2019AJ....157...42S

We consider the problem of inferring the shape of a transiting object's silhouette from its light curve alone, without assuming a physical model for the object. We model the object as a grid of pixels which transits a star; each pixel has an opacity, ranging from transparent to opaque, which we infer from the light curve. We explore three interesting degeneracies inherent to this problem, in which markedly different transiting shapes can produce identical light curves: (i) the "flip" degeneracy, by which two pixels transiting at the same impact parameter on opposite sides of the star's horizontal midplane generate the same light curve; (ii) the "arc" degeneracy, by which opacity can be redistributed along the semicircular arc of pixels which undergoes ingress or egress at the same time without consequence to the light curve; and (iii) the "stretch" degeneracy, by which a wide shape moving fast can produce the same light curve as a narrow shape moving more slowly. By understanding these degeneracies and adopting some additional assumptions, we are able to numerically recover informative shadow images of transiting objects, and we explore a number of different algorithmic approaches to this problem. We apply our methods to real data, including the TRAPPIST-1c/e/f triple transit and two dips of Boyajian's Star. We provide Python code to calculate the transit light curve of any grid and, conversely, infer the image grid which generates any light curve in the software package accompanying this paper, EightBitTransit (https://github.com/esandford/EightBitTransit).