Kepler K2 Campaign 9: I. Candidate short-duration events from the first space-based survey for planetary microlensing

TL;DR

This paper reports the first detection of short-duration microlensing events from the Kepler K2 mission, demonstrating the potential of space-based surveys for planetary microlensing despite observational challenges.

Contribution

It introduces a new pipeline for identifying microlensing events in K2 data and reports the discovery of five new candidate events, including potential free-floating planets.

Findings

Recovered 22 known short-duration events from ground-based surveys.

Discovered 5 new candidate microlensing events, including one binary event.

Demonstrated feasibility of space-based microlensing surveys with Kepler K2.

Abstract

We present the first short-duration candidate microlensing events from the Kepler K2 mission. From late April to early July 2016, Campaign 9 of K2 obtained high temporal cadence observations over a 3.7 square degree region of the Galactic bulge. Its primary objectives were to look for evidence of a free-floating planet (FFP) population using microlensing, and demonstrate the feasibility of space-based planetary microlensing surveys. Though Kepler K2 is far from optimal for microlensing, the recently developed MCPM photometric pipeline enables us to identify and model microlensing events. We describe our blind event-selection pipeline in detail and use it to recover 22 short-duration events with effective timescales of less than 10 days previously announced by the OGLE and KMTNet ground-based surveys. We also announce five new candidate events. One of these is a caustic-crossing binary event, consistent with a bound planet and modelled as such in a companion study. The other four have very short durations (effective timescales less than 0.1 days) typical of an Earth-mass FFP population. Whilst Kepler was not designed for crowded-field photometry, the K2C9 dataset clearly demonstrates the feasibility of conducting blind space-based microlensing surveys towards the Galactic bulge.