Double-slit diffraction pattern

Click the image to view the source page (DESY in Hamburg, Germany).

”The diffraction pattern of a double slit arrangement illustrates the spatial coherence of the photon beam of a free-electron laser.“

Is this indeed what we are seeing (spatial coherence of the laser beam), or is it evidence of the p-wave spacetime structure?
Also, what is causing the three symmetric horizontal regions of lower intensity that are clearly visible in the diffraction pattern?

$double-slit diffraction pattern$