AN X-RAY-METHOD FOR DETECTING SUBSTRUCTURE IN GALAXY CLUSTERS - APPLICATION TO PERSEUS, A2256, CENTAURUS, COMA, AND SERSIC 40/6

We use the moments of the X-ray surface brightness distribution to constrain the dynamical state of a galaxy cluster. Using X-ray observations from the Einstein Observatory Imaging Proportional Counter (IPC), we measure the first moment M1(rBAR), the ellipsoidal orientation angle theta2(rBAR), and the axial ratio eta(rBAR) at a sequence of radii in the cluster. We argue that a significant variation in the image centroid M1(rBAR) as a function of radius is evidence for a nonequilibrium feature in the intracluster medium (ICM) density distribution. In simple terms, centroid shifts indicate that the center of mass of the ICM varies with radius. This variation is a tracer of continuing dynamical evolution. For each cluster, we evaluate the significance of variations in the centroid of the IPC image by computing the same statistics on an ensemble of simulated cluster images. In producing these simulated images we include X-ray point source emission, telescope vignetting, Poisson noise, and characteristics of the IPC. Application of this new method to five Abell clusters (A426 [Perseus], A2256, A3526 [Centaurus], A1656 [Coma], and A3266 [Sersic 40/6]) reveals that the core of each one has significant substructure. In addition, we find significant variations in the orientation angle theta2(rBAR) and the axial ratio eta(rBAR) for several of the clusters.