CTAO LST–1 observations of magnetar SGR 1935+2154: Deep limits on sub-second bursts and persistent tera-electronvolt emission

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 Université Paris Cité, CNRS, Astroparticule et Cosmologie F-75013 Paris, France
⁵⁶ Josip Juraj Strossmayer University of Osijek, Department of Physics Trg Ljudevita Gaja 6 31000 Osijek, Croatia
⁵⁷ Department of Astronomy and Space Science, Chungnam National University Daejeon 34134, Republic of Korea
⁵⁸ 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
⁵⁹ Chiba University, 1-33, Yayoicho, Inage-ku, Chiba-shi Chiba 263-8522, Japan
⁶⁰ Kitashirakawa Oiwakecho, Sakyo Ward Kyoto 606-8502, Japan
⁶¹ FZU – Institute of Physics of the Czech Academy of Sciences Na Slovance 1999/2 182 21 Praha 8, Czech Republic
⁶² Laboratory for High Energy Physics, École Polytechnique Fédérale CH-1015 Lausanne, Switzerland
⁶³ Astronomical Institute of the Czech Academy of Sciences Bocni II 1401 14100 Prague, Czech Republic
⁶⁴ Faculty of Science, Ibaraki University, 2 Chome-1-1 Bunkyo Mito Ibaraki 310-0056, Japan
⁶⁵ Sorbonne Université, CNRS/IN2P3, Laboratoire de Physique Nucléaire et de Hautes Energies, LPNHE 4 Place Jussieu 75005 Paris, France
⁶⁶ Graduate School of Science and Engineering, Saitama University, 255 Simo-Ohkubo Sakura-ku Saitama city Saitama 338-8570, Japan
⁶⁷ Institute of Particle and Nuclear Studies, KEK (High Energy Accelerator Research Organization) 1-1 Oho Tsukuba 305-0801, Japan
⁶⁸ INFN Sezione di Trieste and Università degli Studi di Trieste Via Valerio 2 I 34127 Trieste, Italy
⁶⁹ Escuela Politécnica Superior de Jaén, Universidad de Jaén, Campus Las Lagunillas s/n Edif. A3 23071 Jaén, Spain
⁷⁰ Saha Institute of Nuclear Physics, A CI of Homi Bhabha National Institute Kolkata 700064 West Bengal, India
⁷¹ Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences 72 boul. Tsarigradsko chaussee 1784 Sofia, Bulgaria
⁷² Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics Clemson SC 29634, USA
⁷³ Institut de Fisica d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology, Campus UAB 08193 Bellaterra (Barcelona), Spain
⁷⁴ Grupo de Electronica, Universidad Complutense de Madrid Av. Complutense s/n 28040 Madrid, Spain
⁷⁵ Macroarea di Scienze MMFFNN, Università di Roma Tor Vergata Via della Ricerca Scientifica 1 00133 Rome, Italy
⁷⁶ 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
⁷⁷ Department of Physics, Konan University 8-9-1 Okamoto Higashinada-ku Kobe 658-8501, Japan
⁷⁸ School of Allied Health Sciences, Kitasato University, Sagamihara Kanagawa 228-8555, Japan
⁷⁹ RIKEN, Institute of Physical and Chemical Research, 2-1 Hirosawa Wako Saitama 351-0198, Japan
⁸⁰ Charles University, Institute of Particle and Nuclear Physics V Holešovičkách 2 180 00 Prague 8, Czech Republic
⁸¹ Division of Physics and Astronomy, Graduate School of Science, Kyoto University, Sakyo-ku Kyoto 606-8502, Japan
⁸² Institute for Space-Earth Environmental Research, Nagoya University, Chikusa-ku Nagoya 464-8601, Japan
⁸³ Kobayashi-Maskawa Institute (KMI) for the Origin of Particles and the Universe, Nagoya University, Chikusa-ku Nagoya 464-8602, Japan
⁸⁴ Graduate School of Technology, Industrial and Social Sciences, Tokushima University 2-1 Minamijosanjima Tokushima 770-8506, Japan
⁸⁵ INFN Sezione di Pisa, Edificio C – Polo Fibonacci Largo Bruno Pontecorvo 3 56127 Pisa, Italy
⁸⁶ Gifu University, Faculty of Engineering 1-1 Yanagido Gifu 501-1193, Japan
⁸⁷ Department of Physical Sciences, Aoyama Gakuin University, Fuchinobe, Sagamihara Kanagawa 252-5258, Japan
⁸⁸ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano Via A. Corti 12 20133 Milano, Italy
⁸⁹ Cherenkov Telescope Array Observatory gGmbH, Via Piero Gobetti 93/3 40129 Bologna, Italy

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Received: 7 May 2025
Accepted: 29 October 2025

Abstract

Context. The Galactic magnetar SGR 1935+2154 has exhibited prolific high-energy (HE) bursting activity in recent years.

Aims. Investigating its potential tera-electronvolt counterpart could provide insights into the underlying mechanisms of magnetar emission and very high-energy (VHE) processes in extreme astrophysical environments. We aim to search for a possible tera-electronvolt counterpart to both its persistent and sub-second-scale burst emission.

Methods. We analysed over 25 hour of observations from the Large-Sized Telescope prototype (LST−1) of the Cherenkov Telescope Array Observatory (CTAO) during periods of HE activity from SGR 1935+2154 in 2021 and 2022 to search for persistent emission. For bursting emission, we selected and analysed nine 0.1 s time windows centred around known short X-ray bursts, targeting potential sub-second-scale tera-electronvolt counterparts in a low-photon-statistics regime.

Results. While no persistent or bursting emission was detected in our search, we establish upper limits for the tera-electronvolt emission of a short magnetar burst simultaneous to its soft gamma-ray flux. Specifically, for the brightest burst in our sample, the ratio between tera-electronvolt and X-ray flux is ≲10⁻³.

Conclusions. The non-detection of either persistent or bursting tera-electronvolt emission from SGR 1935+2154 suggests that if such components exist, they may occur under specific conditions not covered by our observations. This aligns with theoretical predictions of VHE components in magnetar-powered fast radio bursts and the detection of MeV–GeV emission in giant magnetar flares. These findings underscore the potential of magnetars, fast radio bursts, and other fast transients as promising candidates for future observations in the low-photon-statistics regime with Imaging Atmospheric Cherenkov Telescopes, particularly with the CTAO.