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Galaxy clusters grow primarily through the continuous accretion of group-scale haloes. Group galaxies experience preprocessing during their journey into clusters. A star-bursting compact group, the Blue Infalling Group (BIG), is plunging into the nearby cluster A1367. Previous optical observations reveal rich tidal features in the BIG members, and a long H$alpha$ trail behind. Here we report the discovery of a projected $sim 250$ kpc X-ray tail behind the BIG using Chandra and XMM-Newton observations. The total hot gas mass in the tail is $sim 7times 10^{10} {rm M}_odot$ with an X-ray bolometric luminosity of $sim 3.8times 10^{41}$ erg s$^{-1}$. The temperature along the tail is $sim 1$ keV, but the apparent metallicity is very low, an indication of the multi-$T$ nature of the gas. The X-ray and H$alpha$ surface brightnesses in the front part of the BIG tail follow the tight correlation established from a sample of stripped tails in nearby clusters, which suggests the multiphase gas originates from the mixing of the stripped interstellar medium (ISM) with the hot intracluster medium (ICM). Because thermal conduction and hydrodynamic instabilities are significantly suppressed, the stripped ISM can be long lived and produce ICM clumps. The BIG provides us a rare laboratory to study galaxy transformation and preprocessing.
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The interaction processes in galaxy clusters between the hot ionized gas (ICM) and the member galaxies are of crucial importance in order to understand the dynamics in galaxy clusters, the chemical enrichment processes and the validity of their hydrostatic mass estimates. Recently, several X-ray tails associated to gas which was partly stripped of galaxies have been discovered. Here we report on the X-ray tail in the 3 keV galaxy cluster Zwicky 8338, which might be the longest ever observed. We derive the properties of the galaxy cluster environment and give hints on the substructure present in this X-ray tail, which is very likely associated to the galaxy CGCG254-021. The X-ray tail is extraordinarily luminous ($2times10^{42}$ erg/s), the thermal emission has a temperature of 0.8 keV and the X-ray luminous gas might be stripped off completely from the galaxy. From the assumptions on the 3D geometry we estimate the gas mass fraction (< 0.1%) and conclude that the gas has been compressed and/or heated.
We investigate the X-ray variability characteristics of hard X-ray selected AGNs (based on Swift/BAT data) in the soft X-ray band using the RXTE/ASM data. The uncertainties involved in the individual dwell measurements of ASM are critically examined and a method is developed to combine a large number of dwells with appropriate error propagation to derive long duration flux measurements (greater than 10 days). We also provide a general prescription to estimate the errors in variability derived from rms values from unequally spaced data. Though the derived variability for individual sources are not of very high significance, we find that, in general, the soft X-ray variability is higher than those in hard X-rays and the variability strengths decrease with energy for the diverse classes of AGN. We also examine the strength of variability as a function of the break time scale in the power density spectrum (derived from the estimated mass and bolometric luminosity of the sources) and find that the data are consistent with the idea of higher variability at time scales longer than the break time scale.
The main results from a deep X-ray observation of M82 are summarised: spatially-dependent chemical abundances, temperature structure of the gas, charge-exchange emission lines in the spectrum. We also present an update of the chemical bundances, based on a more refined extraction of spectra.
Neutron star X-ray binaries emit a compact, optically thick, relativistic radio jet during low-luminosity, usually hard states, as Galactic black-hole X-ray binaries do. When radio emission is bright, a hard power-law tail without evidence for an exponential cutoff is observed in most systems. We have developed a jet model that explains many spectral and timing properties of black-hole binaries in the states where a jet is present. Our goal is to investigate whether our jet model can reproduce the hard tail, with the correct range of photon index and the absence of a high-energy cutoff, in neutron-star X-ray binaries. We have performed Monte Carlo simulations of the Compton upscattering of soft, accretion-disk or boundary layer photons, in the jet and computed the emergent energy spectra, as well as the time lag of hard photons with respect to softer ones as a function of Fourier frequency. We demonstrate that our jet model explains the observed power-law distribution with photon index in the range 1.8-3. With an appropriate choice of the parameters, the cutoff expected from Comptonization is shifted to energies above ~300 keV, producing a pure power law without any evidence for a rollover, in agreement with the observations. Our results reinforce the idea that the link between the outflow (jet) and inflow (disk) in X-ray binaries does not depend on the nature of the compact object, but on the process of accretion. Furthermore, we address the differences of jets in black-hole and neutron-star X-ray binaries and predict that the break frequency in the spectral energy distribution of neutron-star X-ray binaries, as a class, will be lower than that of black-hole binaries.
We present abundances for seven stars in the (extremely) low-metallicity tail of the Sculptor dwarf spheroidal galaxy, from spectra taken with X-shooter on the ESO VLT. Targets were selected from the Ca II triplet (CaT) survey of the Dwarf Abundances and Radial Velocities Team (DART) using the latest calibration. Of the seven extremely metal-poor candidates, five stars are confirmed to be extremely metal-poor (i.e., [Fe/H]<-3 dex), with [Fe/H]=-3.47 +/- 0.07 for our most metal-poor star. All are around or below [Fe/H]=-2.5 dex from the measurement of individual Fe lines. These values are in agreement with the CaT predictions to within error bars. None of the seven stars is found to be carbon-rich. We estimate a 2-13% possibility of this being a pure chance effect, which could indicate a lower fraction of carbon-rich extremely metal-poor stars in Sculptor compared to the Milky Way halo. The [alpha/Fe] ratios show a range from +0.5 to -0.5, a larger variation than seen in Galactic samples although typically consistent within 1-2sigma. One star seems mildly iron-enhanced. Our program stars show no deviations from the Galactic abundance trends in chromium and the heavy elements barium and strontium. Sodium abundances are, however, below the Galactic values for several stars. Overall, we conclude that the CaT lines are a successful metallicity indicator down to the extremely metal-poor regime and that the extremely metal-poor stars in the Sculptor dwarf galaxy are chemically more similar to their Milky Way halo equivalents than the more metal-rich population of stars.
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