  The supernovae (SNe) associated with gamma-ray bursts (GRBs) are generally
seen as a homogenous population, but at least one exception exists, both in
terms of luminosity as well as Spectral Energy Distribution (SED). However,
this event, SN 2011kl, was associated with an ultra-long GRB 111209A. Do such
outliers also exist for more typical GRBs? 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. We hereby aim to show
this bump is significantly more luminous and blue than usual SNe following
GRBs. We compile all available data from the literature, and add a full
analysis of the Swift UVOT data, which allows us to trace the light curve from
the first minutes all the way to the host galaxy, as well as construct a broad
SED of the afterglow that extends the previous SED analysis based on
ground-based spectroscopy. We find robust evidence for a late-time bump/plateau
following the afterglow which shows evidence for a strong colour change, with
the spectral slope becoming flatter in the blue region of the spectrum. This
bump can be interpreted as a luminous SN bump which is spectrally dissimilar to
typical GRB-SNe. Correcting it for the large line-of-sight extinction results
in extreme values which make 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. While not supported by spectroscopic
evidence, it is likely the blue bump following GRB 140506A is the signature of
a SN which 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 thought so far, and can reach luminosities
comparable to those of superluminous SNe.
