South-West extension of the hard X-ray emission from the Coma cluster

TL;DR

This study analyzes deep hard X-ray observations of the Coma cluster, revealing an extended emission in the SW direction likely linked to a subcluster or diffuse source, clarifying the nature of the hard X-ray excess.

Contribution

It provides the first detailed morphological analysis of the hard X-ray emission from the Coma cluster using INTEGRAL data, identifying an extended SW component.

Findings

The hard X-ray emission extends approximately 17 arcminutes in the SW direction.

The emission centroid in 18-30 keV is displaced SW compared to 1-10 keV.

The morphology is best described by an elliptical diffuse source, possibly with an additional point source.

Abstract

We explore the morphology of hard (18-30 keV) X-ray emission from the Coma cluster of galaxies. We analyze a deep (1.1 Ms) observation of the Coma cluster with the ISGRI imager on board the \emph{INTEGRAL} satellite. We show that the source extension in the North-East to South-West (SW) direction ($\sim 17'$) significantly exceeds the size of the point spread function of ISGRI, and that the centroid of the image of the source in the 18-30 keV band is displaced in the SW direction compared to the centroid in the 1-10 keV band. To test the nature of the SW extension we fit the data assuming different models of source morphology. The best fit is achieved with a diffuse source of elliptical shape, although an acceptable fit can be achieved assuming an additional point source SW of the cluster core. In the case of an elliptical source, the direction of extension of the source coincides with the direction toward the subcluster falling onto the Coma cluster. If the SW excess is due to the presence of a point source with a hard spectrum, we show that there is no obvious X-ray counterpart for this additional source, and that the closest X-ray source is the quasar EXO 1256+281, which is located $6.1'$ from the centroid of the excess. The observed morphology of the hard X-ray emission clarifies the nature of the hard X-ray "excess" emission from the Coma cluster, which is due to the presence of an extended hard X-ray source SW of the cluster core.