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Register a new userThe Swift BAT Perspective on Non-thermal Emission in HIFLUGCS Galaxy Clusters
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The search for diffuse non-thermal, inverse Compton (IC) emission from galaxy clusters at hard X-ray energies has been underway for many years, with most detections being either of low significance or controversial. In this work, we investigate 14-195 keV spectra from the Swift Burst Alert Telescope (BAT) all-sky survey for evidence of non-thermal excess emission above the exponentially decreasing tail of thermal emission in the flux-limited HIFLUGCS sample. To account for the thermal contribution at BAT energies, XMM-Newton EPIC spectra are extracted from coincident spatial regions so that both thermal and non-thermal spectral components can be determined simultaneously. We find marginally significant IC components in six clusters, though after closer inspection and consideration of systematic errors we are unable to claim a clear detection in any of them. The spectra of all clusters are also summed to enhance a cumulative non-thermal signal not quite detectable in individual clusters. After constructing a model based on single-temperature fits to the XMM-Newton data alone, we see no significant excess emission above that predicted by the thermal model determined at soft energies. This result also holds for the summed spectra of various subgroups, except for the subsample of clusters with diffuse radio emission. For clusters hosting a diffuse radio halo, a relic, or a mini-halo, non-thermal emission is initially detected at the sim5-sigma confidence level - driven by clusters with mini-halos - but modeling and systematic uncertainties ultimately degrade this significance. In individual clusters, the non-thermal pressure of relativistic electrons is limited to sim10% of the thermal electron pressure, with stricter limits for the more massive clusters, indicating that these electrons are likely not dynamically important in the central regions of clusters.
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The relevance of non-thermal cluster studies and the importance of combining observations of future radio surveys with WFXT data are discussed in this paper.
The existence of cosmic rays and weak magnetic fields in the intracluster volume has been well proven by deep radio observations of galaxy clusters. However a detailed physical characterization of the non-thermal component of large scale-structures, relevant for high-precision cosmology, is still missing. I will show the importance of combining numerical and theoretical works with cluster observations by a new-generation of radio, Gamma- and X-ray instruments.
We present an ensemble X-ray analysis of systematic perturbations in the central hot gas properties for a sample of 28 nearby strong cool-core systems selected from the HIghest X-ray FLUx Galaxy Cluster Sample (HIFLUGCS). We analyze their cool-core features observed with the Chandra X-ray Observatory. All individual systems in our sample exhibit at least a pair of positive and negative excess perturbations in the X-ray residual image after subtracting the global brightness profile. We extract and analyze X-ray spectra of the intracluster medium (ICM) in the detected perturbed regions. To investigate possible origins of the gas perturbations, we characterize thermodynamic properties of the ICM in the perturbed regions and characterize their correlations between positive and negative excess regions. The best-fit relations for temperature and entropy show a clear offset from the one-to-one relation, $T_mathrm{neg}/T_mathrm{pos}=1.20^{+0.04}_{-0.03}$ and $K_mathrm{neg}/K_mathrm{pos}=1.43pm 0.07$, whereas the best-fit relation for pressure is found to be remarkably consistent with the one-to-one relation $P_mathrm{neg}=P_mathrm{pos}$, indicating that the ICM in the perturbed regions is in pressure equilibrium. These observed features in the HIFLUGCS sample are in agreement with the hypothesis that the gas perturbations in cool cores are generated by gas sloshing. We also analyze synthetic observations of perturbed cluster cores created from binary merger simulations, finding that the observed temperature ratio agrees with the simulations, $T_mathrm{neg}/T_mathrm{pos}sim 1.3$. We conclude that gas sloshing induced by infalling substructures plays a major role in producing the characteristic gas perturbations in cool cores. The ubiquitous presence of gas perturbations in cool cores may suggest a significant contribution of gas sloshing to suppressing runaway cooling of the ICM.
(Abridged) The main purpose of this paper is to consider the contribution of all three non-thermal components to total mass measurements of galaxy clusters: cosmic rays, turbulence and magnetic pressures. To estimate the thermal pressure we used public XMM-textit{Newton} archival data of 5 Abell clusters. To describe the magnetic pressure, we assume a radial distribution for the magnetic field, $B(r) propto rho_{g}^{alpha}$, to seek generality we assume $alpha$ within the range of 0.5 to 0.9, as indicated by observations and numerical simulations. For the turbulent component, we assumed an isotropic pressure, $P_{rm turb} = {1/3}rho_{rm g}(sigma_{r}^{2}+sigma_{t}^{2})$. We also consider the contribution of cosmic ray pressure, $P_{cr}propto r^{-0.5}$. It follows that a consistent description for the non-thermal component could yield variation in mass estimates that vary from 10% up to $sim$30%. We verified that in the inner parts of cool-core clusters the cosmic ray component is comparable to the magnetic pressure, while in non cool-core cluster the cosmic ray component is dominant. For cool-core clusters the magnetic pressure is the dominant component, contributing with more than 50% of total mass variation due to non-thermal pressure components. However, for non cool-core clusters, the major influence comes from the cosmic ray pressure that accounts with more than 80% of total mass variation due to non-thermal pressure effects. For our sample, the maximum influence of the turbulent component to total mass variation can be almost 20%. We show that this analysis can be regarded as a starting point for a more detailed and refined exploration of the influence of non-thermal pressure in the intra-cluster medium (ICM).
A diffuse non-thermal component has now been observed in massive merging clusters. To better characterise this component, and to extend analyses done for massive clusters down to a lower mass regime, we are conducting a statistical analysis over a large number of X-ray clusters (from ROSAT based catalogues). By means of their stacked radio and X-ray emissions, we are investigating correlations between the non-thermal and the thermal baryonic components. We will present preliminary results on radio-X scaling relations with which we aim to probe the mechanisms that power diffuse radio emission ; to better constrain whether the non-thermal cluster properties are compatible with a hierarchical framework of structure formation ; and to quantify the non-thermal pressure.
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