A Soft X-Ray Excess in the A754 Cluster

We have analyzed the Chandra, BeppoSAX, ASCA, and ROSAT PSPC observations of A754 and report evidence of a soft, diffuse X-ray component. A radial analysis shows that it is detected within a region that extends out to 8' from the X-ray center and that the emission is higher in the central region of the cluster. Fitting a thermal model to the combined BeppoSAX and PSPC spectra show excess emission below 1 keV in the PSPC and above 100 keV in the BeppoSAX PDS. The source, 26W20, is in the field of view of the PDS. The addition of a power law, with the spectral parameters measured by Silverman et al. in 1998 for 26W20, successfully models the hard component in the PDS. The excess soft emission can be attributed to a low-temperature, 0.77-1.21 keV, component. The soft excess is also modeled with a power law, although the 90% uncertainty for the normalization of the power law is consistent with zero. Either component added to a hot thermal component provides a statistically significant improvement over a single hot thermal component. The Chandra temperature map provides a detailed description of the thermal state of the gas on a scale of 100 kpc and larger and does not show any region cooler than 5.9 keV (90% confidence) within the region where the cool component was detected. Calculations of the expected emission from one or more groups randomly embedded in a hot gas component were performed that demonstrate that groups are a plausible source of ~1 keV emission, in that they can match the measured cool-component luminosity without violating the spatial temperature constraints provided by the temperature map. The cool component is centrally peaked in the cluster, and the gas density and temperature are relatively high, arguing against the warm hot intergalactic medium as the source of the X-ray emission. Furthermore, because the cool component is centrally peaked, the groups are likely embedded in the intracluster gas, rather than in the intercluster gas. The typical X-ray emission from early-type galaxies is not high enough to provide the total cool-component luminosity, 2.1×1043 ergs s-1. The peak of the cool component is located between the low-frequency radio halos, thus arguing against a nonthermal interpretation for the emission based on the synchrotron inverse Compton model, which requires that the nonthermal X-ray and radio emission be cospatial. Thus, we conclude that emission from embedded groups is the most likely origin of the cool component in A754.