Séminaires

Searching for Dark Matter Substructures with Strong Lensing
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)

A key prediction of the Cold Dark Matter (CDM) model is the existence of a large population of low-mass dark matter haloes, many of which are too small to host visible galaxies. Strong gravitational lensing provides a powerful way to detect these otherwise invisible structures through the perturbations they induce in lensed arcs. However, these signals are subtle, and robust detections require careful control of modeling systematics.

In this talk, I will first briefly introduce the basic picture of strong gravitational lensing and how it can be used to probe dark matter substructures. I will then present our recent progress in improving strong-lensing modeling for dark matter substructure searches, including flexible lens-light models, improved pixelized source reconstructions, and methods for controlling complexity in the lens mass distribution.

Finally, I will take the 'Jackpot' Lens, SDSSJ0946+1006, as a particularly interesting example, where the detected low-mass perturber has been inferred to be remarkably concentrated and difficult to reconcile with standard CDM expectations. I will revisit the nature of this perturber and show how a more detailed treatment can lead to a different physical picture, highlighting both the complexity of drawing physical conclusions from such systems and the importance of careful lens modeling.

The Dark Side of the Universe Seen Through Strong Gravitational Lenses
Martin Makler (Brazilian Center for Physics Research & International Center for Advanced Studies, ICIFI, UNSAM)

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)

Gravitational lensing has become a powerful tool for studying both the dark and visible components of the Universe. In particular, this effect allows us to detect dark matter, study dark energy, and test alternatives to general relativity. In this talk I will briefly review the phenomenology of strong gravitational lensing, which produces large magnifications, multiple images and distortions of astronomical objects, ranging from the microlensing of stars to giant arcs and Einstein rings from the lensing of galaxies. The coming era of wide-field time-domain surveys, in particular, the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST), will open a new window on lensing, with discoveries of strongly lensed transients and variable objects (such as quasars, supernovae, and potentially counterparts of gravitational-wave events), as well as large samples of microlensing events. I will discuss the LaStBeRu (Last Stand Before Rubin) compilation of strong-lens candidates from current wide-field surveys, developed as a precursor to Rubin, and present new results on constraints on modified gravity from strong lensing. I will also discuss microlensing follow-up observations at the CASLEO observatory in Argentina, a pilot project using a skipper CCD camera at this observatory and future plans for this technology. These developments illustrate how new surveys, data-analysis methods, and detector technologies are coming together to explore the dark side of the Universe through gravitational lensing.

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.

Worlds Next Door: Unlocking a New Frontier of Cool Gas Giant Studies with JWST
Aniket Sanghi (Caltech)