Letter to the Editor

Detection of the Geminga pulsar with MAGIC hints at a power-law tail emission beyond 15 GeV

¹ Inst. de Astrofísica de Canarias, 38200 La Laguna, Spain
² Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
³ Università di Udine and INFN Trieste, 33100 Udine, Italy
⁴ National Institute for Astrophysics (INAF), 00136 Rome, Italy
⁵ ETH Zürich, 8093 Zürich, Switzerland
⁶ Japanese MAGIC Group: Institute for Cosmic Ray Research (ICRR), The University of Tokyo, Kashiwa, 277-8582 Chiba, Japan
⁷ Technische Universität Dortmund, 44221 Dortmund, Germany
⁸ Croatian MAGIC Group: University of Zagreb, Faculty of Electrical Engineering and Computing (FER), 10000 Zagreb, Croatia
⁹ IPARCOS Institute and EMFTEL Department, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹⁰ Centro Brasileiro de Pesquisas Físicas (CBPF), 22290-180 URCA Rio de Janeiro (RJ), Brazil
¹¹ University of Lodz, Faculty of Physics and Applied Informatics, Department of Astrophysics, 90-236 Lodz, Poland
¹² Università di Siena and INFN Pisa, 53100 Siena, Italy
¹³ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹⁴ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, 28040 Madrid, Spain
¹⁵ Istituto Nazionale Fisica Nucleare (INFN), 00044 Frascati (Roma), Italy
¹⁶ Max-Planck-Institut für Physik, 80805 München, Germany
¹⁷ Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), 08193 Bellaterra (Barcelona), Spain
¹⁸ Università di Padova and INFN, 35131 Padova, Italy
¹⁹ Università di Pisa and INFN Pisa, 56126 Pisa, Italy
²⁰ Universitat de Barcelona, ICCUB, IEEC-UB, 08028 Barcelona, Spain
²¹ Armenian MAGIC Group: A. Alikhanyan National Science Laboratory, Yerevan, Armenia
²² Croatian MAGIC Group: University of Rijeka, Department of Physics, 51000 Rijeka, Croatia
²³ Universität Würzburg, 97074 Würzburg, Germany
²⁴ Finnish MAGIC Group: Finnish Centre for Astronomy with ESO, University of Turku, 20014 Turku, Finland
²⁵ Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
²⁶ Armenian MAGIC Group: ICRANet-Armenia at NAS RA, Yerevan, Armenia
²⁷ Croatian MAGIC Group: University of Split, Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture (FESB), 21000 Split, Croatia
²⁸ Croatian MAGIC Group: Josip Juraj Strossmayer University of Osijek, Department of Physics, 31000 Osijek, Croatia
²⁹ Japanese MAGIC Group: RIKEN, Wako, Saitama 351-0198, Japan
³⁰ Japanese MAGIC Group: Department of Physics, Kyoto University, 606-8502 Kyoto, Japan
³¹ Japanese MAGIC Group: Department of Physics, Tokai University, Hiratsuka, 259-1292 Kanagawa, Japan
³² Saha Institute of Nuclear Physics, HBNI, 1/AF Bidhannagar, Salt Lake, Sector-1, Kolkata 700064, India
³³ Inst. for Nucl. Research and Nucl. Energy, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
³⁴ Finnish MAGIC Group: Astronomy Research Unit, University of Oulu, 90014 Oulu, Finland
³⁵ Croatian MAGIC Group: Ruđer Bošković Institute, 10000 Zagreb, Croatia
³⁶ Now at University of Innsbruck, Innsbruck, Austria
³⁷ Also at Port d’Informació Científica (PIC), 08193 Bellaterra (Barcelona), Spain
³⁸ Also at Dipartimento di Fisica, Università di Trieste, 34127 Trieste, Italy
³⁹ Also at INAF-Trieste and Dept. of Physics & Astronomy, University of Bologna, Bologna, Italy
⁴⁰ Academia Sinica, Institute of Astronomy and Astrophysics (ASIAA), PO Box 23-141 Taipei, Taiwan
⁴¹ Department of Physics and Laboratory for Space Research, University of Hong Kong, Pokfulam Road, Pok Fu Lam, Hong Kong
⁴² Also at Santa Cruz Institute for Particle Physics, University of California, Santa Cruz, CA 95064, USA

Received: 9 August 2020
Accepted: 18 September 2020

Abstract

We report the detection of pulsed gamma-ray emission from the Geminga pulsar (PSR J0633+1746) between 15 GeV and 75 GeV. This is the first time a middle-aged pulsar has been detected up to these energies. Observations were carried out with the MAGIC telescopes between 2017 and 2019 using the low-energy threshold Sum-Trigger-II system. After quality selection cuts, ∼80 h of observational data were used for this analysis. To compare with the emission at lower energies below the sensitivity range of MAGIC, 11 years of Fermi-LAT data above 100 MeV were also analysed. From the two pulses per rotation seen by Fermi-LAT, only the second one, P2, is detected in the MAGIC energy range, with a significance of 6.3σ. The spectrum measured by MAGIC is well-represented by a simple power law of spectral index Γ = 5.62 ± 0.54, which smoothly extends the Fermi-LAT spectrum. A joint fit to MAGIC and Fermi-LAT data rules out the existence of a sub-exponential cut-off in the combined energy range at the 3.6σ significance level. The power-law tail emission detected by MAGIC is interpreted as the transition from curvature radiation to Inverse Compton Scattering of particles accelerated in the northern outer gap.