[en] X-ray observations of the Galactic X-ray binaries (XRBs) revealed numerous highly ionized metal absorption lines. However, it is unclear whether such lines are produced by the hot interstellar medium (ISM) or the circumstellar medium intrinsic to the binaries. Here we present a Chandra X-ray absorption line study of 28 observations of 12 XRBs, with a focus on the Ne IX and Fe XVII lines. We report the first detections of these lines in a significant amount of observations. We do not find a significant dependence of the line equivalent width on the distance of the XRBs, but we do see a weak dependence on the source X-ray luminosity. We also find 2 out of 12 selected targets show strong temporal variation of the Ne IX absorbers. While the line ratio between the two ion species suggests a temperature consistent with the previous predictions of the ISM, comparing with two theoretical models of the ISM shows the observed column densities are significantly higher than predictions. On the other hand, photoionization by the XRBs provides a reasonably good fit to the data. Our findings suggest that a significant fraction of these X-ray absorbers may originate in the hot gas intrinsic to the XRBs, and that the ISM makes small, if not negligible, contribution. We briefly discuss the implications to the study of the Milky Way hot gas content.

[en] X-ray observations of highly ionized metal absorption lines at z = 0 provide critical information on the hot gas distribution in and around the Milky Way. We present a study of more than 10 yr of Chandra and XMM-Newton observations of 3C 273, one of the brightest extragalactic X-ray sources. Compared with previous works, we obtain much tighter constraints on the physical properties of the X-ray absorber. We also find a large, non-thermal velocity at ∼100-150 km s^–1, the main reason for the higher line equivalent width when compared with other sightlines. Using joint analysis with X-ray emission and ultraviolet observations, we derive a size of 5-15 kpc and a temperature of (1.5-1.8) × 10⁶ K for the X-ray absorber. The 3C 273 sightline passes through a number of Galactic structures, including radio loops I and IV, the North Polar Spur, and the neighborhood of the newly discovered ''Fermi bubbles''. We argue that the X-ray absorber is unlikely to be associated with the nearby radio loops I and IV; however, the non-thermal velocity can be naturally explained as the result of the expansion of the ''Fermi bubbles''. Our data imply a shock-expansion velocity of 200-300 km s^–1. Our study indicates a likely complex environment for the production of the Galactic X-ray absorbers along different sightlines, and highlights the significance of probing galactic feedback with high resolution X-ray spectroscopy

[en] Cosmological simulations of galaxy clusters typically find that the weight of a cluster at a given radius is not balanced entirely by the thermal gas pressure of the hot intracluster medium (ICM), with theoretical studies emphasizing the role of random turbulent motions to provide the necessary additional pressure support. Using a set of high-resolution, hydrodynamical simulations of galaxy clusters that include radiative cooling and star formation and are formed in a cold dark matter (CDM) universe, we find instead that in the most relaxed clusters rotational support exceeds that from random turbulent motions for radii 0.1-0.5 r ₅₀₀, while at larger radii, out to 0.8 r ₅₀₀, they remain comparable. We also find that the X-ray images of the ICM flatten prominently over a wide radial range, 0.1-0.4 r ₅₀₀. When compared with the average ellipticity profile of the observed X-ray images computed for nine relaxed nearby clusters, we find that the observed clusters are much rounder than the relaxed CDM clusters within ∼0.4 r ₅₀₀. Moreover, while the observed clusters display an average ellipticity profile that does not vary significantly with radius, the ellipticity of the relaxed CDM clusters declines markedly with increasing radius, suggesting that the ICM of the observed clusters rotates less rapidly than that of the relaxed CDM clusters out to ∼0.6 r ₅₀₀. When these results are compared with those obtained from a simulation without radiative cooling, we find a cluster ellipticity profile in much better agreement with the observations, implying that overcooling has a substantial impact on the gasdynamics and morphology out to larger radii than previously recognized. It also suggests that the 10-20% systematic errors from nonthermal gas pressure support reported for simulated cluster masses, obtained from fitting simulated X-ray data over large radial ranges within r ₅₀₀, may need to be revised downward. These results demonstrate the utility of X-ray ellipticity profiles as a probe of ICM rotation and overcooling which should be used to constrain future cosmological cluster simulations.

[en] We present a multi-wavelength analysis of the host galaxy of short-duration gamma-ray burst (GRB) 150101B. Follow-up optical and X-ray observations suggested that the host galaxy, 2MASX J12320498-1056010, likely harbors low-luminosity active galactic nuclei (AGNs). Our modeling of the spectral energy distribution has confirmed the nature of the AGN, making it the first reported GRB host that contains an AGN. We have also found the host galaxy is a massive elliptical galaxy with stellar population of ∼5.7 Gyr, one of the oldest among the short-duration GRB hosts. Our analysis suggests that the host galaxy can be classified as an X-ray bright, optically normal galaxy, and the central AGN is likely dominated by a radiatively inefficient accretion flow. Our work explores an interesting connection that may exist between GRB and AGN activities of the host galaxy, which can help in understanding the host environment of the GRB events and the roles of AGN feedback.

[en] The Milky Way appears to be missing baryons, as the observed mass in stars and gas is well below the cosmic mean. One possibility is that a substantial fraction of the Galaxy's baryons are embedded within an extended, million-degree hot halo, an idea supported indirectly by observations of warm gas clouds in the halo and gas-free dwarf spheroidal satellites. X-ray observations have established that hot gas does exist in our Galaxy beyond the local hot bubble; however, it may be distributed in a hot disk configuration. Moreover, recent investigations into the X-ray constraints have suggested that any Galactic corona must be insignificant. Here we re-examine the observational data, particularly in the X-ray and radio bands, in order to determine whether it is possible for a substantial fraction of the Galaxy's baryons to exist in ∼10⁶ K gas. In agreement with past studies, we find that a baryonically closed halo is clearly ruled out if one assumes that the hot corona is distributed with a cuspy Navarro-Frenk-White profile. However, if the hot corona of the galaxy is in an extended, low-density distribution with a large central core, as expected for an adiabatic gas in hydrostatic equilibrium, then it may contain up to 10¹¹ M _☉ of material, possibly accounting for all of the missing Galactic baryons. We briefly discuss some potential avenues for discriminating between a massive, extended hot halo and a local hot disk.

[en] The existence of ≳10⁹ M _⊙ supermassive black holes (SMBHs) at redshift z > 6 raises the problem of how such SMBHs can grow up within the cosmic time (<1 Gyr) from small seed BHs. In this Letter, we use the observations of 14 quasars at z > 6.5 with mass estimates to constrain their seeds and early growth, by self-consistently considering the spin evolution and the possibility of super-Eddington accretion. We find that spin plays an important role in the growth of early SMBHs, and the constraints on seed mass and super-Eddington accretion fraction strongly depend on the assumed accretion history. If the accretion is coherent with single (or a small number of) episode(s), leading to high spins for the majority of accretion time, then the SMBH growth is relatively slow; and if the accretion is chaotic with many episodes and in each episode the total accreted mass is much less than the SMBH mass, leading to moderate/low spins, then the growth is relatively fast. The constraints on the seed mass and super-Eddington accretion fraction are degenerate. A significant fraction (≳0.1%–1% in linear scale but ∼3–4 dex in logarithmic scale for 10³–10⁴ M _⊙ seeds) of super-Eddington accretion is required if the seed mass is not ≫10⁵ M _⊙, and the requirements of high seed mass and/or super-Eddington accretion fraction are moderately relaxed if the accretion is chaotic.

[en] We present XMM Reflection Grating Spectrometer (RGS) and Chandra Low-Energy Transmission Grating (LETG) observations of the blazar, H 2356-309, located behind the Sculptor Wall, a large-scale galaxy structure expected to harbor high-density Warm-Hot Intergalactic Medium (WHIM). Our simultaneous analysis of the RGS and LETG spectra yields a 3σ detection of the crucial redshifted O VII Kα line with a column density (∼>10¹⁶ cm^-2) consistent with similar large-scale structures produced in cosmological simulations. This represents the first detection of nonlocal WHIM from X-ray absorption studies where XMM and Chandra data are analyzed simultaneously and the absorber redshift is already known, thus providing robust evidence for the expected repository of the 'missing baryons'.

[en] ''Missing baryons'', in the form of warm-hot intergalactic medium (WHIM), are expected to reside in cosmic filamentary structures that can be traced by signposts such as large-scale galaxy superstructures. The clear detection of an X-ray absorption line in the Sculptor Wall demonstrated the success of using galaxy superstructures as a signpost to search for the WHIM. Here we present an XMM -Newton Reflection Grating Spectrometer observation of the blazar Mkn 501, located in the Hercules Supercluster. We detected an O VII Kα absorption line at the 98.7% level (2.5σ) at the redshift of the foreground Hercules Supercluster. The derived properties of the absorber are consistent with theoretical expectations of the WHIM. We discuss the implication of our detection for the search for the ''missing baryons''. While this detection shows again that using signposts is a very effective strategy to search for the WHIM, follow-up observations are crucial both to strengthen the statistical significance of the detection and to rule out other interpretations. A local, z ∼ 0 O VII Kα absorption line was also clearly detected at the 4σ level, and we discuss its implications for our understanding of the hot gas content of our Galaxy

[en] Highly ionized, z = 0 metal absorption lines detected in the X-ray spectra of background active galactic nuclei (AGNs) provide an effective method to probe the hot (T ∼ 10⁶ K) gas and its metal content in and around the Milky Way. We present an all-sky survey of the $K_{\mathrm{\alpha <!--\; ??\; -->}}$ transition of the local O vii absorption lines obtained by Voigt-profile fitting archival XMM-Newton observations. A total of 43 AGNs were selected, among which 12 are BL Lac-type AGNs, and the rest are Seyfert 1 galaxies. At above the $3\mathrm{\sigma <!--\; ??\; -->}$ level the local O vii absorption lines were detected in 21 AGNs, among which 7 were newly discovered in this work. The sky covering fraction, defined as the ratio between the number of detections and the sample size, increases from about 40% for all targets to 100% for the brightest targets, suggesting a uniform distribution of the O vii absorbers. We correlate the line equivalent width with the Galactic coordinates and do not find any strong correlations between these quantities. Some AGNs have warm absorbers that may complicate the analysis of the local X-ray absorber since the recession velocity can be compensated by the outflow velocity, especially for nearby targets. We discuss the potential impact of warm absorbers on our analysis. A comprehensive theoretical modeling of the X-ray absorbers will be presented in a later paper.

[en] We present a pair of high-resolution SPH (smoothed particle hydrodynamics) simulations that explore the nature of cool gas infall into galaxies, and the physical conditions necessary to support the type of gaseous halos that seem to be required by observations. Observations of local X-ray absorbers, high-velocity clouds, and distant quasar absorption line systems suggest that a significant fraction of baryons may reside in multi-phase, low-density, extended, ∼100 kpc, gaseous halos around normal galaxies. The two simulations are identical other than their initial gas density distributions: one is initialized with a standard hot gas halo that traces the cuspy profile of the dark matter, and the other is initialized with a cored hot halo with a high central entropy, as might be expected in models with early pre-heating feedback. Galaxy formation proceeds in dramatically different fashions in these two cases. While the standard cuspy halo cools rapidly, primarily from the central region, the cored halo is quasi-stable for ∼4 Gyr and eventually cools via the fragmentation and infall of clouds from ∼100 kpc distances. After 10 Gyr of cooling, the X-ray luminosity of the standard halo is ∼100 times current limits and the resultant disk galaxy is twice as massive as the Milky Way. In contrast, the cored halo has an X-ray luminosity that is in line with observations, an extended cloud population reminiscent of the high-velocity cloud population of the Milky Way, and a disk galaxy with half the mass and ∼50% more specific angular momentum than the disk formed in the low-entropy simulation