# RATIR Followup of LIGO/Virgo Gravitational Wave Events

## TL;DR

This paper discusses the development and testing of a followup observation strategy and image-processing pipeline for gravitational wave events, demonstrating effective false alarm reduction and robustness in optical/NIR transient detection.

## Contribution

It introduces a robust, multi-band image subtraction pipeline for RATIR that enhances gravitational wave followup capabilities and reduces false alarms.

## Key findings

- Simultaneous detection in two bands reduces false alarms by a factor of 10.
- The pipeline maintains low false alarm rates under varying observing conditions.
- Expanding to three or more bands achieves about 1% false alarm rate with 90% efficiency.

## Abstract

Recently we have witnessed the first multi-messenger detection of colliding neutron stars through Gravitational Waves (GWs) and Electromagnetic (EM) waves (GW170817), thanks to the joint efforts of LIGO/Virgo and Space/Ground-based telescopes. In this paper, we report on the RATIR followup observation strategies and show the results for the trigger G194575. This trigger is not of astrophysical interest; however, is of great interests to the robust design of a followup engine to explore large sky error regions. We discuss the development of an image-subtraction pipeline for the 6-color, optical/NIR imaging camera RATIR. Considering a two band ($i$ and $r$) campaign in the Fall of 2015, we find that the requirement of simultaneous detection in both bands leads to a factor $\sim$10 reduction in false alarm rate, which can be further reduced using additional bands. We also show that the performance of our proposed algorithm is robust to fluctuating observing conditions, maintaining a low false alarm rate with a modest decrease in system efficiency that can be overcome utilizing repeat visits. Expanding our pipeline to search for either optical or NIR detections (3 or more bands), considering separately the optical $riZ$ and NIR $YJH$ bands, should result in a false alarm rate $\approx 1\%$ and an efficiency $\approx 90\%$. RATIR's simultaneous optical/NIR observations are expected to yield about one candidate transient in the vast 100 $\mathrm{deg^2}$ LIGO error region for prioritized followup with larger aperture telescopes.