Discovery of large scale shock fronts correlated with the radio halo and radio relic in the A2163 galaxy cluster

Imprints of galaxy cluster formation processes are visible in the intracluster medium and can arise in shock fronts, which are detectable via discontinuities in e.g. the gas temperature and density profiles. In this study, we investigate the X-ray properties of the intracluster gas and the radio morphology of the extraordinary cluster A2163. This cluster shows an irregular morphology in various wavelengths and has one of the most luminous and extended known radio halos. Additionally, it is one of the hottest clusters known. We analyze two Suzaku observations of A2163, one in the north-east (NE) and one in the south-west (SW) direction, and use archival XMM-Newton data to remove point sources in the field of view. To compare our findings in the X-ray regime with the radio emission, we obtain radio images of the cluster from an archival VLA observation at 20cm. We identify three shock fronts in A2163 in our spectral X-ray study. A clear shock front lies in the NE direction at a distance of 1.4Mpc from the center, with a Mach number of $M=1.7_{-0.2}^{+0.3}$, estimated from the temperature discontinuity. This shock coincides with the position of a known radio relic. We identify two additional shocks in the SW direction, one with $M=1.5_{-0.3}^{+0.5}$ at a distance of 0.7Mpc, which is likely related to a cool core remnant, and a strong shock with $M=3.2_{-0.7}^{+0.6}$ at a distance of 1.3Mpc, which also closely matches the radio contours. The complex structure of A2163 as well as the different Mach numbers and shock velocities suggest a merging scenario with two unequal merging constituents, where two shock fronts emerged in an early stage of the merger and traveled outwards while an additional shock front developed in front of the merging cluster cores.Comment: 9 pages, 7 figures, accepted by A&

Investigating the hard X-ray emission from the hottest Abell cluster A2163 with Suzaku

We present the results from Suzaku observations of the hottest Abell galaxy cluster A2163 at $z=0.2$. To study the physics of gas heating in cluster mergers, we investigated hard X-ray emission from the merging cluster A2163, which hosts the brightest synchrotron radio halo. We analyzed hard X-ray spectra accumulated from two-pointed Suzaku observations. Non-thermal hard X-ray emission should result from the inverse Compton (IC) scattering of relativistic electrons by the CMB photons. To measure this emission, the dominant thermal emission in the hard X-ray band must be modeled in detail. To this end, we analyzed the combined broad-band X-ray data of A2163 collected by Suzaku and XMM-Newton, assuming single- and multi-temperature models for thermal emission and the power-law model for non-thermal emission. From the Suzaku data, we detected significant hard X-ray emission from A2163 in the 12-60 keV band at the $28\sigma$ level (or at the $5.5\sigma$ level if a systematic error is considered). The Suzaku HXD spectrum alone is consistent with the single-T thermal model of gas temperature $kT=14$ keV. From the XMM data, we constructed a multi-T model including a very hot ($kT=18$ keV) component in the NE region. Incorporating the multi-T and the power-law models into a two-component model with a radio-band photon index, the 12-60 keV energy flux of non-thermal emission is constrained within $5.3 \pm 0.9 (\pm 3.8)\times 10^{-12}~{\rm erg\, s^{-1} cm^{-2}}$. The 90% upper limit of detected IC emission is marginal ($< 1.2\times 10^{-11}~{\rm erg\, s^{-1} cm^{-2}}$ in the 12-60 keV). The estimated magnetic field in A2163 is $B > 0.098~{\rm \mu G}$. While the present results represent a three-fold increase in the accuracy of the broad band spectral model of A2163, more sensitive hard X-ray observations are needed to decisively test for the presence of hard X-ray emission due to IC emission.Comment: 7 pages, 7 figures, A&A accepted. Minor correctio

Radio Galaxy NGC 1265 unveils the Accretion Shock onto the Perseus Galaxy Cluster

We present a consistent 3D model for the head-tail radio galaxy NGC 1265 that explains the complex radio morphology and spectrum by a past passage of the galaxy and radio bubble through a shock wave. Using analytical solutions to the full Riemann problem and hydrodynamical simulations, we study how this passage transformed the plasma bubble into a toroidal vortex ring. Adiabatic compression of the aged electron population causes it to be energized and to emit low-surface brightness and steep-spectrum radio emission. The large infall velocity of NGC 1265 and the low Faraday rotation measure values and variance of the jet strongly argue that this transformation was due to the accretion shock onto Perseus situated roughly at R_200. Estimating the volume change of the radio bubble enables inferring a shock Mach number of M = 4.2_{-1.2}^{+0.8}, a density jump of 3.4_{-0.4}^{+0.2}, a temperature jump of 6.3_{-2.7}^{+2.5}, and a pressure jump of 21.5 +/- 10.5 while allowing for uncertainties in the equation of state of the radio plasma and volume of the torus. Extrapolating X-ray profiles, we obtain upper limits on the gas temperature and density in the infalling warm-hot intergalactic medium of kT < 0.4 keV and n < 5e-5 / cm^3. The orientation of the ellipsoidally shaped radio torus in combination with the direction of the galaxy's head and tail in the plane of the sky is impossible to reconcile with projection effects. Instead, this argues for post-shock shear flows that have been caused by curvature in the shock surface with a characteristic radius of 850 kpc. The energy density of the shear flow corresponds to a turbulent-to-thermal energy density of 14%. The shock-injected vorticity might be important in generating and amplifying magnetic fields in galaxy clusters. Future LOFAR observations of head-tail galaxies can be complementary probes of accretion shocks onto galaxy clusters.Comment: 14 pages, 4 figures, ApJ, in print; v3: typos corrected to match the published version; v2: improved presentation, added 2D numerical simulations and exact solution to the 1D Riemann problem of a shock overrunning a spherical bubble that gets transformed into a vortex rin

A deep cluster survey in Chandra archival data. First results

I present the first results of a search for clusters of galaxies in Chandra ACIS pointed observations at high galactic latitude with exposure times larger than 10 ks. The survey is being carried out using the Voronoi Tessellation and Percolation technique, which is particularly suited for the detection and accurate quantification of extended and/or low surface brightness emission in X-ray imaging observations. A new catalogue of 36 cluster candidates has been created from 5.55 square degrees of surveyed area. Five of these candidates have already been associated to visible enhancements of the projected galaxy distribution in low deepness DSS-II fields and are probably low-to moderate redshift systems. Three of the candidates have been identified in previous ROSAT-based surveys. I show that a significative fraction (30-40%) of the candidate clusters are probably intermediate to high redshift systems. In this paper I publish the catalogue of these first candidate clusters. I also derive the number counts of clusters and compare it with the results of deep ROSAT-based cluster surveys.Comment: 28 pages, 20 figure

Chandra detection of diffuse X-ray emission from the globular cluster Terzan 5

Terzan 5, a globular cluster (GC) prominent in mass and population of compact objects, is searched for diffuse X-ray emission, as proposed by several models. We analyzed the data of an archival Chandra observation of Terzan 5 to search for extended diffuse X-ray emission outside the half-mass radius of the GC. We removed detected point sources from the data to extract spectra from diffuse regions around Terzan 5. The Galactic background emission was modeled by a 2-temperature thermal component, which is typical for Galactic diffuse emission. We detected significant diffuse excess emission above the particle background level from the whole field-of-view. The surface brightness appears to be peaked at the GC center and decreases smoothly outwards. After the subtraction of particle and Galactic background, the excess spectrum of the diffuse emission between the half-mass radius and 3' can be described by a power-law model with photon index $\Gamma$ = 0.9$\pm$0.5 and a surface flux of F$_X$ = (1.17$\pm$0.16) 10$^{-7}$ erg s$^{-1}$ cm$^{-2}$ sr$^{-1}$ in the 1--7 keV band. We estimated the contribution from unresolved point sources to the observed excess to be negligible. The observations suggest that a purely thermal origin of the emission is less likely than a non-thermal scenario. However, from simple modeling we cannot identify a clearly preferred scenario.Comment: 6 pages, 4 figures, accepted for publication by A&

Non-parametric modeling of the intra-cluster gas using APEX-SZ bolometer imaging data

We demonstrate the usability of mm-wavelength imaging data obtained from the APEX-SZ bolometer array to derive the radial temperature profile of the hot intra-cluster gas out to radius r_500 and beyond. The goal is to study the physical properties of the intra-cluster gas by using a non-parametric de-projection method that is, aside from the assumption of spherical symmetry, free from modeling bias. We use publicly available X-ray imaging data from the XMM-Newton observatory and our Sunyaev-Zel'dovich Effect (SZE) imaging data from the APEX-SZ experiment at 150 GHz to de-project the density and temperature profiles for the relaxed cluster Abell 2204. We derive the gas density, temperature and entropy profiles assuming spherical symmetry, and obtain the total mass profile under the assumption of hydrostatic equilibrium. For comparison with X-ray spectroscopic temperature models, a re-analysis of the recent Chandra observation is done with the latest calibration updates. Using the non-parametric modeling we demonstrate a decrease of gas temperature in the cluster outskirts, and also measure the gas entropy profile. These results are obtained for the first time independently of X-ray spectroscopy, using SZE and X-ray imaging data. The contribution of the SZE systematic uncertainties in measuring T_e at large radii is shown to be small compared to the Chandra systematic spectroscopic errors. The upper limit on M_200 derived from the non-parametric method is consistent with the NFW model prediction from weak lensing analysis.Comment: Replaced with the published version; A&A 519, A29 (2010