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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, theirγ-ray signatures remain poorly constrained. Aims.To further constrain the underlying mechanism, we carried out a systematic search for giga-electronvoltγ-ray emission using theFermiLarge 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 putativeγ-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 significantγ-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 theγ-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 andγ-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 strongγ − γabsorption. 

ABSTRACT A deep survey of the Large Magellanic Cloud at ∼0.1–100 TeV photon energies with the Cherenkov Telescope Array is planned. We assess the detection prospects based on a model for the emission of the galaxy, comprising the four known TeV emitters, mock populations of sources, and interstellar emission on galactic scales. We also assess the detectability of 30 Doradus and SN 1987A, and the constraints that can be derived on the nature of dark matter. The survey will allow for fine spectral studies of N 157B, N 132D, LMC P3, and 30 Doradus C, and half a dozen other sources should be revealed, mainly pulsar-powered objects. The remnant from SN 1987A could be detected if it produces cosmic-ray nuclei with a flat power-law spectrum at high energies, or with a steeper index 2.3–2.4 pending a flux increase by a factor of >3–4 over ∼2015–2035. Large-scale interstellar emission remains mostly out of reach of the survey if its >10 GeV spectrum has a soft photon index ∼2.7, but degree-scale 0.1–10 TeV pion-decay emission could be detected if the cosmic-ray spectrum hardens above >100 GeV. The 30 Doradus star-forming region is detectable if acceleration efficiency is on the order of 1−10 per cent of the mechanical luminosity and diffusion is suppressed by two orders of magnitude within <100 pc. Finally, the survey could probe the canonical velocity-averaged cross-section for self-annihilation of weakly interacting massive particles for cuspy Navarro–Frenk–White profiles.