  In this work we continue a line of inquiry begun in Kanner et al. which
detailed a strategy for utilizing telescopes with narrow fields of view, such
as the Swift X-ray Telescope (XRT), to localize gravity wave (GW) triggers from
LIGO/Virgo. If one considers the brightest galaxies that produce ~50% of the
light, then the number of galaxies inside typical GW error boxes will be
several tens. We have found that this result applies both in the early years of
Advanced LIGO when the range is small and the error boxes large, and in the
later years when the error boxes will be small and the range large. This
strategy has the beneficial property of reducing the number of telescope
pointings by a factor 10 to 100 compared with tiling the entire error box.
Additional galaxy count reduction will come from a GW rapid distance estimate
which will restrict the radial slice in search volume. Combining the bright
galaxy strategy with a convolution based on anticipated GW localizations, we
find that the searches can be restricted to about 18+/-5 galaxies for 2015,
about 23+/-4 for 2017, and about 11+/-2 for 2020. This assumes a distance
localization at or near the putative NS-NS merger range for each target year,
and these totals are integrated out to the range. Integrating out to the
horizon would roughly double the totals. For nearer localizations the totals
would decrease. The galaxy strategy we present in this work will enable
numerous sensitive optical and X-ray telescopes with small fields of view to
participate meaningfully in searches wherein the prospects for rapidly fading
afterglow place a premium on a fast response time.
