SuperBoRG: Exploration of point sources at $z\sim8$ in HST parallel
  fields
  To extend the search for quasars in the epoch of reionization beyond the tip
of the luminosity function, we explore point source candidates at redshift
$z\sim8$ in SuperBoRG, a compilation of $\sim$0.4deg$^2$ archival medium-deep
($m_{\rm F160W}\sim 26.5$ABmag, 5$\sigma$) parallel IR images taken with the
Hubble Space Telescope (HST). Initial candidates are selected by using the
Lyman-break technique. We then carefully analyze source morphology, and
robustly identify 3 point sources at $z\sim8$. Photometric redshift analysis
reveals that they are preferentially fit by extra-galactic templates, and we
conclude that they are unlikely to be low-$z$ interlopers, including brown
dwarfs. A clear IRAC ch2 flux excess is seen in one of the point sources, which
is expected if the source has strong H$\beta$+[O III] emission with rest-frame
equivalent width of $\sim3000$AA. Deep spectroscopic data taken with
Keck/MOSFIRE, however, do not reveal Ly$\alpha$ emission from the object. In
combination with the estimated H$\beta$+[O III] equivalent width, we place an
upper limit on its Ly$\alpha$ escape fraction $f_{\rm esc, Ly\alpha}< 2 \%$. We
estimate the number density of these point sources
$\sim1\times10^{-6}$Mpc$^{-3}$mag$^{-1}$ at $M_{\rm UV}\sim-23$mag. The final
interpretation of our results remains inconclusive: extrapolation from low-$z$
studies of $faint$ quasars suggests that $>100\times$ survey volume may be
required to find one of this luminosity. The James Webb Space Telescope will be
able to conclusively determine the nature of our luminous point source
candidates, while the Roman Space Telescope will probe $\sim 200$ times the
area of the sky with the same observing time considered in this HST study.
