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H.E.S.S. observations of the 2021 periastron passage of PSR B1259-63/LS 2883
No full textInternational audiencePSR B1259-63 is a gamma-ray binary system that hosts a pulsar in an eccentric orbit, with a 3.4 year period, around an O9.5Ve star. At orbital phases close to periastron passages, the system radiates bright and variable non-thermal emission. We report on an extensive VHE observation campaign conducted with the High Energy Stereoscopic System, comprised of ~100 hours of data taken from days to days around the system's 2021 periastron passage. We also present the timing and spectral analyses of the source. The VHE light curve in 2021 is consistent with the stacked light curve of all previous observations. Within the light curve, we report a VHE maximum at times coincident with the third X-ray peak first detected in the 2021 X-ray light curve. In the light curve -- although sparsely sampled in this time period -- we see no VHE enhancement during the second disc crossing. In addition, we see no correspondence to the 2021 GeV flare in the VHE light curve. The VHE spectrum obtained from the analysis of the 2021 dataset is best described by a power law of spectral index , a value consistent with the previous H.E.S.S. observations of the source. We report spectral variability with a difference of at 95% c.l., between sub-periods of the 2021 dataset. We also find a linear correlation between contemporaneous flux values of X-ray and TeV datasets, detected mainly after days, suggesting a change in the available energy for non-thermal radiation processes. We detect no significant correlation between GeV and TeV flux points, within the uncertainties of the measurements, from days to days. This suggests that the GeV and TeV emission originate from different electron populations
Interplay between the non-resonant streaming instability and self-generated pressure anisotropies
No full textInternational audienceThe non-thermal particles escaping from collision-less shocks into the surrounding medium can trigger a non-resonant streaming instability that converts parts of their drift kinetic energy into large amplitude magnetic field perturbations, and promote the confinement and acceleration of high energy cosmic rays. We present simulations of the instability using an hybrid-Particle-in-Cell approach including Monte Carlo collisions, and demonstrate that the development of the non-resonant mode is associated with important ion pressure anisotropies in the background plasma. Depending on the initial conditions, the anisotropies may act on the instability by lowering its growth and trigger secondary micro-instabilities. Introducing collisions with neutrals yield a strong reduction of the magnetic field amplification as predicted by linear fluid theory. In contrast, Coulomb collisions in fully ionized plasmas are found to mitigate the self-generated pressure anisotropies and promote the growth of the magnetic field
ComPair-2: a next-generation medium-energy gamma-ray telescope prototype
No full textInternational audienceMany questions posed in the Astro2020 Decadal survey in both the New Messengers and New Physics and the Cosmic Ecosystems science themes require a gamma-ray mission with capabilities exceeding those of existing (e.g. Fermi, Swift) and planned (e.g. COSI) observatories. ComPair, the Compton Pair telescope, is a prototype of such a next-generation gamma-ray mission. It had its inaugural balloon flight from Ft. Sumner, New Mexico in August 2023. To continue the goals of the ComPair project to develop technologies that will enable a future gamma-ray mission, the next generation of ComPair (ComPair-2) will be upgraded to increase the sensitivity and low-energy transient capabilities of the instrument. These advancements are enabled by AstroPix, a silicon monolithic active pixel sensor, in the tracker and custom dual-gain silicon photomultipliers and front-end electronics in the calorimeter. This effort builds on design work for the All-sky Medium Energy Gamma-ray Observatory eXplorer (AMEGO-X) concept that was submitted the 2021 MIDEX Announcement of Opportunity. Here we describe the ComPair-2 prototype design and integration and testing plans to advance the readiness level of these novel technologies
Role of FLR effects in magnetopause equilibrium
No full textInternational audienceThe Earth magnetopause, when sufficiently plane and stationary at a local scale, can be considered as a "quasi-tangential" discontinuity, since the normal component of the magnetic field B n is typically very small but not zero. Contrary to observations, the "Classic Theory of Discontinuities" (CTD) predicts that rotational and compressional jumps should be mutually exclusive in the general case Bn diff 0, but allows only one exception: the tangential discontinuity provided that B n is strictly zero. Here we show that Finite Larmor Radius (FLR) effects play an important role in the quasi-tangential case, whenever the ion Larmor radius is not fully negligible with respect to the magnetopause thickness. By including FLR effects, the results suggest that a rotational discontinuity undergoes a change comparable to the change of a Shear Alfvén into a Kinetic Alfvén wave when considering linear modes. For this new kind of discontinuity, the co-existence of rotational and compressional variations at the magnetopause does no more imply that this boundary is a strict tangential discontinuity, even in 1D-like regions far from X-lines if any. This result may lead to important consequences concerning the oldest and most basic questions of magnetospheric physics: how can the magnetopause be open, where and when? The role of FLR being established theoretically, the paper then shows that it can be proved experimentally. For that, we make use of MMS data and process them with the most recent available 4 spacecraft tools. First, we present the different processing techniques that we use to estimate spatial derivatives, such as grad(B) and div(P), and the magnetopause normal direction.We point out why this normal direction must be determined with extremely high accuracy to make the conclusions unambiguous. Then, the results obtained by these techniques are presented in a detailed case study and on a statistical basis
A spherical and heterogeneous radiative transfer code for near-infrared remote sensing: application to Titan
No full textInternational audienceThe Cassini mission has gathered extensive near-infrared spectra and imagery of Titan, which pose challenges for analysis due to the significant scattering effects of its atmosphere with potential heterogeneous radiative properties. Traditional radiative transfer models used to analyze Cassini data often employ plane-parallel or pseudo-spherical approximations and assume that atmospheric layers are horizontally uniform. To address these limitations, a new radiative transfer code that solves a spherical and heterogeneous model based on the Monte Carlo method has been developed [1]. This new radiative transfer solver takes into account the geometry of a "ground" of arbitrary shape, described by a triangular mesh, with the possibility of using an arbitrary number of materials. The radiative properties of a gas mixture are provided on a tetrahedral mesh using the k-distribution spectral model. The radiative properties of an arbitrary number of aerosols and clouds can also be provided on their individual meshes. Furthermore, to retrieve atmospheric (e.g., haze density, top of cloud altitude, etc.) or surface properties, not only the intensity but also its gradient (the so-called sensitivities) are needed. Since these gradient calculations cannot be done with finite differences with the Monte Carlo approach, we need to differentiate the Monte Carlo estimator of intensity to build the Monte Carlo estimator of sensitivities. By conserving the same random paths for the estimation of intensity and all sensitivities, the extra calculation time to estimate all these quantities is minimized. A vectorized Monte Carlo estimator is then built, which can estimate simultaneously the intensity and its gradient. Currently, the estimation of sensitivities of mixed gas radiative properties is not available because of its complexity, which needs further development. The methodology is applied to Titan's atmosphere, where we perform sensitivity analysis. The results show that because of the strong scattering effect in the atmosphere, the heterogeneity of the surface and atmosphere impacts a lot of the observable (i.e., the measured intensity), which cannot be investigated by classical parallel-plane and homogeneous layers radiative transfer solvers. For instance, we show that at around 933 nm, the surface reflectivity at 250 km away from the targeting point still has an unnegligible effect on the measured signal. Similarly, sensitivity analysis is also performed on the aerosol density in the atmosphere. These sensitivity analyses help understand the scattering effect in Titan's atmosphere and help design the inversion process of atmospheric properties. This project was funded by French National Research Agency (ANR-21-CE49-0020).[1] htrdr, Meso-Star https://www.meso-star.com/projects/htrdr/htrdr.htm
Cross-Observatory Coordination with tilepy: A Novel Tool for Observations of Multi-Messenger Transient Events
No full textInternational audienceTime-domain astrophysics has leaped forward with the direct discovery of gravitational waves and the emergence of new generation instruments for multi-messenger studies. The capacity of the multi-messenger multi-wavelength community to effectively pursue follow-up observations is hindered by the suboptimal localization of numerous transient events and the escalating volume of alerts. Thus, we have developed an effective tool to overcome the observational and technical hurdles inherent in the emerging field of multi-messenger astrophysics. We present tilepy, a Python package for the automatic scheduling of follow-up observations of poorly localized transient events. It is ideally suited to tackle the challenge of complex follow-up in mid and small-FoV telescope campaigns, with or without human intervention. We demonstrate the capabilities of tilepy in the realm of multi-observatory, multi-wavelength campaigns, to cover the localization uncertainty region of various events ultimately aiming at pinpointing the source of the multi-messenger emission. The tilepy code is publicly available on GitHub and is sufficiently flexible to be employed either automatically or in a customized manner, tailored to collaboration and individual requirements. tilepy is also accessible via a public API and through the Astro-COLIBRI platform
Mean-field limit of particle systems with absorption
No full textWe consider a particle system of singular interaction where particles are removed from the system once they hit some barrier. We show the wellposedness of the particle system and its mean-field limit and prove the propagation of chaos
Interventions politiques dans l'économie numérique : Essais empiriques
No full textThis thesis empirically investigates policy and managerial interventions in the digital economy and analyses their effectiveness and potential side effects.The first chapter uncovers an unintended policy effect of a public support scheme for firms in economically lagging regions in Germany. It exploits variation in the eligibility and the size of potential investment subsidies of firms in order to analyse the relationship with the firms' propensity to adopt cloud services. The empirical results demonstrate that the higher the potential subsidy for investments, the lower the incentive for firms to adopt cloud services. This unintended policy effect is highly relevant for policy makers, as the adoption of cloud services has been linked to various benefits on the firm as well as on the aggregate economy level.The second chapter analyses the pioneering online hate speech regulation on the social media platform X. It exploits the regulation, the German Network Enforcement Act, in a quasi-experimental approach to measure the causal impact of the law on the prevalence of hateful content in a target group of the German-speaking segment of X. The results imply a significant and robust decrease in the intensity and volume of hate speech in tweets tackling sensitive migration and religion related topics. Importantly, tweets tackling other topics as well as the tweeting style of users are not affected by the regulation, which is in line with its aim. This chapter highlights that legislation for combating harmful online content can significantly reduce the prevalence of hate speech and contributes to understanding the perspective implications of the European Digital Services Act.The third chapter investigates mechanisms of the Creator Economy, which capitalizes on a multi-sided business model, connecting content creators, users, and (sometimes) advertisers. Matching the needs of these different stakeholders is a complex challenge, as evidenced by the impact of the YouTube “Adpocalypse” in 2017, when major advertisers fled YouTube due to concerns about their ads appearing alongside objectionable content. This chapter exploits YouTube's subsequent content moderation efforts following the Adpocalypse to measure the cross-platform responses of content creators and content consumers on Patreon. Focusing on content creators that multi-home on YouTube and Patreon, the theoretical model and empirical evidence of this chapter confirm that these content creators respond strategically and shift their efforts toward Patreon. As a result, consumers also increase their use of Patreon through memberships, comments, and likes. However, we also find that YouTube's content moderation and the shift by content creators and consumers that follows, results in an increase in toxicity on Patreon. These findings indicate that governance rules of a single platform cannot be looked at in isolation. Instead, platform managers and policy makers need to consider potential cross-platform reactions of all stakeholders.The three chapters of this thesis demonstrate that interventions in the digital economy can be effective in shaping online discussions, but digital particularities such as rapidly changing technologies and substitution effects need to be carefully considered.Cette thèse étudie de manière empirique les interventions politiques et managériales dans l'économie numérique et analyse leur efficacité et leurs effets secondaires potentiels.Le premier chapitre met en évidence un effet politique involontaire d'un régime d'aide publique aux entreprises, mis en place dans les régions allemandes ayant un retard sur le plan économique. Nous exploitons des variations dans les conditions d'éligibilité et dans les sommes des potentielles subventions, afin d'analyser leur influence sur la propension des entreprises à adopter des services du Cloud. Les résultats empiriques montrent que plus la subvention potentielle à l'investissement est élevée, moins les entreprises sont incitées à adopter des services du Cloud. Cet effet involontaire de la politique est très pertinent pour les décideurs politiques, car l'adoption des services du Cloud a été associée à divers bienfaits pour l'entreprise, ainsi que pour l'économie dans son ensemble.Le deuxième chapitre analyse une réglementation pionnière en matière de discours haineux en ligne sur la plateforme de médias sociaux X (ex Twitter). Nous y exploitons l'introduction d'une régulation en Allemagne, qui s'applique aux réseaux sociaux, dans une approche quasi-expérimentale pour mesurer l'impact causal de la loi sur la prévalence des contenus haineux dans un groupe cible du segment germanophone de X. Les résultats impliquent une diminution significative et robuste de l'intensité et du volume des discours de haine dans les tweets abordant des sujets sensibles liés à l'immigration et à la religion. Il est important de noter que les tweets abordant d'autres sujets, ainsi que le style de tweet des utilisateurs ne sont pas affectés par le règlement, ce qui est conforme à son objectif. Ce chapitre montre que la législation visant à lutter contre les contenus en ligne préjudiciables peut réduire de manière significative la prévalence du discours de haine. Par ailleurs, il contribue à comprendre les potentiels effets que pourrait avoir le Digital Service Act, instauré par l'Union Européenne.Le troisième chapitre étudie les mécanismes de l'«économie des créateurs». Elle s'appuie sur un modèle de plateforme multi-face, mettant en relation les créateurs de contenu, les utilisateurs et (parfois) les annonceurs. Répondre aux besoins de ces différents partis est un défi complexe, comme le montre l'impact de l'«Adpocalypse» de YouTube en 2017. Lors de cet épisode, de grands annonceurs ont quitté YouTube parce qu'ils craignaient que leurs publicités soit associées à des contenus répréhensibles. Ce chapitre exploite les efforts de modération de contenu déployés par YouTube à la suite de l'Adpocalypse pour mesurer les réponses multiplateformes des créateurs et des consommateurs de contenu sur Patreon. Le modèle théorique et les données empiriques de ce chapitre confirment que les créateurs de contenu qui pratiquent le multi-homing sur YouTube et Patreon réagissent de manière stratégique et concentrent leurs efforts sur Patreon.Par conséquent, les consommateurs augmentent leur utilisation de Patreon par le biais d'adhésions, de commentaires et d'appréciations. Toutefois, nous constatons également que la modération du contenu de YouTube et le changement de comportement des créateurs et des consommateurs qui en découle entraînent une augmentation de la toxicité sur Patreon. Ces résultats indiquent que les règles de gouvernance d'une seule plateforme ne peuvent pas être considérées de manière isolée. Au contraire, les gestionnaires de plateformes et les décideurs politiques doivent prendre en compte les réactions potentielles de toutes les parties prenantes.Les trois chapitres de cette thèse démontrent que les interventions dans l'économie numérique peuvent être efficaces pour façonner les discussions en ligne, mais les particularités numériques telles que l'évolution rapide des technologies et les effets de substitution doivent être soigneusement prises en compte