Detection of the Geminga pulsar at energies down to 20 GeV with the LST-1 of CTAO

¹ Department of Physics, Tokai University, 4-1-1, Kita-Kaname, Hiratsuka, Kanagawa 259-1292, Japan
² Institute for Cosmic Ray Research, University of Tokyo, 5-1-5, Kashiwa-no-ha, Kashiwa, Chiba 277-8582, Japan
³ INFN and Università degli Studi di Siena, Dipartimento di Scienze Fisiche, della Terra e dell’Ambiente (DSFTA), Sezione di Fisica, Via Roma 56, 53100 Siena, Italy
⁴ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, F-91191 Gif-sur-Yvette Cedex, France
⁵ FSLAC IRL 2009, CNRS/IAC, La Laguna, Tenerife, Spain
⁶ Departament de Física Quàntica i Astrofísica, Institut de Ciències del Cosmos, Universitat de Barcelona, IEEC-UB, Martí i Franquès, 1, 08028 Barcelona, Spain
⁷ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁸ Department of Astronomy, University of Geneva, Chemin d’Ecogia 16, CH-1290 Versoix, Switzerland
⁹ INFN Sezione di Napoli, Via Cintia, ed. G, 80126 Napoli, Italy
¹⁰ INAF – Osservatorio Astronomico di Roma, Via di Frascati 33, 00040 Monteporzio Catone, Italy
¹¹ Max-Planck-Institut für Physik, Boltzmannstraße 8, 85748 Garching bei München, Germany
¹² INFN Sezione di Padova and Università degli Studi di Padova, Via Marzolo 8, 35131 Padova, Italy
¹³ Instituto de Astrofísica de Canarias and Departamento de Astrofísica, Universidad de La Laguna, C. Vía Láctea, s/n, 38205 La Laguna, Santa Cruz de Tenerife, Spain
¹⁴ Univ. Savoie Mont Blanc, CNRS, Laboratoire d’Annecy de Physique des Particules - IN2P3, 74000 Annecy, France
¹⁵ Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
¹⁶ Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
¹⁷ IPARCOS-UCM, Instituto de Física de Partículas y del Cosmos, and EMFTEL Department, Universidad Complutense de Madrid, Plaza de Ciencias, 1. Ciudad Universitaria, 28040 Madrid, Spain
¹⁸ Faculty of Science and Technology, Universidad del Azuay, Cuenca, Ecuador
¹⁹ Centro Brasileiro de Pesquisas Físicas, Rua Xavier Sigaud 150, RJ 22290-180 Rio de Janeiro, Brazil
²⁰ CIEMAT, Avda. Complutense 40, 28040 Madrid, Spain
²¹ University of Geneva – Département de physique nucléaire et corpusculaire, 24 Quai Ernest Ansernet, 1211 Genève 4, Switzerland
²² INFN Sezione di Bari and Politecnico di Bari, via Orabona 4, 70124 Bari, Italy
²³ Institut de Fisica d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology, Campus UAB, 08193 Bellaterra, (Barcelona), Spain
²⁴ INAF – Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milano, Italy
²⁵ Faculty of Physics and Applied Informatics, University of Lodz, ul. Pomorska 149-153, 90-236 Lodz, Poland
²⁶ INAF – Osservatorio di Astrofisica e Scienza dello spazio di Bologna, Via Piero Gobetti 93/3, 40129 Bologna, Italy
²⁷ Dipartimento di Fisica e Astronomia (DIFA) Augusto Righi, Università di Bologna, via Gobetti 93/2, I-40129 Bologna, Italy
²⁸ Lamarr Institute for Machine Learning and Artificial Intelligence, 44227 Dortmund, Germany
²⁹ INFN Sezione di Trieste and Università degli studi di Udine, via delle scienze 206, 33100 Udine, Italy
³⁰ INAF – Istituto di Astrofisica e Planetologia Spaziali (IAPS), Via del Fosso del Cavaliere 100, 00133 Roma, Italy
³¹ Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France
³² University of Alcalá UAH, Departamento de Physics and Mathematics, Pza. San Diego, 28801 Alcalá de Henares, Madrid, Spain
³³ INFN Sezione di Bari and Università di Bari, via Orabona 4, 70126 Bari, Italy
³⁴ INFN Sezione di Torino, Via P. Giuria 1, 10125 Torino, Italy
³⁵ Dipartimento di Fisica – Universitá degli Studi di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
³⁶ Palacky University Olomouc, Faculty of Science, 17. listopadu 1192/12, 771 46 Olomouc, Czech Republic
³⁷ Dipartimento di Fisica e Chimica ’E. Segrè’ Università degli Studi di Palermo, via delle Scienze, 90128 Palermo, Italy
³⁸ INFN Sezione di Catania, Via S. Sofia 64, 95123 Catania, Italy
³⁹ IRFU, CEA, Université Paris-Saclay, Bât 141, 91191 Gif-sur-Yvette, France
⁴⁰ Port d’Informació Científica, Edifici D, Carrer de l’Albareda, 08193 Bellaterrra (Cerdanyola del Vallès), Spain
⁴¹ INFN Sezione di Bari, via Orabona 4, 70125 Bari, Italy
⁴² Department of Physics, TU Dortmund University, Otto-Hahn-Str. 4, 44227 Dortmund, Germany
⁴³ University of Rijeka, Department of Physics, Radmile Matejcic 2, 51000 Rijeka, Croatia
⁴⁴ Institute for Theoretical Physics and Astrophysics, Universität Würzburg, Campus Hubland Nord, Emil-Fischer-Str. 31, 97074 Würzburg, Germany
⁴⁵ Department of Physics and Astronomy, University of Turku, Finland, FI-20014 University of Turku, Finland
⁴⁶ Department of Physics, TU Dortmund University, Otto-Hahn-Str. 4, 44227 Dortmund, Germany
⁴⁷ INFN Sezione di Roma La Sapienza, P.le Aldo Moro, 2, 00185 Rome, Italy
⁴⁸ ILANCE, CNRS – University of Tokyo International Research Laboratory, University of Tokyo, 5-1-5 Kashiwa-no-Ha Kashiwa City, Chiba 277-8582, Japan
⁴⁹ Physics Program, Graduate School of Advanced Science and Engineering, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima City, Hiroshima 739-8526, Japan
⁵⁰ INFN Sezione di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy
⁵¹ University of Split, FESB, R. Boškovića 32, 21000 Split, Croatia
⁵² Department of Physics, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata-shi 990-8560, Japan
⁵³ Institut für Theoretische Physik, Lehrstuhl IV: Plasma-Astroteilchenphysik, Ruhr-Universität Bochum, Universitätsstraße 150, 44801 Bochum, Germany
⁵⁴ Sendai College, National Institute of Technology, 4-16-1 Ayashi-Chuo, Aoba-ku, Sendai city, Miyagi 989-3128, Japan
⁵⁵ Josip Juraj Strossmayer University of Osijek, Department of Physics, Trg Ljudevita Gaja 6, 31000 Osijek, Croatia
⁵⁶ INFN Dipartimento di Scienze Fisiche e Chimiche – Università degli Studi dell’Aquila and Gran Sasso Science Institute, Via Vetoio 1, Viale Crispi 7, 67100 L’Aquila, Italy
⁵⁷ Chiba University, 1-33, Yayoicho, Inage-ku, Chiba-shi, Chiba 263-8522, Japan
⁵⁸ Kitashirakawa Oiwakecho, Sakyo Ward, Kyoto 606-8502, Japan
⁵⁹ FZU – Institute of Physics of the Czech Academy of Sciences, Na Slovance 1999/2, 182 21 Praha 8, Czech Republic
⁶⁰ Laboratory for High Energy Physics, Ècole Polytechnique Fédérale, CH-1015 Lausanne, Switzerland
⁶¹ Astronomical Institute of the Czech Academy of Sciences, Bocni II 1401, 14100 Prague, Czech Republic
⁶² Faculty of Science, Ibaraki University, 2 Chome-1-1 Bunkyo, Mito, Ibaraki 310-0056, Japan
⁶³ Sorbonne Université, CNRS/IN2P3, Laboratoire de Physique Nucléaire et de Hautes Energies, LPNHE, 4 place Jussieu, 75005 Paris, France
⁶⁴ Graduate School of Science and Engineering, Saitama University, 255 Simo-Ohkubo, Sakura-ku, Saitama city, Saitama 338-8570, Japan
⁶⁵ Institute of Particle and Nuclear Studies, KEK (High Energy Accelerator Research Organization), 1-1 Oho, Tsukuba 305-0801, Japan
⁶⁶ INFN Sezione di Trieste and Università degli Studi di Trieste, Via Valerio 2 I, 34127 Trieste, Italy
⁶⁷ Escuela Politécnica Superior de Jaén, Universidad de Jaén, Campus Las Lagunillas s/n, Edif. A3, 23071 Jaén, Spain
⁶⁸ Saha Institute of Nuclear Physics, Sector 1, AF Block, Bidhan Nagar, Bidhannagar, Kolkata, West Bengal 700064, India
⁶⁹ Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, 72 boul. Tsarigradsko chaussee, 1784 Sofia, Bulgaria
⁷⁰ Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC 29634, USA
⁷¹ Institut de Fisica d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology, Campus UAB, 08193 Bellaterra, (Barcelona), Spain
⁷² Grupo de Electronica, Universidad Complutense de Madrid, Av. Complutense s/n, 28040 Madrid, Spain
⁷³ Institute of Space Sciences (ICE, CSIC), and Institut d’Estudis Espacials de Catalunya (IEEC), and Institució Catalana de Recerca I Estudis Avançats (ICREA), Campus UAB, Carrer de Can Magrans, s/n, 08193 Bellatera, Spain
⁷⁴ Department of Physics, Konan University, 8-9-1 Okamoto, Higashinada-ku Kobe 658-8501, Japan
⁷⁵ School of Allied Health Sciences, Kitasato University, Sagamihara, Kanagawa 228-8555, Japan
⁷⁶ RIKEN, Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
⁷⁷ Charles University, Institute of Particle and Nuclear Physics, V Holešovičkách 2, 180 00 Prague 8, Czech Republic
⁷⁸ Division of Physics and Astronomy, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
⁷⁹ Institute for Space-Earth Environmental Research, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan
⁸⁰ Kobayashi-Maskawa Institute (KMI) for the Origin of Particles and the Universe, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan
⁸¹ Graduate School of Technology, Industrial and Social Sciences, Tokushima University, 2-1 Minamijosanjima, Tokushima 770-8506, Japan
⁸² INFN Sezione di Pisa, Edificio C – Polo Fibonacci, Largo Bruno Pontecorvo 3, 56127 Pisa, Italy
⁸³ Gifu University, Faculty of Engineering, 1-1 Yanagido, Gifu 501-1193, Japan
⁸⁴ Department of Physical Sciences, Aoyama Gakuin University, Fuchinobe, Sagamihara, Kanagawa 252-5258, Japan
⁸⁵ Macroarea di Scienze MMFFNN, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy

^⋆ Corresponding authors: lst-contact@cta-observatory.org

Received: 3 March 2025
Accepted: 21 May 2025

Abstract

Context. Geminga is the third gamma-ray pulsar firmly detected by imaging atmospheric Cherenkov telescopes (IACTs) after the Crab and the Vela pulsars. Most of its emission is expected at tens of giga-electronvolts, and, out of the planned telescopes of the upcoming Cherenkov Telescope Array Observatory (CTAO), the Large-Sized Telescopes (LSTs) are the only ones with optimised sensitivity at these energies.

Aims. We aim to characterise the gamma-ray pulse shape and spectrum of Geminga as observed by the first LST (hereafter LST-1) of the Northern Array of CTAO. Furthermore, this study confirms the great performance and the improved energy threshold of the telescope, as low as 10 GeV for pulsar analysis, with respect to current-generation Cherenkov telescopes.

Methods. We analysed 60 hours of good-quality data taken by the LST-1 between December 2022 and March 2024 at zenith angles below 50°. Additionally, a new Fermi-LAT analysis of 16.6 years of data was carried out to extend the spectral analysis down to 100 MeV. Lastly, a detailed study of the systematic effects was performed.

Results. We report the detection of Geminga in the energy range between 20 and 65 GeV. Of the two peaks of the phaseogram, the second one, P2, is detected with a significance of 12.2σ, while the first (P1) reaches a significance level of 2.6σ. The best-fit model for the spectrum of P2 was found to be a power law with a spectral index of Γ = (4.5 ± 0.4_stat)_−0.6sys^+0.2_sys, compatible with the previous results obtained by the MAGIC Collaboration. No evidence of curvature is found in the LST-1 energy range. The joint fit with Fermi-LAT data confirms a preference for a sub-exponential cut-off over a pure exponential, even though both models fail to reproduce the data above several tens of giga-electronvolts. The overall results presented in this paper prove that the LST-1 is an excellent telescope for the observation of pulsars, and improved sensitivity is expected to be achieved with the full CTAO Northern Array.