diff --git a/views.py b/views.py
index 24392d0..d8c60ad 100644
--- a/views.py
+++ b/views.py
@@ -910,6 +910,7 @@ class StackedUpperLimitView(APIView):
 
         # Create a single list to hold all Healpix IDs
         all_map_pixels = []
+        source_indices = []
 
         # Iterate over source vectors and get all unique Healpix IDs
         for index, src_vec in enumerate(src_vec_list):
@@ -921,6 +922,7 @@ class StackedUpperLimitView(APIView):
                 radius=(map_radius * u.arcsecond).to(u.radian).value,
             )
             all_map_pixels.extend(pixels)
+            source_indices.extend([index] * len(pixels))
 
         # Get only unique Healpix IDs
         unique_map_pixels = list(set(all_map_pixels))
@@ -953,6 +955,7 @@ class StackedUpperLimitView(APIView):
             "healpix": map_healpix_list,
             "counts": map_counts_list,
             "exposure": map_exposure_list,
+            "source_index": source_indices,
             # "contaminated": map_contaminated_list,
             "nside": map_nside,
             "order": map_order,