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

¹ INAF/IASF-Milano, via E. Bassini 15, 20133 Milano, Italy
e-mail: eckert@iasf-milano.inaf.it
² University of California at Irvine, 4129, Frederick Reines Hall, Irvine, CA, 92697-4575, USA

Received: 25 January 2011
Accepted: 4 March 2011

Abstract

Aims. We reassess the properties of the ICM at large radii in the galaxy cluster PKS 0745-191 in light of recent Suzaku measurements.

Methods. We analyze an archival 10.5 ks 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 by combining the ROSAT density profile and the published temperature profiles from different instruments.

Results. We find that the ROSAT surface-brightness profile is statistically inconsistent (7.7σ) 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 for studying 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).

Conclusions. 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 measurements 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 on PKS 0745-191 should be considered with caution.