PhD 2024-2027 – 4D mapping of blazars: from optical to gamma-ray emission


Julien Peloton:  <>

Jonathan Biteau: <>

Scientific context

The multi-messenger universe is one of the most fruitful areas of research in astronomy. The last fifteen years have seen in particular i) the unveiling of the gamma-ray sky at GeV energies in space and at TeV energies from the ground; ii) the discovery of neutrinos of astrophysical origin up to PeV energies; iii) the direct observation of gravitational waves; iv) the discovery of the anisotropy in the arrival directions of cosmic rays beyond EeV energies. These discoveries marked the rise of the multi-messenger field.

A particular class of astrophysical sources of interest to multi-messenger studies are active galactic nuclei (AGNs). Jetted AGNs account for 80% of the ~1,600 sources detected in gamma rays above 10 GeV by Fermi-LAT over the full sky. These non-thermal sources emit across the entire electromagnetic spectrum with high bolometric luminosity, linked to the relativistic motion of the radiative region along the jets. With jets almost aligned with the line of sight, blazars that host a supermassive black hole of millions to billions of solar masses can persistently or transiently outshine the billions of stars in the host galaxy. Blazars display a variable flux on timescales ranging from several years to a few minutes. The rapid variability of these sources provides constraints on the size of the emission region and on its overall speed of approach to the observer. Their flux distributions are often heavy-tailed and relatively well modelled by log-normal distributions, with debated conclusions on the origin of flux variation: cascade propagation of fluctuations in the accretion disk or variations in the acceleration rate within the jet. While theoretical work is being actively developed to predict these variations, an unbiased census of blazar variability remains out of reach. This lack of knowledge prevents us from drawing firm conclusions about the association of blazars with messengers such as TeV-PeV neutrinos and EeV cosmic rays.

Blazars observed in the gamma-ray range by Fermi-LAT are also observed in optical surveys. Their optical and gamma-ray emissions share common properties, as expected from broadband emission models. With the advent of time-domain astronomy, optical surveys will soon make it possible to map the variability of most of the transient sky. For example, the ZTF survey currently generates ~10⁵ optical alerts per night, and the upcoming Vera C. Rubin Observatory with its deep and wide survey of the sky will revolutionise the time domain astronomy reaching dozens of millions alerts per night between 2025 and 2035. These data are sorted by alert managers (brokers) such as Fink, which, in addition to a high-frequency processing capabilities, can ingest and redistribute alerts openly, cross-reference them with those from other observatories and correlate them with catalogues of known sources. We have already configured Fink to ingest real-time data streams from Fermi-LAT blazars. This gamma-ray data stream is the best path-finder for alerts to be sent and received by the Cherenkov Telescope Array (CTA), the next-generation gamma-ray observatory.

This PhD project takes advantage of the latest developments in time-domain astronomy, with a combination of optical and gamma-ray surveys to tackle the challenge of blazar variability. In particular, the first year of the PhD work will see the start of operations at Rubin/LSST and CTA-North, to which the PhD supervisors are actively contributing. The aim is to establish a coherent mathematical framework to characterise altogether the optical and gamma-ray properties of blazars. The PhD work will focus on constructing a four-dimensional (direction, distance, time) mapping of blazars which will serve as the basis to deduce the duty cycle of blazars and automate the detection of transient events from these sources. Such an approach will continuously benefit from new information obtained (automatic and/or human learning), eventually leading to the discovery of new transient events from blazars, and ultimately opening a new window on the transient sky.

Thesis program

The thesis program will focus on optical and gamma-ray extragalactic astronomy, with data from several observatories. Depending on the candidate’s interests and skills developed throughout the project, key developments are expected on:

A) Fermi-LAT / ZTF

Initially, the work will focus on the study of the blazar population in the gamma-ray range using data from the Fermi space observatory (LAT instrument). The current dataset, comprising around ~1,300 jetted AGNs detected in gamma rays above 10 GeV by Fermi-LAT, will serve as a starting point for constructing the distribution function characterising the gamma-ray population and its temporal properties. We will then look for counterparts to these sources in the ZTF data received by the Fink broker since 2019 (over 160 million alerts), to finally build a first version of the four-dimensional mapping that includes both optical and gamma-ray data.

B) Fermi-LAT / Rubin

From 2025, the Rubin observatory will begin observations in the optical range from Chile. Rubin’s frequency coverage will be greater than that of ZTF (6 frequency bands: u, g, r, i, z, and Y), the depth will be greater (up to a cosmological redshift of around 3), and the precision of photometric data will be better (around 0.01 magnitude). The PhD student will have first access to the Rubin data. The model initially built from ZTF optical data will rapidly benefit from Rubin data to improve understanding of blazar variability. Particular attention will be paid to the understanding and rejection of contaminants, given the expected rate of alerts (> 10 million per night).

C) CTA / Rubin

From 2026, the first CTA telescopes will be operational. The most interesting alerts selected from Rubin observatory data will be sent in real time to the CTA telescopes for gamma-ray monitoring. Work will focus on synchronising the alert systems (interoperable tools) and reducing the first data taken by CTA to complete the mathematical model including all the optical and gamma-ray properties of the blazars. Work will also focus on the study of new transients revealed by the Rubin & CTA observatories. Finally, all the results of this work will be made available to the community in an open science approach, already at the heart of the strategy of all the participants (Fink, Rubin, CTA).


International exchanges and collaborations constitute a key stone of the proposed PhD program. The PhD student will join the CTA Consortium and as such will participate in CTA groups and consortium meetings. The PhD will also join the Rubin Observatory, as well as be a full member of the Fink broker. The student will participate in workshops and conferences in Europe. A close collaboration, including possible exchanges of a few weeks, can be envisioned with collaborating groups, more particularly in Germany and in the United States. 

Prerequisites and skills acquired

Candidates to the present PhD program should demonstrate, as prerequisites:

  • a collaborative mindset;
  • basic skills in Linux systems, Python, and scientific computing (statistics, signal processing, data science);
  • fundamental notions in high-energy astrophysics, astroparticle physics, and cosmology;
  • Master’s degree in astronomy / cosmology / astroparticle, or equivalent level in another area of science.

Throughout the course of this project, the PhD student will acquire:

  • an expertise in astronomy and astroparticle physics;
  • synthesis skills, through the writing of notes, conference proceedings, and publications;
  • presentation skills, through oral presentations to the international community;
  • the ability to lead a project from end to end;
  • possibly teaching skills, should the student be interested.

Supervising team

The successful candidate will carry out her/his PhD program in the A2C department (J. Biteau) and the IT department (J. Peloton) of the Irène Joliot-Curie 2 Infinities Physics Laboratory – IJCLab.

The Astrophysics, Astroparticles and Cosmology (A2C) department comprises 60 researchers studying various facets of the Universe, from the solar system to large-scale structures and the most violent phenomena. The teams’ work ranges from phenomenological developments to the design, construction and operation of large observatories. The successful candidate will join the high-energy astroparticle team of A2C, which comprises five permanent members. They are involved in the Pierre Auger Collaboration (since 2000), in the CTA Collaboration (since 2015), and in the DAMIC project (since 2018). As of 2023, the group comprises three postdoctoral researchers and two PhD students. The group acquired a world-wide recognized expertise in the study of extragalactic gamma-ray sources (coordination of the CTA extragalactic group in 2018-19), development of calibration devices for gamma-ray astronomy (coordination of the NectarCAM associated work package since 2018), as well as reconstruction of cosmic-ray events, analysis of anisotropies and cosmic-ray physics at the Pierre Auger Observatory.

IJCLab’s IT department employs over 50 engineers and technicians working in various fields of physics. The development team specialises in scientific computing, such as parallel computing, machine learning, big data manipulation and visualisation. The department is in charge of the VirtualData cloud of the Université Paris-Saclay. In 2023, the group comprises one postdoctoral researcher and two PhD students. The department contributes to all the laboratory’s projects, including the technical and scientific management of the Fink project. The department also manages the VirtualData cloud infrastructure at the Université Paris-Saclay (10,000 vCPUs, 2 PB of storage).

Bibliography – publications of the group on topics related to the PhD program

Variability studies of active galactic nuclei from the long-term monitoring program with the Cherenkov Telescope Array
G. Grolleron, …, J. Biteau et al. for CTA, arXiv:2309.12157
PoS ICRC2023 (2023), 856

Cosmographic model of the astroparticle skies
J. Biteau et al., arXiv:2108.10775
PoS ICRC2021 (2021), 1012

Stellar Mass and Star Formation Rate within a Billion Light-years
J. Biteau, arXiv:2105.11345
Astrophys.J.Supp. 256 (2021) 1, 15

Fink, a new generation of broker for the LSST community
A. Möller, J. Peloton, E. E. O. Ishida et al, arXiv:2009.10185
MNRAS, Volume 501, Issue 3, March 2021, Pages 3272–3288

Progress in unveiling extreme particle acceleration in persistent astrophysical jets
J. Biteau et al., arXiv:2001.09222
Nature Astron. 4 (2020) no.2, 124-131

KSP: Active Galactic Nuclei
A. Zech, D. Mazin, J. Biteau et al., arXiv:1709.07997
Chap. 12 in “Science with the Cherenkov Telescope Array” (Research book), Ed. CTA Consortium, World Scientific (2019), ISBN #9789813270091

Gamma-rays From the Quasar PKS 1441+25: Story of an Escape
VERITAS, SPOL, ASAS-SN, OVRO, NuSTAR and CRTS Collaborations (J. Biteau et al.), arXiv:1512.04434
Astrophys.J.Lett. 815 (2015) no.2, L22

The minijets-in-a-jet statistical model and the RMS-flux correlation
J. Biteau, B. Giebels, arXiv:1210.2045
Astron.Astrophys. 548 (2012) A123