Add a note on neg. flux values
@@ -104,6 +104,20 @@ n_sigma_ul Number of sigma used for upper limit estimation
ts_threshold_ul TS threshold to define the use of an upper limit
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+A note on negative flux and upper limit values:
+
+.. note::
+
+ Gammapy allows for negative flux values and upper limits by default.
+ While those values are physically not valid solutions, they are still
+ valid statistically. Negative flux values either hint at overestimated
+ background levels or underestimated systematic errors in general. Or in
+ case of many measurements, such as pixels in a flux map, they are even
+ statistically expected. In future versions of Gammapy it will be possible
+ to account for systematic errors in the likelihood as well. For now the
+ correct interpretation of the results is left to the user.
+
+
Getting started
===============
