Rencontre du groupe de travail « Formes d'onde » (GdR Ondes Gravitationnelles)

lundi 20 mai 2019, 08:55 → 18:00 Europe/Paris

Amphithéâtre (Institut d'Astrophysique de Paris)

1 Waveform of compact binaries and dimensional regularization

Compact binaries are modelled as systems of point particles. In high post-Newtonian calculations of the motion and radiation UV and IR divergencies appear. These are cured using the dimensional regularization. We discuss recent aspects of this problem.

Orateur: Luc Blanchet (C.N.R.S.)

2 Weakly eccentric waveform of compact binaries at the third post-Newtonian order

We present the recent calculation of the so-called hereditary terms in the waveform of a binary system of compact objects at the third post-Newtonian (3PN) order, expanded in powers of eccentricity. Those results, combined with earlier ones, are used to build the full gravitational-wave modes with the same approximations.

Orateur: Guillaume Faye (Institut d'Astrophysique de Paris)

3 Higher-order tidal effects in binary neutron-star systems

The detection of GW170817 coming from the coalescence of two neutron stars allowed us, by combining the analytical models with the analysis of the signal, to constrain the equation of state (EOS) of neutron stars. The post-Newtonian formalism played a crucial role in the signal analysis but more precision on the tidal effects entering the waveform is required for comparison with numerical relativity or future data processing. The goal of this talk is to present some recent work aimed at increasing the precision of the tidal effects within binary systems that appear in the wave phase, so as to better constrain the EOS of neutron stars.

Orateur: Quentin Henry (Institut d'Astrophysique de Paris)

10:30 Pause

4 Imaging black holes with the Event Horizon Telescope

One of the most fundamental predictions of general relativity are black holes. Their defining feature is the event horizon, the surface that even light cannot escape. When illuminated by ambient light, the event horizon of black holes will cast a dark shadow on the emitting region that is detectable under certain circumstances with global interferometers operating at mm- and submm-wavelengths. Recently the Event Horizon Telescope has detected this shadow feature in the radio galaxy M87, providing a first glimpse at scales surrounding the event horizon. Models invoking general relativity and magnetized plasma hydrodynamics are able to reproduce the appearance of the shadow and of the powerful jet launched at these scales. This provides strong support for the existence of supermassive black holes in the universe and sheds light on how they work. To improve the imaging quality further more telescopes should be added to the array, in particular in Africa. The more distant future will belong to higher frequencies and space-based interferometry. The talk will review the latest results of the Event Horizon Telescope, its scientific implications and future expansions of the array.

Orateur: Heino Falcke (Institut de Mathématiques, Astrophysique et Physique des particules, Université Radboud de Nimègue)

5 Gravitational waves in LISA band from bodies orbiting the Galactic Center black hole

A fully relativistic study of gravitational radiation from bodies in circular equatorial orbits around the massive black hole at the Galactic Center, Sgr A*, has been performed, taking into account the Roche limit induced by tidal forces in the Kerr metric. The signal-to-noise ratio for LISA, as well as the time spent in LISA band, has been computed. It is found that brown dwarfs, main sequence stars and compact objects are all detectable in one year of LISA data with a signal-to-noise ratio above 10 during at least 10^5 yr in the slow inspiral towards either the innermost stable circular orbit (compact objects) or the Roche limit (main-sequence stars and brown dwarfs). The longest times in-band, of the order of 10^6 yr, are achieved for primordial black holes (mass < 10^{-3} solar mass), as well as for brown dwarfs, just followed by white dwarfs and low mass main-sequence stars.

Orateur: Éric Gourgoulhon (LUTH, Observatoire de Paris)

6 Modeling the strong-field dynamics of binary neutron star merger

The observation of gravitational and electromagnetic waves from a binary neutron star merger in August 2017 conveyed key information on the nature of matter at supranuclear densities, on the origin of short-gamma ray burst, on the production site of of heavy elements via r-process nucleosynthesis, and on cosmography. Thus, multimessenger observations of compact binary mergers hold the promise to unprecedent insights on some of the most fundamental physics questions. A crucial and necessary ingredient to intepret such observations is the precise knowledge of the dynamics of the sources. I will talk about recent developments on the modeling of neutron star mergers using numerical simulations in general relativity. I will focus on the numerical exploration of the the merger remnant and mass ejecta and their dependence on the binary parameters. I will discuss detailed models of the gravitational waves and kilonova light curves, highlighting the prospect of using them in joint analysis of multimessenger data.

Orateur: Sebastiano Bernuzzi (Université d'Iéna)

7 The formation of BH-HMXBs and BBHs: Impact in Gravitationl Waves Astrophysics and Cosmology

Theoretical models and observations suggest that primordial Stellar Black Holes (Pop-III-BHs) were prolifically formed in HMXBs, which are powerful relativistic jet sources of synchrotron radiation called Microquasars (MQs). Large populations of BH-HMXB-MQs at cosmic dawn produce a smooth synchrotron cosmic radio background (CRB) that could account for the possible excess amplitude of atomic hydrogen absorption at z~17, recently reported by EDGES. BH-HMXB-MQs at cosmic dawn precede supernovae, neutron stars and dust. BH-HMXB-MQs promptly inject hard X-rays and relativistic jets into the IGM, which overtake the slower expanding HII regions ionized by progenitor Pop-III stars, heating and partially ionizing the IGM over larger distance scales. BH-HMXBs are channels for the formation of Binary-Black-Holes (BBHs). The large masses of BBHs detected by gravitational waves, relative to the masses of BHs detected by X-rays, and the high rates of BBH-mergers, are consistent with high formation rates of BH-HMXBs and BBHs in the early universe.

Orateur: Felix Mirabel (CEA)

15:30 Pause

8 Discussion