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Accueil > Thèses, Stages, Formation et Enseignement > Propositions de thèses antérieures > Propositions de thèses 2022 > Testing Lorentz invariance with high energy astrophysical sources : the dawn of a new era

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Testing Lorentz invariance with high energy astrophysical sources : the dawn of a new era

Title : Separation between Lorentz invariance violation effects and source intrinsic effects in high-energy gamma-ray astronomy: first population study with multi-type high-energy gamma-ray sources and preparation of CTA analyses

Supervisor : : Julien Bolmont

Co-supervisor : Hélène Sol (LUTh Laboratory)

Team : Cosmic radiation and dark matter ; H.E.S.S. and CTA groups

Funded PhD : Yes, already funded

Description :

Gamma-ray astronomy is the study of photons emitted by astrophysical sources in an energy range typically between a few hundred kiloelectronvolts (keV) and a few hundred teraelectronvolts (TeV). This discipline took off in the 60s and since then, thousands of gamma-ray emitting astrophysical sources have been observed. Their common feature is to be produced by violent phenomena such as explosions of massive stars (supernovæ) or by the resulting compact bodies: neutron stars or black holes.

An example of such sources are Active Galactic Nuclei (AGN). About 10% of the observed galaxies are AGN: they show a particularly bright central region emitting in a large domain of energies, from radio to X and gamma domains. This phenomenon is due to the presence of a supermassive black hole which accretes gas and dust from its close surroundings. Part of this gas and dust are ejected in highly-relativistic plasma jets. When such a jet points in a direction close to Earth, the high-energy emission appears particularly bright to the observer. These particular AGN are called “blazars”. Blazars have the particularity to be highly variable: they can be extremely bright at some times, with the emission of bunches of photons over durations ranging from a few minutes to several hours. These episodes with high brightness are called “flares”.

The variability shown by blazars, but also by other sources such as Gamma-ray Bursts (GRB) and pulsars, is a precious way to infer some properties of the source. For example, the faster the variability is, the more compact the emitting region is. Variability can also be used to measure time-lags between photons of different energies, the so-called “spectral lags”. These spectral lags can have different origins. The first one is the source itself, or rather the emission of photons and the acceleration of the primary particles. These are the so-called “source intrinsic effects”, or simply “intrinsic effects”. The second possible origin can be related to the fact photon propagation is altered during the travel between the source and the Earth. These are called “propagation effects”. An example of a propagation effect will be of particular interest for the proposed thesis. Some theoretical models developed with the aim to unify the infinitely small of Quantum Mechanics and the infinitely large of General relativity predict such a propagation effect. These models of Quantum Gravity predict that the quantum nature of spacetime at very small scales (typically the Planck scale, 10-35 m) could alter the propagation of photons in such a way that their velocity would depend on their energy, resulting in a violation of Lorentz invariance (VLI).

The magnitude of VLI propagation effects should increase systematically with the distance of the sources, while the one of intrinsic effects should not. Both effects can add up or cancel each other. As a result, they must be studied in parallel. In addition, it is necessary to study as many sources as possible to search for these effects in so-called “population studies”.

The groups of LPNHE and LUTh have developed a strong expertise in both intrinsic effects and propagation effects. They have been collaborating on these topics for years. The expertise of the LPNHE group lies in the analysis methods, the techniques to look for spectral lags (including VIL effects) in the data of gamma-ray experiments, in particular of ground-based Cherenkov telescopes. The LUTh group is specialized in the physics of AGN and relativistic jets. Several simulation codes are available for stationary and time-dependent modeling of blazars. Both groups are involved in the experiments H.E.S.S. and CTA. H.E.S.S. is an array of five Cherenkov telescopes located in Namibia. CTA is a big array of tens of Cherenkov telescopes which is currently under construction in La Palma (Canarias). It should take its first data in 2024. The performance of CTA will surpass the one of H.E.S.S. and the sensitivity to intrinsic and VLI effects will greatly improve.

Objectives of the thesis

The PhD will be divided in two parts, to be developed in parallel.
In a first part, the student will carry out the first large population study for the search of VLI effects at high and very high energies. He/She will analyze all available sources observed by H.E.S.S. and participate in the joint working group already existing and gathering researchers from H.E.S.S., and other collaborations (MAGIC and VERITAS). The goal of this working group is to perform this population study gathering AGN, GRB and pulsars observed by different ground-based experiments. Including space-based experiments such as the Fermi satellite is also a possibility. This work will lead to a high-impact publication.
In a second part, the student will focus on possible methods to disentangle source effects and propagation effects in a coherent framework. He/She will take into account recent progress in AGN jet modeling in the context of VLI searches. In particular, she/he will simulate “virtual data” to explore the capabilities of CTA to firmly detect spectral delays from typical AGN flares and will study the CTA performance to separate VLI and intrinsic delays. He/she will make proposals on observational strategies to optimize the chances of achieving this objective. A second publication is foreseen about this study. The PhD student will possibly also contribute to the analysis of first data from CTA at the end of the thesis.

Additional information

The student will be a member of both LPNHE and LUTh groups as well as a member of the H.E.S.S. Collaboration and the CTA Consortium. She/He will have the opportunity to participate in shifts at the H.E.S.S. site in Namibia. She/He will have the opportunity to present her/his work at international conferences.

Work location: LPNHE, Paris

Possible trips: The selected candidate will have to make regular visits to the Paris Observatory in Meudon. He/She will have the opportunity to participate in the data-taking of the H.E.S.S. experiment in Namibia. He/She will have to attend the H.E.S.S. and CTA collaboration meetings.

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