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\begin{document}
\title{The \Fermi-LAT experiment}

\author{
M. Kuss$^{1}$,
F. Longo$^{2}$,
on behalf of the \Fermi LAT collaboration}

\address{$^{1}$ INFN Sezione di Pisa, Pisa, IT}
\address{$^{2}$ University of Trieste and INFN Sezione di Trieste, Trieste, IT}
\ead{michael.kuss@pi.infn.it}

\begin{abstract} 
The \Fermi Large Area Telescope is a current generation experiment dedicated to gamma-ray astrophysics.
\end{abstract}

\section{The \Fermi LAT Experiment}

The Large Area Telescope (LAT) is the primary instrument on the \emph{Fermi Gamma-ray Space Telescope} mission, launched on June 11, 2008.
It is the product of an international collaboration between DOE, NASA and academic US institutions as well as international partners in France,
Italy, Japan and Sweden.
The LAT is a pair-conversion detector of high-energy gamma rays covering the energy range from 20 MeV to more than 300 GeV \cite{LATinstrument}.
It has been designed to achieve a good position resolution ($&lt;$10 arcmin) and an energy resolution of $\sim$10 \%. 
Thanks to its wide field of view ($\sim$2.4 sr at 1 GeV), the LAT has been routinely monitoring the gamma-ray sky and has shed light on the extreme, non-thermal Universe.
This includes gamma-ray sources such as active galactic nuclei, gamma-ray bursts, galactic pulsars
and their environment, supernova remnants, solar flares, etc.

% ref:
% triggers: last roll-over integral + telemetry trending of LHKGEMSENT
% events: datacat integral
% photons: current value on https://fermi.gsfc.nasa.gov/cgi-bin/ssc/LAT/LATDataQuery.cgi
% 2018: I took Rob's presentation at the collaboration meeting

By end of 2018, the LAT had registered 640 billion triggers (1800 Hz average trigger rate).
An on-board filter analyses the event topology and discards about 80\%.
Of the 128 billion events that were transferred to ground 1.2 billion were classified as photons.
All photon data are made public almost immediately.
Downlink, processing, preparation and storage take about 8 hours.

\section{Scientific Achievements Published in 2018}

% source: Fermi LAT publications page and links there and Peter's presentation
In 2018, 48 collaboration papers (Cat.\ I and II) were published, keeping the impressive pace of about 50 - 60 per year since the launch in 2008.
Independent publications by LAT collaboration members (Cat.\ III) amount to 18.
% source: NASA Fermi Bibliography Search Tool
Also external scientists are able to analyse the \Fermi public data, resulting in about 100 external publications.
Also the latter number is constant over the years, demonstrating the continuous interest in LAT science by the astrophysics community.

\section{The Computing Model}

The \Fermi-LAT offline processing system is hosted by the LAT ISOC (Instrument Science Operations Center)
based at the SLAC National Accelerator Laboratory in California.
The \Fermi-LAT data processing pipeline (e.g.\ see \cite{LATp2} for a detailed technical description) 
was designed with the focus on allowing the management of arbitrarily complex work flows and handling multiple tasks simultaneously 
(e.g., prompt data processing, data reprocessing, MC production, and science analysis).
The available resources are used for specific tasks: the SLAC batch farm for data processing, high level analysis, and smaller MC tasks,
the batch farm of the CC-IN2P3 at Lyon and the grid resources for large MC campaigns.
The grid resources \cite{CHEP2013} are accessed through a DIRAC (Distributed Infrastructure with Remote Agent Control) \cite{DIRAC} interface to the LAT data pipeline \cite{LAT-DIRAC}.
This setup was put into production mode in April 2014.

\section{Conclusions and Perspectives}
\label{conclusions}

The prototype setup based on the DIRAC framework described in the INFN-CNAF Annual Report 2013 \cite{CNAF2013} proved to be successful.
In 2014 we transitioned into production mode.
However, \Fermi{} is in its full maturity and doesn't plan major upgrades
to be supported by massive MC simulations.
The regular data processing and possible re-processing is managed by SLAC.
The not regular MC production is currently taken care by the Lyon farm.
Note however that we expect that our usage may rise again,
partially due to changes in the computing model associated with long term mission plans of Fermi-LAT,
and partially to satisfy specialized simulation needs.

\section*{References}
\begin{thebibliography}{99}
\bibitem{LATinstrument} Atwood W B et al.\ 2009 {\it Astrophysical Journal} {\bf 697}, 1071
\bibitem{LATp2} Dubois R 2009 {\it ASP Conference Series} {\bf 411} 189
\bibitem{CHEP2013} Arrabito L et al.\ 2013 CHEP 2013 conference proceedings arXiv:1403.7221
\bibitem{DIRAC} Tsaregorodtsev A et al.\ 2008 {\it Journal of Physics: Conference Series} {\bf 119}, 062048
\bibitem{LAT-DIRAC} Zimmer S et al.\ 2012 {\it Journal of Physics: Conference Series} {\bf 396}, 032121
\bibitem{CNAF2013} Arrabito L et al.\ 2014 {\it INFN-CNAF Annual Report 2013}, edited by L.\ dell'Agnello, F.\ Giacomini, and C.\ Grandi, pp.\ 46

\end{thebibliography}

\end{document}
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