E5: Numerical Observatory of Violent Accretion systems NOVAs strong gravity and beyond

> Read the articles connected to the project.



    first GR-HD simulation of a thin disk equilibrium GYOTO view of a GR accretion disk

    Collaboration between AIM, APC and LUTH


    The advent of high energy observation facilities in the last decades has proven the existence of powerful mechanisms emitting photons up to gamma-rays. It is now commonly admitted that the most energetic events are associated with compact objects believed to be relics of massive stars. These objects are prone to the most extreme gravity fields and are likely efficient attractors of the plasma present in their vicinity. The motion of plasma in the close neighborhood of compact objects is only properly described in the framework of general relativistic magnetohydrodynamic (GR- MHD). The equations governing GR-MHD are so complex that the only way to solve them is trough large-scale numerical simulations.

    The topic of the project is to sustain a computational effort dedicated to GRMHD simulations of accretion flows near compact objects and to link them to synthetic observations of the associated violent events, one of the major themes within LabeX UnivEarths.


    General Objective of the Project


    The quest to unveil the nature of the compact objects detected in the center of radio-loud galaxies, in X-ray binaries and also at the center of the Milky Way, is at the crossroads between high resolution, time-dependent, multi-wavelength observations and the coming of age of numerical GR-MHD codes. The project plans to exploit this opportunity by setting a long- standing collaboration between members of three teams involved in the LabeX UnivEarths which are already tackling, individually, the problem from the different angles of analytical and numerical studies, observations, and high-performance code development.


    While the french community is, at the moment, lacking a numerical code able to describe the dynamics of a magnetized plasma in a full GR framework, the LabeX UnivEarths benefits from a unique conjunction where plasma numericists, high-energy observers and GR numericists are present in the same area and have had a good track-record on common past projects.


    By joining forces, our aim is to sustain the development of such a GR-MHD code in order to perform large-scale simulations of plasma accretion onto compact objects and subsequently create an ”observation” from it. Another innovative feature of our code is stem from its ability to handle any kind of GR metrics, not only conventional metric such as Schwarzschild or Kerr, but also metrics of alternative, non-GR, gravitation theories which generally do not have any analytical expression.


  • The three teams and interaction

    Our team, while approaching a new subject linking their respective research, is composed of people used to working together on a variety of topics such as GRBs and planet formation. For this project we plan on using our distinct approaches to tackle the creation the first full GR- MHD code coupled to a ray-tracing code able to provide synthetic observations of the environment of compact objects.

    WP leader CASSE Fabien APC MCF (Univ. Paris Diderot)
    WP co-leader RODRIGUEZ Jérôme AIM Ing. Chercheur, CEA
    WP co-leader MELIANI Zakaria LUTh AA, CNAP
    WP member VINCENT Frédéric LUTh CR, CNRS
    WP member VAN MARLE A.-J. APC Postdoc (17/11-17/12)
    WP member DEMIDEM Camilia APC PhD student (started 2016/10)
    WP member CANGEMI Floriane AIM PhD student (student 2017/10)


    AIM – high-energy observers axis led by J.Rodriguez. They are expert of multi-wavelengths observations of compact objects and have access (through dedicated and collaborative programs) to a wide range of observations. This broad range of data have allowed them to tackle the problems of accretion-ejection connections and mechanisms prevailing in those systems, including also the study of rapid X-ray variability.


    APC – ADAMIS simulation axis led by F. Casse and P. Varniere. Their expertise ranges from analytical to MHD numerical studies of the accretion-ejection system. They also do the extra step of linking their results with observations. They are developers in the mpi-amrvac project.

    LUTH – Relativistic fluid simulations axis, led by Z.Meliani and E.Gourgoulhon. ZM is long term developer on the mpi-amrvac project is now fully concentrating on implementing and using GR-MHD to study compact objects. EG is a leader in strong gravity calculation.



    Over the last four years, we have managed to develop a new general relativistic (GR) fluid code aiming at studying the behavior of plasmas prone to extreme gravitational fields, namely in the vicinity of any kind of compact objects, and fully coupled it with ray-tracing to get spectral and timing synthetic observations. The numerical progress we made during the first years have opened the door to new astrophysical fluid studies while we carry on efforts in data processing in order to access the physical conditions prevailing in accretion flows orbiting around compact objects. It is noteworthy that we are now harvesting scientific results as the number of refereed papers stemming from WP NOVAs has reached 23 over the last 4 years.


    The main results of this WP focus on the variability from compact objects, in particular the Quasi-Periodic Oscillations, that are detected in the Power Density Spectrum. We have two models that cater to the low-frequency and high-frequency QPOs observed in those sources and using simulations, numerical models combined with ray-tracing we are using those QPO to infer what is happening in the source.


    Exploring alternate compact object models

    While microquasars are thought to be black-hole binaries, the exact nature of the compact object remain open. The recent detection of gravitational waves whose signal is consistent with coalescent binary black holes strongly supports such assumption. However, we cannot yet entirely rule out other possibilities. Amongst the alternative models, one of the most popular ones is the boson star model.



    Instability in accretion flows around black holes

    In the context of accretion disks orbiting around black holes we addressed the physics of a fluid instability which may explain observational features occurring in the vicinity of compact object systems. Indeed, the Rossby wave instability (RWI) has been proposed to explain radiative emission variability in microquasars as well as supermassive black hole such as SgrA* but this was done in the Pseudo-Newtonian approach. Using our code we performed the first in-depth study of this instability in a general relativistic framework. The first step was to follow the instability from a Newtonian distance and getting closer and closer to the last stable orbit of the disk in the case of a null spin. Such study allows us to investigate the minute changes in the behaviour of the instability (Casse et al MNRAS 2017).



  • Impact of the gravity of a Schwarzschild black hole upon the Rossby wave instability

    Casse, P.Varniere & Z. Meliani, 2017, MNRAS Vol. 464, 3704


    Shocks in relativistic transverse stratified jets, a new paradigm for radio-loud AGN.

    Hervet, Z. Meliani et al., 2017, A&A (in press)


    On tidal disruption of clouds and disk formation near boson stars

    Meliani, F. Casse, P. Grandclement, E. Gourgoulhon, 2017, Class. & Quant. Gravity (in press)


    On magnetic field amplification and particle acceleration near non-relativistic astrophysical shocks:

    Particles in MHD Cells simulations

    A.J. van Marle, F.Casse & A. Marcowith, 2017, MNRAS (in press)


    Single-dish and VLBI observations of Cygnus X-3 during the 2016 giant flare episode

    Egron, E. et al, 2017, MNRAS Vol. 471, 2703


    Reproducing the Correlations of Type C Low-frequency Quasi-periodic Oscillation Parameters in

    XTE J1550-564 with a Spiral Structure

    Varniere & F. Vincent, 2017, ApJ Vol. 834, 188


    Refereed proceedings :


    On magnetic field amplification and particle acceleration near non-relativistic collisionless shocks:

    Particles in MHD Cells simulations

    Casse, A.J. van Marle, A. Marcowith, 2017, Plasma Physics & Controlled Fusion (in press), Invited talk at the 44th euopean conference on Plasma Physics (Belfast, UK), June 2017.


    Using a combined PIC-MHD code to simulate particle acceleration in astrophysical shocks

    A.J. van Marle, F. Casse & A. Marcowith, 2017, Proceedings of Science (in press), Talk at the 35th International Cosmic Ray Conference (ICRC) , Busan, South Korea, July 2017