X-ray observations of the lunar occultation of Abell 401.

The A401 cluster of galaxies, classified as distance class 3 and richness class 2 by Abell (1958), is one of the more distant clusters identified as a source of X-ray emission. The paper presents new information on the position and extent of the X-ray source, using the results of an observation made by the HEAO A-1 instrument during a lunar occultation of the cluster. A401 was occulted by the moon during two satellite orbits on 14 February 1978. The results obtained are more consistent than earlier results with the hypothesis that the X-ray emission is concentric with the distribution of galaxies, especially if Abell's result placing the center at the cD galaxy is adopted. In conclusion, the study confirms the Uhuru and Ariel 5 identification of an X-ray source with A401, and it is shown that the extent of the emission is at least 18 arcmin or 2.4 Mpc.