A magnetar powers the luminous supernova 2023pel, which is associated with a long gamma-ray burst

Aims. We explore supernova (SN) 2023pel, the most recent event associated with gamma-ray bursts (GRBs), specifically GRB 230812B. SN 2023pel has a high luminosity (∼1.5 × 1043 erg s−1 at the peak) and low expansion velocities (v ∼ 16 000 km s−1 at the peak) compared to other GRB-SNe. These properties seem difficult to reconcile with a single nickel power source. We searched for models that can explain the properties of this event. Methods. We calculated a grid of hydrodynamic models based on pre-SN structures derived from evolutionary calculations. We compared our models with observations of SN 2023pel and selected our preferred model using statistical analysis, taking both light curves and expansion velocities into account. This allowed us to derive a set of physical properties for SN 2023pel. Results. Our models suggest that the most probable scenario involves a millisecond magnetar as the primary power source, supplemented by energy from radioactive decay. Our preferred model has a spin period of P = 3.2 ms, a magnetic field of B = 28 × 1014 G, an explosion energy of 2.3 foe, a nickel mass of MNi = 0.24 M⊙, and an ejected mass of 3.4 M⊙. Alternatively, we find that a purely nickel-powered model also provides a good match with the observations, though MNi ≥ 0.8 M⊙ are always required. However, the combination of such high values of MNi and low Mej is difficult to reconcile, indicating that this scenario is less probable. We have also identified a specific region within the peak luminosity-velocity plane where an additional energy source beyond nickel may be necessary to power SNe with characteristics similar to SN 2023pel. Conclusions. Our study indicates that an additional energy source beyond radioactive decay is essential to explain the high brightness and relatively low expansion velocities of SN 2023pel. A magnetar-powered model, similar to the models proposed for the very luminous GRB-SN 2011kl, aligns well with these characteristics.