Tiling strategies for optical follow-up of gravitational-wave triggers by telescopes with a wide field of view

¹ Department of Astrophysics, IMAPP, Radboud University, 6500 GL, Nijmegen, The Netherlands
e-mail: shaon@astro.ru.nl
² LIGO Laboratory, California Institute of Technology, Pasadena, CA 91125, USA
³ Nikhef − National Institute for Subatomic Physics, Science Park 105, 1098 XG Amsterdam, The Netherlands
⁴ Institute of Astronomy, KU Leuven, Belgium

Received: 7 November 2015
Accepted: 23 May 2016

Abstract

Aims. Binary neutron stars are among the most promising candidates for joint gravitational-wave and electromagnetic astronomy. The goal of this work is to investigate various observing strategies that telescopes with wide field of view might incorporate while searching for electromagnetic counterparts of gravitational-wave triggers.

Methods. We examined various strategies of scanning the gravitational-wave sky localizations on the mock 2015−16 gravitational-wave events. First, we studied the performance of the sky coverage using a naive tiling system that completely covers a given confidence interval contour using a fixed grid. Then we propose the ranked-tiling strategy where we sample the localization in discrete two-dimensional intervals that are equivalent to the telescope’s field of view and rank them based on their sample localizations. We then introduce an optimization of the grid by iterative sliding of the tiles. Next, we conducted tests for all the methods on a large sample of sky localizations that are expected in the first two years of operation of the Laser interferometer Gravitational-wave Observatory (LIGO) and Virgo detectors. We investigated the performance of the ranked-tiling strategy for telescope arrays and compared their performance against monolithic telescopes with a giant field of view. Finally, we studied the ability of optical counterpart detection by various types of telescopes.

Results. Our analysis reveals that the ranked-tiling strategy improves the localization coverage over the contour-covering method. The improvement is more significant for telescopes with larger fields of view. We also find that while optimizing the position of the tiles significantly improves the coverage compared to contour-covering tiles. For ranked-tiles the same procedure leads to negligible improvement in the coverage of the sky localizations. We observed that distributing the field of view of the telescopes into arrays of multiple telescopes significantly improves the coverage efficiency, by as much as 50% over a single telescope with a large field of view in 2016 localizations while scanning ~100 deg². Finally, through analyzing a range telescopes with wide field of view, we discovered that counterpart detection can be improved by sacrificing coverage of localization in order to achieve a greater observation depth for telescopes with very large field of view and small aperture, especially if the intrinsic brightness of the optical counterparts is weak.