The Kepler supernova remnant (SNR) is the last historic supernova remnant lacking significant gamma-ray detection, a probe to understand particle acceleration. A recent analysis of Fermi-LAT data by Xiang et al., 2021 reported a likely gamma-ray candidate in the direction of the SNR.

Using approximately the same dataset but with an optimized analysis configuration, we confirm the gamma-ray candidate to a solid >6 sigma detection.
The gamma-ray excess is not significantly extended and is fully compatible with the radio, infra-red or X-ray spatial distributions.

The multi-wavelength emission was successfully modeled in a scenario, coherent with what we know from Kepler, where the GeV gamma-rays
are emitted from the North-West of the SNR, where accelerated particles interact with the dense circumstellar material and radiate through pi0 decay.
The X-ray synchrotron and inverse-Compton emission mostly stem from the fast shocks in the southern regions with a high magnetic field B~100 muG or higher.

The latest Fermi-LAT source catalog constructed from scratch (4FGL: 5065 sources above 50 MeV) was based on eight years (2008 - 2016) of Pass 8 data. Since neither the event reconstruction (Pass 8) nor the interstellar emission model (gll_iem_v07) has evolved since then, we decided we will provide incremental 4FGL releases at regular intervals of two years, until one of those two key ingredients changes. The first of those incremental catalogs, 4FGL DR2 covering 10 years of LAT data, was released in 2020.

The main goals of these releases are to go a little deeper, report transients that became active only recently and provide a reasonable model for background sources. I will describe precisely what we mean by incremental and what products are provided. I will then introduce the second incremental 4FGL catalog, DR3 covering 12 years of data, which benefits from a few analysis improvements and provides additional information.

Plus de 10 ans après sa découverte, l'excès de Fermi reste un sujet de recherche captivant. L'explication favorite avancée pour expliquer cet excès serait qu'une population non résolue de pulsars millisecondes (ou MSPs pour "millisecond pulsars"), émetteurs de rayons gamma, soit située au centre Galactique. Les données du Fermi-LAT ont été minutieusement étudiées et afin de tester les différentes origines proposées, une approche multi-longueurs d'onde est désormais nécessaire. Dans cette présentation, je parlerai de nos récents résultats [1], obtenus en étudiant la capacité du télescope à rayons X Chandra à détecter une population de MSPs du bulbe Galactique. Pour cela, nous avons créé une population synthétique de MSPs en se basant sur des modèles ajustés à des données observationnelles et construit une relation empirique entre les émissions gamma et X, basée sur les propriétés de sources observées. Comparer notre population simulée à la sensibilité de Chandra en direction du centre Galactique nous a permis de conclure qu'un nombre non négligeable de MSPs devraient être détectables en rayons X. Nous avons ainsi sélectionné des candidats parmi les sources non identifiées du dernier catalogue de sources de Chandra en exploitant les informations spectrales disponibles et des données astrométriques fournies par Gaia. Nous avons trouvé qu'une part significative des sources Chandra pourrait être des MSPs. Enfin, je présenterai également des développements plus récents ayant pour but de réduire davantage le nombre de candidats MSPs et de faire des prédictions pour de futures observations radio.

[1] arXiv:2012.03580

Modeling the extragalactic astroparticle skies involves reconstructing the 3D distribution of the most extreme sources in the Universe. Full-sky tomographic surveys at near-infrared wavelengths have already enabled the astroparticle community to bind the density of sources of astrophysical neutrinos and ultra-high cosmic rays (UHECRs), constrain the distribution of binary black-hole mergers and identify some of the components of the extragalactic gamma-ray background. This contribution will present the efforts of cleaning and complementing the stellar mass catalogs developed by the gravitational-wave and near-infrared communities, in order to obtain a cosmographic view on stellar mass (𝑀∗) and star formation rate (SFR). Unprecedented cosmography is offered by a sample of about 400,000 galaxies within 350 Mpc, with a 50-50 ratio of spectroscopic and photometric distances, 𝑀∗, SFR and corrections for incompleteness with increasing distance and decreasing Galactic latitude. The inferred 3D distribution of 𝑀∗ and SFR is consistent with cosmic flows. The 𝑀∗ and SFR densities converge towards values compatible with deep-field observations beyond 100 Mpc, suggesting a close-to-isotropic distribution of more distant sources. In addition to discussing relevant applications for the four astroparticle communities, this contribution will highlight the distribution of magnetic fields at Mpc scales deduced from the 3D distribution of matter, which is believed to be crucial in shaping the ultra-high-energy sky. These efforts provide a new basis for modeling UHECR anisotropies, which bodes well for the identification of their long-sought sources.

I will present the Virgo "Service National d'Observation". I will explicitly describe its goals, its current organization and our mid-term strategy to reinforce the participation of the FRench astronomy community in multi-messenger analysis.

The LISA mission will observe gravitational wave emission from tens of thousands of galactic binaries, in particular white dwarf systems. These objects are known to have intense magnetic fields. Generally, these effects are not taken into account in the orbital evolution of the dynamical system, as they are considered weak and are taken into account in the spin evolution of the objects. It turns out that magnetic fields modify the orbits, in particular their geometry with respect to the observer. In this work, we revisit the issue and show how the magnetic fields of the companions of a binary, in the magnetostatic approximation, generate periodic variations in inclination, leading to modifications of the gravitational waveforms potentially detectable by LISA.

Particle acceleration in relativistic shocks is quenched in the presence of a transverse magnetic field, even for a moderately low upstream magnetization. Pulsar wind nebulae form downstream of an ultra-relativistic magnetized shock; yet these objects are one of the most efficient particle accelerators known in the Galaxy. We propose that the key to this striking discrepancy lies in the anisotropic nature of the magnetic field profile in the pulsar wind. Using particle-in-cell simulations, we show that it has a dramatic impact on the structure and evolution of the shock. The formation of a current sheet in the equatorial plane, combined with a large-scale velocity shear flow lead to strong plasma turbulence and efficient particle acceleration near the Bohm limit. A strinking and unexpected feature is the formation of a cavity at the equator drilling through the upstream medium that is blown by the most energetic particles contained in the simulation. These properties are robust and may apply to other astrophysical environments such as gamma-ray bursts or the hotspots and lobes of AGN jets.

A variety of astrophysical phenomena can only be explained as being powered by black holes. In particular, accreting supermassive black holes are responsible for launching relativistic plasma jets and for accelerating ultra-energetic particles. The mechanism that channels energy from the black hole to the particles remains a mystery.  

Observations have come to help lately. The Event Horizon Telescope collaboration has been able to image the shadow of the supermassive black hole M87*, probing the magnetic structure almost down to the event horizon. The GRAVITY collaboration detected a hot spot in infrared orbiting Sgr A*, indicating the presence of a large-scale poloidal magnetic field. These observations give new clues to constrain theoretical models that are able to explain jets and particle acceleration.

This problem involves complex interactions between collisionless plasma dynamics in high gravitational field, and pair creation due to the interaction of energetic particles with surrounding photons from the accretion flow. Only kinetic simulations can capture all these effects.

In this talk, I will present general relativistic particle-in-cell simulations of axisymmetric black-hole magnetospheres, which include self-consistent plasma supply and radiative processes. I will put the emphasis on extracting synthetic observables from these simulations, such as gamma-ray lightcurves and millimeter images, which can directly be compared to actual observations. This allows us to model very accurately the non-thermal radiation emitted from the innermost regions of black-hole magnetospheres.

Particle acceleration in stationary and moving internal shocks in AGN relativistic jets is one of the most plausible ways to explain the variability observed from the radio up to the gamma-ray band.
Using the SRMHD code MPI-AMRVAC, coupled with a treatment of radiative transfer, we study the emergence of radio flares in a moving shock, standing shock interaction.
To understand the impact of the magnetic configuration on a structured jet to visualized the emission regions, we generate light curves for four cases : turbulent, toroidal, poloidal and helical.
We reproduce a standing shock structure inside a two-component jet surround by an unmagnetized ambient medium. An over-density is injected at the base of the jet and generates a moving shock wave. The injection of relativistic particles, their radio synchrotron emission and the radiative transfer are calculated in post-treatment for given observation angles. 
We demonstrate that the presence of a toroidal or a poloidal magnetic field component affect differently cohesion of the standing shock, its intensity and morphology. This leads to an observed profile of flares characterized by interactions of moving and standing shocks of varying strengths.
A qualitative comparison with VLBI and single-dish radio data from the quasar 3C 273 will be discussed. 

Several active galactic nuclei show correlated variations in the UV/optical range, with time delays increasing at longer wavelengths. Thermal reprocessing of the X-rays illuminating the accretion disk has been proposed as a viable explanation. In this scenario, the variable X-ray flux irradiating the accretion disk is partially reflected in X-rays and partially absorbed, thermalised, and reemitted with some delay by the accretion disk at longer wavelengths. I will present a new model that explains the observed continuum time lags in the UV/optical range in AGN, within the context of thermal reverberation. I will then discuss how thermal reverberation can help us understand the properties of the accretion flow and the corona in AGN, and be potentially used as a new technique to constrain BH spins in AGN.

The precise mechanism for the prompt emission of gamma-ray bursts (GRBs) is still largely debated. Polarization measurements of the prompt gamma-ray emission could help model this phenomenon, and lead to a broader understanding of GRBs and astrophysical relativistic jets. COMCUBE is a project of the European programme AHEAD2020 aiming at the development of a Compton polarimeter CubeSat mission to measure the polarization of bright GRBs. The launch of a constellation of 6U CubeSat will allow the detection of several hundred GRBs per year and make possible a full-time monitoring of the gamma-ray sky for time-domain and multi-messenger astronomy. In addition, COMCUBE will accurately measure the polarization of the prompt emission of several GRBs per year. An extensive simulation work has been conducted to design the scientific payload, and is ongoing to quantify the sensitivity of the instruments to GRBs and GRB polarization. Simultaneously, a reduced prototype of the Compton polarimeter is being developed. It uses double-sided silicon stripped detectors and inorganic scintillators read by SiPMs, and it will be tested during a stratospheric balloon flight in 2023.

While electromagnetic (near-IR, X-ray) flares are regularly detected from Sagittarius A*, the supermassive black hole at the center of our galaxy, no model has yet gained wide acceptance. This is especially true concerning the intra-flare variability of the light curve seen in the longer flares. One of the models aiming at explaining this variability involves an instability developing close to the innermost stable orbit and excited by accreted gas following a total or partial tidal disruption of a cloud or a star. Indeed, such scenarios have be shown to match some of the observed flux properties (e.g., Falanga al, 2007). Unfortunately, uncertainties remain on the computed period, derived in pseudo-Newtonian gravity, and on the link between the accreted material and the flare properties. In the present work, I relax the pseudo-Newtonian approximation and intend to link realistic simulations and observations.

Using 3D GR magneto-hydrodynamical simulations and a GR ray-tracing code, I aim to investigate how a post-disruption circularized annulus of gas hosts the development of the Rossby Wave Instability (Tagger & Varniere, 2006), and how the subsequent accretion bursts trigger electromagnetic flares similar to those observed. In particular, I will show how the annulus properties in terms of mass and spatial extent (partly inherited from its parent object, star or cloud) impact the observed energy spectrum and light curve. Following this, our final goal is to be able to reproduce the variety of detected observational signatures.

A deep absorption in the 21-cm line of atomic hydrogen (HI) , redshifted to the epoch of cosmic dawn (z ~ 20), was reported by the EDGES experiment. To explain that absorption trough it has been proposed that either an additional exotic cooling mechanism, or a brighter radio background emission previously unaccounted for is needed. Here we discuss the possibility that the required cosmic radio background could be produced by non-thermal emission from a prolific population of black holes formed at cosmic dawn. We conclude that unless black holes formed at that epoch are radically different from those observed in the local Universe, the radio emission is orders of magnitude below the required levels.

The last three decades have seen the development and launch of numerous X-rays observatories, providing a long temporal baseline to use to search for long term transients and test new transient detection methods. We present a systematic study of the cross-correlation of 5 different X-rays source catalogs (4XMM-DR10, 4XMM-DR10 Stacked, CSC2, 2SXPS and XMM Slew 2), also taking into account upper limits in the case of non-detections, to search for X-ray sources that have varied over the last 30 years. We also use complimentary multi-wavelength data to identify the sources.

This method has already allowed us to find several variable Ultra Luminous X-ray sources (ULXs), changing-look AGNs, some Hyper Luminous X-ray sources and tidal disruption event candidates. Two major interests of such a method are the wealth of different variable sources that can still be uncovered in the archival data, and the future use of this method in the XMM-Newton pipeline to alert the community in quasi real time to sources undergoing strong variability. This will allow a prompt reaction and follow-up from the community, improving our knowledge of the X-ray transient sky.

Understanding the mechanisms of accretion-ejection in compact objects has been a problem for decades. What is the origin of the change of the X-ray spectral states during the X-ray binaries (XrB) outbursts? What is the link with the appearance/disappearance of the radio emission (signature of jet)? How does the AGN zoology fit in the XrB spectral states? In 2018, Marcel et al. developed a two-temperature plasma code computing the spectrum of hybrid disks composed of a truncated outer Standard Accretion Disk (SAD, Shakura & Sunyaev, 1973) and an inner Jet Emitting Disk (JED, Ferreira et al 2006). In this paradigm, the JED plays the role of the hot corona while simultaneously explaining the presence of a radio jet (Marcel et al. 2018a,b, 2019, 2020).

By applying this model to X-ray and radio observations of 4 outbursts of GX339-4 during the 2000-2010 decade (RXTE PCA X-ray spectra), we were able to constrain simultaneously the radiative properties of the accretion flow and the jet. We also show that the functional dependency of the radio emission on the model parameters (mainly the accretion rate and the transition radius between the JED and the SAD) is different between the rising and decaying phases, suggesting a change in the radiative and/or dynamical properties of the ejection between the beginning and the end of the outburst (Barnier et al. submitted).

On the other hand, we also apply the JED-SAD model to the UV/X-ray/radio correlation of AGNs (Lusso+2020, Merloni+2003), signature of universal accretion-ejection mechanisms at work in these objects. We globally reproduce the observed correlations, putting constraints on the physical parameters of our model.

We will present our results, discuss the potential scaling between the physics at work in stellar mass and supermassive black holes and their implications in our understanding of accretion-ejection processes.

Serendipitous X-ray surveys have been proven to be an efficient way to find rare objects – tidal disruption events, galaxy clusters, binary quasars, etc. As X-ray astronomy slowly enters the era of Big Data, an automated classification of X-ray sources becomes increasingly valuable.
I present a revisited Naive Bayes Classification of the X-ray sources in the Swift-XRT and XMM-Newton catalogues which amongst other objects identifies different types of AGN, stars and X-ray binaries − based on their spatial, spectral and variability properties at different timescales and their multiwavelength counterparts. An outlier measure is used to identify objects of other nature. I will show the reliability of the method developed and demonstrate its suitability to data mining purposes, even at faint fluxes. In the last part of my talk, I will show how we can address the very small populations in some object classes using citizen science, with the development of a new platform designed for the classification of XMM sources by volunteers.

The planet-size network of millimetric antennas Event Horizon Telescope (EHT) has recently delivered images of the surroundings of the supermassive compact object M87* at the center of the galaxy Messier 87. Such images are crucial to better understand the physics at play in a strong gravitational field environment. They might also allow to probe the extreme relativistic effects on the radiation emitted close to the compact object.

In this talk, I will present a simple semi-analytic model of the accretion flow surrounding M87* and discuss images of this flow. In particular, I will focus on the highly-lensed part of the image generally refered to as the « photon ring ». After providing a definition of this feature, I will discuss the prospect of using it as a probe of the underlying spacetime. I will also discuss how the highly-lensed part of the image changes when the compact object is not a black hole but a more exotic alternative object.

The final goal of my talk is to discuss to what extent can EHT-like images help constrain the nature of the compact objects at the center of M87 or our Milky Way.