Early science and commissioning preparation with the first telescopes of CTA

The CTA (Cherenkov Telescope Array) is a worldwide project to construct the next generation ground based very high energy gamma ray instrument [1]-[2]. CTA will use tens of Imaging Air Cherenkov Telescopes (IACT) of three different sizes (mirror diameter of 4 m, 12 m and 23 m) deployed on two sites, one on each hemisphere (La Palma in the Canary Islands and Paranal in Chile). The observatory will detect gamma-rays with energy ranging from 20 GeV up to 300 TeV by imaging the Cherenkov light emitted from the charged particle shower produced by the interaction of the primary gamma ray in the upper atmosphere.

The unconventional capabilities of CTA will address, among others, the intriguing question of the origin of the very high energy galactic cosmic rays by the search for galactic sources capable of accelerating cosmic rays up to the PeV energies, called PeVatrons. The last few years have been extremely exciting for the PeVatron search since the large field of view detector LHAASO has detected several ultra high energy gamma-ray sources (E??>100 TeV) proving that PeVatrons exist in our Galaxy [3]-[4]. Nevertheless the nature of these sources is still unknown and CTA, thanks to its excellent angular and energy resolution, will be able to precisely study these PeVatrons and to disclose their hadronic or leptonic nature.

The construction of the CTA observatory has started and a first Large-Sized Telescope (LST-1) is already installed and taking data in La Palma. Three more LST telescopes and one Medium-Sized Telescope (MST) will be installed in the next 1-2 years. The camera of the first MST telescope on La Palma (NectarCAM) is fully equipped and should be installed on the structure in 2025.

The PhD project will be divided in two parts. A first part will be devoted to the preparation of the commissioning of NectarCAM and science verification measurements of the Crab nebula which is the standard calibration source for very high energy gamma-ray observations. To this end the candidate will perform the full simulation of the observation that consists in particle shower and telescope Monte Carlo simulations. A detailed simulation of the camera has been developed in the past and will have to be adapted comparing the simulation results to real data taken in the laboratory and on-sky. For the data analysis of both simulation and on sky data the official dataPipe pipeline of the CTA Observatory will be used. The main goal of this part will be to predict the expected performance of the MST telescope in detecting the Crab and prepare the data analysis of on-sky data.

The second part of the PhD project will be focused on the analysis of the data of the coming observation campaign of LST-1 of PeVatron sources detected by LHAASO. Some of these sources are unidentified with no very high energy counterpart. Even if LST-1 cannot reach enough sensitivity to access energies above 10-50 TeV, it should be able to detect some of them in the 100 GeV-10 TeV energy region for the first time or to provide stringent upper limits contributing significantly to the understanding of these intriguing sources. In the case of detection, thanks to the good angular resolution of the telescope, energy dependent morphology study can be performed. A possible extension of the measurement could be to observe the source at large zenith angle maximizing the detection efficiency at very high energy. The latter would allow to explore the energy region above 10 TeV and to extend to higher energies the study of energy dependent morphology to understand the nature of the source.

The project will include the participation to the LST-1 observation campaign with stays of four weeks in the Roque de los Muchachos Observatory in La Palma.

The CPPM CTA group works since several years both in the building of the NectarCAM camera for MST and in the building and commissioning of the LST-1 telescope. The group also works on the preparatory studies for the research of galactic PeVatrons with CTA [6] and is leading the observation campaign with LST-1 and MAGIC of SNR G106.3-2, which is one of the PeVatron LHAASO sources.

Candidates should send their CV and motivation letter as well as grades (Bachelor, M1, M2) to costantini@cppm.in2p3.fr and cassol@cppm.in2p3.fr. Applications will be selected on the base of qualifications and an oral interview.

[1] Science with the Cherenkov Telescope Array: https://arxiv.org/abs/1709.07997

[2] https://www.cta-observatory.org/

[3] Z. Cao et al. Nature, 594, 33–36 (2021)

[4] Z. Cao et al. (2023) https://arxiv.org/abs/2305.17030

[5] F. Acero et al., Astroparticle Physics, 2023, 150, pp.102850.