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We present a deep (100 ks) Chandra observation of IDCS J1426.5+3508, aspectroscopically confirmed, infrared-selected galaxy cluster at $z = 1.75$.This cluster is the most massive galaxy cluster currently known at $z > 1.5$,based on existing Sunyaev-Zel'dovich (SZ) and gravitational lensing detections.We confirm this high mass via a variety of X-ray scaling relations, including$T_X$-M, $f_g$-M, $Y_X$-M and $L_X$-M, finding a tight distribution of massesfrom these different methods, spanning M$_{500}$ = 2.3-3.3 $\times 10^{14}$M$_{\odot}$, with the low-scatter $Y_X$-based mass $M_{500,Y_X} =2.6^{+1.5}_{-0.5} \times 10^{14}$ M$_\odot$. IDCS J1426.5+3508 is currently theonly cluster at $z > 1.5$ for which X-ray, SZ and gravitational lensing massestimates exist, and these are in remarkably good agreement. We find arelatively tight distribution of the gas-to-total mass ratio, employing totalmasses from all of the aforementioned indicators, with values ranging from$f_{gas,500}$ = 0.087-0.12. We do not detect metals in the intracluster medium(ICM) of this system, placing a 2$\sigma$ upper limit of $Z(r < R_{500}) < 0.18Z_{\odot}$. This upper limit on the metallicity suggests that this system maystill be in the process of enriching its ICM. The cluster has a dense,low-entropy core, offset by $\sim$30 kpc from the X-ray centroid, which makesit one of the few "cool core" clusters discovered at $z > 1$, and the firstknown cool core cluster at $z > 1.2$. The offset of this core from thelarge-scale centroid suggests that this cluster has had a relatively recent($\lesssim$500 Myr) merger/interaction with another massive system.

IDCS J1426.5+3508: The Most Massive Galaxy Cluster at $z > 1.5$