Séminaires

Active galaxies and supermassive black holes.
Professor Dhruba Saikia (National Centre for Radio Astrophysics (NCRA) of the Tata Institute of Fundamental Research (TIFR))

0/60 (R52) au B4, Amphithéâtres de l'Europe
Bâtiment B4, Quartier Agora, Boulevard du Rectorat, 3, B-4000 Liège 1 (Sart-Tilman)

Galaxies, which are the fundamental building blocks of the Universe, are often much more than the billions of stars and the medium between them, which are all embedded in massive dark matter halos. They harbour supermassive blackholes in their nuclear regions with masses ranging from millions to billions of solar masses. These active galactic nuclei are responsible for some of the most energetic phenomena in the Universe with luminosities up to a hundred trillion times that of our Sun. They can eject jets of relativistic plasma over millions of light years, spanning a distance of up to about 200 times the size of our own Galaxy, the Milky Way, at nearly the velocity of light. In this talk, I will introduce you to this fascinating world of active galaxies, our current understanding of these objects and highlight a couple of new interesting results.

The effect of stellar surface granulation on spectral line variability
Cis Lagae (University of Warwick, UK)

Salle de réunion AGO (local -1/14), Institut d'Astrophysique et de Géophysique
Bâtiment B5c, Quartier Agora, Allée du 6 Août, 19C, B-4000 Liège 1 (Sart-Tilman)

Radial velocity noise from the exoplanet hosting star, originating from stellar activity such as faculae and granulation, hampers the detection and characterization of earth-mass exoplanets. A profound understanding of how spectral lines behave in the presence of these stellar components is necessary to eliminate such radial velocity noise from observations. In this context, we have built a framework that isolates and characterizes the radial velocity signature due to granulation for individual, synthetic, spectral lines computed from 3D stellar models. Building on this, we expanded the method to larger spectral regions, including tens of spectral lines, with the goal of mimicking real disk-integrated stellar observations. Using this method, we have quantified how granulation affects the properties and radial velocities of spectral lines differentially, from disk center to the stellar limb. We found that certain groups of lines behave radically differently than others, based on their atomic parameters and line depth. In addition, we investigated how different methods of computing radial velocities affect these relationships. Ultimately, this work will enable us to identify key granulation noise diagnostics across the optical spectrum that can be used to reduce the granulation-induced radial velocity noise from observations.

TBD
Quihan He (Durham University or Heidelberg)

Salle de réunion AGO (local -1/14), Institut d'Astrophysique et de Géophysique
Bâtiment B5c, Quartier Agora, Allée du 6 Août, 19C, B-4000 Liège 1 (Sart-Tilman)

TBD
Martin Makler (TBD)

Salle de réunion AGO (local -1/14), Institut d'Astrophysique et de Géophysique
Bâtiment B5c, Quartier Agora, Allée du 6 Août, 19C, B-4000 Liège 1 (Sart-Tilman)

TBD

From Quiescence to Glow: Unveiling the Pre-Perihelion Activity of Interstellar Comet 3I/ATLAS through Multi-Epoch Spectroscopy
Rohan Rahatgaonkar (Pontificia Universidad Católica de Chile)

Salle de réunion AGO (local -1/14), Institut d'Astrophysique et de Géophysique
Bâtiment B5c, Quartier Agora, Allée du 6 Août, 19C, B-4000 Liège 1 (Sart-Tilman)

The discovery of 3I/ATLAS (C/2025 N1) in July 2025 marked the detection of the third confirmed interstellar object passing through our Solar System, and the first ISO discovered several months before perihelion. With an extreme hyperbolic orbit (e ≈ 6.14) and an estimated age older than the Solar System, 3I/ATLAS provides a unique window into the chemistry and conditions of ancient extrasolar planetary systems. We present spectroscopic observations from SOAR and VLT facilities tracking 3I from 4.4 to 2.85 AU, revealing its evolution from quiescence to activity. Our data show a red continuum spectrum with complex organics, the dramatic onset of CN emission at 3.65 AU, and remarkably, strong Ni I emission beginning at 3.78 AU with no detectable Fe I. The unusually steep heliocentric scaling of production rates (Q(Ni) ∝ r_h^(-7.7) and Q(CN) ∝ r_h^(-6.7)) suggests low-activation-energy release mechanisms inconsistent with simple volatile sublimation, potentially involving metal carbonyls or metalated organics. I will discuss these findings in the context of 3I's ancient origins and our campaigns to monitor its evolution through pre-perihelion. 

Genesis: the ESA mission to measure Earth down to the millimeter
Gilles Wautelet (LPAP, STAR Institute, ULiège)

Salle de réunion AGO (local -1/14), Institut d'Astrophysique et de Géophysique
Bâtiment B5c, Quartier Agora, Allée du 6 Août, 19C, B-4000 Liège 1 (Sart-Tilman)

Genesis is an ESA space geodetic mission that will be launched in early 2029. It aims at improving and homogenizing the International Terrestrial Reference Frame (ITRF), with a target accuracy for station position and velocity of 1mm and 0.1mm/year, respectively. In order to achieve this high-accuracy level, Genesis will collocate, for the first time ever, the four space geodetic techniques on a single platform: the spacecraft will host a Global Navigation Satellite Systems (GNSS) receiver, a Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) receiver, a Satellite Laser Ranging (SLR) retroreflector and a Very Long Baseline Interferometry (VLBI) transmitter. Because these instruments use radio signals (except SLR) that travel into the Earth’s ionosphere and plasmasphere, the contribution of these ionized layers of the atmosphere must be accurately modeled and mitigated to ensure precise and reliable measurements that will be used to compute the ITRF. After the presentation of the Genesis mission and the related geodetic framework, we investigate the magnitude and the variability of the ionospheric contribution on the future Genesis observables. Then, based on simulations of GNSS satellite visibility from Genesis, we explore the possibility to take benefit from Genesis GNSS observations to remotely sense the ionosphere and the plasmasphere using the radio-occultation technique.