GROWTH on S190426c II: GROWTH-India Telescope search for an optical counterpart with a custom image reduction and candidate vetting pipeline

TL;DR

This paper describes a systematic optical search for counterparts to a neutron star-black hole merger candidate using the GROWTH-India Telescope, including a new image processing pipeline, resulting in no kilonova detections but providing valuable insights for future searches.

Contribution

Development of a custom image subtraction and candidate vetting pipeline with high efficiency for transient detection in the context of gravitational wave follow-up.

Findings

Multiple transients detected, none match kilonova models.

The pipeline achieved ~94% efficiency in transient detection.

Results inform future follow-up strategies during O4 run.

Abstract

S190426c / GW190426_152155 was the first probable neutron star - black hole merger candidate detected by the LIGO-Virgo Collaboration. We undertook a tiled search for optical counterparts of this event using the 0.7m GROWTH-India Telescope. Over a period of two weeks, we obtained multiple observations over a 22.1 deg^2 area, with a 17.5% probability of containing the source location. Initial efforts included obtaining photometry of sources reported by various groups, and a visual search for sources in all galaxies contained in the region. Subsequently, we have developed an image subtraction and candidate vetting pipeline with ~ 94% efficiency for transient detection. Processing the data with this pipeline, we find several transients, but none that are compatible with kilonova models. We present the details of our observations, working of our pipeline, results from the search, and our interpretations of the non-detections that will work as a pathfinder during the O4 run of LVK.