Highly luminous supernovae associated with gamma-ray bursts

II. The luminous blue bump in the afterglow of GRB 140506A

¹ Hessian Research Cluster ELEMENTS, Giersch Science Center, Max-von-Laue-Stra βe 12, Goethe University Frankfurt, Campus Riedberg, 60438 Frankfurt am Main, Germany
² Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, 40129 Bologna, Italy
e-mail: andrea.rossi@inaf.it
⁴ Department of Physics, Lancaster University, Lancs LA1 4YB, UK
⁵ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁶ Centro Astronómico Hispano en Andalucía, Observatorio de Calar Alto, Sierra de los Filabres, 04550 Gérgal, Almería, Spain
⁷ Artemis, Université de la Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, 06304 Nice, France
⁸ Aix Marseille Univ, CNRS, LAM Marseille, France
⁹ Astronomical Institute, Czech Academy of Sciences, Fričova 298, Ondřejov, Czech Republic
¹⁰ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, 1800 Sherman Ave., Evanston, IL 60201, USA

Received: 30 September 2021
Accepted: 30 January 2024

Abstract

Context. The supernovae (SNe) associated with gamma-ray bursts (GRBs) are generally seen as a homogeneous population, but at least one exception exists: the highly luminous SN 2011kl associated with the ultra-long GRB 111209A. Such outliers may also exist for more typical GRBs.

Aims. Within the context of a systematic analysis of photometric signatures of GRB-associated SNe, we found an anomalous bump in the late-time transient following GRB 140506A at redshift z = 0.889. We hereby aim to show this bump is significantly more luminous and blue than usual SNe following GRBs.

Methods. We compiled all available data from the literature and added a full analysis of the Swift/UVOT data, which allowed us to trace the light curve from the first minutes all the way to the host galaxy and to construct a broad spectral energy distribution (SED) of the afterglow that extends the previous SED analysis based on ground-based spectroscopy.

Results. We find robust evidence of a late-time bump following the afterglow that shows evidence of a strong color change, with the spectral slope becoming flatter in the blue region of the spectrum. This bump can be interpreted as a luminous SN bump that is spectrally dissimilar to typical GRB-SNe. Correcting it for the large line-of-sight extinction makes the SN associated with GRB 140506A the most luminous detected so far. Even so, it would be in agreement with a luminosity-duration relation of GRB-SNe.

Conclusions. While not supported by spectroscopic evidence, it is likely the bump following GRB 140506A is the signature of an SN that is spectrally dissimilar to classical GRB-SNe and more similar to SN 2011kl – while being associated with an average GRB, indicating the GRB-SN population is more diverse than previously thought and can reach luminosities comparable to those of superluminous SNe.