Require sky maps to be non-negative in overlap integral

ligo/raven/search.py

@@ -259,6 +259,10 @@ def skymap_overlap_integral(se_skymap, exttrig_skymap=[],
     se_order = 'nested' if se_nested or any(se_skymap_uniq) else 'ring'
     ext_order = 'nested' if ext_nested or any(ext_skymap_uniq) else 'ring'
 
+    # Enforce the sky maps to be non-negative
+    se_skymap = np.abs(se_skymap)
+    exttrig_skymap = np.abs(exttrig_skymap)
+
     if not any(exttrig_skymap) and not (ra and dec):
         # Raise error if external info not given
         raise AssertionError('Provide external sky map or ra/dec')