Several investigations of the X-ray variability of active galactic nuclei (AGN) using the normalised excess variance (${sigma^2_{rm NXS}}$) parameter have shown that variability has a strong anti-correlation with black hole mass ($M_{rm BH}$) and X-ray luminosity ($L_{rm X}$). In this study we confirm these previous correlations and find no evidence of a redshift evolution. Using observations from XMM-Newton, we determine the ${sigma^2_{rm NXS}}$ and $L_{rm X}$ for a sample of 1091 AGN drawn from the XMM-Newton Cluster Survey (XCS) - making this the largest study of X-ray spectral properties of AGNs. We created light-curves in three time-scales; 10 ks, 20 ks and 40 ks and used these to derive scaling relations between ${sigma^2_{rm NXS}}$, $L_{rm X}$ (2.0-10 keV range) and literature estimates of $M_{rm BH}$ from reverberation mapping. We confirm the anti-correlation between $M_{rm BH}$ and ${sigma^2_{rm NXS}}$ and find a positive correlation between $M_{rm BH}$ and $L_{rm X}$. The use of ${sigma^2_{rm NXS}}$ is practical only for pointed observations where the observation time is tens of kiloseconds. For much shorter observations one cannot accurately quantify variability to estimate $M_{rm BH}$. Here we describe a method to derive $L_{rm X}$ from short duration observations and used these results as an estimate for $M_{rm BH}$. We find that it is possible to estimate $L_{rm X}$ from observations of just a few hundred seconds and that when correlated with $M_{rm BH}$, the relation is statistically similar to the relation of $M_{rm BH}$-$L_{rm X}$ derived from a spectroscopic analysis of full XMM observations. This method may be particularly useful to the eROSITA mission, an all-sky survey, which will detect $>$10$^{6}$ AGN.
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