partitions the infinite plane into convex polygon cells according to a list of starting locations (named Voronoi sites) in the plane. Each cell contains only one Voronoi site and includes all points that are closer to this site than to any other. The positions of all occupied pixels are assumed as Voronoi sites, and the local photon density value associated to each occupied pixel is computed as the number of photons in the occupied pixel divided by the Voronoi cell area. In this way, the raw image is converted into the local photon density map, in which high-density regions show up by a direct comparison between the photon density map and a background map. Then, the FOF method is used to identify source candidates.

The VT+FOF method has two immediate advantages. First, it is applied to the X-ray photon event list or to the original unbinned image, and it preserves the full angular resolution at each step, at variance with other methods, where image-binning is often adopted to avoid oversampling of the PSF and improve the low count statistics. Second, the VT+FOF algorithm does not require assumptions about the shape and size of the structure one is looking for. In 