Field of research

My first field of research has been the study of Active Galactic Nuclei (AGN), which was the subject of my Ph.D. thesis (Vuillaume, 2015). I am particularly interested in the modeling of the high-energy emission of these objects.

An Active Galactic Nucleus (AGN) is a compact object situated at the center of a galaxy and showing very bright emission at some or at every wavelength. The only known phenomenon capable of producing enough energy to explain such luminosities is the accretion of matter onto a supermassive black hole. One can also observe relativistic jets of sizes comparable to that of the host galaxy. In models I am interested in, the bright emission is directly due to these jets. My work consists in describing the physical processes occurring in the jet in order to explain and describe the observed emission.

High energy observations

High energy observations are particularly important in my work and can be done by different telescopes: satellites observing in X-rays (XMM) and gamma rays (Fermi) or ground-based telescopes, such as Cherenkov telescopes (HESS, CTA).

Picture of the galaxy NGC 4261 from Hubble archives. On the left, a composite image of the galaxy NGC 4261. Both jets coming from the central core are visible in radio. On the right, a zoom on the central core by the Hubble Space Telescope where we can see the accretion disk and the dusty torus.

Scheme of the central region (extracted from Trevor C. Weekes, Very High Energy Gamma-Ray Astronomy). A lot of material (mainly gas and dust) gravitates around the central black hole. The jet emanates from the central regions of the AGN, either from the ergosphere of the BH or from the accretion disc.

The two-flow model

During my Ph.D. thesis, supervised by Gilles Henri and Pierre-Olivier Petrucci at IPAG, I worked on improving the two-flow model. In this model, the jet is composed of two fluids:

• a MHD sheath mildly relativistic and carrying most of the power
• an inner, highly relativistic, leptonic jet responsible for the non-thermal emission observed

I contributed to the development of a numerical model corresponding to the two-flow. An important part of my work has been to find analytical and numerical approximations to increase the efficiency of the model and its usability in an acceptable computing time.

The Compton rocket

Interesting results include the study of the acceleration of the inner jet via the Compton rocket. I showed that the complex photon field of an AGN created by the accretion disc, the dusty torus and the broad line region induces variation of the speed of the flow along the jet (with acceleration and deceleration zones). These variations are crucial as they can result in modulation of the emission in space and in time.

This work has been published in (Vuillaume et al., 2015).

Modeling AGN

In order to optimize the model on actual data, I had to develop an optimization algorithm. I chose genetic algorithms for their ability to deal with complex behavior in numerous dimensions. Here is a little animation of an optimization of the model (orange thick line) compared to data (crosses). Generation after generation, the model improves itself by finding parameters that better fit the data.

Fit of 3C 273 SED by the genetic algorithm GaJet I developed.

Modeling of 3C 273 by JetModel. The modeled SED fits 30 years of data from Türler et al. Different zones of the structured jet emit at different wavelengths, thus explaining the radiation from radio to gamma-rays. This work has been published in (Vuillaume et al., 2018).