Description du sujet de thèse

The field of gamma-ray astrophysics is about to enter a new major area with the advent of CTA (Cherenkov Telescope Array). The proposed thesis will cover two main axis that are complementary: the first part will be linked to instrumentation, and software development dedicated to the camera NectarCAM ; the second part will be dedicated to the study of extended gamma-ray sources, with a size of the order of the field of view of CTA, and the development of dedicated tools, taking up the challenge of background estimation and rejection for example.
The LP2iB is involved in the assembly, calibration and commissioning of the NectarCAM, which is one type of camera that will equip the 9 middle size telescopes on the northern hemisphere site of CTA in La Palma. The first camera is at the moment in the dark room at CEA (Paris Saclay) where it has been assembled. In this preliminary phase, LP2iB is in charge of coordinating the elements of the low-level software that will be in the end integrated to the general CTA pipeline. The Ph.D. candidate will gain expertise in several aspects such as the calibration of a camera for a Cherenkov telescope and the characterisation of the quality of the low-level data. These knowledge is really important to understand how the instrument is working especially over the first years of data taking. The Ph.D. student will be involved in the development of the nectarchain software2 dedicated to the calibration and reduction of NectarCAM data, participate in the commissioning of the NectarCAM on site and the analysis of the first data.
The scientific part of the Ph.D. thesis will be dedicated to extended gamma-ray sources, for which Cherenkov telescopes like H.E.S.S. and CTA are not designed to have the best performances. Indeed, with their very good angular resolution, they are sensitive to point-like or slightly extended sources (a few tenths of a degree), for which it will be possible to study sub-structures. However, in the case of very extended sources (degree scale) with diffuse emission and low surface brightness, the development of analysis tools dedicated to this type of source is needed. The critical point is the modelling of the astrophysical background, to be able to extract the signal. For this purpose, we can think about optimising the classical approach, but also explore new ways to implement it using artificial intelligence methods. For this purpose, we will collaborate with the IMS laboratory, specialised in signal and image processing. These algorithms will be tested on H.E.S.S. data, where we expect to find yet undetected extended sources.
Amongst very extended sources, TeV halos around pulsars are particularly interesting because they are the place of direct injection in the interstellar medium of cosmic rays, that are the final goal of our study to better understand our galaxy. Pulsar halos are believed to be the advanced stage of a pulsar wind nebulae ( ≳ 100 kyr) and they exhibit very low diffusion coefficient, but it is not clear if it is the fate of all pulsars, nor if their environment plays a role. The first pulsar halos have been detected a few years ago by the HAWC observatory, and since then only a handful of candidates have been highlighted. A recent study showed that pulsar halos could be a non- negligible fraction of the TeV gamma-ray sources and that several dozens would be accessible to CTA in the galactic plane. However, specific detection and analysis tools are required to extract the relevant parameters of these objects and their surroundings because CTA's performances, on the contrary of HAWC, are not suited for this kind of object. Outside our galaxy, another type of halo, e+/e- pair halos around blazars, yet undetected object, would also benefit from these algorithms. They are one of the most promising ways to probe intergalactic magnetic fields. Even though it will be challenging, it has been speculated that under some conditions CTA may have the sensitivity to detect them, for example around M87. Finally, these algorithms could also be used for star forming regions, large scale diffuse emission as well as the Fermi bubbles.

Contexte de travail

The PhD student will be integrated into LP2iB's Astroparticles group, whose activities focus on the study of galactic sources emitting high-energy radiation through the Fermi, Hess, CTA and SWGO instruments.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.