Event Horizon Telescope | EPFL Graph Search

vignette|redresse=1.3|Sagittarius A*. L’Event Horizon Telescope (en abrégé EHT, littéralement « Télescope de l'horizon des évènements ») est un réseau de radiotélescopes terrestres combinant les données de stations d’interférométrie à très longue base situées sur la Terre afin d’étudier notamment l’environnement immédiat de Sagittarius A*, le trou noir supermassif du centre de la Voie lactée, et de M87*, celui de M87, avec un pouvoir de résolution permettant d'observer leur horizon. Description vignette|gauche|Montage d'images représentant le grossissement atteint par l’Event Horizon Telescope, équivalent à l'observation d'une balle de tennis sur la Lune. Le grossissement part de l'image en haut à gauche (champ de vue approximatif d'un œil humain depuis la Terre), dans le sens antihoraire, et montre le trou noir en haut à droite. L’Event Horizon Telescope (EHT) est composé de plusieurs observatoires radio, ou télescopes radio, autour du monde reliés pour créer un télescope

Gravitational wave signal from primordial magnetic fields in the Pulsar Timing Array frequency band

Andrii Neronov

The NANOGrav, Parkes, European, and International Pulsar Timing Array (PTA) Collaborations have reported evidence for a common-spectrum process that can potentially correspond to a stochastic gravitational wave background (SGWB) in the 1-100 nHz frequency range. We consider the scenario in which this signal is produced by magnetohydrodynamic (MHD) turbulence in the early Universe, induced by a nonhelical primordial magnetic field at the energy scale corresponding to the quark confinement phase transition. We perform MHD simulations to study the dynamical evolution of the magnetic field and compute the resulting SGWB. We show that the SGWB output from the simulations can be very well approximated by assuming that the magnetic anisotropic stress is constant in time, over a time interval related to the eddy turnover time. The analytical spectrum that we derive under this assumption features a change of slope at a frequency corresponding to the GW source duration that we confirm with the numerical simulations. We compare the SGWB signal with the PTA data to constrain the temperature scale at which the SGWB is sourced, as well as the amplitude and characteristic scale of the initial magnetic field. We find that the generation temperature is constrained to be in the 1-200 MeV range, the magnetic field amplitude must be >1% of the radiation energy density at that time, and the magnetic field characteristic scale is constrained to be >10% of the horizon scale. We show that the turbulent decay of this magnetic field will lead to a field at recombination that can help to alleviate the Hubble tension and can be tested by measurements in the voids of the Large Scale Structure with gamma-ray telescopes like the Cherenkov Telescope Array.

AMER PHYSICAL SOC2022

Potential of Radio Telescopes as High-Frequency Gravitational Wave Detectors

Valerie Fiona Domcke

In the presence of magnetic fields, gravitational waves are converted into photons and vice versa. We demonstrate that this conversion leads to a distortion of the cosmic microwave background (CMB), which can serve as a detector for MHz to GHz gravitational wave sources active before reionization. The measurements of the radio telescope EDGES can be cast as a bound on the gravitational wave amplitude, h(c) < 10(-21) (10(-12)) at 78 MHz, for the strongest (weakest) cosmic magnetic fields allowed by current astrophysical and cosmological constraints. Similarly, the results of ARCADE 2 imply h(c) < 10(-24) (10(-14)) at 3-30 GHz. For the strongest magnetic fields, these constraints exceed current laboratory constraints by about 7 orders of magnitude. Future advances in 21 cm astronomy may conceivably push these bounds below the sensitivity of cosmological constraints on the total energy density of gravitational waves.

2021

Data-Driven Convergence Prediction of Astrobots Swarms

Francesco Basciani, Denis Gillet, Jean-Paul Richard Kneib, Matin Macktoobian

Astrobots are robotic artifacts whose swarms are used in astrophysical studies to generate the map of the observable universe. These swarms have to be coordinated with respect to various desired observations. Such coordination\footnote{\z{A coordination sample of the astrobots may be found in \url{http://y2u.be/MpXWvpz4h00.}}} are so complicated that distributed swarm controllers cannot always coordinate enough astrobots to fulfill the minimum data desired to be obtained in the course of observations. Thus, a convergence verification is necessary to check the suitability of a coordination before its execution. However, a formal verification method does not exist for this purpose. In this paper, we instead use machine learning to predict the convergence of astrobots swarm. \z{As the first solution to this problem}, we propose a weighted

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-NN-based algorithm which \rr{requires the initial status of a swarm as well as its observational targets to predict its convergence. Our algorithm learns to predict based on the coordination data obtained from previous coordination of the desired swarm. This method first generates a convergence probability for each astrobot based on a distance metric. Then, these probabilities are transformed to either a complete or an incomplete categorical result.} The method is applied to two typical swarms including 116 and 487 astrobots. It turns out that the correct prediction of successful coordination may be up to 80% of overall predictions. Thus, these results witness the efficient accuracy of our predictive convergence analysis strategy.

2021

Data-Driven Convergence Prediction of Astrobots Swarms

Francesco Basciani, Denis Gillet, Jean-Paul Richard Kneib, Matin Macktoobian

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2021