  Detections of coalescing binary black holes by LIGO have opened a new window
of transient astronomy. With increasing sensitivity of LIGO and participation
of the Virgo detector in Cascina, Italy, we expect to soon detect coalescence
of compact binary systems with one or more neutron stars. These are the prime
targets for electromagnetic follow-up of gravitational wave triggers, which
holds enormous promise of rich science. However, hunting for electromagnetic
counterparts of gravitational wave events is a non-trivial task due to the
sheer size of the error regions, which could span hundreds of square degrees.
The Zwicky Transient facility (ZTF), scheduled to begin operation in 2017, is
designed to cover such large sky-localization areas. In this work, we present
the strategies of efficiently tiling the sky to facilitate the observation of
the gravitational wave error regions using ZTF. To do this we used simulations
consisting of 475 binary neutron star coalescences detected using a mix of two-
and three-detector networks. Our studies reveal that, using two overlapping
sets of ZTF tiles and a (modified) ranked-tiling algorithm, we can cover the
gravitational-wave sky-localization regions with half as many pointings as a
simple contour-covering algorithm. We then incorporated the ranked-tiling
strategy to study our ability to observe the counterparts. This requires
optimization of observation depth and localization area coverage. Our results
show that observation in r-band with ~600 seconds of integration time per
pointing seems to be optimum for typical assumed brightnesses of
electromagnetic counterparts, if we plan to spend equal amount of time per
pointing. However, our results also reveal that we can gain by as much as 50%
in detection efficiency if we linearly scale our integration time per pointing
based on the tile probability.

Instrumentation and Methods for Astrophysics