Gamma-ray signature of superluminous supernovae: Fermi-LAT GeV detection of SN 2017egm and evidence of a central engine

¹ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, F-91191 Gif-sur-Yvette Cedex, France
² FSLAC IRL 2009, CNRS/IAC, La Laguna, Tenerife, Spain
³ Center for Cosmology and Particle Physics Phenomenology, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark
⁴ Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, I-06123 Perugia, Italy
⁵ Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC 29634-0978, USA
⁶ Dipartimento di Fisica, Università di Trieste, I-34127 Trieste, Italy
⁷ Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, I-34127 Trieste, Italy
⁸ Università di Pisa, Dipartimento di Fisica E. Fermi, I-56127 Pisa, Italy
⁹ Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa, Italy
¹⁰ Istituto Nazionale di Fisica Nucleare, Sezione di Bari, I-70126 Bari, Italy
¹¹ Università degli studi di Trento, Via Calepina 14, 38122 Trento, Italy
¹² Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 Padova, Italy
¹³ Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova, Via F. Marzolo, 8, I-35131 Padova, Italy
¹⁴ Center for Space Studies and Activities “G. Colombo”, University of Padova, Via Venezia 15, I-35131 Padova, Italy
¹⁵ Instituto de Astrofísica de Canarias and Universidad de La Laguna, Dpto. Astrofísica, 38200 La Laguna, Tenerife, Spain
¹⁶ Dipartimento di Fisica “M. Merlin” dell’Università e del Politecnico di Bari, Via Amendola 173, I-70126 Bari, Italy
¹⁷ Istituto Nazionale di Fisica Nucleare, Sezione di Torino, I-10125 Torino, Italy
¹⁸ Dipartimento di Fisica, Università degli Studi di Torino, I-10125 Torino, Italy
¹⁹ Laboratoire Leprince-Ringuet, CNRS/IN2P3, École polytechnique, Institut Polytechnique de Paris, 91120 Palaiseau, France
²⁰ Deutsches Elektronen Synchrotron DESY, D-15738 Zeuthen, Germany
²¹ Institut für Theoretische Physik and Astrophysik, Universität Würzburg, D-97074 Würzburg, Germany
²² W. W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305, USA
²³ INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via E. Bassini 15, I-20133 Milano, Italy
²⁴ Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, I-00133 Roma, Italy
²⁵ Space Science Data Center – Agenzia Spaziale Italiana, Via del Politecnico, snc, I-00133 Roma, Italy
²⁶ Dipartimento di Fisica, Università La Sapienza, Piazzale A. Moro, 2, I-00185 Roma, Italy
²⁷ Dipartimento di Fisica, Università degli Studi di Perugia, I-06123 Perugia, Italy
²⁸ Italian Space Agency, Via del Politecnico snc, 00133 Roma, Italy
²⁹ Space Science Division, Naval Research Laboratory, Washington, DC 20375-5352, USA
³⁰ Friedrich-Alexander Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
³¹ Friedrich-Alexander-Universität, Erlangen-Nürnberg, Schlossplatz 4, 91054 Erlangen, Germany
³² INAF Istituto di Radioastronomia, I-40129 Bologna, Italy
³³ Grupo de Altas Energías, Universidad Complutense de Madrid, E-28040 Madrid, Spain
³⁴ Université Bordeaux, CNRS, LP2I Bordeaux, UMR 5797, F-33170 Gradignan, France
³⁵ Università di Pisa and Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa, Italy
³⁶ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), 44780 Bochum, Germany
³⁷ Department of Physical Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
³⁸ Dipartimento di Fisica, Università di Roma “Tor Vergata”, I-00133 Roma, Italy
³⁹ Dipartimento di Fisica e Geologia, Università degli Studi di Perugia, Via Pascoli snc, I-06123 Perugia, Italy
⁴⁰ Department of Fundamental Physics, University of Salamanca, Plaza de la Merced s/n, E-37008 Salamanca, Spain
⁴¹ Université Paris Cité, Université Paris-Saclay, CEA, CNRS, AIM, F-91191 Gif-sur-Yvette, France
⁴² The George Washington University, Department of Physics, 725 21st St, NW, Washington, DC 20052, USA
⁴³ Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁴⁴ University of North Florida, Department of Physics, 1 UNF Drive, Jacksonville, FL 32224, USA
⁴⁵ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, China
⁴⁶ NASA Postdoctoral Program Fellow, USA
⁴⁷ Department of Astronomy, University of Science and Technology of China, Hefei 230026, China
⁴⁸ School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China
⁴⁹ Institute of Astrophysics, Foundation for Research and Technology-Hellas, Heraklion GR-70013, Greece
⁵⁰ The Aerospace Corporation, 14745 Lee Rd, Chantilly, VA 20151, USA
⁵¹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Magrans s/n, E-08193 Barcelona, Spain
⁵² Center for Space Science and Technology, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁵³ Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁵⁴ Vatican Observatory, Castel Gandolfo, V-00120, Vatican City State, Italy
⁵⁵ Department of physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA
⁵⁶ INAF-Astronomical Observatory of Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy
⁵⁷ Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, A-6020 Innsbruck, Austria
⁵⁸ Instituto de Física Teórica UAM/CSIC, Universidad Autónoma de Madrid, E-28049 Madrid, Spain
⁵⁹ Departamento de Física Teórica, Universidad Autónoma de Madrid, 28049 Madrid, Spain
⁶⁰ Santa Cruz Institute for Particle Physics, Department of Physics and Department of Astronomy and Astrophysics, University of California at Santa Cruz, Santa Cruz, CA 95064, USA
⁶¹ NYCB Real-Time Computing Inc., Lattingtown, NY 11560-1025, USA
⁶² Purdue University Northwest, Hammond, IN 46323, USA
⁶³ Nagoya University, Institute for Space-Earth Environmental Research, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
⁶⁴ Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Japan
⁶⁵ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁶⁶ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Magrans s/n, E-08193 Barcelona, Spain and Institut d’Estudis Espacials de Catalunya (IEEC), E-08034 Barcelona, Spain and Institució Catalana de Recerca i Estudis Avançats (ICREA), E-08010 Barcelona, Spain
⁶⁷ Praxis Inc., Alexandria, VA 22303, resident at Naval Research Laboratory, Washington, DC 20375, USA
⁶⁸ Center for Astrophysics and Cosmology, University of Nova Gorica, Nova Gorica, Slovenia
⁶⁹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Magrans s/n, E-08193 Barcelona, Spain; and Institut d’Estudis Espacials de Catalunya (IEEC), E-08034 Barcelona, Spain
⁷⁰ Department of Physics and Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
⁷¹ Center for Computational Astrophysics, Flatiron Institute, 162 5th Ave, New York, NY 10010, USA
⁷² The Oskar Klein Centre, Department of Astronomy, Stockholm University, Albanova University Center, SE 106 91 Stockholm, Sweden
⁷³ Astrophysics Division, National Centre for Nuclear Research, Pasteura 7, 02-093 Warsaw, Poland
⁷⁴ Tartu Observatory, University of Tartu, Tõravere, 61602, Tartumaa, Estonia

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Received: 12 December 2025
Accepted: 7 March 2026

Abstract

Context. Superluminous supernovae (SLSNe) are a rare class of transients with peak luminosities 10–100 times greater than those of standard core-collapse supernovae (SNe). The mechanisms powering their extreme brightness remain debated, with circumstellar medium (CSM) interaction, or energy injection from a central engine like a magnetar wind nebula being the most plausible scenarios. While the optical properties of SLSNe are extensively studied, their γ-ray signatures remain poorly constrained.

Aims. To further constrain the underlying mechanism, we carried out a systematic search for giga-electronvolt γ-ray emission using the Fermi Large Area Telescope (LAT) from a sample of nearby hydrogen-poor (Type I) and hydrogen-rich (Type II) SLSNe over the past 16 years. Our objective is to test predictions from CSM and magnetar models, and to assess the prospects for future detections with the Cherenkov Telescope Array Observatory (CTAO).

Methods. For the six targets of this sample, we studied the time variability of a putative γ-ray signal at the optical position of the SLSN on a six-month timescale, and in the case of SN 2017egm, we further investigated variability on 15-day intervals and applied a Bayesian block algorithm to characterize the time variability of the signal. We then compared the temporal evolution and spectral properties to the predictions from a magnetar and CSM interaction model.

Results. Among the sample, only SN 2017egm shows significant γ-ray emission, with likelihood test statistic (TS) values of 26–33 (i.e., > 5σ) depending on the adopted time window. The signal arises between 50 and 160 days after explosion and is well described by a power-law spectrum with index Γ = 2.17 ± 0.23. The emission is consistent both in terms of its light curve and its spectrum, with predictions from magnetar models requiring either low nebular magnetization or faster spin-down than dipole losses. The CSM shell interaction scenario can reproduce the observed flux level but not the observed timing of the γ-ray signal. In addition, the observed ratio, L_γ/L_opt ∼ 1, is inconsistent with theoretical expectations and not in line with ratio measurements in other interacting CSM-dominated objects (e.g., novae or SNe) where this ratio is less than 10⁻².

Conclusions. Our study strongly suggests that a central engine like a magnetar plays a key role in this SLSN and could explain the bulk of the optical and γ-ray light curves properties. In order to explain the observed late-time bumps in the optical light curve of SN 2017egm, we require either: a hybrid picture combining magnetar and multiple CSM shells for the optical bumps or a pure magnetar model with infalling matter on an accretion disk. Finally, simulations of 50 hours of CTAO observations indicate that a SN 2017egm-like event would be detectable up to ∼140 Mpc in the magnetar model but not in the CSM model due to strong γ − γ absorption.