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Institut d'Astrophysique et
de Géophysique (Bât. B5c)

Quartier Agora
Allée du 6 août, 19C
B-4000 Liège 1 (Sart-Tilman)

Tel.: 04.366.9774
Fax: 04.366.9729
Recul rapide Année précédente Séminaires : Archives 2021    
Jan Fév Mar Avr Mai Jun Jul Aoû Sep Oct Nov Déc
Mois précédent Mois suivant Janvier 2021
Séminaire suivant  1er séminaire : jeudi 21 janvier, 16h00
Stochastic GWSearch for a Stochastic Gravitational Waves Background using a Network of Detectors
Anirban Ain (INFN, Pisa)

The direct detection of Gravitational Waves (GW) by LIGO and Virgo is one of the most significant discoveries of modern times. The network of operational GW detectors is expected to observe a stochastic gravitational waves background (SGWB) in the near future. This all-sky source is made up of unresolved astrophysical sources and remnant GW from cosmological phenomena. Detection of the SGWB can be challenging because the signal is much smaller than the detector noise and being a stochastic signal there is no optimized filter to match-filter the data. Instead, an aperture synthesis technique is used to search for SGWB where data from different detectors are correlated to find signals and long stretches of data is added to build up significance. I have been working on the improvement of SGWB search pipelines. In recent years we implemented data folding and some new techniques to make the pipeline a few thousand times faster. I will be discussing SGWB mapping techniques and how they are optimized.
Séminaire précédant Séminaire suivant 2ème séminaire : jeudi 28 janvier, 16h00
Probing Early universe with Gravitational waves at nano-frequencies
Rome Samanta (CEICO, Institute of Physics, Czech Academy of Sciences, Prague.)

The discovery of Gravitational Waves (GWs) by LIGO and Virgo collaboration has unequivocally opened up a new cosmic frontier for the multi-frequency study of stochastic GW background (SGWB). There are several sources in the Early Universe (EU) that can produce detectable SGWB. Interestingly, recently the NANOGrav pulsar timing array (PTA) collaboration have reported strong evidence for a stochastic common-spectrum process over independent red-noises across 45 pulsars at a frequency f~1/yr. If this is interpreted as GWs, several interesting mechanisms such as models of inflation, primordial black hole dark matter, and baryogenesis can be probed. In this talk, I will briefly summarise some of the mechanisms/sources that can produce GW at nano-frequencies and therefore testable by the PTAs. I will particularly give emphasis on the models of baryogenesis and discuss how PTAs serve as novel probes of baryogenesis-one of the unresolved cosmological puzzles to date.
Mois précédent Mois suivant Février 2021
Séminaire précédant Séminaire suivant 3ème séminaire : jeudi 25 février, 16h00
Degeneracies in gravitational lens modeling: Can implicit assumptions about galaxy structure introduce problems for H_0?
Matt Gomer (STAR)

Gravitational lens modelling is subject to a number of degeneracies, meaning the same observables are reproducible with different lens characteristics. To make a decision between many possible solutions, a lens model is informed by assumptions about galaxy structure, such as that the radial profile being a power law or the lens having an ellipse shape. What if the lens does not correspond to these simple assumptions? This talk explores some possible ramifications for H_0 measurement when lens models are oversimplified.
Mois précédent Mois suivant Mars 2021
Séminaire précédant Séminaire suivant 4ème séminaire : jeudi 04 mars, 16h00
Seasonal cycle of Arctic Ocean circulation inferred from satellite altimetry
Francesca Doglioni (Alfred-Wegener Institute, Climate Sciences | Physical Oceanography of the Polar Seas)

Francesca Doglioni1, Benjamin Rabe1, Robert Ricker1, Torsten Kanzow1,2

1 Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany.
2 Department of Physics and Electrical Engineering, Universität Bremen Germany

In recent decades, the retreat of the Arctic sea ice has modified vertical momentum fluxes from the atmosphere to the ice and the ocean, in turn affecting the surface circulation. Satellite altimetry has contributed in the past ten years to understand these changes. Most oceanographic datasets are however to date limited either to open ocean or to ice-covered regions, given that different techniques are required to track sea surface height over these two surfaces. Hence, efforts to generate unified Arctic-wide datasets are still required to further basin-wide studies of the Arctic Ocean surface circulation.

We present here the assessment and the seasonality of a new Arctic-wide gridded dataset of the Sea Level Anomaly (SLA) and SLA-derived geostrophic velocities. This dataset is based on Cryosat-2 observations over ice-covered and open ocean areas in the Arctic during 2011 to 2018.

We compare the SLA and geostrophic currents derived hereof to in situ observations of ocean bottom pressure, steric height and near-surface ocean velocity, in three regions: the Fram Strait, the shelf break north of the Arctic Cape and the Laptev Sea continental slope. Good agreement in SLA is shown at seasonal time scales, with the dominant component of SLA variability being steric height both in Fram Strait and at the Arctic Cape. On the other hand, ocean bottom pressure dominates SLA changes at the Laptev Sea site. The comparison of velocity at two mooring transects, one in Fram Strait and the other at the Laptev Sea continental slope, reveals that the correlation is highest at the moorings closest to the shelf break, where currents are faster and the seasonal cycle is enhanced.

The seasonal cycle of SLA and geostrophic currents as derived from the altimetric product is in favourable agreement with previous results. A quasi-simultaneous occurrence of the SLA maximum happens between October and January; similar phase has been found in steric height seasonal cycle by studies using hydrographic profiles in several regions of the Arctic Ocean. We thereby find the highest SLA amplitude over the shelves, which other studies point to be possibly related to winter-enhanced shoreward water mass transport. Seasonal variability in the geostrophic currents is most pronounced along the shelf edges, representing a basin wide, coherent seasonal acceleration of the Arctic slope currents in winter and a deceleration in summer. This is consistent with the shelf-amplified SLA seasonal cycle described above. Density driven coastal currents near Siberia are strongest in spring/summer and weakest in winter, consistent with the cycle of river runoff. Enhanced south-western limb of the Beaufort Gyre in early winter is in agreement with a combination between the Beaufort High buildup and relatively thin sea ice.

In summary, we provide evidence that the altimetric data set has skills to reproduce the seasonal cycle of SLA and geostrophic currents consistently with in situ data and findings from other studies. We suggest that this dataset could be used not only for large scale studies but also to study Arctic boundary currents.
Séminaire précédant Séminaire suivant 5ème séminaire : jeudi 25 mars, 16h00
Maximising the resolution of large aperture telescopes to study planets close to their birth
Matt Willson (STAR)

Occupying the resolution space between coronagraphic imaging and long baseline interferometry, sparse aperture masking or non-redundant masking provides the ability to probe resolution spaces inaccessible to either. Using precise and self calibrating triangles of baselines, we are able to recover companions with contrasts of ~6-8mag on the lambda/D scale, and in some cases even closer. Additionally we can provide the missing short baselines for image restoration of long baseline interferometry data sets, adding in the necessary context to fully understand fine structure on the mas scale. I will describe results of a survey of young stars aimed at resolving the most promising regions for planet formation, discovering four potential protoplanets in their natal disc. I will then further present the results of our follow up efforts to reobserve and characterise these objects, including the potential discovery of a sub-stellar mass companion at an angular separation of 0.25lambda/D within the complex disc surrounding the Herbig star, V1247 Orionis.
Mois précédent Mois suivant Avril 2021
Séminaire précédant Séminaire suivant 6ème séminaire : jeudi 01 avril, 16h00
Deep learning searches for gravitational waves stochastic backgrounds
Andreï Utina (Nikhef)

The background of gravitational waves has long been studied and remains one of the most exciting aspects in the observation and analysis of gravitational radiation. The work focuses on the study of the background of gravitational waves using deep neural networks. An astrophysical background due to the presence of many binary black hole coalescences was simulated for Advanced LIGO O3 sensitivity and the Einstein Telescope design sensitivity. The detection pipeline targets signal data out of the noisy detector background. Its architecture comprises of simulated whitened data as input to three classes of deep neural networks algorithms: a 1D and a 2D convolutional neural network (CNN) and a Long Short Term Memory (LSTM) network. It was found that all three algorithms could distinguish signals from noise with high precision for the ET sensitivity, but the current sensitivity of LIGO is too low to permit the algorithms to learn signal features from the input vectors.
Séminaire précédant Séminaire suivant 7ème séminaire : jeudi 29 avril, 16h00
Seasonal Hydrography of the west Antarctic Peninsula: The Oceanic Forcing to Glacier Retreat
Borja Aguiar González (Physical Oceanography and Applied Geophysics, Universidad de Las Palmas de Gran Canaria)

Seasonal Hydrography of the west Antarctic Peninsula: The Oceanic Forcing to Glacier Retreat [to be completed]
Mois précédent Mois suivant Mai 2021
Séminaire précédant Séminaire suivant 8ème séminaire : jeudi 06 mai, 16h00
A bright future for cometary science
Cyrielle Opitom (The Royal Observatory, Edinburgh)

Comets are some of the most pristine relics of the early solar system. Our understanding of their formation, structure, and composition has made huge progresses in the last decade, in large part thanks to the Rosetta mission. However, many questions are still left to answer. In this talk, I will go through future mission, instrumentation, and areas of research that will allow us to tackle these questions in the future. I will first give a brief overview of the newly selected ESA Comet Interceptor mission, that will for the first time visit a comet coming directly from the Oort cloud. I then will talk about CUBES, the future near-UV spectrograph to be installed on the Very Large Telescope, which has a huge potential for the study of comets. Finally, in the last part of the talk, I will tell the story of interstellar comets. The first interstellar object was discovered only a few years ago, and they provide incredible opportunities to probe material that was formed around different stars and compare it to our own solar system comets.
Séminaire précédant 9ème séminaire : jeudi 20 mai, 16h00 
Recent tests of gravity theories with pulsar timing
Paulo Freire (Max-Planck-Institut für Radioastronomie)

In this talk, I describe how the precise timing of pulsars in several binary systems and one triple system have in the last two years yielded far more stringent tests tests of fundamental properties of space-time and gravity. We concentrate on a few tests of the universality of free fall (UFF), which include tests of the fundamental nature of gravitational waves. In general relativity, the latter have a purely quadrupolar structure, but in alternative gravity theories they can have a large dipolar component. All experiments we have made confirm, within experimental precision, the purely quadrupolar structure of gravitational waves, and more broadly the UFF. These results can be seen as tests of specific gravity theories. In particular, the radiative test with the ``double pulsar'' system, J0737-3039, confirms the general relativistic quadrupole emission formula to a precision of 0.00013 (95 % C. L.), which is 25 times more precise than the previous best radiative test with the the original binary (Hulse-Taylor) pulsar. General relativity passes all the new pulsar tests, and several classes of alternative theories of gravity are either ruled out or severely constrained.
Université de Liège > Faculté des Sciences > Département d'Astrophysique, Géophysique et Océanographie : CoWebAGO, Juin 2009.