The unique microlensing event Kojima-1: Keck’s aperture masking observations

We present observations and analysis of the nearby microlensing event in the anti-bulge direction: Kojima-1 (TCP J05074264+2447555). Kojima-1Lb is the first planet discovered in a microlensing event found by an amateur astronomer (i.e., Tadashi Kojima), and it has the brightest host star of any planetary microlensing event. Kojima-1Lb will soon be a good target for host star spectra, and is a typical microlensing planet with the mass ratio of q ~ 1.e-4, which is close to the peak of the exoplanet distribution found by microlensing. This is the first planetary event with the Einstein radius and the relative proper motion measured by interferometry, and the first using the VLTI/GRAVITY instrument.

As the field density is much lower in the anti-bulge direction, aperture masking is the optimal tool to observe this event. We observed this event using the aperture masking technique with the near-infrared instrument NIRC2 at Keck, almost exactly two years after the event. Our results in terms of separation, PA, K-band magnitude difference, and relative proper motion are similar with previous findings, and our results confirm the Gravity ones — even though the Gravity data were affected by a fringe-tracking issue (Dong et al. 2019).

Future calculations will include inferred details of the planetary lens system, and further comparison with previous observations. A growing number of microlensing events in less crowded fields is being found through observation e.g., with the ZTF (Zwicky Transient Factory, e.g., Rodriguez et al. 2021) or Gaia; and in the future the Vera C. Rubin Observatory will find even more.