The starburst galaxy NGC 253 revisited by H.E.S.S. and Fermi-LAT

• Verfasst von: Abdalla, Hassan [VerfasserIn]
• Verfasst von: Glawion, Dorit [VerfasserIn]
• Verfasst von: Jankowsky, Felix [VerfasserIn]
• Verfasst von: Mohamed, Mahmoud [VerfasserIn]
• Verfasst von: Quirrenbach, Andreas [VerfasserIn]
• Verfasst von: Rieger, Frank M. [VerfasserIn]
• Verfasst von: Schwemmer, Stephanie [VerfasserIn]
• Verfasst von: Wagner, Stefan [VerfasserIn]
• Titel: The starburst galaxy NGC 253 revisited by H.E.S.S. and Fermi-LAT
• Körperschaft: H.E.S.S. Collaboration [VerfasserIn]
• Verf.angabe: H.E.S.S. Collaboration, H. Abdalla, D. Glawion, F. Jankowsky, M. Mohamed, A. Quirrenbach, F. Rieger, S. Schwemmer, S.J. Wagner [und weitere]
• E-Jahr: 2018
• Jahr: 20 September 2018
• Umfang: 7 S.
• Fussnoten: Gesehen am 12.06.2019
• Titel Quelle: Enthalten in: Astronomy and astrophysics
• Ort Quelle: Les Ulis : EDP Sciences, 1969
• Jahr Quelle: 2018
• Band/Heft Quelle: 617(2018) Artikel-Nummer A73, 7 Seiten
• ISSN Quelle: 1432-0746
• DOI: doi:10.1051/0004-6361/201833202
• Datenträger: Online-Ressource
• Sprache: eng
• K10plus-PPN: 1586730053

Abstract

<i>Context.<i/> NGC 253 is one of only two starburst galaxies found to emit <i>γ<i/>-rays from hundreds of MeV to multi-TeV energies. Accurate measurements of the very-high-energy (VHE; <i>E<i/> > 100 GeV) and high-energy (HE; <i>E<i/> > 60 MeV) spectra are crucial to study the underlying particle accelerators, probe the dominant emission mechanism(s) and to study cosmic-ray interaction and transport.<i>Aims.<i/> The measurement of the VHE <i>γ<i/>-ray emission of NGC 253 published in 2012 by H.E.S.S. was limited by large systematic uncertainties. Here, the most up to date measurement of the <i>γ<i/>-ray spectrum of NGC 253 is investigated in both HE and VHE <i>γ<i/>-rays. Assuming a hadronic origin of the <i>γ<i/>-ray emission, the measurement uncertainties are propagated into the interpretation of the accelerated particle population.<i>Methods.<i/> The data of H.E.S.S. observations are reanalysed using an updated calibration and analysis chain. The improved <i>Fermi<i/>-LAT analysis employs more than 8 yr of data processed using pass 8. The cosmic-ray particle population is evaluated from the combined HE-VHE <i>γ<i/>-ray spectrum using NAIMA in the optically thin case.<i>Results.<i/> The VHE <i>γ<i/>-ray energy spectrum is best fit by a power-law distribution with a flux normalisation of (1.34 ± 0.14<sup>stat<sup/> ± 0.27<sup>sys<sup/>) × 10<sup>−13<sup/> cm<sup>−2<sup/> s<sup>−1<sup/> TeV<sup>1<sup/> at 1 TeV - about 40% above, but compatible with the value obtained in . The spectral index Γ = 2.39 ± 0.14<sup>stat<sup/> ± 0.25<sup>sys<sup/> is slightly softer than but consistent with the previous measurement within systematic errors. In the <i>Fermi<i/> energy range an integral flux of <i>F<i/>(<i>E<i/> > 60 MeV) = (1.56 ± 0.28<sup>stat<sup/> ± 0.15<sup>sys<sup/>) × 10<sup>−8<sup/> cm<sup>−2<sup/> s<sup>−1<sup/> is obtained. At energies above ∼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. with an overall spectral index Γ = 2.22 ± 0.06<sup>stat<sup/>.<i>Conclusions.<i/> Two scenarios for the starburst nucleus are tested, in which the gas in the starburst nucleus acts as either a thin or a thick target for hadronic cosmic rays accelerated by the individual sources in the nucleus. In these two models, the level to which NGC 253 acts as a calorimeter is estimated to a range of <i>f<i/><sub>cal<sub/> = 0.1 to 1 while accounting for the measurement uncertainties. The presented spectrum is likely to remain the most accurate measurements until the Cherenkov Telescope Array (CTA) has collected a substantial set of data towards NGC 253.