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\begin{document}
\title{The Cherenkov Telescope Array}

\author{L. Arrabito$^1$, C. Bigongiari$^2$, F. Di Pierro$^3$ and P. Vallania$^{3,4}$}

\address{$^1$ Laboratoire Univers et Particules de Montpellier, Universit\'e de Montpellier II Place Eug\`ene Bataillon - CC 72, CNRS/IN2P3,
 F-34095 Montpellier, France}
\address{$^2$ INAF Osservatorio Astronomico di Roma - Via Frascati 33, 00040, Monte Porzio Catone (RM), Italy}
\address{$^3$ INFN Sezione di Torino - Via Pietro Giuria 1, 10125, Torino (TO), Italy}
\address{$^4$ INAF Osservatorio Astrofisico di Torino - Via Pietro Giuria 1, 10125, Torino (TO), Italy}

\ead{arrabito@in2p3.fr, ciro.bigongiari@oa-roma.inaf.it, federico.dipierro@to.infn.it, piero.vallania@to.infn.it}


\begin{abstract}

The Cherenkov Telescope Array (CTA) is an ongoing worldwide project to build a new generation ground based observatory for Very High Energy (VHE) gamma-ray astronomy. 
CTA will feature two arrays of Imaging Atmospheric Cherenkov Telescopes (IACTs), one in each Earth hemisphere, to ensure the full sky coverage and will be operated as an open observatory to maximize its scientific yield.  
Each array will be composed of tens of IACTs of different sizes to achieve a ten-fold improvement in sensitivity, 
with respect to current generation facilities, over an unprecedented energy range which extends from a few tens of GeV to a hundred of TeV. 
Imaging Cherenkov telescopes have already discovered tens of VHE gamma-ray emitters providing plentiful of valuable data and clearly demonstrating the power of the imaging Cherenkov technique. 
The much higher telescope multiplicity provided by CTA will drive to highly improved angular and energy resolution,  which will permit more accurate morphological and spectrographical studies of VHE gamma-ray sources. CTA project combines therefore guaranteed scientific return, in the form of high precision astrophysics, with considerable potential for major discoveries in astrophysics, cosmology and fundamental physics.

\end{abstract}

\section{Introduction}

Since the discovery of the first VHE gamma-ray source, the Crab Nebula \cite{CrabDiscovery} by the Whipple collaboration in 1989, ground-based gamma-ray astronomy has undergone an impressive development which drove to the discovery of more than 190 gamma-ray sources in less than 30 years \cite{TevCat}. 
Whenever a new generation of ground-based gamma-ray observatory came into play gamma-ray astronomy experienced a major step in the number of discovered sources as well as in the comprehension of the astrophysical phenomena involved in the emission of VHE gamma radiation.  
Present generation facilities like H.E.S.S. \cite{HESS}, MAGIC \cite{MAGIC} and VERITAS \cite{VERITAS} already provided a deep insight into the non-thermal processes which are responsible of the high energy emission by many astrophysical sources, like Supernova Remnants, Pulsar Wind Nebulae, Micro-quasars and Active Galactic Nuclei, clearly demonstrating the huge physics potential of this field, which is not restricted to pure astrophysical observations, but allows significant contributions to particle physics and cosmology too, see \cite{DeNauroisMazin2015,LemoineGoumard2015} for recent reviews. The impressive physics achievements obtained with the present generation instruments as well as the technological developments regarding mirror production
and new photon-detectors triggered many projects for a new-generation gamma-ray observatory by groups of astroparticle physicists around the world which later merged to form the CTA consortium \cite{CtaConsortium}. 
CTA members are carrying on a worldwide effort to provide the scientific community with a state-of-the-art ground-based gamma-ray observatory, allowing exploration of cosmic radiation in the very high energy range with unprecedented accuracy and sensitivity. 


\begin{figure}[ht]
\includegraphics[width=\textwidth]{CTA_ProjectTimeline_Nov2018.eps}
\caption{\label{CtaTimeline} CTA project time line.}
\end{figure}


VHE gamma-rays can be produced in the collision of highly relativistic particles with surrounding gas clouds or in their interaction with low energy photons or magnetic fields. Possible sources of such energetic particles include jets emerging from active galactic nuclei, remnants of supernova explosions, and the environment of rapidly spinning neutron stars. High-energy gamma-rays can also be produced in top-down scenarios by the decay of heavy particles such as hypothetical dark matter candidates or cosmic strings.
The CTA observations will be used for detailed studies of above-mentioned astrophysical sources as well as for fundamental physics measurements, such as the indirect search of dark matter, searches for high energy violation of Lorentz invariance and searches for axion-like particles. 
High-energy gamma-rays can be used moreover to trace the populations of high-energy particles, thus providing insightful information about the sources of cosmic rays. 
Close cooperation with observatories of other wavelength ranges of the electromagnetic spectrum, and those using cosmic rays, neutrinos and gravitational waves are foreseen.

To achieve a full sky-coverage the CTA observatory will consist of two arrays of IACTs, one in both Earth hemispheres. The northern array will be placed at the Observatorio del Roque de Los Muchachos on La Palma Island, Spain, while the southern array will be located in Chile at the ESO site close to the Cerro Paranal. 
The two sites were selected after years of careful consideration of extensive studies of the environmental conditions, simulations of the science performance and assessments of construction and operation costs.
Each array will be composed by IACTs of different sizes to achieve an overall ten-fold improvement in sensitivity with respect to current IACT arrays while extending the covered energy range from about 20 GeV to about 300 TeV. 
The southern hemisphere array will feature telescopes of three different sizes to cover the full energy range for a detailed investigation of galactic sources, and in particular of the Galactic center, without neglecting observations of extragalactic objects. 
The northern hemisphere array instead will consist of telescopes of two different sizes only covering the low energy end of the above-mentioned range (up to some tens of TeV) and will be dedicated mainly to northern extragalactic objects and cosmology studies.  

The CTA observatory with its two arrays will be operated by one single consortium and a significant and increasing fraction of the observation time will be open to the general astrophysical community to maximize CTA scientific return. 
 
The CTA project has entered the pre-construction phase. The first Large Size Telescope has been inaugurated in October 2018, accordingly to the schedule (see Fig. \ref{CtaTimeline}), in the La Palma CTA Northern Site. During 2019 the construction of 3 more LSTs will start. In December 2018 another telescope prototype, the Dual Mirror Medium Size Telescope has been also inaugurated at the Mount Whipple Observatory (Arizona, US). 
Meanwhile detailed geophysical characterization of the southern site is ongoing and the agreement between the hosting country and the CTA Observatory has been signed. 
First commissioning data from LST1 have started to be acquired at the end of 2018, in 2019 the first gamma-rays observations are expected.
CTA Observatory is expected to become fully operational by 2025 but precursors mini-arrays are expected to operate already in 2020. 

A detailed description of the project and its expected performance can be found in a dedicated volume of the Astroparticle Physics journal \cite{CtaApP}, while an update on the project status can be found in \cite{Ong2017}. 
CTA is included in the 2008 roadmap of the European Strategy Forum on Research Infrastructures (ESFRI), 
is one of the Magnificent Seven of the European strategy for astroparticle physics by ASPERA, 
and highly ranked in the strategic plan for European astronomy of ASTRONET.



\section{Computing Model}

In the pre-construction phase the available computing resources are used mainly for the simulation of atmospheric showers and their interaction with the Cherenkov telescopes of the CTA arrays to evaluate the expected performance and optimize many construction parameters. 
The simulation of the atmospheric shower development, performed with Corsika \cite{Corsika}, is followed by the simulation of the detector response with sim\_telarray \cite{SimTelarray}, a code developed within the CTA consortium.
It is worthwhile to notice that thanks to the very high rejection of hadronic background achieved with the IACT technique, huge samples of simulated hadronic events are needed to achieve statistically significant estimates of the CTA performance. 
About $10^{11}$ cosmic ray induced atmospheric showers for each site are needed to properly estimate the array sensitivity, energy and angular resolution requiring extensive computing needs in term of both disk space and CPU power. Given these large storage and computing requirements, the Grid approach was chosen to pursue this task and a Virtual Organization for CTA was created in 2008 and is presently supported by 20 EGI sites and one ARC site spread over 7 countries, with more than 3.6 PB of storage, about 7000 available cores on average and usage peaks as high as 12000 concurrent running jobs.

The CTA production system currently in use \cite{Arrabito2015} is based on the DIRAC framework \cite{Dirac}, which has been originally developed to support the production activities of the LHCb (Large Hadron Collider Beauty) experiment and today is extensively used by several particle physics and biology communities. DIRAC offers powerful job submission functionalities and can interface with a palette of heterogeneous resources, such as grid sites, cloud sites, HPC centers, computer clusters and volunteer computing platforms. Moreover, DIRAC provides a layer for interfacing with different types of resources, like computing elements, catalogs or storage systems. 

A massive production of simulated data has been carried on in 2018 to estimate the expected performance with improved telescopes' models and with different night-sky background levels. A simulation dedicated to the detailed comparison of different Small Size Telescope versions was also carried on. Simulated data have been analyzed with two different analysis chains to crosscheck the results and have been also used for the development of the new official CTA reconstruction and analysis pipeline. 

\begin{figure}[ht]
\includegraphics[width=\textwidth]{cpu-days-used-2018-bysite.eps}
\caption{\label{CPU} CPU power provided in 2018 by Grid sites in the CTA Virtual Organization.}
\end{figure}

About 2.7 million of GRID jobs have been executed in 2018 for such task corresponding to about 206.4 millions of HS06 hours of CPU power and 10 PB of data transferred. 
CNAF contributed to this effort with about 16.8 millions of HS06 hours and 790 TB of disk space corresponding to 8\% of the overall CPU power used and the 17\% of the disk space resulting the second contributor in terms of storage and the fourth in terms of CPU time (see Fig. \ref{CPU}-\ref{Disk}).

\begin{figure}[ht]
\includegraphics[width=0.8\textwidth]{normalized-cpu-used-2018-bysite-cumulative.eps}
\caption{\label{CPU-cumu} Cumulative normalized CPU used in 2018 by Grid sites in the CTA Virtual Organization.}
\end{figure}

\begin{figure}[ht]
\includegraphics[width=0.8\textwidth]{transfered-data-2018-bysite.eps}
\caption{\label{Disk} Total transferred data in 2018, for the Grid sites in the CTA Virtual Organization.}
\end{figure}

\clearpage

\section*{References}
\begin{thebibliography}{19}
\bibitem{CrabDiscovery} Weekes T C {\it et al.} 1898 ``Observation of TeV gamma rays from the Crab nebula using the atmospheric Cerenkov imaging technique''
{\it ApJ} {\bf 342} 379-95 

\bibitem{TevCat} TevCat web page http://tevcat.uchicago.edu

\bibitem{HESS} H.E.S.S. web page https://www.mpi-hd.mpg.de/hfm/HESS/

\bibitem{MAGIC} MAGIC web page https://magic.mppmu.mpg.de

\bibitem{VERITAS} VERITAS web page http://veritas.sao.arizona.edu

\bibitem{DeNauroisMazin2015} de Naurois M and Mazin D ``Ground-based detectors in very-high-energy gamma-ray astronomy'' 
Comptes Rendus - Physique {\bf 16}  Issue 6-7, 610-27

\bibitem{LemoineGoumard2015} Lemoine-Goumard M 2015 ``Status of ground-based gamma-ray astronomy'' Conf. Proc of $34^{th}$ International Conference on C, 2015, The Hague, 
PoS ICRC2015 (2016) 012

\bibitem{CtaConsortium} CTA web page https://www.cta-observatory.org/about/cta-consortium/

\bibitem{CtaApP} Hinton J, Sarkar S, Torres D and Knapp J 2013 ``Seeing the High-Energy Universe with the Cherenkov Telescope Array. The Science Explored with the CTA'' {\it Astropart. Phys.} {\bf 43}  1-356

%\bibitem{Bigongiari2016} Bigongiari C 2016 ``The Cherenkov Telescope Array'' Proc. of Cosmic Ray International Seminar (CRIS2015), %2015, Gallipoli,
% {\it Nucl. Part. Phys. Proc.} {\bf 279–281} 174-81

\bibitem{Ong2017} Ong R A et al. 2017 ``Cherenkov Telescope Array: The Next Generation Gamma-Ray Observatory'' 
Proc. of 35th Int. Cosmic Ray Conf. - ICRC2017, 10-20 July, 2017, Busan, Korea (arXiv:1709.05434v1)

\bibitem{Corsika} Heck D, Knapp J, Capdevielle J N, Schatz G and Thouw T 1998 ``CORSIKA: a Monte Carlo code to simulate extensive air showers'' 
Forschungszentrum Karlsruhe GmbH, Karlsruhe (Germany), Feb 1998, V + 90 p., TIB Hannover, D-30167 Hannover (Germany)

\bibitem{SimTelarray} Bernlh{\"o}r K 2008 ``Simulation of imaging atmospheric Cherenkov telescopes with CORSIKA and sim\_telarray'' {\it Astropart. Phys} {\bf 30} 149-58 

\bibitem{Arrabito2015} Arrabito L, Bregeon J, Haupt A, Graciani Diaz R, Stagni F and Tsaregorodtsev A 2015 ``Prototype of a production system for Cherenkov Telescope Array with DIRAC'' Proc. of $21^{st}$ Int. Conf.e on Computing in High Energy and Nuclear Physics (CHEP2015), 2015, Okinawa, 
{\it J. Phys.: Conf. Series} {\bf 664} 032001

\bibitem{Dirac} Tsaregorodtsev A {\it et al.} 2014 ``DIRAC Distributed Computing Services'' Proc. of $20^{st}$ Int. Conf.e on Computing in High Energy and Nuclear Physics (CHEP2013) 
{\it J. Phys.: Conf. Series} {\bf 513} 032096 


\end{thebibliography}

\end{document}


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