اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدSpectral analysis and interpretation of the gamma-ray emission from the Starburst galaxy NGC 253
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Very-high-energy (VHE; E >100 GeV) and high-energy (HE; 100 MeV < E < 100 GeV) data from gamma-ray observations performed with the H.E.S.S. telescope array and the Fermi-LAT instrument, respectively, are analysed in order to investigate the non-thermal processes in the starburst galaxy NGC 253. The VHE gamma-ray data can be described by a power law in energy with differential photon index Gamma=2.14 pm 0.18_stat pm 0.30_sys and differential flux normalisation at 1 TeV of F_0 = (9.6 pm 1.5_stat (+5.7,-2.9)_sys) x 10^{-14} TeV^{-1} cm^{-2} s^{-1}. A power-law fit to the differential HE gamma-ray spectrum reveals a photon index of Gamma=2.24 pm 0.14_stat pm 0.03_sys and an integral flux between 200 MeV and 200 GeV of F(0.2-200 GeV) = (4.9 pm 1.0_stat pm 0.3_sys) x 10^{-9} cm^{-2} s^{-1}. No evidence for a spectral break or turnover is found over the dynamic range of both the LAT instrument and the H.E.S.S. experiment: a combined fit of a power law to the HE and VHE gamma-ray data results in a differential photon index Gamma=2.34 pm 0.03 with a p-value of 30%. The gamma-ray observations indicate that at least about 20% of the energy of the cosmic rays capable of producing hadronic interactions is channeled into pion production. The smooth alignment between the spectra in the HE and VHE gamma-ray domain suggests that the same transport processes dominate in the entire energy range. Advection is most likely responsible for charged particle removal from the starburst nucleus from GeV to multiple TeV energies. In a hadronic scenario for the gamma-ray production, the single overall power-law spectrum observed would therefore correspond to the mean energy spectrum produced by the ensemble of cosmic-ray sources in the starburst region.
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re crucial to study the underlying particle accelerators and cosmic-ray interaction and transport. Aims. The measurement of the VHE $gamma$-ray emission of NGC 253 published in 2012 by H.E.S.S. was limited by large systematic uncertainties. Here, a measurement of the $gamma$-ray spectrum of NGC 253 is investigated in both HE and VHE $gamma$-rays. Methods. The data of H.E.S.S. observations are reanalysed using an updated calibration and analysis chain. The $Fermi$-LAT analysis employs more than 8 years of data processed using pass 8. The cosmic-ray particle population is evaluated from the combined HE--VHE $gamma$-ray spectrum using NAIMA. Results. The VHE $gamma$-ray energy spectrum is best fit by a power-law with a flux normalisation of $(1.34,pm,0.14^{mathrm{stat}},pm,0.27^{mathrm{sys}}) times 10^{-13} mathrm{cm^{-2} s^{-1} TeV^{-1}}$ at 1 TeV -- about 40 % above, but compatible with the value obtained in Abramowski et al. (2012). The spectral index $Gamma = 2.39 pm 0.14^{mathrm{stat}} pm 0.25^{mathrm{sys}}$ is slightly softer than but consistent with the previous measurement. At energies above $sim$3 GeV the HE spectrum is consistent with a power-law ranging into the VHE part of the spectrum measured by H.E.S.S. Conclusions. Two scenarios for the starburst nucleus are tested, in which the gas in the starburst nucleus acts as a target for hadronic cosmic rays. In these two models, the level to which NGC,253 acts as a calorimeter is estimated to a range of $f_{rm cal} = 0.1$ to $1$ while accounting for the measurement uncertainties.
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