X-ray observations of PKS 0745-191 at the virial radius: Are we there yet?

TL;DR

This study compares X-ray observations of galaxy cluster PKS 0745-191 from ROSAT and Suzaku, revealing significant discrepancies likely due to background modeling issues, and highlights the uncertainties in measuring ICM properties at the virial radius.

Contribution

The paper provides a critical reassessment of ICM properties at large radii using archival ROSAT data, highlighting systematic uncertainties in Suzaku measurements and emphasizing ROSAT's sensitivity in low surface-brightness regimes.

Findings

ROSAT surface brightness profile is statistically inconsistent with Suzaku results.

Suzaku temperature and mass profiles conflict with other satellites like XMM-Newton and Swift.

Systematic errors of about 25% affect estimates of the virial radius properties.

Abstract

We wish to reassess the properties of the ICM at large radii in the galaxy cluster PKS 0745-191 in light of the recent Suzaku measurements. We analyze an archival 10.5 ksec ROSAT/PSPC observation to extract the surface-brightness profile of PKS 0745-191 and infer the deprojected density profile. We then compare the ROSAT surface-brightness profile with the Suzaku result. We perform a mass analysis combining the ROSAT density profile and the published temperature profiles from different instruments. We find that the ROSAT surface-brightness profile is statistically inconsistent (7.7 sigma) with the Suzaku result around and beyond the value of r200 estimated by Suzaku. We argue that, thanks to its large field of view and low background, ROSAT/PSPC is to the present day the most sensitive instrument to low surface-brightness X-ray emission in the 0.4-2.0 keV band. We also note that the Suzaku temperature and mass profiles are at odds with the results from at least two other satellites (XMM-Newton and Swift). The difference in surface brightness between ROSAT and Suzaku is most likely explained by the existence of additional foreground components at the low Galactic latitude of the source, which were not taken into account in the Suzaku background modeling. In light of our mass analysis, we conclude that any estimate of the fraction of the virial radius reached by X-ray measures is affected by systematic errors of the order of 25%. As a result, the properties of the ICM at the virial radius are still uncertain, and the Suzaku results should be considered with caution.