Photometry of K2 Campaign 9 bulge data

¹ Department of Astronomy, Ohio State University, 140 W. 18th Ave., Columbus, OH 43210, USA
e-mail: poleski.1@osu.edu
² Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
e-mail: rpoleski@astrouw.edu.pl
³ Astrophysics Science Division, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁴ NASA Ames Research Center, Moffett Blvd, Mountain View, CA 94035, USA
⁵ Bay Area Environmental Research Institute, 625 2nd St., Ste. 209, Petaluma, CA 94952, USA
⁶ Korea Astronomy and Space Science Institute, 776 Daedukdae-ro Yuseong-gu, Daejeon 34055, Korea
⁷ Max-Planck-Institute for Astronomy, Köigstuhl 17, 69117 Heidelberg, Germany

Received: 31 October 2018
Accepted: 13 May 2019

Abstract

In its Campaign 9, K2 observed dense regions toward the Galactic bulge in order to constrain the microlensing parallaxes and probe for free-floating planets. Photometric reduction of the K2 bulge data poses a significant challenge due to a combination of the very high stellar density, large pixels of the Kepler camera, and the pointing drift of the spacecraft. Here we present a new method to extract K2 photometry in dense stellar regions. We extended the Causal Pixel Model developed for less-crowded fields, first by using the pixel response function together with accurate astrometric grids, second by combining signals from a few pixels, and third by simultaneously fitting for an astrophysical model. We tested the method on two microlensing events and a long-period eclipsing binary. The extracted K2 photometry is an order of magnitude more precise than the photometry from other method.