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

Context. Superluminous supernovae (SLSNe) are a rare class of transients with peak luminosities 10–100 times greater than those of standard corecollapse 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 gamma-ray signatures remain poorly constrained. Aims. To further constrain the underlying mechanism, we carried out a systematic search for giga-electronvolt gamma-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 gamma-ray signal at the optical position of the SLSN on a sixmonth 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 gamma-ray emission, with likelihood test statistic (TS) values of 26–33 (i.e., >5 sigma) 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 gamma = 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 gamma-ray signal. In addition, the observed ratio, Lgamma / Lopt = 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 10e-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 gamma-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 approx 140 Mpc in the magnetar model but not in the CSM model due to strong gamma-gamma absorption.