Detection of RS Oph with LST-1 and modelling of its HE/VHE gamma-ray 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 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
¹⁰ INFN Sezione di Napoli, Via Cintia, ed. G, 80126 Napoli, Italy
¹¹ INAF – Osservatorio Astronomico di Roma, Via di Frascati 33, 00040 Monteporzio Catone, Italy
¹² Max-Planck-Institut für Physik, Föhringer Ring 6, 80805 München, Germany
¹³ INFN Sezione di Padova and Università degli Studi di Padova, Via Marzolo 8, 35131 Padova, Italy
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 Faculty of Science and Technology, Universidad del Azuay, Cuenca, Ecuador
¹⁸ Centro Brasileiro de Pesquisas Físicas, Rua Xavier Sigaud 150, RJ 22290-180 Rio de Janeiro, Brazil
¹⁹ 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
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 Lamarr Institute for Machine Learning and Artificial Intelligence, 44227 Dortmund, Germany
²⁸ INFN Sezione di Trieste and Università degli studi di Udine, Via delle scienze 206, 33100 Udine, Italy
²⁹ INFN Sezione di Catania, Via S. Sofia 64, 95123 Catania, 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 Archirafi 36, 90123 Palermo, Italy
³⁸ IRFU, CEA, Université Paris-Saclay, Bât 141, 91191 Gif-sur-Yvette, France
³⁹ Port d’Informació Científica, Edifici D, Carrer de l’Albareda, 08193 Bellaterrra, (Cerdanyola del Vallès), Spain
⁴⁰ INFN Sezione di Bari, Via Orabona 4, 70125 Bari, Italy
⁴¹ Department of Physics, TU Dortmund University, Otto-Hahn-Str. 4, 44227 Dortmund, Germany
⁴² University of Rijeka, Department of Physics, Radmile Matejcic 2, 51000 Rijeka, Croatia
⁴³ Institute for Theoretical Physics and Astrophysics, Universität Würzburg, Campus Hubland Nord, Emil-Fischer-Str. 31, 97074 Würzburg, Germany
⁴⁴ Department of Physics and Astronomy, University of Turku, Finland, FI-20014 University of Turku, Finland
⁴⁵ INFN Sezione di Roma La Sapienza, P.le Aldo Moro, 2, 00185 Rome, Italy
⁴⁶ ILANCE, CNRS – University of Tokyo International Research Laboratory, University of Tokyo, 5-1-5 Kashiwa-no-Ha Kashiwa City, Chiba 277-8582, Japan
⁴⁷ Physics Program, Graduate School of Advanced Science and Engineering, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima City, Hiroshima 739-8526, Japan
⁴⁸ INFN Sezione di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy
⁴⁹ University of Split, FESB, R. Boškovića 32, 21000 Split, Croatia
⁵⁰ Department of Physics, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata-shi 990-8560, Japan
⁵¹ Institut für Theoretische Physik, Lehrstuhl IV: Plasma-Astroteilchenphysik, Ruhr-Universität Bochum, Universitätsstraße 150, 44801 Bochum, Germany
⁵² Sendai College, National Institute of Technology, 4-16-1 Ayashi-Chuo, Aoba-ku, Sendai city, Miyagi 989-3128, Japan
⁵³ Josip Juraj Strossmayer University of Osijek, Department of Physics, Trg Ljudevita Gaja 6, 31000 Osijek, Croatia
⁵⁴ 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
⁵⁵ Chiba University, 1-33, Yayoicho, Inage-ku, Chiba-shi, Chiba 263-8522, Japan
⁵⁶ Kitashirakawa Oiwakecho, Sakyo Ward, Kyoto 606-8502, Japan
⁵⁷ FZU – Institute of Physics of the Czech Academy of Sciences, Na Slovance 1999/2, 182 21 Praha 8, Czech Republic
⁵⁸ Laboratory for High Energy Physics, École Polytechnique Fédérale, CH-1015 Lausanne, Switzerland
⁵⁹ Astronomical Institute of the Czech Academy of Sciences, Bocni II 1401, 14100 Prague, Czech Republic
⁶⁰ Faculty of Science, Ibaraki University, 2 Chome-1-1 Bunkyo, Mito, Ibaraki 310-0056, Japan
⁶¹ Faculty of Science and Engineering, Waseda University, 3 Chome-4-1 Okubo, Shinjuku City, Tokyo 169-0072, Japan
⁶² Sorbonne Université, CNRS/IN2P3, Laboratoire de Physique Nucléaire et de Hautes Energies, LPNHE, 4 place Jussieu, 75005 Paris, France
⁶³ Institute of Particle and Nuclear Studies, KEK (High Energy Accelerator Research Organization), 1-1 Oho, Tsukuba 305-0801, Japan
⁶⁴ INFN Sezione di Trieste and Università degli Studi di Trieste, Via Valerio 2 I, 34127 Trieste, Italy
⁶⁵ Escuela Politécnica Superior de Jaén, Universidad de Jaén, Campus Las Lagunillas s/n, Edif. A3, 23071 Jaén, Spain
⁶⁶ Saha Institute of Nuclear Physics, Sector 1, AF Block, Bidhan Nagar, Bidhannagar, Kolkata, West Bengal 700064, India
⁶⁷ Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, 72 boul. Tsarigradsko chaussee, 1784 Sofia, Bulgaria
⁶⁸ Dipartimento di Fisica e Astronomia (DIFA) Augusto Righi, Università di Bologna, Via Gobetti 93/2, I-40129 Bologna, Italy
⁶⁹ Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC 29634, USA
⁷⁰ Dipartimento di Fisica e Chimica ‘E. Segrè’ Università degli Studi di Palermo, Via delle Scienze, 90128 Palermo, Italy
⁷¹ Grupo de Electronica, Universidad Complutense de Madrid, Av. Complutense s/n, 28040 Madrid, Spain
⁷² 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
⁷³ Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁷⁴ School of Allied Health Sciences, Kitasato University, Sagamihara, Kanagawa 228-8555, Japan
⁷⁵ RIKEN, Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
⁷⁶ Charles University, Institute of Particle and Nuclear Physics, V Holešovičkách 2, 180 00 Prague 8, Czech Republic
⁷⁷ Division of Physics and Astronomy, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
⁷⁸ Institute for Space-Earth Environmental Research, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan
⁷⁹ Kobayashi-Maskawa Institute (KMI) for the Origin of Particles and the Universe, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan
⁸⁰ Graduate School of Technology, Industrial and Social Sciences, Tokushima University, 2-1 Minamijosanjima, Tokushima 770-8506, Japan
⁸¹ INFN Sezione di Pisa, Edificio C – Polo Fibonacci, Largo Bruno Pontecorvo 3, 56127 Pisa, Italy
⁸² 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
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 Graduate School of Science and Engineering, Saitama University, 255 Simo-Ohkubo, Sakura-ku, Saitama city, Saitama 338-8570, Japan
⁸⁶ Department of Physics, Konan University, 8-9-1 Okamoto, Higashinada-ku Kobe 658-8501, Japan

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

Received: 1 October 2024
Accepted: 15 January 2025

Abstract

Context. The recurrent nova RS Ophiuchi (RS Oph) underwent a thermonuclear eruption in August 2021. In this event, RS Oph was detected by the High Energy Stereoscopic System (H.E.S.S.), the Major Atmospheric Gamma Imaging Cherenkov (MAGIC), and the first Large-Sized Telescope (LST-1) of the future Cherenkov Telescope Array Observatory (CTAO) at very-high gamma-ray energies above 100 GeV. This means that novae are a new class of very-high-energy (VHE) gamma-ray emitters.

Aims. We report the analysis of the RS Oph observations with LST-1. We constrain the particle population that causes the observed emission in hadronic and leptonic scenarios. Additionally, we study the prospects of detecting further novae using LST-1 and the upcoming LST array of CTAO-North.

Methods. We conducted target-of-opportunity observations with LST-1 from the first day of this nova event. The data were analysed in the framework of cta-lstchain and Gammapy, the official CTAO-LST reconstruction and analysis packages. One-zone hadronic and leptonic models were considered to model the gamma-ray emission of RS Oph using the spectral information from Fermi-LAT and LST-1, together with public data from the MAGIC and H.E.S.S. telescopes.

Results. RS Oph was detected at 6.6σ with LST-1 in the first 6.35 hours of observations following the eruption. The hadronic scenario is preferred over the leptonic scenario considering a proton energy spectrum with a power-law model with an exponential cutoff whose position increases from (0.26 ± 0.08) TeV on day 1 up to (1.6 ± 0.6) TeV on day 4 after the eruption. The deep sensitivity and low energy threshold of the LST-1/LST array will allow us to detect faint novae and increase their discovery rate.