Add sf error

Signed-off-by: cgalelli <[email protected]>

gammapy/stats/tests/test_variability.py

@@ -172,7 +172,7 @@ def test_structure_function():
         * u.s
     )
 
-    sf, distances = structure_function(flux, flux_err, time)
+    sf, sf_error, distances = structure_function(flux, flux_err, time)
 
     assert_allclose(
         sf,

gammapy/stats/variability.py

@@ -265,6 +265,7 @@ def structure_function(flux, flux_err, time, tdelta_precision=5):
     shape = distances.shape + flux.shape[1:]
     factor = np.zeros(shape)
     norm = np.zeros(shape)
+    error = np.zeros(shape)
 
     for i, distance in enumerate(distances):
         indexes = np.array(np.where(dist_matrix == distance))
@@ -273,10 +274,16 @@ def structure_function(flux, flux_err, time, tdelta_precision=5):
             w = (flux[index[1], ...] / flux_err[index[1], ...]) * (
                 flux[index[0], ...] / flux_err[index[0], ...]
             )
+
             f = np.nan_to_num(f)
             w = np.nan_to_num(w)
             factor[i] = factor[i] + f * w
             norm[i] = norm[i] + w
-    sf = factor / norm
 
-    return sf, distances
+            err = (w**2) * (25 * f - 2 * flux[index[1], ...] * flux[index[0], ...])
+            err = np.nan_to_num(err)
+
+            error[i] = error[i] + np.abs(err)
+    sf = factor / norm
+    sf_error = np.sqrt(error / norm)
+    return sf, sf_error, distances