excellent experiment?

In addition, the improved energy coverage and resolution will make CTA an excellent experiment for other fundamental physics questions, such as searches for axion-like particles, effects of quantum gravity and other violations of Lorentz invariance. 

Q5. What are the main elements that ensure the smooth running of the CTA observatory?

The main elements that guarantee the smooth running of the CTA observatory are (i) the Science Operation Centre, which is in charge of the organisation of observations, (ii) the Array Operation Centre, which conducts the operation, monitors the telescopes and the atmosphere, and provides all calibration and environmental data necessary for the analysis, and (iii) the Science Data Centre, which provides and disseminates data and analysis software to the science community at large, using common astronomical standards and existing computing infrastructures. 

Q6. What is the way to measure the distance of the image from the camera centre to the telescope?

A relatively large FoV is mandatory for the widely spaced telescopes of the high-energy array, since the distance of the image from the camera centre scales with the distance of the impact point of the air shower to the telescope. 

Q7. What is the recent generation of gamma-ray instruments?

The latest generation of ground-based gamma-ray instruments have enabled the imaging, photometry and spectroscopy of VHE gamma-ray sources and have propelled their studies into a genuine branch of astronomy. 

Q8. How many LSTs are used to collect the Cherenkov light?

The CTA design assumes a small number (≤4) of closely placed large-size telescopes (LSTs), with a mirror diameter of about 23 m, to collect as many Cherenkov photons as possible from the low energy showers. 

Q9. What is the purpose of the VHE gamma-ray science gateway?

Access to data, support services, software and data center infrastructures will be provided through a single access point, the “VHE gamma-ray Science Gateway”. 

Q10. How can CTA reach a resolution of better than 2 arc minutes?

By selecting a subset of gamma-ray induced cascades detected simultaneously by many of its telescopes, CTA can reach angular resolutions of better than 2 arc minutes for energies above 1 TeV, a factor of 5 better than the typical values for current instruments. 

Q11. How many MBytes of data per night will be needed to process the data?

It will be necessary to reduce a volume of typically 10 TB of raw data per night to a few tens of MBytes of high-level data within a couple of hours. 

Q12. How many sources can be detected by CTA?

To achieve a substantially improved sensitivity at the highest energies, CTA requires a collection area of the order of 10 km2 which means spreading numerous3 

Q13. How many orders of magnitude can be achieved?

With Fermi and CTA operating simultaneously, an unprecedented seamless coverage of more than seven orders of magnitude in energy can be achieved. 

Q14. What is the advantage of air shower detectors?

Compared to Cherenkov telescopes, air shower detectors such as Tibet AS-gamma or ARGO-YBJ have the advantage of a large duty cycle, as they can also observe during daytime, and of large instantaneous sky coverage. 

Q15. What is the angular resolution of a single gamma ray?

A typical angular resolution is 0.1◦ or slightly better for a single gamma ray, but sufficiently intense point sources can be located with a precision of 10–20 arc seconds. 

Q16. What is the advantage of a sub-cluster?

While it is not immediately obvious which option offers the best cost/performance ratio at high energies, the sub-cluster concept with larger telescopes has the advantage of providing additional high-quality shower images towards lower energies, for impact positions near the sub-cluster.