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

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 deg2 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 time-scales teff < 10 d 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, modelled in a companion study. The other four have very short durations (teff < 0.1 d) typical of an Earth-mass FFP population. Whilst Kepler was not designed for crowded-field photometry, the K2C9 data set clearly demonstrates the feasibility of conducting blind space-based microlensing surveys towards the Galactic bulge.

Kepler K2Campaign 9 – II. First space-based discovery of an exoplanet using microlensing

“…Despite this intensive ground-based campaign, none of the groundbased surveys flagged K2-2016-BLG-0005 in advance of the blind K2C9 data search by McDonald et al (2021). Only after this study presented this event did the survey teams extract their data for this event.…”

“…Calibrated K2C9 image data were obtained from the Mikulski Archive for Space Telescopes 2 . A detailed description of the photometric reduction and candidate selection that led to the discovery of this event is given in McDonald et al (2021).…”

“…Space-based microlensing surveys would also have sub-Earth mass sensitivity to planetary-mass objects that are unbound from any host, objects often referred to as free-floating planets (Johnson et al 2020). McDonald et al (2021) recently used data from K2 Campaign 9 of the Kepler mission (hereafter K2C9, Henderson et al 2016) to conduct a blind search for short timescale microlensing signals. The search revealed four new ultra-short candidate events consistent with free-floating planets of around Earth mass.…”

Kepler K2 Campaign 9: II. First space-based discovery of an exoplanet using microlensing

“…The selected sub-region of the Kepler field, known as the "superstamp" (Figure 2), was predicted to exhibit a large number of microlensing events (e.g., Ban et al 2016). The data were blindly searched by McDonald et al (2021) for short-timescale microlensing events, indicative of free-floating planets. They found five new candidate microlensing events, together with 22 additional events that had been previously catalogued by ground-based survey teams.…”

“…Space-based microlensing surveys are also have sub-Earth mass sensitivity to planetary-mass objects that are unbound from any host, objects often referred to as free-floating planets (Johnson et al 2020). McDonald et al (2021) recently used data from K2 Campaign 9 of the Kepler mission (hereafter K2C9, Henderson et al 2016) to conduct a blind search for short timescale microlensing signals. The search revealed four new ultra-short candidate events consistent with free-floating planets of around Earth mass.…”