Searching for Binary Asteroids in Pan-STARRS1 Archival Images

Abstract

We developed two different point-spread function (PSF) analysis techniques for discovering wide separation binary asteroids in wide-field surveys. We then applied these techniques to images of main belt asteroids in the 4–60 km size range captured by Pan-STARRS1. Johnston lists fewer than 10 known binaries in this size range with separations greater than 10% of the primary’s Hill radius, so discovering more wide binary asteroids is crucial for understanding the limits of binary stability and improving our knowledge of asteroid masses. We analyzed each image by (i) comparing the major axis orientation of the asteroid’s elliptical PSF to its nonsidereal rate on the sky and (ii) comparing the one-dimensional median profile created by collapsing the image along the asteroid’s direction of motion to that of nearby field stars. For both methods, we flagged any results that deviated significantly from the expected measurements of single asteroids, and those targets with the most flags were identified as binary candidates for confirmation with high-acuity imaging.