A search for gravitational lensing in 38 X-ray selected clusters of galaxies

We present the results of a CCD imaging survey for gravitational lensing in a sample of 38 X-ray-selected clusters of galaxies. Our sample consists of the most X-ray luminous (L-x greater than or equal to 2 10(44) ergs-l) clu sters selected from the Einstein, Observatory Extended Medium Sensitivity S urvey (EMSS) that are observable from Mauna Kea (delta > -40 degrees). The sample spans a redshift range of 0.15 less than or equal to z less than or equal to 0.823 and includes 5 clusters with z > 0.5. CCD images of the clus ters were obtained in excellent seeing. There is evidence of strong gravita tional lensing in the form of giant arcs (length l greater than or equal to 8 ", axis ratio l/w greater than or equal to 10) in 8 of the 38 clusters. Two additional clusters contain shorter arclets, and 6 more clusters contai n candidate arcs that require follow-up observations to confirm their lensi ng origin. Since the survey does not have a uniform surface brightness limi t we do not draw any conclusion based on the statistics of the arcs found. We note, however, that 60% (3 of 5) of the clusters with L-x > 10(45) ergs( -1), and none of the 15 clusters with L-x < 4 1044 ergs(-1) contain giant a rcs, thereby confirming that high X-ray luminosity does identify the most m assive systems, and thus X-ray selection is the preferred method for findin g true, rich clusters at intermediate and high redshifts. The observed geom etry of the arcs, most of which are thin, have large axis ratios (l/w >10), and are aligned orthogonal to the optical major axes of the clusters, indi cate the cluster core mass density profiles must be compact (steeper than i sothermal). In several cases, however, there is also some evidence, in the form of possible radial arcs, for density profiles with finite core radii.