Gas temperature profiles in galaxy clusters with Swift XRT: observations and capabilities to map near R₂₀₀

¹ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate ( LC), Italy
e-mail: alberto.moretti@brera.inaf.it
² INAF, via E. Bassini 15, 20133 Milano, Italy
³ INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ INFN, Sezione di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy

Received: 21 October 2010
Accepted: 21 December 2010

Abstract

Aims. We investigate the possibility of using the X-ray telescope (XRT) on board the Swift satellite to improve the current accuracy of the intra-cluster medium (ICM) temperature measurements in the region close to the virial radius of nearby clusters.

Methods. We present the spectral analysis of the Swift XRT observations of 6 galaxy clusters and their temperature profiles in the regions within 0.2−0.6 r₂₀₀. Four of them are nearby famous and very well studied objects (Coma, Abell 1795, Abell 2029 and PKS0745-19). The remaining two, SWJ1557+35 (Abell 2141) and SWJ0847+13, at redshift z = 0.16 and z = 0.36, were serendipitously observed by Swift-XRT. We accurately quantify the temperature uncertainties, with particular focus on the impact of the background scatter (both instrumental and cosmic). We extrapolate these results and simulate a deep observation of the external region of Abell 1795 which is assumed here as a case study. In particular we calculate the expected uncertainties in the temperature measurement as far as r₂₀₀.

Results. We find that, with a fairly deep observation (300 ks), the Swift XRT would be able to measure the ICM temperature profiles in the external regions as far as the virial radius, significantly improving the best accuracy among the previous measurements. This can be achieved thanks to the unprecedented combination of good PSF over the full field of view and very accurate control of the instrumental background.

Conclusions. Somehow unexpectedly we conclude that, among currently operating telescope, the Swift-XRT is the only potentially able to improve the current accuracy in plasma temperature measurement at the edges of the cluster potential. This will be true until a new generation of low-background and large field of view telescopes, aimed to the study of galaxy clusters, will operate. These observations would be of great importance in developing the observing strategy for such missions.