The XMM Cluster Survey: An independent demonstration of the fidelity of
  the eFEDS galaxy cluster data products and implications for future studies
  We present the first comparison between properties of clusters of galaxies
detected by the eROSITA Final Equatorial-Depth Survey (eFEDS) and the XMM
Cluster Survey (XCS). We have compared, in an ensemble fashion, properties from
the eFEDS X-ray cluster catalogue with those from the Ultimate XMM
eXtragaLactic (XXL) survey project (XXL-100-GC). We find the distributions of
redshift and X-ray temperature ($T_{\rm X}$) to be broadly similar between the
two surveys, with a larger proportion of clusters above 4 keV in the XXL-100-GC
sample. We find 62 eFEDS cluster candidates with XMM data (eFEDS-XMM sample);
10 do not have good enough XMM data to confirm or deny, 11 are classed as
sample contaminants, and 4 have their X-ray flux contaminated by another
source. The majority of eFEDS-XMM sources have a longer exposure in XMM than
eFEDS, and the majority of eFEDS positions are within 100 kpc of XCS positions.
Our eFEDS-XCS sample of 37 clusters is used to calculate minimum sample
contamination fractions of ${\sim}$18% and ${\sim}$9% in the eFEDS X-ray and
optically confirmed samples respectively, in general agreement with eFEDS
findings. We compare 29 X-ray luminosities ($L_{\rm X}$) measured by eFEDS and
XCS, which are in excellent agreement. Eight clusters have a $T_{\rm X}$
measured by XMM and eROSITA, and we find that XMM temperatures are 25$\pm$9%
larger than their eROSITA counterparts. Finally, we construct $L_{\rm X}$ -
$T_{\rm X}$ scaling relations based on eFEDS and XCS measurements, which are in
tension; the tension is decreased when we measure a third scaling relation with
calibrated XCS temperatures.
