Top: averaged window for 10 days for a virtual source near the Crab. Crab steps are at bins ~47and ~72. Middle: filtered data with the central portion fit to a polynomial (green), and the outer background portion fit to a spline function (red). Large dips from Crab steps result in negative amplitude for the virtual source. Bottom: filtered data refit with central portion only ± f_a instead of the normal ± (2f_a + f_b).