SEARCH FOR DIFFUSE EMISSIONS AND SEARCHES IN VERY-HIGH-ENERGY GAMMA RAYS AND FUNDAMENTAL PHYSICS WITH H.E.S.S. AND CTAO
Observations in very-high-energy (VHE, E>100 GeV) gamma rays are crucial for understanding the most violent non-thermal phenomena at work in the Universe. The central region of the Milky Way is a complex region active in VHE gamma rays. Among the VHE gamma sources are the supermassive black hole Sagittarius A* at the heart of the Galaxy, supernova remnants and even star formation regions. The Galactic Center (GC) houses a cosmic ray accelerator up to energies of PeV, diffuse emissions from GeV to TeV including the “Galactic Center Excess” (GCE) whose origin is still unknown, potential variable sources at TeV, as well as possible populations of sources not yet resolved (millisecond pulsars, intermediate mass black holes). The GC should be the brightest source of annihilations of massive dark matter particles of the WIMPs type. Lighter dark matter candidates, axion-like particles (ALP), could convert into photons, and vice versa, in magnetic fields leaving an oscillation imprint in the gamma-ray spectra of active galactic nuclei (AGN).
The H.E.S.S. observatory located in Namibia is composed of five atmospheric Cherenkov effect imaging telescopes. It is designed to detect gamma rays from a few tens of GeV to several tens of TeV. The Galactic Center region is observed by H.E.S.S. for twenty years. These observations made it possible to detect the first Galactic Pevatron and place the strongest constraints to date on the annihilation cross section of dark matter particles in the TeV mass range. The future CTA observatory will be deployed on two sites, one in La Palma and the other in Chile. The latter composed of more than 50 telescopes will provide an unprecedented scan of the region on the Galactic Center.
The proposed work will focus on the analysis and interpretation of H.E.S.S observations. carried out in the Galactic Center region for the search for diffuse emissions (populations of unresolved sources, massive dark matter) as well as observations carried out towards a selection of active galactic nuclei for the search for ALPs constituting dark matter. These new analysis frameworks will be implemented for the future CTA analyses. Involvement in taking H.E.S.S. data. is expected.
STUDY OF THE MULTI-SCALE VARIABILITY OF THE VERY HIGH ENERGY GAMMA-RAY SKY
Very high energy gamma ray astronomy observes the sky above a few tens of GeV. This emerging field of astronomy has been in constant expansion since the early 1990s, in particular since the commissioning of the H.E.S.S. array in 2004 in Namibia. IRFU/CEA-Paris Saclay is a particularly active member of this collaboration from the start. It is also involved in the preparation of the future CTAO observatory (Cherenkov Telescope Array Observatory), which is now being installed. The detection of gamma rays above a few tens of GeV makes it possible to study the processes of charged particles acceleration within objects as diverse as supernova remnants or active galactic nuclei. Through this, H.E.S.S. aims in particular at answering the century-old question of the origin of cosmic rays.
H.E.S.S. allows measuring the direction, the energy and the arrival time of each detected photon. The time measurement makes it possible to highlight sources which present significant temporal or periodic flux variations. The study of these variable
Direction de la Recherche Fondamentale
Institut de recherche
sur les lois fondamentales de l’univers
emissions (transient or periodic), either towards the Galactic Center or active nuclei of galaxies (AGN) at cosmological distance allows for a better understanding of the emission processes at work in these sources. It also helps characterizing the medium in which the photons propagate and testing the validity of some fundamental physical laws such as Lorentz invariance. It is possible to probe a wide range of time scales variations in the flux of astrophysical sources. These time scales range from a few seconds (gamma ray bursts, primordial black holes) to a few years (binary systems of high mass, active galaxy nuclei).
One of the major successes of H.E.S.S.'s two decades of data-taking. was to conduct surveys of the galactic and extragalactic skies in the very-high energy range. These surveys combine observations dedicated to certain sources, such as the Galactic Center or certain remains of supernovae, as well as blind observations for the discovery of new sources. The thesis subject proposed here concerns an aspect of the study of sources which remains to be explored: the research and study of the variability of very-high energy sources. For variable sources, it is also interesting to correlate the variability in other wavelength ranges. Finally, the source model can help predict its behavior, for example its “high states” or its bursts.