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The grand vision of the Cherenkov Telescope Array

Image of an artist's impression of the Cherenkov Telescope Array.
An artist's impression of the final constructed Cherenkov Telescope Array. Image courtesy G. Perez, SMM, IAC.

On 1st July 2012, the Royal Society in London, UK, is celebrating the centenary to mark the discovery of cosmic rays by Victor Francis Hess, an Austrian physicist. He won the coveted Nobel Prize of Physics for his radiation measurement experiments of the atmosphere, which included taking his equipment up with him in a balloon. At the event, a number of projects will be on show, which include the Pierre Auger Observatory and the High Energy Stereoscopic System. One is an ambitious design study that represents the largest investigation into high-energy cosmic particles that has ever been undertaken, albeit it will be on the ground this time.

The Cherenkov Telescope Array (CTA) will consist of two arrays of telescopes in two different hemispheres, allowing full coverage of the sky. The south CTA will cover about three square kilometers (1.16 square miles) of land with around 60 telescopes that will monitor all the energy ranges in the center of the Milky Way's galactic plane. The north CTA will cover one square kilometer (0.39 square miles) and be composed of 30 telescopes. These telescopes will be targeted at extragalactic astronomy.

"CTA opens a new window of essentially unexplored photon energies," said Giovanni Bignami, president of the Italian National Institute for Astrophysics (INAF). "Its potential impact is enormous: part of it, we imagine, will consist in discovering thousands of new [very-high-energy photon] sources, and part of it will be surprises. It's the surprises we like best, and it's the surprises that will most appeal to the public at large."

The CTA started as a European led project, which now, not only, involves European research groups, but researchers from Africa, Argentina, Brazil, India, Japan, Mexico, and the US. It is a global effort and there are currently 27 countries, and over 1,000 scientists involved.

Preparing for the flood

This distributed telescope array represents a grand data management challenge too. The expected data rates from the CTA are very large according to Giovanni Lamanna, CTA international data management coordinator, and leader of the HESS, and CTA research team of LAPP - IN2P3.

What is the goal of the CTA?

The project will be composed of a collection of Cherenkov telescopes that will scan the universe at very-high-energy gamma-rays from 100 giga-electronvolts to about 100 tera-electronvolts; energies which are one hundred billion to one hundred trillion times higher than of visible light.The CTA will also investigate cosmic processes that create particles travelling close to the speed of light.

The CTA combines the fields of astronomy, astrophysics, and fundamental physics research. Studies will include the origin of cosmic rays and their impact on other bodies within the universe. Researchers will investigate galactic particle accelerators, black holes, extragalactic gamma rays, dark matter, and the effects of quantum gravity.

This international collaboration is also listed on the European Strategy Forum on Research Infrastructures (ESFRI) roadmap and is currently funded by the EC in a FP7-funded preparatory phase, which will end in 2014. This will conclude with the production of a final technical design report which will allow the CTA international consortium to be ready for a construction phase in 2015. The construction of the infrastructure will last three to four years after 2015, while scientific research will start immediately after the deployment of the first telescope. Possible sites that will host the CTA observatory are South Africa, Latin America, Central America, and the Canary Islands.

"The total expected raw data rate for a specific array configuration can vary from 0.3 to 4.0 gigabytes per second for the southern array, and from 0.1 to 1.3 gigabytes per second for the northern one. This translates to an archived raw data volume of between 2 and 25 petabytes per year," said Lamanna. The top end of this data rate is equivalent to the amount of data that the Large Hadron Collider's WLCG currently stores per year, which is approximately 23 petabytes.

The ASPERA (AStroParticle ERAnet) project is a FP7-funded multi-national project that has coordinated a number of workshops to address the problems of data collection, data storage, and data mining for the next generation of large-scale projects such as CTA. The organizers will release a white paper at the end of this year.

"CTA will be an open observatory, with a science data center, which will provide pre-processed data to the user, as well as the tools necessary for the most common analyses,"Lamanna said. "The software tools will provide easy-to-use and well-defined data access. CTA data will be accessible through an online virtual observatory, with varying interfaces matched to different levels of user expertise."

The start of a new grid?

Once fully operational, a huge amount of simulations will be required to optimize the CTA telescopes for the best configuration, performance, and for analysis of data.

"Performance depends on a large number of parameters, including general layout of the installation, telescope sizes and locations, and many more technical aspects," said Lamanna.

Huge numbers of background showers have to be simulated before conclusions on the performance of a telescope configuration can be made: 100 billion proton, gamma, and electron particle shower simulations are needed. When the telescope array is constructed, Monte Carlo simulations will help researchers study CTA detector response to gamma rays and background cosmic particle events.

A distributed infrastructure approach is seen as the most suitable for the CTA project and a feasibility study of a dedicated computing grid that will service thousands of users is underway. Graphical Processor Units may also be used for real-time data analysis.

For now, CTA researchers are supported by a virtual organization coordinated by the European Grid Infrastructure (EGI). This CTA virtual organization is supported by 19 grid sites, in 6 countries (Czech Republic, France, Germany, Greece, Poland, and Spain). It has thousands of available CPUs and more than 600 terabytes of storage. More computing centers and countries are continuously joining the CTA virtual organization.

Currently, there are 75 registered users to the CTA virtual organization on EGI, and 100 more scientists working on simulation data in the grid.

Combining research & the next two decades

A CGI video showing the north and south telescope locations of the CTA project when construction is finished. Both arrays will have telescopes of three different classes for the study of three different energy ranges. This includes the detection of gamma rays. Image courtesy G. Perez, SMM, IAC

"The aims of the CTA can be roughly grouped into three main themes: understanding the origin of cosmic rays and their role in the universe, understanding the nature and variety of particle accelerations around black holes, and searching for the ultimate nature of matter and physics beyond the Standard Model," Lamanna said.

According to Lamanna, the CTA project not only joins two of Earth's hemispheres together, but it unifies astrophysicists and particle physicists too.

The multi-national CTA collaboration is working across international borders and will enable young researchers to acquire not only physics skills, but also communication and management skills, data processing and data mining techniques, and programming of complex control and monitoring systems, and electronic designs.

It is an exciting time for researchers to be in astronomy.

Today, astronomy is facing an avalanche of data. The International Center for Radio Astronomy Research recently announced that the Square Kilometer Array (SKA) project will generate one exabyte of data alone when it searches the sky. That's equivalent to the total amount of computer data generated by the entire world in a whole year.

While there is no current collaboration between the CTA and SKA, both projects represent the new frontier of radio and gamma-ray energy observations, and they are both developing services which will feed astrophysical researches for the next two decades.

The CTA data management working group will develop services and tools for access and analysis of this data. "This will be integrated into the Very-High-Energy Gamma-Ray Science Gateway. The gateway will be the official gateway to increase public awareness and scientific interest," Lamannasaid. "The gateway will combine a number of web services to allow any user to upload or access the data archive, and access software, and hardware computing infrastructures for analysis workflows, and to publish results." This gateway may be connected to Europe, Africa, and Latin America via fast gigabit-per-second research networks such as GÉANT.

The CTA gateway may even combine with other large astronomy projects such as the Google Sky project, which is working with a number of large observatories to bring the public free, detailed maps of the universe at different wavelengths. Now CTA researchers are hard at work on the R&D, prototypes, and designs that will culminate with a final design construction report in mid-2014.

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