SN 2016gsd: An unusually luminous and linear type II supernova with high
  velocities
  We present observations of the unusually luminous Type II supernova (SN)
2016gsd. With a peak absolute magnitude of V = $-$19.95 $\pm$ 0.08, this object
is one of the brightest Type II SNe, and lies in the gap of magnitudes between
the majority of Type II SNe and the superluminous SNe. Its light curve shows
little evidence of the expected drop from the optically thick phase to the
radioactively powered tail. The velocities derived from the absorption in
H$\alpha$ are also unusually high with the blue edge tracing the fastest moving
gas initially at 20000 km s$^{-1}$, and then declining approximately linearly
to 15000 km s$^{-1}$ over $\sim$100 d. The dwarf host galaxy of the SN
indicates a low-metallicity progenitor which may also contribute to the
weakness of the metal lines in its spectra. We examine SN 2016gsd with
reference to similarly luminous, linear Type II SNe such as SNe 1979C and
1998S, and discuss the interpretation of its observational characteristics. We
compare the observations with a model produced by the JEKYLL code and find that
a massive star with a depleted and inflated hydrogen envelope struggles to
reproduce the high luminosity and extreme linearity of SN 2016gsd. Instead, we
suggest that the influence of interaction between the SN ejecta and
circumstellar material can explain the majority of the observed properties of
the SN. The high velocities and strong H$\alpha$ absorption present throughout
the evolution of the SN may imply a circumstellar medium configured in an
asymmetric geometry.
