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تسجيل مستخدم جديدMeasurement of the EBL spectral energy distribution using the VHE gamma-ray spectra of H.E.S.S. blazars
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Very high-energy gamma-rays (VHE, E>100 GeV) propagating over cosmological distances can interact with the low-energy photons of the extragalactic background light (EBL) and produce electron-positron pairs. The transparency of the universe to VHE gamma-rays is then directly related to the spectral energy distribution (SED) of the EBL. The observation of features in the VHE energy spectra of extragalactic sources allows the EBL to be measured, which otherwise is very difficult to determine. An EBL-model independent measurement of the EBL SED with the H.E.S.S. array of Cherenkov telescopes is presented. It is obtained by extracting the EBL absorption signal from the reanalysis of high-quality spectra of blazars. From H.E.S.S. data alone the EBL signature is detected at a significance of 9.5 sigma, and the intensity of the EBL obtained in different spectral bands is presented together with the associated gamma-ray horizon.
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The vast majority of extragalactic sources detected in the very high energy (E> 100 GeV) domaine are active galactic nuclei (AGN) located at cosmological distances. During their travel towards Earth, the emitted gamma-rays suffer from absorption by t
he extragalactic background light (EBL). The density of the EBL is not very well constrained by direct or indirect measurement which leads to uncertainties on the intrinsic spectrum of the sources. High-quality AGN spectra obtained with the High Energy Stereoscopic System (H.E.S.S.) have been used to perform a model-independent measurement of the EBL spectral energy distribution. While the precision of this measurement remains limited, it reflects the sensitivity of H.E.S.S. alone to the EBL and offers the possibility to access the intrinsic spectra of AGNs in a consistent manner, taking into account the derived uncertainties on the EBL spectral energy distribution. In this contribution, we study the intrinsic spectra as measured by H.E.S.S. and by the Fermi Large Area Telescope (LAT) of several blazars of the H.E.S.S. sky. This provides the opportunity to have new insight into the emission processes at play in the jets of AGN. The data presented consist of monitoring data of quiescent state of blazars and also bright blazar flares, such as PKS~2155-304, Mrk~421, recorded by H.E.S.S.
The extragalactic background light (EBL) is the diffuse radiation with the second highest energy density in the Universe after the cosmic microwave background. The aim of this study is the measurement of the imprint of the EBL opacity to gamma-rays o
n the spectra of the brightest extragalactic sources detected with the High Energy Stereoscopic System (H.E.S.S.). The originality of the method lies in the joint fit of the EBL optical depth and of the intrinsic spectra of the sources, assuming intrinsic smoothness. Analysis of a total of ~10^5 gamma-ray events enables the detection of an EBL signature at the 8.8 std dev level and constitutes the first measurement of the EBL optical depth using very-high energy (E>100 GeV) gamma-rays. The EBL flux density is constrained over almost two decades of wavelengths (0.30-17 microns) and the peak value at 1.4 micron is derived as 15 +/- 2 (stat) +/- 3 (sys) nW / m^2 sr.
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