Soft and hard X-ray excess emission in abell 3112 observed with Chandra

Chandra ACIS-S observations of the galaxy cluster Abell 3112 feature the presence of an excess of X-ray emission above the contribution from the diffuse hot gas, which can be equally well modeled with an additional nonthermal power-law model or with a low-temperature thermal model of low metal abundance. We show that the excess emission cannot be due to uncertainties in the background subtraction or in the Galactic H I column density. Calibration uncertainties in the ACIS detector that may affect our results are addressed by comparing the Chandra data to XMM-Newton MOS and PN spectra. While differences between the three instruments remain, all detect the excess in similar amounts, providing evidence against an instrumental nature of the excess. Given the presence of nonthermal radio emission near the center of Abell 3112, we argue that the excess X-ray emission is of nonthermal nature and distributed throughout the entire X-ray bandpass, from soft to hard X-rays. The excess can be explained with the presence of a population of relativistic electrons with similar to 7% of the cluster's gas pressure. We also discuss a possible thermal nature of the excess and examine the problems associated with such interpretation.