SuperBoRG: Search for The Brightest of Reionizing Galaxies and Quasars
  in HST Parallel Imaging Data
  The Hubble Space Telescope (HST) has been providing tremendous survey
efficiency via its pure-parallel mode, by observing another field in parallel
with the primary instrument in operation for the primary observation. In this
study, we present a new archival project, SuperBoRG, which aims at compiling
data taken in extragalactic parallel programs of HST with WFC3 in the past
decade; including pure-parallel (BoRG, HIPPIES, and COS-GTO) and
coordinated-parallel (CLASH and RELICS) programs. The total effective area
reaches $\sim0.41$deg$^2$ from 4.1Msec, or 47days, of observing time, which is
the largest collection of optical-NIR imaging data of HST for extragalactic
science. We reduce all data in a consistent manner with an updated version of
our data reduction pipeline. When available, infrared imaging data from the
Spitzer Space Telescope are included in photometric analyses. The dataset
consists of 316 independent sightlines and is highly effective for
identification of high-$z$ luminous sources ($M_\mathrm{UV}<-21$mag) at
$z\sim7$ to $12$, helping to minimize the effects of cosmic variance. As a
demonstration, we present three new $z>7$ source candidates, including one
luminous galaxy candidate at $z_\mathrm{phot}\sim10.4$ with
$M_\mathrm{UV}\sim-21.9$ mag; for this object the best-fit spectral energy
distribution implies a large amount of stellar mass ($\log M_*/M_\odot \sim
10$) and moderate dust attenuation ($A_V \sim 1.4$mag), though the possibility
of it being a low-$z$ interloper cannot completely be rejected ($\sim23\%$)
with the current dataset. The dataset presented in this study is also suited
for intermediate and low-$z$ science cases.
