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

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, 40129 Bologna, Italy
² INAF, Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Naples, Italy
³ DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark
⁴ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
⁵ Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK
⁶ Cosmic Dawn Center (DAWN)
⁷ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
⁸ Department of Astrophysics/IMAPP, Radboud University, 6525 AJ, Nijmegen, The Netherlands
⁹ Hessian Research Cluster ELEMENTS, Giersch Science Center, Max-von-Laue-Stra βe 12, Goethe University Frankfurt, Campus Riedberg, D-60438 Frankfurt am Main, Germany
¹⁰ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate (LC), Italy
¹³ Artemis, Université de la Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, 06304 Nice, France
¹⁴ Aix Marseille Univ, CNRS, LAM, Marseille, France
¹⁵ Department of Physics, Royal Holloway – University of London, Egham TW20 0EX, U.K.
¹⁶ Lomonosov Moscow State University, 119234 Moscow, Universitetskiy prospekt, 13, Moscow, Russia
¹⁷ School of Physics and Centre for Space Research, University College Dublin, Belfield D04 V1W8 Dublin, Ireland
¹⁸ INAF, Osservatorio Astronomico di Roma, Via Frascati 33, I-00078 Monte Porzio Catone (Roma), Italy
¹⁹ Department of Physics, Lancaster University, Lancs LA1 4YB, UK
²⁰ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, 1800 Sherman Avenue, Evanston, IL 60201, USA
²¹ School of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
²² Space Science Data Center (SSDC) – Agenzia Spaziale Italiana (ASI), Via del Politecnico snc, I-00133 Roma, Italy
²³ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
²⁴ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
²⁵ The Oskar Klein Centre, Department of Astronomy, AlbaNova SE-106 91 Stockholm, Sweden
²⁶ School of Mathematical and Physical Sciences, Macquarie University, NSW 2109, Australia

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 13 October 2025
Accepted: 16 January 2026

Abstract

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.