  Rapid identification of the optical counterparts of Neutron Star (NS) merger
events discovered by gravitational wave detectors may require observing a large
error region and sifting through a large number of transients to identify the
object of interest. Given the expense of spectroscopic observations, a question
arises: How can we utilize photometric observations for candidate
prioritization, and what kinds of photometric observations are needed to
achieve this goal? NS merger kilonova exhibits low ejecta mass (~5x10^-2 solar
mass) and a rapidly evolving photospheric radius (with a velocity ~0.2c). As a
consequence, these sources display rapid optical-flux evolution. Indeed,
selection based on fast flux variations is commonly used for young supernovae
and NS mergers. In this study, we leverage the best currently available
flux-limited transient survey - the Zwicky Transient Facility Bright Transient
Survey - to extend and quantify this approach. We focus on selecting transients
detected in a 3-day cadence survey and observed at a one-day cadence. We
explore their distribution in the phase space defined by g-r, g-dot, and r-dot.
Our analysis demonstrates that for a significant portion of the time during the
first week, the kilonova AT 2017gfo stands out in this phase space. It is
important to note that this investigation is subject to various biases and
challenges; nevertheless, it suggests that certain photometric observations can
be leveraged to identify transients with the highest probability of being
fast-evolving events. We also find that a large fraction (~0.75) of the
transient candidates with |g-dot|>0.7 mag/day, are cataclysmic variables or
active galactic nuclei with radio counterparts.
