Early Warning for Lensed Gravitational Wave Counterparts from Time Delays of Their Host Galaxies Observed in the Optical

Abstract Gravitational lensing of gravitational waves can be leveraged to provide early-warning times of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">O</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">hours</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">O</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">days</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> before the merger of binary neutron stars (BNSs) and neutron star black holes (NSBHs). This in turn could enable electromagnetic (EM) telescopes to capture emissions surrounding the time of the merger. In this work, we assess the practicability of lensing-driven early warning by analysing optical images of the lensed host galaxy to predict the arrival time of subsequent BNS/NSBH signals following the observation of the first signal. We produce mock lenses with image quality and resolution similar to images taken with the Hubble Space Telescope (HST) and the ground-based Hyper Suprime-Cam (HSC) on the Subaru telescope. We compare the time delay uncertainties between these two cases for typical lensed image configurations and multiplicity. These include doubles and quads and, among quads, the fold, cusp, and cross-image configurations. We find that time delay uncertainties for doubles are comparable for both HST and HSC mocks. On the other hand, quads tend to provide accurate time-delay predictions (typical relative error ∼0.1) with HST. Analysis of a real lens led to a difference in time-delay estimates of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">O</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">days</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> between the predictions derived from HST and HSC data. Our work therefore strongly advocates the need for high-resolution EM observations of lensed host galaxies to feasibly enable lensing-driven early-warning.