Introduction

Cherenkov Telescope Array

The Cherenkov Telescope Array CTA, is the next generation of ground-based Cherenkov telescopes observing the gamma-ray sky in the energy range 20 GeV - 300 TeV. The array will be composed of imaging atmospheric Cherenkov telescopes of three different sizes distributed into two sites, one in the northern hemisphere in the Canary Island of La Palma (Spain) and another located in the southern hemisphere at Paranal Observatory (Chile).

The prototype for CTA of the Large-Sized Telescope LST-1, located at the Observatorio del Roque de Los Muchachos (ORM) in La Palma, is presently going through its commissioning phase. It is placed next to the two MAGIC (Major Atmospheric Gamma Imaging Cherenkov) telescopes, which is an advantage for the operation, maintenance and calibration of the telescope. A total of four LSTs, among other different-size telescopes, will operate together at ORM as part of the CTA North (CTA-N) site.

LSTOSA

LSTOSA is born out of experience gained on On-Site Analysis (OSA) of the MAGIC processing pipeline. Due to the large size of the daily recorded data, transferring the raw data through the network connection from La Palma island to continental Europe in due time is an issue for the LST-1. Therefore a fast LST On-Site Analysis (LSTOSA) chain is being developed, aimed at performing a reduction of the raw data at the LST-1 site, so that the low and intermediate analysis products are available to the LST Collaboration and delivered by internet to the CTA data centers. The pipeline also performs data quality checks to debug potential problems. To ensure reproducibility LSTOSA tracks the provenance of the analysis products.

Data reduction steps

Data analysis steps implemented in lstchain are summarized in Fig. 1:

Fig. 1 Data reduction steps, starting from raw uncalibrated waveform signals to selected photon lists.

Computing infrastructure

An IT Container housing a compact data center, placed next to the telescope, allows us to record and process the data acquired by the telescope (data acquisition rate 3 TB per hour of observation), including LSTOSA pipeline data processing. The data center provides 55 computing nodes, each one with 32 cores, for a total of 1760 cores and 3.5 PB of disk space. This cluster uses the CentOS operating system, administers the work load through the SLURM batch scheduling system and implements the Fujitsu Scalable File System FEFS based on Lustre.

Once the data have been recorded and processed, they are copied via the network to the computing center PIC (Port d’Informació Científica) located in Barcelona. The members of the LST Collaboration have access to the so called IT Container and use it for the commissioning of the telescopes and preliminary astrophysics analysis. The vast computing power available in the IT Container is key to make possible the processing of LST-1 data.