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Stellar physics at sub-nanoradian angular resolution
No full textInternational audienceMany stars -- if they could be imaged with enough angular resolution -- would exhibit features expected from theory but not possible to extract from spectra. We may group these by increasing complexity as follows. First, smooth variations in brightness across the surface, resembling solar limb darkening but much more prominent and involving more processes in stars with fast spin or external tides. Next, there are periodic features: not only oscillations, but also convective cells and starspots, which appear to transit across a star as its spins, and exoplanets that really do transit across the star. Then, there are transients like flares. Current optical interferometers provide synthetic apertures of a few hundred metres and angular resolutions down to about nanoradian (milliarcsecond), enough to resolve some of the above features on the nearest upper main-sequence stars, giants and supergiants. Ongoing projects aims to km-scale synthetic apertures, enough to measure the radius of the nearest white dwarf. In this White Paper we briefly discuss what could be observed with synthetic apertures over km -- resolving detail on white dwarfs at the level currently possible on supergiants
Combining the second data release of the European Pulsar Timing Array with low-frequency pulsar data
No full textInternational audienceLow-frequency radio data improve the sensitivity of pulsar timing arrays (PTAs) to propagation effects such as dispersion measure (DM) variations, enabling better noise characterization essential for detecting the stochastic gravitational wave background (GWB). We combined LOFAR (100-200 MHz) and NenuFAR (30-90 MHz) observations with the recent European and Indian PTA release (DR2new+) into a new dataset, DR2low, spanning ~11 years for 12 pulsars. DR2low allows updated noise models, increasing PTA sensitivity to the GWB. Using Libstempo and Enterprise, we applied standard noise models including red noise (RN) and time-variable DM (DMv) as power laws, and performed Bayesian model selection over RN, DMv, and an additional chromatic noise term (CN4). Compared to DR2new+, DR2low improves DM constraints and separates DM and RN contributions. We found that the RN is required in the final model for 10 out of 12 pulsars, compared to only 5 in the DR2new+ dataset. The improved sensitivity to plasma effects provided by DR2low also favors the identification of significant CN4 in eight pulsars, while none showed such evidence in DR2new+. The analysis also reveals unmodelled solar wind effects, particularly near solar conjunction, with residual delays absorbed into the DM component, highlighting the importance of accurately modelling the solar wind in PTA datasets
Strong Lensing Model and Dust Extinction Maps of the Host Galaxy of Type Ia Supernova H0pe
No full textInternational audienceStrong gravitational lensing by massive galaxy clusters offers particularly rare opportunities to observe multiple images of distant () Type Ia supernovae (SNe) and resolve the properties of their host galaxies. A recent outstanding example is the Type Ia SN "H0pe" (), discovered in James Webb Space Telescope (JWST) NIRCam images when it was still triply imaged by the galaxy cluster PLCK G165.7+67.0 (G165, ). In this work we build a new strong lensing model of G165, first by using only the position of multiple images of background galaxies. We then increase significantly the number of constraints around the position of SN H0pe by modeling the extended surface brightness of the SN host galaxy. The average uncertainty on mass model parameters is reduced by more than an order of magnitude. We also study the spatial distribution of dust in the arc to estimate the dust extinction at the position of SN H0pe. We find good statistical agreement of the extinction estimate at with three fully independent methods based on spectral energy distribution fitting. Moreover, our extended-image lens model of G165 allows us to map the dust distribution of the host galaxy from the image plane to the source plane. Supernova H0pe exploded in a region with a relatively high extinction of at around from its host center. This work shows that extended image modeling in lensing clusters simultaneously reduces the uncertainty on lens model parameters and enables spatially resolved analyses of lensed transients host galaxies. Such modeling advances are expected to play an important role in future cosmological analyses using strongly lensed SNe
MEGATRON: Reproducing the Diversity of High-Redshift Galaxy Spectra with Cosmological Radiation Hydrodynamics Simulations
No full textInternational audienceWe present the MEGATRON suite of cosmological radiation hydrodynamics simulations following the formation of Milky Way-mass galaxies from the earliest cosmic epochs when Population III stars form to Cosmic Noon. The suite represents the first set of cosmological simulations that couples a vast non-equilibrium thermochemistry network of primordial species, metals, and molecules to multifrequency, on-the-fly radiation transport, allowing us to directly predict the spectral properties of early galaxies. By initializing the simulations at zero metallicity, resolving haloes well below the atomic cooling threshold, reaching parsec-scale resolution, and modeling a Milky Way-mass environment, we aim to address four key science themes: 1) Star formation at cosmic dawn, 2) Galaxy formation and the interstellar medium in the epoch of reionization, 3) The circumgalactic medium towards cosmic noon, and 4) Reionization in a local volume environment and near-field cosmology. In this introductory work, we present an overview of the physical characteristics of high-redshift MEGATRON galaxies and their environment at . We present a library of simulated galaxy spectra and demonstrate how the diversity of galaxy spectra seen by JWST is naturally reproduced in the context of a CDM cosmology. This project represents a step towards making more direct comparisons between simulations and observations and will enable future work to both optimize methods for inferring galaxy properties from observations and to elucidate the physics that governs galaxy formation in the early Universe
Slowly rotating Black Holes in DHOST Theories
No full textInternational audienceWe study slowly rotating black hole solutions within Degenerate Higher Order Scalar Tensor (DHOST) theories. Starting from a static, spherically symmetric metric solution of a DHOST theory, we employ the Hartle-Thorne ansatz to model a slowly rotating spacetime. We show that the differential equation governing the frame-dragging function (which is supposed to depend on the radial coordinate only) is integrable for any DHOST theory allowing us to obtain its explicit form. We also consider angular dependence in and show that regularity at the horizon and at infinity forbids it, as in General Relativity. As an illustration of the formalism introduced here, we study the slowly-rotating version of black hole solutions with primary hair obtained recently, examining the influence of the rotation on the Innermost Stable Circular Orbit (ISCO) and on the circular light trajectories in the equatorial plane
The Simons Observatory: forecasted constraints on primordial gravitational waves with the expanded array of Small Aperture Telescopes
No full textInternational audienceWe present updated forecasts for the scientific performance of the degree-scale (0.5 deg FWHM at 93 GHz), deep-field survey to be conducted by the Simons Observatory (SO). By 2027, the SO Small Aperture Telescope (SAT) complement will be doubled from three to six telescopes, including a doubling of the detector count in the 93 GHz and 145 GHz channels to 48,160 detectors. Combined with a planned extension of the survey duration to 2035, this expansion will significantly enhance SO's search for a -mode signal in the polarisation of the cosmic microwave background, a potential signature of gravitational waves produced in the very early Universe. Assuming a noise model with knee multipole and a moderately complex model for Galactic foregrounds, we forecast a (or 68% confidence level) constraint on the tensor-to-scalar ratio of , assuming no primordial -modes are present. This forecast assumes that 70% of the -mode lensing signal can ultimately be removed using high resolution observations from the SO Large Aperture Telescope (LAT) and overlapping large-scale structure surveys. For more optimistic assumptions regarding foregrounds and noise, and assuming the same level of delensing, this forecast constraint improves to . These forecasts represent a major improvement in SO's constraining power, being a factor of around 2.5 times better than what could be achieved with the originally planned campaign, which assumed the existing three SATs would conduct a five-year survey
Seismogenic and rheological behaviours from time-dependent analysis of earthquake depth distribution in the Corinth Rift
No full textInternational audienceUnderstanding earthquake depth distribution is critical for improving seismogenesis models. While the spatiotemporal pattern of earthquakes is well studied, transient changes in depth distribution remain poorly explored. In this study, we investigate how crustal rheological parameters influence the depth of earthquakes through time, focusing on the Corinth rift, a well-monitored region experiencing a high-level seismic activity in a homogeneous extensional stress field.To calculate crustal yield strength profiles, we compile geophysical and geological data, including heat flow, rock compositions and properties, Moho depth and strain rate. These estimates are then compared to a high-quality 11-year seismic catalogue of the region. An inversion approach is applied to identify crustal layers associated with persistent versus sporadic seismicity defined here instead of the conventional background versus clustered seismicity.Our time analysis reveals that the persistent seismicity nicely matches the theoretical brittle-ductile transition and allows us to confidently define the seismogenic thickness, while sporadic seismicity is clustered at depths associated with swarm occurrences. Both distributions are subject to kilometre-scale changes after magnitude 4.0 -5.5 earthquakes, evidencing a relaxation process even after moderate magnitude events. We conclude that in specific case studies aiming to compare depth distribution and yield strength in the crust, the application of declustering methods may not be optimal for examining the potential rheological controls on earthquake depth distribution and their temporal variations. Instead, the analysis of persistent and sporadic seismicity defined in this study is more accurate and reliable than a declustering approach and offers new and valuable insights for this comparison.</div
Next-Generation Data Assimilation Methods for Polar Ionospheric Electrodynamics
No full textInternational audienceAccurately specifying polar ionospheric electrodynamics is essential for understanding energy and momentum exchange between space and the upper atmosphere and for improving simulations of the ionosphere and the thermosphere. Statistical models are commonly used to provide input for global circulation models (GCMs). However, maps derived from simultaneous multi-instrument observations better represent the actual state of the system. Such maps integrate measurements from ground-based magnetometers and radars, in situ plasma and magnetic field sensors at low-Earth orbit, and optical and particle observations of auroral precipitation. However, ionospheric data assimilation remains in its early stages. Current methods rely on restrictive assumptions to simplify equations and stabilize inverse problems, but these constraints limit applicability beyond polar regions, hinder the inclusion of time-dependent processes, and prevent independent estimation of ionospheric conductance. This review examines the physical foundations of ionospheric data assimilation, evaluates the limitations of existing approaches, and explores pathways toward more accurate and flexible techniques. Specifically, we discuss approaches to: (1) use a common dataset to estimate conductance and fields in a single inversion; (2) incorporate neutral winds instead of assuming they are zero; (3) account for a realistic main magnetic field geometry instead of assuming radial field lines; (4) eliminate a sharp boundary between polar and low-latitude regions; (5) use F-region density measurements to capture the history of ionospheric conductance and plasma transport; (6) account for the magnetic field of ground-induced currents in a more realistic way; (7) include ionospheric induction effects to stabilize time-dependent inversions; and (8) couple ionospheric electrodynamics with global magnetosphere simulations to model the physics of time variations
Flood pulse monitoring in wetlands with multi-temporal Sentinel-1 interferometric coherence data: Application to the Okavango Delta (Botswana)
No full textInternational audienceFlood-pulsed wetlands are characterized by significant seasonal water fluctuations, which play a critical role in the dynamics of these sensitive ecosystems. Among the growing number of existing remote sensing products, we explore the potential of interferometric (InSAR) coherence time series, derived from Sentinel-1 synthetic-aperture radar images, to characterize the hydrological dynamics of the Okavango Delta, a vast flood-pulsed wetland. Interferometric coherence reflects changes in surface conditions, making it a powerful tool for detecting flood propagation. By fitting harmonic functions, we produce parameters that quantify the seasonality of coherence time series with short isotemporal baselines (12 days). In particular, we developed a normalized seasonal index based on the ratio between the seasonal amplitude and the root-mean-square error of the fitted harmonic function, to map the seasonality of the coherence time series. A multi-annual analysis of coherence time series reveals a strong relationship between their seasonality, land cover, and flood frequency. Unsupervised clustering applied to statistical and seasonal metrics of coherence time series yields consistent classifications that map the variability of flood frequencies across wetland areas and clearly distinguish wetlands from dry zones. Similarly thresholds applied to normalized seasonal indices delineate the year-to-year extent of flood pulses with accuracy around 79 %. We show that coherence time series in never flooded areas exhibit a pronounced seasonal pattern driven by rainfall cycle, whereas this seasonality is disrupted by flood pulses in wetlands. Building on this, developed a change-detection approach to map the floods by identifying the date when coherence time series diverge from their seasonal pattern. The resulting flood arrival dates achieve 74–83 % accuracy compared to a reference dataset derived from optical data. Our results highlight the potential of coherence time series as a robust indicator of seasonal variations in inundation extent in flood-pulsed wetlands