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

¹ Centre for Space Research, North-West University, Potchefstroom 2520, South Africa
² Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
³ Max-Planck-Institut für Kernphysik, PO Box 103980, 69029 Heidelberg, Germany
⁴ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place Dublin 2, Ireland
⁵ National Academy of Sciences of the Republic of Armenia, Marshall Baghramian Avenue, 24, 0019 Yerevan, Republic of Armenia
⁶ Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
⁷ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
⁸ University of Namibia, Department of Physics, Private Bag 13301 Windhoek, Namibia
⁹ GRAPPA, Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
¹⁰ Department of Physics and Electrical Engineering, Linnaeus University, 351 95 Växjö, Sweden
¹¹ Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
¹² Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
¹³ School of Physical Sciences, University of Adelaide, Adelaide 5005, Australia
¹⁴ LUTH, Observatoire de Paris, PSL Research University, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
¹⁵ Sorbonne Université, Université Paris Diderot, Sorbonne Paris Cité, CNRS/IN2P3, Laboratoire de Physique Nucléaire et de Hautes Energies, LPNHE, 4 Place Jussieu, 75252 Paris, France
¹⁶ Laboratoire Univers et Particules de Montpellier, Université Montpellier, CNRS/IN2P3, CC 72, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
¹⁷ IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
¹⁸ Astronomical Observatory, The University of Warsaw, Al. Ujazdowskie 4 00-478 Warsaw, Poland
¹⁹ Aix Marseille Université, CNRS/IN2P3, CPPM, Marseille, France
²⁰ Instytut Fizyki Ja¸drowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland
²¹ School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, 2050 Johannesburg, South Africa
²² Laboratoire d’Annecy de Physique des Particules, Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, LAPP, 74000 Annecy, France
²³ Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
²⁴ Université Bordeaux, CNRS/IN2P3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France
²⁵ Oskar Klein Centre, Department of Physics, Stockholm University, Albanova University Center, 10691 Stockholm, Sweden
²⁶ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
²⁷ Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/ IN2P3, 91128 Palaiseau, France
²⁸ APC, AstroParticule et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, Sorbonne Paris Cité, 10, rue Alice Domon et Léonie Duquet, 75205, Paris Cedex 13, France
²⁹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³⁰ Department of Physics and Astronomy, The University of Leicester, University Road Leicester LE1 7RH, UK
³¹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
³² Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, 14476 Potsdam, Germany
e-mail: choische@uni-potsdam.de
³³ Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
³⁴ DESY, 15738 Zeuthen, Germany
³⁵ Obserwatorium Astronomiczne, Uniwersytet Jagiello´nski, ul. Orla 171, 30-244 Kraków, Poland
³⁶ Centre for Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland
³⁷ Department of Physics, University of the Free State, PO Box 339 9300, Bloemfontein, South Africa
³⁸ Heisenberg Fellow (DFG), ITA Universität Heidelberg, Germany
³⁹ Department of Physics, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan
⁴⁰ Japan Aerospace Exploration Agency (JAXA), Institute of Space and Astronautical Science (ISAS), 3-1-1 Yoshinodai, Chuo-ku, Kanagawa, Sagamihara, 229-8510, Japan
⁴¹ Now at Instituto de Física de São Carlos, Universidade de São Paulo, Av. Trabalhador São-carlense, 400 – CEP 13566-590 São Carlos, SP, Brazil

Received: 10 April 2018
Accepted: 24 May 2018

Abstract

Context. NGC 253 is one of only two starburst galaxies found to emit γ-rays from hundreds of MeV to multi-TeV energies. Accurate measurements of the very-high-energy (VHE; E > 100 GeV) and high-energy (HE; E > 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.

Aims. The measurement of the VHE γ-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 γ-ray spectrum of NGC 253 is investigated in both HE and VHE γ-rays. Assuming a hadronic origin of the γ-ray emission, the measurement uncertainties are propagated into the interpretation of the accelerated particle population.

Methods. The data of H.E.S.S. observations are reanalysed using an updated calibration and analysis chain. The improved Fermi–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 γ-ray spectrum using NAIMA in the optically thin case.

Results. The VHE γ-ray energy spectrum is best fit by a power-law distribution with a flux normalisation of (1.34 ± 0.14^stat ± 0.27^sys) × 10⁻¹³ cm⁻² s⁻¹ TeV¹ at 1 TeV – about 40% above, but compatible with the value obtained in Abramowski et al. (2012). The spectral index Γ = 2.39 ± 0.14^stat ± 0.25^sys is slightly softer than but consistent with the previous measurement within systematic errors. In the Fermi energy range an integral flux of F(E > 60 MeV) = (1.56 ± 0.28^stat ± 0.15^sys) × 10⁻⁸ cm⁻² s⁻¹ 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^stat.

Conclusions. 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 f_cal = 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.