PhD Candidate (M/F) - Time delays in AGN astrophysics: interplay between intrinsic source effects and propagation effects

Job Description

Thesis Subject

Context

About 10% of galaxies host an active galactic nucleus (AGN), powered by accretion onto a supermassive black hole, and in roughly 10% of these systems relativistic jets are launched. This fraction can grow as high as 50% for the most powerful AGNs. When the jets are aligned with the line of sight, these objects are called blazars, and are among the most luminous, broadband and rapidly variable extragalactic sources. Their minute-to-hour high flux states, called 'flares', provide a unique laboratory to probe relativistic jet physics and to measure spectral delays—time differences between photons of different energies. Such delays may originate either from source intrinsic effects (SIE), linked to emission and particle acceleration processes, or from propagation effects accumulating during the photons' cosmological journey. An example of the latter arise in several quantum gravity (QG) frameworks predicting modified photon dispersion relations at very high energy (see [1] for a review), leading to an energy-dependent speed of light in vacuum and thus to Lorentz invariance violation (LIV). These effects are expected to grow with distance in the most studied LIV scenarios. Disentangling intrinsic and propagation-induced delays is a central challenge for LIV searches. Addressing it requires population studies of multiple sources at different distances and a coordinated effort combining source modelling centered on SIE and data analysis. This project is embedded in a strong European context, notably the COST Action BridgeQG, a network which federates QG theorists, phenomenologists and observers.

State of the art

Intrinsic delays have been observed in long GRBs (see e.g. [2] for one of the earliest references on the matter) and are expected in other types of sources. For flaring AGN, no significant delay has yet been detected at very high energies, likely due to the limited sensitivity of current instruments. However, neglecting SIE may bias LIV searches or mask genuine effects. To address this issue, the proposers initiated the first dedicated study of SIE in blazars using a simple leptonic one-zone flare model [3,4], establishing a strong collaboration between LPNHE and LUX. These studies demonstrated that intrinsic spectral lags can naturally arise and depend sensitively on model parameters, opening new observational probes of particle acceleration beyond spectral energy distributions alone. To date, no LIV signal has been identified and limits were set on LIV model characteristic energy scale. Constraints reach the Planck scale (∼10^19 GeV) for individual GRBs [5] and ∼10^18 GeV for individual flaring AGN (e.g. [6]), but in all cases SIE are neglected or considered as universal with the same amplitude for all sources (see e.g. [7]). Improving sensitivity of LIV analyses now requires multisource population analyses over extended energy and distance ranges, especially in the TeV domain. Moreover, it also requires explicitly accounting for SIE.

Goals

The ALIVe project lies at the interface of fundamental physics, high-energy astrophysics and advanced gamma-ray observations, leveraging complementary expertise at CNRS-IN2P3 (LIV and QG phenomenology) and CNRS-INSU (AGN jet modelling). Its first objective is to overcome a key limitation of current LIV searches: the difficulty to precisely describe SIE in AGN jets. Going beyond our previous one-zone leptonic studies [3,4], the project will focus on alternative geometrical models of variability, in which flux changes arise only from jet orientation effects and Doppler boosting [8–11]. Such a scenario has been primarily proposed to describe the quasi-periodicity observed in a few AGN, attributed to the helicoidal shape or twisting of the jet. It is expected that in such models intrinsic delays are small and in any case easier to interpret. As a result, they provide a particularly clean probe of propagation-induced effects: in this scenario, intrinsic lag could effectively be disentangled from propagation delays. These geometrical variability models therefore provide a control sample with minimal intrinsic effects, providing a reference baseline against which propagation-induced effects can be robustly identified. A systematic study of SIE in such geometrical flare scenarios, based on adapted existing simulation tools, is expected to yield rapid, high-impact results. The second axis consists of population studies of flaring AGN, exploiting and extending analysis tools developed within the γLIV Working Group [12]. An unprecedented combined analysis of H.E.S.S. data will be performed on at least four blazars (PKS 2155-304, Mkn 501, 3C 279, PG 1553+113) plus any object detected during the thesis, followed by a global analysis including MAGIC, VERITAS and CTAO-LST1 datasets. As such, the analysis strategy relies on a statistically robust sample of existing observations. The goal is either to identify LIV signatures or to set the most stringent constraints on LIV in the photon sector to date, with, for the first time, a robust and thorough analysis of systematics related to SIE. Population-based analyses accounting for intrinsic effects are expected to improve current LIV constraints by a factor of a few compared to single-flare studies, while significantly reducing statistical uncertainties. The PhD student, at the crossroads of modelling and data analysis, will play a central role in this effort. Regardless of whether LIV signatures are detected, the project will deliver robust constraints and a quantitative characterization of intrinsic spectral delays in geometrical flare scenarios, providing lasting value for both fundamental physics and jet astrophysics.

The PhD student will take primary responsibility for the development of geometrical flare simulations and for the coordinated H.E.S.S. population analysis, ensuring excellent visibility across both modeling and analysis communities.

References
[1] A. Addazi et al., Prog. Part. Nucl. Phys. 125, 103948 (2022), arXiv:2111.05659.
[2] J.P . Norris et al., ApJ 459, 393 (1996)
[3] C. Perennes et al., A&A 633, A143 (2020), arXiv:1911.10377.
[4] C. Levy et al., A&A 689, A136 (2024), arXiv:2406.01182.
[5] V. Vasileiou et al., Phys. Rev. D 87, 122001 (2013), arXiv:1305.3463.
[6] A. Abramowski et al., H.E.S.S. Collaboration, Astroparticle Physics 34, 738–747 (2011),
arXiv:1101.3650.
[7] J. Ellis et al., Astropart. Phys. 25, 402 (2006), astro-ph/0510172.
[8] R. Prince et al., A&A 678, A100 (2023), arXiv:2308.11317.
[9] C. M. Raiteri et al., A&A 692, A48 (2024).
[10] P . Peñil et al., A&A 700, A208 (2025), arXiv:2507.03967.
[11] E. Sobacchi et al., MNRAS 465, 161 (2017), arXiv:1610.04709.
[12] J. Bolmont et al., ApJ 930, 75 (2022), arXiv:2201.02087.

Your Work Environment

The LPNHE is a major research unit primarily focused on experimental particle and astroparticle physics. It operates under the joint supervision of three institutions: CNRS, Sorbonne University, and Université Paris Cité, and comprises around 150 members organized into 12 research teams and 5 support services.

You will be hosted within the HESS/CTAO team.

The workplace is located on the Jussieu campus (5th arrondissement of Paris).

Compensation and benefits

Compensation

2300 € gross monthly

Annual leave and RTT

44 jours

Remote Working practice and compensation

Pratique et indemnisation du TT

Transport

Prise en charge à 75% du coût et forfait mobilité durable jusqu’à 300€

About the offer

About the CNRS

The CNRS is a major player in fundamental research on a global scale. The CNRS is the only French organization active in all scientific fields. Its unique position as a multi-specialist allows it to bring together different disciplines to address the most important challenges of the contemporary world, in connection with the actors of change.

CNRS

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