X-ray flux from the 4XMM-DR9 and CSC 2.0 catalogs compared to the eRASS1cat flux for the 7 eRASS1 detected UCDs and the one UCD candidate with an archival XMM-Newton and/or Chandra detection (one object has both an XMM-Newton and a Chandra detection and is marked with a cross-symbol); the values for the archival X-ray fluxes are listed in Table 4. The black line corresponds to equal flux with the two compared instruments. To enable the comparison with the archival fluxes, the eROSITA fluxes were calculated with a power-law model.