  Observable thermodynamical properties of the intracluster medium (ICM)
reflect the complex interplay between AGN feedback and the gravitational
collapse of haloes. Using the large volume TNG300 simulation of the
IllustrisTNG project we provide predictions for X-ray and Sunyaev-Zel'dovich
(SZ) scaling relations for a sample of over 30,000 haloes that cover a wide
mass range from galaxies to massive galaxy clusters ($M_{\rm 500crit}$ $\in
[10^{12}$ M$_{\odot} - 2\times 10^{15}$ M$_{\odot}$]). We produce mock X-ray
observations of simulated haloes using methods that are consistent with
observational techniques. Thus, we investigate the scaling relations between
the soft-band X-ray luminosity, spectroscopic temperature, gas mass fraction,
$Y_{\rm X}$ and $Y_{\rm SZ}$ as a function of halo mass, and we find broad
agreement between IllustrisTNG and the observed relations. Our results
highlight the scatter and bias introduced by estimated masses, and thus the
importance of converting simulated ICM properties to the observable space when
comparing simulations to current X-ray observations. The wide range of halo
masses in our sample provides new insights into the shape of the X-ray and SZ
scaling relations across three orders of magnitude in mass. Our findings show
strong evidence for a break in $z=0$ scaling relations. We introduce a smoothly
broken power law model which robustly captures the location of this break, the
width of the transition region around the break, as well as the slope
dependence on halo mass. Our results inform the next generation of subgrid
black hole feedback models and provide predictions for ongoing and future
observational surveys.
