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The isotropy of the late Universe is an assumption greatly used in astronomy.However, many studies have recently reported deviations from isotropy with adefinitive conclusion yet to be made. New, independent methods to robustly testthe cosmic isotropy are of crucial importance. In this work, we investigate thedirectional behavior of the X-ray luminosity-temperature ($L_X-T$) relation ofgalaxy clusters. A tight correlation exists between the luminosity andtemperature of the X-ray-emitting intracluster medium. While the measuredluminosity depends on the underlying cosmology, the temperature can bedetermined without any cosmological assumptions. By exploiting this propertyone can effectively test the isotropy of cosmological parameters over the fullextragalactic sky. Here, we used 313 homogeneously selected X-ray galaxyclusters from the MCXC catalog and obtained core-excised temperatures for allof them. We find that the behavior of the $L_X-T$ relation heavily depends onthe direction of the sky. Strong anisotropies are detected at a $\sim 4\sigma$level toward $(l,b)\sim (280^{\circ}, -20^{\circ})$. Several X-ray andcluster-related effects that could potentially explain these anisotropies wereexamined, but none did so. Interestingly, two other available cluster samplesappear to have a similar behavior throughout the sky, while being fullyindependent of each other and our sample. Performing a joint analysis of thethree samples, the final anisotropy is further intensified ($\sim 5\sigma$),toward $(l,b)\sim (303^{\circ}, -27^{\circ})$, which is in good agreement withother cosmological probes. This result demonstrates that X-ray galaxy clusterstudies that assume perfect isotropy can produce strongly biased resultswhether the underlying reason is cosmological or related to X-rays. Theidentification of the exact nature of these anisotropies is therefore crucial.

Probing cosmic isotropy with a new X-ray galaxy cluster sample through the $L_{\text{X}}-T$ scaling relation