Name: Atelier API "Ondes gravitationnelles et objets compacts"
Start: 2022-11-14T09:00:00+01:00
End: 2022-11-16T18:00:00+01:00
Location: Observatoire de Paris - site de Meudon

lun. 14 novembre
- Présentation des équipes
  - 1
    
    Welcoming
  - 2
    
    LUTH
    
    presentation_roc.pdf
  - 3
    
    SYRTE
  - 4
    
    LESIA
  - 5
    
    APC (x2)
- 11:00
  
  Pause café
- Projets financés
  - 6
    
    Equation of state representation for neutron star oscillations
    
    The choice of an equation of state is crucial in stellar structure. The most common choice is polytropes as they are good approximations for zero-temperature Fermi gas of electrons or nuclear matter. However, when going to more in-depth description of neutron stars, nuclear equations of state are needed to describe the interactions between baryons at very high densities. Those equations of state are often not analytical and thus come as tables, as given for example in CompOSE. Therefore, using them comes with interpolation schemes for the physical quantities are not limited to tabulated values. This, added to the fact that numerical precision of tables may not be as high as one wants due to the phenomenological nature of the nuclear equations of state, brings numerical noise in the simulation codes. One way to address this issue is to create analytical fits of the tables which not only allows for any desired numerical accuracy but also possibly reduces computation time as all thermodynamical quantities of interest can be computed analytically instead of having to interpolate through tables containing thousands to millions of grid points, which can be computationally expensive. I will describe the idea of pseudo-polytrope which generalizes polytropic equations of state, and present the current version of a code aimed at simulating the post-merger hypermassive neutron star.
    
    Orateur: Gaël Servignat (LUTH)
    
    presentation_API_14nov2022.pdf
  - 7
    
    Analytical representations of unified and modern equations of state
    
    Equations of state of dense matter play a key role in neutron star modelling. Well constructed equations of state must be constrained by nuclear physics experiments, and multi-messenger observations. But they must also be designed with the same nuclear model for the high density, and the low density part of the star. Yet, non-unified equations of state can be abundantly found in the literature and in many simulations.
    In this study, we show that one of the most commonly used analytical representation of cold and catalyzed equations of state is based on non-unified constructions, and leads to errors on macroscopic parameter modelling. In the spirit of Read et al. 2009, we provide piecewise polytropic fits for a set of 50 equations of state which are unified, and well calibrated by nuclear physics and astrophysics data.
    
    Orateur: Lami Suleiman (Laboratoire Univers et Théories)
    
    API_Suleiman_MordernAndUnifiedEOS.pdf
- 12:30
  
  Pause déjeuner
- Einstein Toolkit: Miguel Zilhao
  
  Hands-on session
  - 8
    
    Présentation générale
  - 14:45
    
    Pause
  - 9
    
    Hands-on session
  - 16:00
    
    Pause café
  - 10
    
    Exemples d'utilisation
mar. 15 novembre
- Projets financés
  - 11
    
    Modelling the flares of Sagittarius A* observed by GRAVITY
    
    For the past two decades, flares (i.e. outbursts of radiation) have been observed from the centre of the Milky Way where a massive compact object of 4.2 millions solar masses resides at only 8.3 kpc. This makes this object called Sgr A* the closest supermassive black hole candidate to Earth and an unique laboratory for relativistic astrophysics. Recent observations have shown that the source of these outbursts is close to the event horizon and has an orbital motion around the black hole.
    
    Many scenarios are envisaged to explain this phenomenon without reaching a consensus. Among these scenarios, magnetic reconnection is one of the most promising, supported by many GRMHD and PIC studies. During this presentation I will present two models of flares: a general analytical "hot spot" model taking into account the quiescent state of Sgr A and a more realistic magnetic reconnection model based on kinetic simulations. I will examine the diversity of observables associated to these models and discuss them in the light of the recent VLTI/GRAVITY observations of Sgr A flares.
    
    Orateur: Nicolas Aimar (LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. de Paris, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France)
  - 12
    
    Gravitational waves emitted by the orbit of a massive object around a Boson Star
    
    The future mission LISA will observe gravitational waves in a frequency range going from 10-4 to 10-1 Hz through three satellites orbiting in a heliocentric orbit. It is expected that this mission will detect the orbit of solar mass star orbiting around the compact object at the center of the Milky Way, Sagittarius A. Depending on the orbit and the star’s mass, the gravitational waves emitted by such a system should be observable by LISA, frequency and amplitude-wise. This might help the community in confirming or denying the nature of Sgr A as a supermassive Black Hole. In this project, we worked under the assumption that Sgr A is a supermassive Boson Star with no interaction to matter apart from gravity. Using the GYOTO code, written by the LESIA in collaboration with the LUTH, we have computed the timelike geodesics around the compact object considering a numerically solved metric around a boson star of mass equivalent to Sgr A. Then, with the quadruple formula in the case of an EMRI, we have generated the form of the gravitational waves emitted by numerous orbits and analysed them in order to try and find waveform unique to EMRI’s orbits around a Boson Star.
    
    Orateur: Samy Aoulad Lafkih (Observatoire de Paris)
- 11:00
  
  Pause café
- Contributed presentations
  - 13
    
    GRAVITY+: black holes from here to there
    
    Optical interferometry just recently came to an age with the transformative results brought up by GRAVITY at the VLTI, on topics such as exoplanets, active galactic nuclei (AGN) and the Galactic Center. The next stage is already upcoming with GRAVITY+, a series of upgrades of the instrument and the infrastructure. With laser-assisted, extreme adaptive optics for each unit telescope and wide-angle fringe-tracking, GRAVITY+ will reach targets as faint as K=22, including many AGNs and all of the Galactic plane. I will show how these upgraded performance will open the door to a statistical approach to black hole science, from here to the end of the Universe.
    
    Orateur: Thibaut Paumard (LESIA - CNRS/Observatoire de Paris)
  - 14
    Dark Matter reconstruction from stellar orbits in the Galactic Centre
    
    The continuous monitoring of stellar orbits in the Galactic Centre over the past few decades revealed the massive central object Sgr A at their shared focal point (→ Nobel prize 2020). These and other observations (orbits of hot flares, black hole shadow) are in best agreement with the notion of Sgr A being a massive black hole of about 4 million solar masses. Assuming the black hole model, the orbit of the star S2 (also S-02) also allowed for the observation of relativistic effects (gravitational redshift, Schwarzschild precession).
    
    While we currently do not see any matter between the innermost known S-stars (e.g. S2) and the accretion flow of Sgr A*, observations still allow for the presence of at least a few thousand solar masses of distributed matter, such as a cluster of faint stars and stellar remnants,(particle) Dark Matter or a combination thereof. Depending on the nature of the distributed matter, theory predicts it to attain certain density profiles. In my talk I will first summarise some recent theoretical results as well as observational upper bounds on dark mass distributions around Sgr A*. I will then introduce a novel mass shell model for such matter distributions, which does not assume a specific functional form of the density profile a priori, but rather obtains this form from fitting it to the data. For this I will present a proof of concept.
    
    Orateur: Gernot Heißel (Observatoire de Paris / LESIA)
    
    presentation Gernot.pdf
- 12:30
  
  Pause déjeuner
- GYOTO: Frédéric Vincent
  
  Hands-on session
  - 15
    
    Présentation générale
  - 14:45
    
    Pause
  - 16
    
    Hands-on session
  - 16:00
    
    Pause café
  - 17
    
    Exemples d'utilisation
mer. 16 novembre
- Projets financés
  - 18
    
    QFT methods for GW phyiscs
    
    Recent years have seen a renewed interest in analytical treatments of the two body problem in gravity. This is mainly due to the recent detections of gravitational waves from binary coalescences. Lately, many tools originating from particle physics and QFT have been utilized for the analytic solution of the two-body problem in gravity such as EFT methods and modern methods for Scattering Amplitudes. In this talk, we will present some recent results derived using these techniques.
    
    Orateur: Stavros Mougiakakos (IPhT,CEA-Saclay)
    
    QFT for GW API.pdf
  - 19
    
    Towards multi-messenger observations of accreting binary black holes
    
    The many recent detections of gravitational waves (GWs) of binary black hole (BBHs) mergers have opened the way for future multimessenger campaigns. One expected result, not achieved yet, is the co-detection of electromagnetic (EM) radiation from a BBH merger system together with its GW inspiral emission, detectable for stellar-mass BHs with LIGO/Virgo, intermediate-mass BHs (and stellar-mass BHs) with the Einstein Telescope and supermassive BHs with the space-based detector LISA in 2030+. The latter could result from the future LISA and Athena (X-ray satellite, 2030+) synergies and need to be anticipated. However, the EM signatures of such systems are not firmly identified because few numerical codes are able to model the gravitational impact of the BBH on its accretion disk in General Relativity (GR).
    
    In this talk, I will present e-NOVAs, standing for "extended Numerical Observatory for Violent Accreting systems" as it has been recently extended to work with any type of spacetime. It is the first European code to evolve an analytical BBH metric as it solves the equations of GR-magnetohydrodynamics and to compute synthetic observations in the same metric via GR ray-tracing. Using e-NOVAs, I will study the influence of a BBH in the inspiral regime on their circumbinary disk. First, I will present the accretion structures that could potentially help us distinguishing BBHs from single BHs. Then, I will show if their timing properties computed through ray-tracing in the same BBH spacetime, could be used as an EM signature allowing us to distinguish BBHs from other transient sources in the future.
    
    Orateur: Raphaël Mignon-Risse (CNES/APC)
    
    Mignon-Risse_Raphael_AI.pptx
- 11:00
  
  Pause café
- Discussion: générale
  
  Discussion selon les retours des équipes et des participants lors des premiers jours
- 12:30
  
  Pause déjeuner
- Discussion: thématique
  
  Discussion selon les retours des équipes et des participants lors des premiers jours
- Discussion: Retour sur les discussions thématiques
  
  Discussion selon les retours des équipes et des participants lors des premiers jours
- 15:30
  
  Pause café

Atelier API "Ondes gravitationnelles et objets compacts"

salle du château

Observatoire de Paris - site de Meudon