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Correlation of long-term optical color variability of radio-loud quasars with their VLBI astrometric characteristics
No full textInternational audienceAims. The International Celestial Reference Frame (ICRF) is realized through geodetic very long baseline interferometry (VLBI) positions of thousands of extragalactic objects, mainly radio-loud active galactic nuclei. As previous studies have suggested a link between the optical variability of these sources and optical-radio position offsets and variability, we want to explore the possible relationships between these astrometric quantities and the color variability that characterizes VLBI sources.Methods. We computed the BP/RP and color-magnitude correlations of 2584 ICRF sources using the Gaia DR3 photometric time series at the GBP (BP), GRP (RP), and G bands, and we compared them with astrometric properties (Gaia-VLBI optical-radio offsets and positional variability derived from VLBI coordinate time series). We also searched for possible biases coming from the length of the photometric series, the magnitude, the redshift, and the jet viewing angle when available as well as correlations between the color variability and the central engine properties (black hole mass and accretion luminosity).Results. We find that the most astrometrically stable sources and the sources with smaller optical-radio shifts have lower values of BP/RP correlation and low color-magnitude correlations, corresponding to the undefined-when-brighter (or redder-stable-when-brighter and bluer-stable-when-brighter) classes of optical variability. These preferred astrometric objects often belong to the spectral class FSRQ and have a z of about one and higher. We found a significant correlation for both of the astrometric stability measures with the jet viewing angle. The shorter optical-radio offsets occur in active galactic nuclei that contain more massive black holes and more luminous accretion disks, which may be caused by the tendency of smaller viewing angles to be associated with higher redshifts.Conclusions. Our results strengthen the hypothesis that the Doppler boosting effect on luminosity and geometry is the driving physical mechanism at the origin of the observed optical-radio position offsets and the astrometric variability suggested by previous studies. The results define additional predictors of absolute astrometric performance of reference frame candidates, and they supplement the optical variability criterion. Moreover, this work demonstrates that the relatively short time span (a few years) and irregular sampling of the Gaia light curves are sufficient to predict the astrometric stability of new reference frame objects. This study also shows that absolute astrometry with the geodetic VLBI is a valuable tool to gain new insight into the physical processes responsible for quasar activity in the inner parts of the relativistic engines
Gamma-ray burst prompt emission from the synchrotron radiation of relativistic electrons in a rapidly decaying magnetic field
No full textInternational audienceContext. Synchrotron radiation from accelerated electrons above the photosphere of a relativistic ejecta is a natural candidate for the dominant radiative process for the prompt gamma-ray burst emission. There is, however, a tension between the predicted low-energy spectral index, α = −3/2, in the fast cooling regime and observations.Aims. Radiating electrons have time to travel away from their acceleration site and may experience an evolving magnetic field. We study the impact of a decaying field on the synchrotron spectrum.Methods. We computed the radiation from electrons in a decaying magnetic field, including adiabatic cooling, synchrotron radiation, inverse Compton scatterings, and pair production. We explored the physical conditions in the co-moving frame of the emission region and focused on the fast cooling regime where the radiative timescale of electrons with a Lorentz factor Γm responsible for the peak of the emission, tsyn(Γm), is much shorter than the dynamical timescale tdyn.Results. We find that the effect of the magnetic field decay depends on its characteristic timescale tB: (i) for a slow decay with tB ≳ 10 tsyn(Γm), the effect is very weak and the spectral shape is mostly determined by the impact of the inverse Compton scatterings on the electron cooling, leading to −3/2 ≤ α ≤ −1, and (ii) for a fast decay with 0.1 tsyn(Γm)≲tB ≲ 10 tsyn(Γm), the magnetic field decay has a strong impact, leading naturally to the synchrotron marginally fast cooling regime, where α tends to −2/3, while the radiative efficiency remains high. The high-energy inverse Compton component is enhanced in this regime. (iii) For an even faster decay, the whole electron population is in the slow cooling regime.Conclusions. We conclude that efficient synchrotron radiation in a rapidly decaying magnetic field can reproduce low-energy photon indices ranging from α = −3/2 to −2/3, which is in agreement with the measured value in the majority of gamma-ray burst spectra
A circularly polarized low-frequency radio burst from the exoplanetary system HD 189733
No full textInternational audienceAims. We aim to detect low-frequency radio emission from exoplanetary systems to gain insights into planetary magnetic fields, star–planet interactions, stellar activity, and exo-space weather. The HD 189733 system, hosting a well-studied hot Jupiter, is a prime target for such searches.Methods. We conducted NenuFAR imaging observations in the 15–62 MHz range to cover the entire orbital phase of HD 189733 b. Dynamic spectra were generated for the target and other sources in the field, followed by a transient search in the time-frequency plane. The data processing pipeline incorporated direction-dependent calibration and noise characterization to improve sensitivity. We also searched for periodic signals using a Lomb–Scargle analysis.Results. A highly circularly polarized radio burst was detected at 50 MHz, with a flux density of 1.5 Jy and a significance of 6σ at the position of HD 189733. No counterpart was found in Stokes I, likely because the emission is embedded in confusion noise and remains below the detection threshold. The estimated minimum fractional circular polarization of 38% suggests a coherent emission process. A periodicity search revealed no weaker signals linked to the planet’s orbital period, the star’s rotational period, or the synodic period and harmonic period between them. The burst’s properties are consistent with cyclotron maser instability (CMI) emission, however, the origin remains ambiguous. A comparison with theoretical models suggests star–planet interaction or stellar activity as potential origins. Alternative explanations such as contamination from other sources along the line of sight (e.g. the companion M dwarf) or noise fluctuation are plausible
Euclid: Photometric redshift calibration with self-organising maps
No full textInternational audienceThe Euclid survey aims to trace the evolution of cosmic structures up to redshift 3 and beyond. Its success depends critically on obtaining highly accurate mean redshifts for ensembles of galaxies in all tomographic bins, essential for deriving robust cosmological constraints. However, photometric redshifts (photo-s) suffer from systematic biases arising from various sources of uncertainty. To address these challenges, we utilised self-organising maps (SOMs) with mock samples resembling the Euclid Wide Survey (EWS), to validate Euclid's uncertainty requirement of per tomographic bin, assuming DR3-level data. We observe that defining the redshift tomography using the mean spectroscopic redshift (spec-) per SOM cell, results in none of the ten tomographic redshift bins satisfying the requirement. In contrast, the redshift tomography on the photo-s of the EWS-like sample yields superior results, with eight out of ten bins [] meeting the Euclid requirement. To enhance the realism of our study, we morph our calibration sample to mimic the C3R2 survey in incremental steps. In this context, a maximum of six out of ten bins meet the requirement, strongly advocating the adoption of a redshift tomography defined by the photo-s of individual galaxies rather than the commonly used mean spec- of SOM cells. To examine the impact on the expected biases for , , and measured by Euclid, we perform a Fisher forecast for cosmic shear only, based on our redshift uncertainties. Here, we find that even under an evaluation of the uncertainty where the impact of the redshift bias is substantial, most absolute biases remain below 0.1 in the idealised scenario and below 0.3 in the more realistic case
Initiation Route of Coronal Mass Ejections. II. The Role of Filament Mass
No full textInternational audienceAbstract A thorough understanding of the initiation of coronal mass ejections (CMEs), which is manifested as a slow rise of pre-eruptive structures before the impulsive ejection in kinematics, is key for forecasting solar eruptions. In our previous work, we showed that the slow rise of a hot flux rope with coronal mass density is caused by the moderate magnetic reconnection occurring in the hyperbolic flux tube (HFT) combined with the torus instability. However, it remains unclear how the initiation process varies when a filament is present in the pre-eruptive flux rope. In this work, we reveal the complete initiation route of a CME containing filament mass with a state-of-the-art full-magnetohydrodynamics simulation. The comprehensive analyses show that the filament mass has an important impact on the CME initiation through triggering and driving the slow rise of flux rope with its drainage, besides the contributions of HFT reconnection and torus instability. Finally, in combination with our previous work, we propose that the enhanced drainage of filament mass and various features related to the HFT reconnection, such as the split of pre-eruptive structure and the preflare loops and X-ray emissions, can serve as precursors of CME initiation in observations
Characterization of spurious-electron signals in the double-phase argon TPC of the DarkSide-50 experiment
No full textInternational audienceSpurious-electron signals in dual-phase noble-liquid time projection chambers have been observed in both xenon and argon Time Projection Chambers (TPCs). This paper presents the first comprehensive study of spurious electrons in argon, using data collected by the DarkSide-50 experiment at the INFN Laboratori Nazionali del Gran Sasso (LNGS). Understanding these events is a key factor in improving the sensitivity of low-mass dark matter searches exploiting ionization signals in dual-phase noble liquid TPCs. We find that a significant fraction of spurious-electron events, ranging from 30 to 70% across the experiment's lifetime, are caused by electrons captured from impurities and later released with delays of order 5-50 ms. The rate of spurious-electron events is found to correlate with the operational condition of the purification system and the total event rate in the detector. Finally, we present evidence that multi-electron spurious electron events may originate from photo-ionization of the steel grid used to define the electric fields. These observations indicate the possibility of reduction of the background in future experiments and hint at possible spurious electron production mechanisms
POLARIMETRIE SOLAIRE A LA TOUR DE MEUDON ET A LA LUNETTE TOURELLE DU PIC DU MIDI
No full textNous présentons dans cet article les méthodes mises en place sur le télescope de la Tour de Meudon et sur la Lunette Tourelle du Pic du Midi pour l’analyse de la polarisation de la lumière solaire avant injection dans leurs grands spectrographes respectifs (14 m et 8 m). L’objectif scientifique est d’effectuer des mesures de champ magnétique par effet Zeeman ou d’étudier la polarisation de résonance au limbe de certaines raies et l’effet Hanle associé en champ faible. Nous décrivons les polarimètres à cristaux liquides que nous avons développés et donnons dans l’archive en annexe les mesures de calibration effectuées pour les qualifier
Chemistry and dynamics of the multiphase interstellar medium
No full textUnderstanding the physical processes that govern the state and evolution of the interstellar medium (ISM) is a major topic in galactic and extragalactic physics. The complexity of this task lies in the richness and interplay of the processes involved. The ISM is an inhomogeneous, multi-phase environment, subject to gravity and highly compressible magnetohydrodynamic (MHD) turbulence, and interacting with stellar radiation and cosmic rays. Each of these aspects spans a wide range of spatial and temporal scales, influencing both the dynamics and chemistry of the ISM and ultimately shaping its evolution.We are currently experiencing what feels like a never-ending golden era of observations. The development of space- and ground-based telescopes has enabled the study of the ISM across vast wavelength ranges and spatial scales. These observations have unveiled an unexpected chemical richness, mysterious spatial and chemical correlations, and anomalous excitation properties that challenge our understanding of the microphysical processes that govern the thermodynamic and chemical state of the ISM. Moreover, the ever-growing volume of observational data has enabled the creation of extensive statistical samples, providing new constraints for theoretical studies.My research focuses on developing theoretical models that describe the complex, out-of-equilibrium interactions between dynamics, chemistry, and radiation across nearly all phases of the ISM. The overarching goal is to interpret observational data, provide explanations for the numerous chemical anomalies uncovered by observations, and establish the mass and energy budgets of diverse interstellar environments.Over the years, I have developed a range of models to explore key ISM processes. These include a model of Turbulent Dissipation Regions (TDRs), which describes the thermochemical evolution of interstellar vortices; models of Photodissociation Regions (PDRs) and X-ray Dominated Regions (XDRs); models of atomic and molecular shocks propagating at various velocities in both diffuse and dense gas; and a model of Supernovae Remnants (SNRs) distributed across galactic disks. Additionally, I have led the development of MHD numerical simulations that track the out-of-equilibrium chemical evolution of the multiphase and turbulent diffuse ISM. All these theoretical works have proven instrumental in deciphering atomic and molecular lines observed in absorption and emission, shedding light on the physical and chemical conditions in a variety of astrophysical environments, from planetary nebulae and star-forming regions to the diffuse neutral and ionized phases of the ISM in both the Milky Way and extragalactic systems
Denoising radio pulses from air showers using machine-learning methods
No full textInternational audienceThe Giant Radio Array for Neutrino Detection (GRAND) aims to detect radio signals from extensive air showers (EAS) caused by ultra-high-energy (UHE) cosmic particles. Galactic, hardware-like, and anthropogenic noise are expected to contaminate these signals. To address this problem, we propose training a supervised convolutional network known as an encoder-decoder. This network is used to learn a coded representation of the data and remove specific features from it. This denoiser is trained using high-fidelity air shower simulations specifically tailored to replicate the characteristics of signals detected by GRAND. In this contribution, we describe our machine-learning model and report initial results demonstrating the sensitivity enhancement resulting from our denoising algorithm when applied to realistically simulated GRAND signals with varying signal-to-noise ratios.</div
Le volcanisme explosif sur Mercure à travers la télédétection, les études de laboratoire et l'apprentissage profond en appui à la mission BepiColombo
No full textExploration missions to Mercury have revealed a surface processed over time by volcanism and interactions with space environment. One of the most significant and unexpected findings was the identification of vent morphologies and surrounding deposits, interpreted as products of explosive volcanism. This discovery provides evidence for an accumulation of volatiles in the planet's interior, challenging earlier models of Mercury's formation. This research investigates explosive volcanic deposits on Mercury, leveraging remote sensing spectroscopy, deep learning, and laboratory experiments. The aim is to provide new insights into the planet's volcanic history and to prepare observations and targets for the BepiColombo mission.Observations from MASCS (Mercury Atmospheric and Surface Composition Spectrometer) reveal that pyroclastic deposits exhibit irregular shapes that vary depending on the spectral domain or property evaluated. This complex phenomenon calls upon using deep learning approaches. We apply an unsupervised algorithm to extract the spatial and spectral attributes characterising these deposits. The deep neural network successfully retrieves information highlighting the interior and boundaries of the deposits, enabling us to define the extent of 55 irregular deposits covering 110 vents. This work expands the catalogue of defined faculae and overcomes the limitations of using images instead of spectra. Having defined the extent of pyroclastic deposits, we explore their spectral properties and temporal evolution. While prior research recognises a prolonged period of explosive volcanic activity, the timing of eruptions is unknown. We combine a morphological assessment of the vent degradation with the extracted spectral properties to explore this timing. We find a trend of spectral darkening and flattening over time. We explore three potential processes to explain these variations: space weathering, mixing with the underlying terrain, and changes in pyroclast size over time. We propose that submicroscopic iron accumulating over time as a result of space weathering is expected to be the main contributor to these spectral changes. If this was the case, a large portion of explosive volcanic deposits could be younger than 1.1 Ga, considered “recent” in Mercury's geological history. These conclusions are based on our current understanding of how processes influence Mercury's surface, though many unknowns remain. Laboratory work becomes a key tool in identifying these effects and guiding future BepiColombo operations and target selection.We complement our research with laboratory measurements of Mercury analogues with varying composition and grain sizes, subject to thermal processing and observed under various geometries. These UV-NIR spectral measurements aim to enhance our understanding of remote sensing data and bridge the gap between MESSENGER and BepiColombo. Our results validate the analogues quality, which exhibit spectral shapes similar to young terrains on Mercury. Moreover, we observe that heating and measuring at high phase angles result in spectral reddening and flattening, intensifying with increasing Mg abundance. Regarding the grain sizes, we find that the fine grains expected on Mercury reduce the band depth, even for mixtures with high Mg and Fe content. Our results highlight that multiple factors - physical, chemical, or instrumental - can induce comparable spectral variations, requiring a careful characterisation to disentangle their contributions. Our work hints that, despite the expectations of a volatile-depleted and cooling interior, Mercury's volcanism may have been recently active.Les missions d'exploration de Mercure ont révélé une surface volcanique façonnée au fil du temps par les interactions avec l'environnement spatial. Une des découvertes les plus importantes et inattendues a été l'identification des morphologies de cheminées volcaniques et des dépôts environnants, interprétés comme des produits d'un volcanisme explosif. Cela prouve une accumulation de volatils dans l'intérieur de la planète, ce qui remet en question les modèles antérieurs de sa formation. Cette recherche explore le volcanisme explosif sur Mercure en utilisant les observations spectroscopiques, l'apprentissage profond et des expériences en laboratoire, afin de fournir de nouvelles perspectives sur l'histoire volcanique de la planète et de préparer les observations pour la mission BepiColombo.Les observations du spectrometre MASCS montrent que les dépôts pyroclastiques ont des formes irrégulières qui varient selon le domaine spectral évalué. Nous appliquons un algorithme non supervisé pour extraire les caractéristiques spatiales et spectrales de ces dépôts. L'algorithme extrait avec succès des informations sur l'intérieur et les limites des dépôts, nous permettant de définir l'étendue de 55 dépôts irréguliers couvrant 110 cheminées volcaniques. Après avoir défini l'étendue des dépôts, nous explorons leurs propriétés spectrales et leur évolution temporelle. Nous combinons une évaluation morphologique de la dégradation des cheminées avec les propriétés spectrales extraites. Nous constatons une tendance à l'assombrissement et à l'aplatissement des spectres au fil du temps. Nous proposons trois processus possibles pour expliquer ces variations : l'altération spatiale, le mélange avec le terrain sous-jacent et les changements de taille des pyroclastes. Nous suggérons que l'accumulation de fer submicroscopique due à l'altération spatiale est le facteur principal contribuant à ces changements spectraux. Si tel est le cas, de nombreux dépôts volcaniques explosifs pourraient être plus jeunes que 1,1 milliard d'années, ce qui est considéré comme "récent" dans l'histoire géologique de Mercure. Ces conclusions reposent sur notre compréhension actuelle des processus influençant la surface de Mercure, bien que de nombreuses inconnues demeurent. Les travaux en laboratoire sont essentiels pour identifier ces effets et guider la mission BepiColombo.Nous complétons notre recherche avec des mesures en laboratoire d'analogues de Mercure, avec des compositions et tailles de grains variées, traités thermiquement et observés sous différentes géométries. Ces mesures spectrales de l'ultraviolet au proche infrarouge visent à améliorer notre compréhension des observations de télédétection et à combler l'écart entre MESSENGER et BepiColombo. Nos résultats valident la qualité des analogues, qui présentent des formes spectrales similaires à celles des terrains jeunes sur Mercure. Nous observons que le chauffage et la mesure à des angles de phase élevés entraînent un rougissement spectral et un aplatissement, avec une intensification en fonction de l'abondance en Mg. Concernant les tailles de grains, nous constatons que les grains fins attendus sur Mercure réduisent la profondeur de bande, même pour des mélanges avec une forte teneur en Mg et Fe. Nos résultats montrent que plusieurs facteurs - physiques, chimiques ou instrumentaux - peuvent induire des variations spectrales comparables, nécessitant une caractérisation minutieuse pour démêler leurs contributions.Notre travail suggère que, malgré l'attente d'un intérieur refroidissant et pauvre en volatils, le volcanisme de Mercure pourrait avoir été récemment actif