Source identification in the IGR J17448-3232 field: discovery of the Scorpius galaxy cluster

TL;DR

This study uses X-ray observations to identify a previously unknown galaxy cluster behind the Galactic bulge and clarifies the nature of sources near IGR J17448-3232, including confirming the blazar and ruling out other sources.

Contribution

It reports the discovery of a new galaxy cluster and clarifies the nature of nearby sources, including the blazar, using combined XMM-Newton and Chandra data.

Findings

Discovered a massive galaxy cluster at redshift 0.055.

Confirmed the bright point source as a blazar, likely a flat spectrum radio quasar or BL Lac.

Identified other faint sources, including a possible Galactic X-ray binary.

Abstract

We use a 43-ks XMM-Newton observation to investigate the nature of sources first distinguished by a follow-up Chandra observation of the field surrounding INTEGRAL source IGR J17448-3232, which includes extended emission and a bright point source previously classified as a blazar. We establish that the extended emission is a heretofore unknown massive galaxy cluster hidden behind the Galactic bulge. The emission-weighted temperature of the cluster within the field of view is 8.8 keV, with parts of the cluster reaching temperatures of up to 12 keV; no cool core is evident. At a redshift of 0.055, the cluster is somewhat under-luminous relative to the X-ray luminosity-temperature relation, which may be attributable to its dynamical state. We present a preliminary analysis of its properties in this paper. We also confirm that the bright point source is a blazar, and we propose that it is either a flat spectrum radio quasar or a low-frequency peaked BL Lac object. We find four other fainter sources in the field, which we study and tentatively identify. Only one, which we propose is a foreground Galactic X-ray binary, is hard enough to contribute to IGR J17448-3232, but it is too faint to be significant. We thus determine that IGR J17448-3232 is in fact the galaxy cluster up to $\approx$45 keV and the blazar beyond.