Detecting X-ray filaments in the low redshift Universe with XEUS and Constellation-X

We propose a possible way to detect baryons at low redshifts from the analysis of X-ray absorption spectra of bright AGN pairs. A simple semi-analytical model to simulate the spectra is presented. We model the diffuse warm-hot intergalactic medium (WHIM) component, responsible for the X-ray absorption, using inputs from high-resolution hydro-dynamical simulations and analytical prescriptions. We show that the number of OVII absorbers per unit redshift with column density larger than $10^{13.5}$ cm$^{-2}$ - corresponding to an equivalent width of $\sim$ 1 km/s - which will be possibly detectable by {\it XEUS}, is \magcir 30 per unit redshift. {\it Constellation-X} will detect $\sim 6$ OVII absorptions per unit redshift with an equivalent width of 10 km/s. Our results show that, in a $\Lambda$CDM Universe, the characteristic size of these absorbers at $z\sim 0.1$ is $\sim 1$ $h^{-1}$ Mpc. The filamentary structure of WHIM can be probed by finding coincident absorption lines in the spectra of background AGN pairs. We estimate that at least 20 AGN pairs at separation \mincir 20 arcmin are needed to detect this filamentary structure at a 3$\sigma$ level. Assuming observations of distant sources using {\it XEUS} for exposure times of 500 ksec, we find that the minimum source flux to probe the filamentary structure is $\sim 2\times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$, in the 0.1-2.4 keV energy band. Thus, most pairs of these extragalactic X-ray bright sources have already been identified in the {\it ROSAT} All-Sky Survey. Re-observation of these objects by future missions could be a powerful way to search for baryons in the low redshift Universe.Comment: 18 pages, 10 Figures. Two figures added, Sections 2 and 3 expanded. More optimistic results for Constellation-X. Accepted by MNRA

Weighing simulated galaxy clusters using lensing and X-ray

We aim at investigating potential biases in lensing and X-ray methods to measure the cluster mass profiles. We do so by performing realistic simulations of lensing and X-ray observations that are subsequently analyzed using observational techniques. The resulting mass estimates are compared among them and with the input models. Three clusters obtained from state-of-the-art hydrodynamical simulations, each of which has been projected along three independent lines-of-sight, are used for this analysis. We find that strong lensing models can be trusted over a limited region around the cluster core. Extrapolating the strong lensing mass models to outside the Einstein ring can lead to significant biases in the mass estimates, if the BCG is not modeled properly for example. Weak lensing mass measurements can be largely affected by substructures, depending on the method implemented to convert the shear into a mass estimate. Using non-parametric methods which combine weak and strong lensing data, the projected masses within R200 can be constrained with a precision of ~10%. De-projection of lensing masses increases the scatter around the true masses by more than a factor of two due to cluster triaxiality. X-ray mass measurements have much smaller scatter (about a factor of two smaller than the lensing masses) but they are generally biased low by 5-20%. This bias is ascribable to bulk motions in the gas of our simulated clusters. Using the lensing and the X-ray masses as proxies for the true and the hydrostatic equilibrium masses of the simulated clusters and averaging over the cluster sample we are able to measure the lack of hydrostatic equilibrium in the systems we have investigated.Comment: 27 pages, 21 figures, accepted for publication on A&A. Version with full resolution images can be found at http://pico.bo.astro.it/~massimo/Public/Papers/massComp.pd

A2163: Merger events in the hottest Abell galaxy cluster II. Subcluster accretion with galaxy-gas separation

Located at z = 0.203, A2163 is a rich galaxy cluster with an intra-cluster medium (ICM) that exhibits extraordinary properties, including an exceptionally high X-ray luminosity, average temperature, and a powerful and extended radio halo. The irregular and complex morphology of its gas and galaxy structure suggests that this cluster has recently undergone major merger events that involve two or more cluster components. In this paper, we study the gas structure and dynamics by means of spectral-imaging analysis of X-ray data obtained from XMM-Newton and Chandra observations. From the evidence of a cold front, we infer the westward motion of a cool core across the E-W elongated atmosphere of the main cluster A2163-A. Located close to a galaxy over-density, this gas 'bullet' appears to have been spatially separated from its galaxy (and presumably dark matter component) as a result of high-velocity accretion. From gas brightness and temperature profile analysis performed in two opposite regions of the main cluster, we show that the ICM has been adiabatically compressed behind the crossing 'bullet' possibly because of shock heating, leading to a strong departure of the ICM from hydrostatic equilibrium in this region. Assuming that the mass estimated from the Yx proxy best indicates the overall mass of the system and that the western cluster sector is in approximate hydrostatic equilibrium before subcluster accretion, we infer a merger scenario between two subunits of mass ratio 1:4, leading to a present total system mass of M500 $\propto 1.9 \times 1015 M_{\odot}$. The exceptional properties of A2163 present various similarities with those of 1E0657-56, the so-called 'bullet-cluster'. These similarities are likely to be related to a comparable merger scenario.Comment: A&A, in pres

1WGAJ1226.9+3332: a high redshift cluster discovered by Chandra

We report the detection of 1WGAJ1226.9+3332 as an arcminute scale extended X-ray source with the Chandra X-ray Observatory. The Chandra observation and R and K band imaging strongly support the identification of 1WGAJ1226.9+3332 as a high redshift cluster of galaxies, most probably at z=0.85 +- 0.15, with an inferred temperature kT =10 (+4;-3) keV and an unabsorbed luminosity (in a r=120" aperture) of 1.3 (+0.16;-0.14) x 1e45 erg/s (0.5-10 keV). This indication of redshift is also supported by the K and R band imaging, and is in agreement with the spectroscopic redshift of 0.89 found by Ebeling et al. (2001). The surface brightness profile is consistent with a beta-model with beta=0.770 +- 0.025, rc=(18.1 +-0.9)" (corresponding to 101 +- 5 kpc at z=0.89), and S(0)=1.02 +- 0.08 counts/arcsec**2. 1WGAJ1226.9+3332 was selected as an extreme X-ray loud source with FX/FV>60; this selection method, thanks to the large area sampled, seems to be a highly efficient method for finding luminous high z clusters of galaxies.Comment: 5 pages, 5 figures, 1 table. Accepted for publication in ApJ main journal. Uses emulateapj.st

Temperature and total mass profiles of the A3571 cluster of galaxies

We present BeppoSAX results of a spatially resolved spectral analysis of A3571, a relaxed nearby cluster of galaxies. In the central 2' (130/h_50 kpc) radius the metal abundance is 0.49 +- 0.08 solar and the absorption (1.13 +-0.28) x 10^21 atom/cm^2 whereas elsewhere within an 8'(520/h_50 kpc) radius the abundance is 0.32 \+- 0.05 solar and the absorption consistent with the galactic value of 4.4 x 10^20 atom/cm2. The significant central metal abundance enhancement is consistent with the supernova enrichment scenario. The excess absorption may be attributed to the cooling flow, whose mass flow rate is 80 +- 40 M_Sun/yr from our spectral fit. The BeppoSAX and ASCA radial temperature profiles agree over the entire overlapping radial range r < 25' = 1.6/h_50 Mpc. The combined BeppoSAX and ASCA temperature profile exhibits a constant value out to a radius of 10' (650/h_50 kpc) and a significant decrease (T propto r^-0.55, corresponding to gamma=1.28) at larger radii. These temperature data are used to derive the total mass profile. The best fit NFW dark matter density model results in a temperature profile that is not convectively stable, but the model is acceptable within the uncertainties of the data. The temperature profile is acceptably modeled with a ``core'' model for the dark matter density, consisting of a core radius with a constant slope at larger radii. With this model the total mass and formal 90% confidence errors within the virial radius r_178 (2.5/h_50 Mpc) are 9.1+3.6-1.5 x 10^14 h_50^-1 M_Sun, by a factor of 1.4 smaller than the isothermal value. The gas mass fraction increases with radius, reaching f_gas(r_178) = 0.26+0.05-0.10 x h_50^-3/2. Assuming that the measured gas mass fraction is the lower limit to the primordial baryonic fraction gives Omega_m < 0.4 at 90% confidence

Are we missing baryons in Galaxy Clusters?

The recent constraints on the cosmological parameters put from the observations of the WMAP satellite limit the cosmic baryon fraction in a range that is larger than, and marginally consistent with, what is measured in galaxy clusters. This rises the question whether or not we are considering all the ingredients of cluster baryonic budget. Making a careful weight of the baryons in X-ray emitting plasma and stars in cluster galaxies, I conclude that the cluster baryonic pie is made by 13 (with a 1 sigma range of 8-19) per cent of stars, 70 (56-89) per cent of intracluster hot medium and 17 (0-33) per cent, and a probability of 73 per cent to be larger than 0, of ``other'' baryons, presumably in the form of warm (1e5-1e7 Kelvin) material.Comment: MNRAS in press. Replaced to match proof, correct an error in the determination of the total mass and update reference

Small-scale flows in SUMER and TRACE high-cadence co-observations

We report on the physical properties of small-scale transient flows observed simultaneously at high cadence with the SUMER spectrometer and the TRACE imager in the plage area of an active region. Our major objective is to provide a better understanding of the nature of transient phenomena in the solar atmosphere by using high-cadence imager and spectrometer co-observations at similar spatial and temporal resolution. A sequence of TRACE Fe IX/X 171 A and high-resolution MDI images were analysed together with simultaneously obtained SUMER observations in spectral lines covering a temperature range from 10 000 K to 1 MK. We reveal the existence of numerous transient flows in small-scale loops (up to 30 Mm) observed in the plage area of an active region. These flows have temperatures from 10 000 K (the low temperature limit of our observations) to 250 000 K. The coronal response of these features is uncertain due to a blending of the observed coronal line Mg X 624.85 A. The duration of the events ranges from 60 s to 19 min depending on the loop size. Some of the flows reach supersonic velocities. The Doppler shifts often associated with explosive events or bi-directional jets can actually be identified with flows (some of them reaching supersonic velocities) in small-scale loops. Additionally, we demonstrate how a line-of-sight effect can give misleading information on the nature of the observed phenomena if only either an imager or a spectrometer is used.Comment: 7 pages, 6 figures, accepted by A&

GIADA performance during Rosetta mission scientific operations at comet 67P

The Grain Impact Analyser and Dust Accumulator (GIADA) instrument onboard Rosetta studied the dust environment of comet 67P/Churyumov–Gerasimenko from 3.7 au inbound, through perihelion, to 3.8 au outbound, measuring the dust flow and the dynamic properties of individual particles. GIADA is composed of three subsystems: 1) Grain Detection System (GDS); 2) Impact Sensor (IS); and 3) Micro-Balances System (MBS). Monitoring the subsystems’ performance during operations is an important element for the correct calibration of scientific measurements. In this paper, we analyse the GIADA inflight calibration data obtained by internal calibration devices for the three subsystems during the period from 1 August 2014 to 31 October 2015. The calibration data testify a nominal behaviour of the instrument during these fifteen months of mission; the only exception is a minor loss of sensitivity for one of the two GDS receivers, attributed to dust contamination

Chandra Observation of Abell 2142: Survival of Dense Subcluster Cores in a Merger

We use Chandra data to map the gas temperature in the central region of the merging cluster A2142. The cluster is markedly nonisothermal; it appears that the central cooling flow has been disturbed but not destroyed by a merger. The X-ray image exhibits two sharp, bow-shaped, shock-like surface brightness edges or gas density discontinuities. However, temperature and pressure profiles across these edges indicate that these are not shock fronts. The pressure is reasonably continuous across these edges, while the entropy jumps in the opposite sense to that in a shock (i.e. the denser side of the edge has lower temperature, and hence lower entropy). Most plausibly, these edges delineate the dense subcluster cores that have survived a merger and ram pressure stripping by the surrounding shock-heated gas.Comment: Latex, 9 pages, 5 figures (including color), uses emulateapj.sty. Submitted to Ap