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Register a new userThe Extragalactic sky with the High Energy Stereoscopic System
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D. A. Sanchez
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The number of extragalactic sources detected at very hight energy (VHE, E$>$100GeV) has dramatically increased during the past years to reach more than fifty. The High Energy Stereoscopic System (H.E.S.S.) had observed the sky for more than 10 years now and discovered about twenty objects. With the advent of the fifth 28 meters telescope, the H.E.S.S. energy range extends down to ~30 GeV. When H.E.S.S. data are combined with the data of the Fermi Large area Telescope, the covered energy range is of several decades allowing an unprecedented description of the spectrum of extragalactic objects. In this talk, a review of the extragalactic sources studied with H.E.S.S. will be given together with first H.E.S.S. phase II results on extragalactic sources.
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We present the significant detection of the first extragalactic pulsar wind nebula (PWN) detected in gamma rays, N157B, located in the large Magellanic Cloud (LMC). Pulsars with high spin-down luminosity are found to power energised nebulae that emit gamma rays up to energies of several tens of TeV. N157B is associated with PSRJ0537-6910, which is the pulsar with the highest known spin-down luminosity. The High Energy Stereoscopic System telescope array observed this nebula on a yearly basis from 2004 to 2009 with a dead-time corrected exposure of 46 h. The gamma-ray spectrum between 600 GeV and 12 TeV is well-described by a pure power-law with a photon index of 2.8 pm 0.2(stat) pm 0.3(syst) and a normalisation at 1 TeV of (8.2 pm 0.8(stat) pm 2.5(syst)) times 10^-13 cm^-2s^-1TeV^-1. A leptonic multi-wavelength model shows that an energy of about 4 times 10^49erg is stored in electrons and positrons. The apparent efficiency, which is the ratio of the TeV gamma-ray luminosity to the pulsars spindown luminosity, 0.08% pm 0.01%, is comparable to those of PWNe found in the Milky Way. The detection of a PWN at such a large distance is possible due to the pulsars favourable spin-down luminosity and a bright infrared photon-field serving as an inverse-Compton-scattering target for accelerated leptons. By applying a calorimetric technique to these observations, the pulsars birth period is estimated to be shorter than 10 ms.
This is the index of all contributions of the H.E.S.S. Collaboration to the 37th International Cosmic-Ray Conference, held virtually, July 12 - 23, 2021.
Index of H.E.S.S. conference proceedings to the 35th ICRC, Busan, Korea
We explore the possibility that the recently detected dipole anisotropy in the arrival directions of~$>8$~EeV ultra-high energy cosmic-rays (UHECRs) arises due to the large-scale structure (LSS). We assume that the cosmic ray sources follow the matter distribution and calculate the flux-weighted UHECRs RMS dipole amplitude taking into account the diffusive transport in the intergalactic magnetic field (IGMF). We find that the flux-weighted RMS dipole amplitude is $sim8$% before entering the Galaxy. The amplitude in the [4-8] EeV is only slightly lower $sim 5$%. The required IGMF is of the order of {5-30 nG}, and the UHECR sources must be relatively nearby, within $sim$300 Mpc. The absence of statistically significant signal in the lower energy bin can be explained if the same nuclei specie dominates the composition in both energy bins and diffusion in the Galactic magnetic field (GMF) reduces the dipole of these lower rigidity particles. Photodisintegration of higher energy UHECRs could also reduce somewhat the lower energy dipole.
Ultra-high energy (UHE) photons play an important role as an independent probe of the photo-pion production mechanism by UHE cosmic rays. Their observation, or non-observation, may constrain astrophysical scenarios for the origin of UHECRs and help to understand the nature of the flux suppression observed by several experiments at energies above $10^{19.5}$ eV. Whereas the interaction length of UHE photons above $10^{17}$ eV ranges from a few hundred kpc up to tenths of Mpc, photons can interact with the extragalactic background radiation initiating the development of electromagnetic cascades which affect the fluxes of photons observed at Earth. The interpretation of the current experimental results rely on the simulations of the UHE photon propagation. In this paper, we present the novel Monte Carlo code EleCa to simulate the $Ele$ctromagnetic $Ca$scading initiated by high-energy photons and electrons. We provide an estimation of the surviving probability for photons inducing electromagnetic cascades as a function of their distance from the observer and we calculate the distances within which we expect to observe UHE photons with energy between $10^{17}$ and $10^{19}$ eV. Furthermore, the flux of GZK photons at Earth is investigated in several astrophysical scenarios where we vary both injection spectrum and composition at the source and the intensity of the intervening extragalactic magnetic field. Although the photon propagation depends on several astrophysical factors, our numerical predictions combined with future experimental observations (or non-observations) of UHE photons -- in the energy range between $10^{17.5}$ eV and $10^{20}$ eV -- can help to constrain these scenarios.
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