Chandra and XMM-Newton X-ray observations of AWM 7 - I: Investigating X-ray surface brightness fluctuations

TL;DR

This study analyzes small-scale surface brightness fluctuations in the core of galaxy cluster AWM 7 using Chandra and XMM-Newton data, revealing significant density and pressure variations and identifying radial depressions not explained by smooth models.

Contribution

It introduces a detailed analysis of X-ray surface brightness fluctuations in AWM 7, highlighting the presence of radial depressions and characterizing the fluctuation spectrum with the Delta-variance technique.

Findings

Radial surface brightness depressions of about 4% are observed in the cluster core.

Surface brightness fluctuations have a flatter spectrum than Kolmogorov turbulence, with a large-scale index around -1.7.

3D density and pressure variations are approximately 4% and 2-4%, respectively.

Abstract

We investigate the levels of small scale structure in surface brightness images of the core of the X-ray bright cool-core galaxy cluster AWM 7. After subtraction of a model of the smooth cluster emission, we find a number of approximately radial surface brightness depressions which are not present in simulated images and are seen in both the Chandra and XMM-Newton data. The depressions are most strongly seen in the south of the cluster and have a magnitude of around 4 per cent in surface brightness. We see these features in both an energy band sensitive to the density (0.6 to 5 keV) and a band more sensitive to the pressure (3.5 to 7.5 keV). Histograms of surface brightness in the data, when compared to realisations of a smooth model, reveal stronger surface brightness variations. We use the Delta-variance technique to characterise the magnitude of the fluctuations as a function of length scale. We find that the spectrum in the 0.6 to 5 keV band is flatter than expected for Kolmogorov index fluctuations. If characterised by a power spectrum, on large scales it would have an index around -1.7, rather than -3.7. The implied 3D density fluctuations have a standard deviation of around 4 per cent. The implied 3D pressure variations are at most 4 per cent. Most of the longer-scale power in the density spectrum is contributed by the southern half of the cluster, where the depressions are seen. The density variations implied by the spectrum of the northern sector have a standard deviation of about 2 per cent.