A Soft X‐Ray Excess in the A754 Cluster

We have analyzed the Chandra, BeppoSax, and ROSAT observations of Abell 754and report evidence of a soft, diffuse X-ray component. The emission is peakedin the cluster center and is detected out to 8' from the X-ray center. Fittinga thermal model to the combined BeppoSax and PSPC spectra show excess emissionbelow 1 keV in the PSPC and above 100 keV in the BeppoSax PDS. The source 26W20is in the field of view of the PDS. The addition of a powerlaw with thespectral parameters measured by Silverman et al. (1998) for 26W20 successfullymodels the hard component in the PDS. The remaining excess soft emission can bemodeled by either a low temperature, 0.75 - 1.03 keV component, or by apowerlaw with a steep spectral index, 2.3. Addition of a second thermalcomponent model provides a much better fit to the data than does the additionof a non-thermal component. The Chandra temperature map does not show anyregion cooler than 6.9 keV within the region where the cool component wasdetected. Simulations of the emission from embedded groups were performed andcompared with the Chandra temperature map which show groups are a plausiblesource of ~1 keV emission. The cool component is centrally peaked in thecluster and the gas density and temperature are relatively high arguing againstthe WHIM as the source of the X-ray emission. X-ray emission from ellipticalgalaxies is not high enough to provide the total cool component luminosity,7.0x10^43 ergs s^-1. The peak of the cool component is located between the lowfrequency radio halos arguing against a non-thermal interpretation for theemission. We conclude that emission from embedded groups is the most likelyorigin of the cool component in Abell 754.Comment: Submitted to Ap