X-ray flux–mass relation for

We use a subsample of co-detections of the ACT and MaDCoWS cluster catalogs to verify the predicted relation between the observed X-ray flux FX in the 0.5−2 keV band and the cluster mass M500c for halos at z > 0.6 − 0.7. We modified this relation by introducing a correction coefficient, η, which is supposed to encapsulate factors associated with a particular method of flux estimation, the sample selection function, the definition of the cluster mass. We show that the X-ray flux, being the most basic X-ray observable, serves as a convenient and low-cost mass indicator for distant galaxy clusters with photometric or even missing redshifts (by setting z = 1) as long as it is known that z ≳ 0.6 − 0.7. The correction coefficient η is ≈0.8 if $ M^{\mathrm{UPP}}_{\mathrm{500c}} $ from the ACT-DR5 catalog are used as cluster masses, whereas η ≈ 1.1 if weak-lensing-calibrated masses $ M^{\mathrm{Cal}}_{\mathrm{500c}} $ are used instead.