  We comprehensively model the X-ray luminosity emergent from time dependent
relativistic accretion discs, developing analytical models of the X-ray
luminosity of thermal disc systems as a function of black hole mass $M$, disc
mass $M_d$, and disc $\alpha$-parameter. The X-ray properties of these
solutions will be directly relevant for understanding TDE observations. We
demonstrate an extremely strong suppression of thermal X-ray luminosity from
large mass black holes, $L_X \sim \exp(-m^{7/6})$, where $m$ is a dimensionless
mass, roughly the the black hole mass in unity of $10^6$M$_\odot$. This strong
suppression results in upper-observable black hole mass limits, which we
demonstrate to be of order $M_{\rm lim} \simeq 3 \times 10^7 M_\odot$, above
which thermal X-ray emission will not be observable. This upper observable
black hole mass limit is a function of the remaining disc parameters, and the
full dependence can be described analytically (eq. 82). We demonstrate that the
current population of observed X-ray TDEs is indeed consistent with an upper
black hole mass limit of order $M \sim 10^7M_\odot$, consistent with our
analysis.
