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J. Ruderman (Stanford), H. Ebeling (IfA, UH Manoa)
Chandra X-ray images of clusters of galaxies indicate an excess of X-ray point sources, believed to be AGN cluster members, above the background surface density. We study the point source content of the ACIS-I data for 47 clusters primarily from the MACS survey, which range in redshift from 0.3 to 1.0 and have similar x-ray luminosities, and 7 control fields that do not contain clusters.
We use the Celldetect algorithm, in the 0.5-2.0 keV range, to detect point sources with a three sigma signal-to-noise ratio above the X-ray background, resulting in 789 detections in the cluster fields and 235 point detections in the control fields. Source count rates are converted to unabsorbed energy fluxes (0.5-2.0 keV) using X-Spec software and a scaled power law source spectral model. Based on the resulting logN-logS graphs, we use a flux limit of completeness of 1.1e-14 erg/sec/cm2. We separately analyze eight long-exposure fields in the 0.5 to 0.6 redshift range at a deeper flux limit of 5.9e-15 erg/sec/cm2. Sources above the flux limit are binned into annuli extending 20 arcmin in radius, centered at the cluster center or observation aimpoints, for the cluster and control fields respectively.
The cluster observations exhibit a central excess at the 3-6 sigma level over the densities observed at the edge of the field. No such excess is observed in the control fields. This point source density in the control fields is consistent with the one observed at the edge of the cluster fields.
Future work will focus on the optical counterparts of the X-ray point sources from a subset of the clusters. Color-magnitude diagrams combined with spectroscopic information will allow us to classify the sources as foreground stars, likely cluster members, or gravitationally lensed background galaxies.
This research was funded by the National Science Foundation's Research Experiences for Undergraduates program.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.