Illustration of the long-term X-ray variability revealed by our method. Left panel: distribution of the variability for the multi-instrument sources, including the XMM-Newton upper limits. We only show sources consistent with being variable (i.e., Var_new>1, on the right of the vertical dotted line). The vertical dashed line shows the arbitrary limit for what we consider as significant variability (i.e., pessimistic amplitude above 5). Out of the ~135 000 sources with Var_new>1, only ~16 000 have Var_new>5. Right panel: distribution of improvement of variability between all the initial single-catalog sources for which a variability estimate was available, and the final multi-instrument source. The vertical dotted line signifies the limit between single-catalog sources for which the new variability is larger than the prior estimate (~49 000 sources out of ~95 000), and the ones where the new method does not improve the variability estimate (~46 000).