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X-Ray Point Source Excess of MACS Clusters

Joshua Ruderman
Mentor: Harald Ebeling

Researchers have published several papers over the past several years discussing the apparent excess of X-ray point sources, beyond the background rate, in Chandra images of clusters of galaxies. These X-ray point sources are believed to be Active Galactic Nuclei (AGN) members of the cluster which emit X-rays due to Compton scattering in the accretion disk. Another possibility is that X-ray point sources are due to gravitational lensing of background galaxies. All previous studies have looked at only one to two clusters and therefore have poor statistics for characterizing the distribution of this point source excess. This project is a study of the X-ray point sources of 40 clusters, which range in redshift from z = 0.3 to 0.6, improving on past statistics by an order of magnitude.

After reprocessing the raw data to remove instrumental artifacts and background flares, the Celldetect algorithm from the CIAO software package is used to detect point sources with a 3 σ signal-to-noise ratio above the background, resulting in 719 point source detections in the 40 clusters. Source count rates are converted to unabsorbed energy fluxes (0.5–10 keV) using X-Spec software and a scaled power law source spectral model. Luminosities are similarly computed in the 0.5–10 keV range, accounting for K-correction. Based on NLog-SLog graphs and source exposure times, a global luminosity limit of 2.4 e43 erg/sec is established, leaving 235 X-ray point sources. Using the assumption that the point sources are at the cluster redshift, the sources are binned into annuli of varying distances from the center of the diffuse cluster emission. The spatial detection density is determined to have 5.7, 3.1, and 2.1 σ excesses out to 3.33 Mpc for the z = 0.5–0.6, 0.4–0.5, and 0.3–0.4 clusters, grouped separately, indicating cluster evolution. The first statistically significant profile of the sources is shown to have a continuous profile peaked at the diffuse emission center and reaching the background level at a distance of 5 Mpc from the cluster center.

 




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