  We determine the bright end of the rest-frame UV luminosity function (UVLF)
at $z=8-10$ by selecting bright $z\gtrsim 8$ photometric candidates from the
largest systematic compilation of HST (pure-)parallel observations to date, the
Super-Brightest-of-Reionizing-Galaxies (SuperBoRG) data set. The data set
includes $\sim300$ independent sightlines from WFC3 observations, totalling
$800-1300$ arcmin$^2$ (depending on redshift). We identify 31 $z\gtrsim8$
candidates via colour selection and photo-$z$ analysis with observed magnitude
($24.1< H_{160} <26.6$) and $1\sigma$ range probability of $58-95\%$ of being
true high-z galaxies. Following detailed completeness and source recovery
simulations, as well as modelling of interloper contamination, we derive
rest-frame UVLFs at $z=8-10$ down to $M_{UV}\simeq-23$. We find that the bright
end of the galaxy luminosity function can be described both by a Schechter and
by a double power-law function, with our space-based large area determination
showing some tentative discrepancies with the luminosity functions derived from
ground-based observations at the same redshifts. We identify two possible
explanations. The first is that the UVLF includes a substantial contribution
from AGNs at magnitudes $M_{UV}<-22$. The UVLF we derive at $z=8$ is consistent
with no evolution of the bright end from $z=6-7$ UVLFs that include AGNs. An
alternative (and non-exclusive) possibility is that the dust content of the
observed galaxies decreases with redshift, thereby countering the naturally
expected decrease in the UV luminosity functions because of a decreased star
formation rate with increasing redshift. Both scenarios raise interesting
prospects to further understand galaxy formation in extreme objects during the
epoch of reionisation.
