GRB 180728A and SN 2018fip: The nearest high-energy cosmological gamma-ray burst with an associated supernova

Context. The long gamma-ray burst GRB 180728A at a redshift of z = 0.1171 stands out due to its high isotropic energy of E_γ,iso ≈ 2.5 × 10⁵¹ erg, in contrast with most events at redshift z < 0.2, but it is comparable to the bulk of luminous bursts more common at higher redshift.
Aims. We aim to study the properties of GRB 180728A’s prompt emission, afterglow, and associated supernova (SN 2018fip), comparing them with other GRB-SN events.
Methods. This study employs a dense photometric and spectroscopic follow-up of the afterglow and the SN up to 80 days after the burst. We used image subtraction to remove the presence of a nearby bright star, and modelled both the afterglow and the supernova.
Results. This event lies on the E_p,i–E_γ,iso plane occupied by classical collapsar events, and the prompt emission is one of the most energetic at z < 0.2 after GRB 030329 and GRB 221009A. The afterglow of GRB 180728A is less luminous than that of most long GRBs, showing a shallow early phase that steepens after about 5 hours (0.2 days). The GRB exploded in an irregular low-mass blue star-forming galaxy, which is typical of low-z collapsar events. Because of the relatively faint afterglow, the light curve bump of SN 2018fip dominates the optical emission already after approximately 3 days and is one of the best sampled to date. The strong suppression below ∼4000 Å and a largely featureless continuum in the early 6–9 day spectra favour aspherical two-component ejecta with a high-velocity collimated component (>20 000 km s⁻¹), that is dominant early on and a more massive low-velocity component that dominates at much later epochs.
Conclusions. Our findings indicate that asymmetries need to be considered in order to better understand GRB-SNe. In any case, SN 2018fip shares many characteristics with typical GRB-SNe. Its kinetic energy is below the common range of 10⁵²–10⁵³ erg and does not correlate with the high energy of the GRB, highlighting the complexity and diversity of the GRB-SN energy budget partition.