Upper panel: a section of a typical M 31 difference image from the iPTF pipeline containing the bulge. Lower panel: outer part of the same difference image, approximately half a degree northward of the center of M 31. The masked areas are assigned large negative values. North is upward and east is left. The center of M 31 is marked by the red cross in the upper panel. The vertical sharp edges seen in both panels arise from the boundary of blocks that have been used to match the background before the image subtraction in the iPTF DI pipeline.