iPTF Search for an Optical Counterpart to Gravitational Wave Trigger GW150914

TL;DR

This paper describes the rapid optical follow-up observations by iPTF of the GW150914 gravitational wave event, including candidate identification, classification, and multi-wavelength follow-up, demonstrating the effectiveness of such campaigns for future gravitational wave detections.

Contribution

First comprehensive optical follow-up campaign of a gravitational wave event using iPTF, showcasing rapid response, candidate filtering, and multi-wavelength observations.

Findings

No optical counterpart associated with GW150914 was found.

Eight transient candidates were spectroscopically classified, including a superluminous supernova.

The campaign demonstrates readiness for future gravitational wave follow-up efforts.

Abstract

The intermediate Palomar Transient Factory (iPTF) autonomously responded to and promptly tiled the error region of the first gravitational wave event GW150914 to search for an optical counterpart. Only a small fraction of the total localized region was immediately visible in the Northern night sky, due both to sun-angle and elevation constraints. Here, we report on the transient candidates identified and rapid follow-up undertaken to determine the nature of each candidate. Even in the small area imaged of 126 sq deg, after extensive filtering, 8 candidates were deemed worthy of additional follow-up. Within two hours, all 8 were spectroscopically classified by the Keck II telescope. Curiously, even though such events are rare, one of our candidates was a superluminous supernova. We obtained radio data with the Jansky Very Large Array and X-ray follow-up with the Swift satellite for this transient. None of our candidates appear to be associated with the gravitational wave trigger, which is unsurprising given that GW150914 came from the merger of two stellar-mass black holes. This end-to-end discovery and follow-up campaign bodes well for future searches in this post-detection era of gravitational waves.