Most of the baryons from galaxies have been "missing" and several studies have attempted to map the circumgalactic medium (CGM) of galaxies in their quest. We report on X-ray observations made with the Chandra X-ray Observatory probing the warm-hot phase of the CGM of our Milky Way at about 10 6 K. We detect O vii and O viii absorption lines at z = 0 in extragalactic sight lines and measure accurate column densities using both Kα and Kβ lines of O vii. We then combine these measurements with the emission measure of the Galactic halo from literature to derive the density and the pathlength of the CGM. We show that the warm-hot phase of the CGM is massive, extending over a large region around the Milky Way, with a radius of over 100 kpc. The mass content of this phase is over ten billion solar masses, many times more than that in cooler gas phases and comparable to the total baryonic mass in the disk of the Galaxy. The missing mass of the Galaxy appears to be in this warm-hot gas phase.

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We present a study of the variations in the absorbing column density of 25 X-ray defined Seyfert 2 galaxies, as inferred from hard X-ray observations, on timescales from months to several years. We show that a significant variation of N H (from 20% to 80%) is observed in almost all (22/25) the sources with multiple X-ray observations, although X-ray absorption never vanishes. For a subsample of 11 sources observed at least five times the typical variation time, as defined by a structure function, is less than one year for both heavily absorbed (N H ∼ 10 23 cm −2 ) and moderately absorbed (N H ∼ 10 22 cm −2 ) sources. These variations rule out the simplest version of the unified models, based on a homogeneous obscuring torus, and suggest the presence of clumpy circumnuclear material on a scale well below a parsec. We propose a modification of the torus model in which an overabundance of slightly dusty BELR clouds obscures the BELR. The BELR needs, like the torus, to have an axisymmetric structure. This model is closely related to that of Elvis (2000) for type 1 AGN. For lightly obscured AGN (N H ∼ 10 22 cm −2 ) the structure function shows an increase at a timescale of ∼5 yr, indicating a second absorber, most probably on a 5-10 pc scale associated with the host galaxy.

The CIAO (Chandra Interactive Analysis of Observations) software package was first released in 1999 following the launch of the Chandra X-ray Observatory and is used by astronomers across the world to analyze Chandra data as well as data from other telescopes. From the earliest design discussions, CIAO was planned as a generalpurpose scientific data analysis system optimized for X-ray astronomy, and consists mainly of command line tools (allowing easy pipelining and scripting) with a parameter-based interface layered on a flexible data manipulation I/O library. The same code is used for the standard Chandra archive pipeline, allowing users to recalibrate their data in a consistent way.We will discuss the lessons learned from the first six years of the software's evolution. Our initial approach to documentation evolved to concentrate on recipe-based "threads" which have proved very successful. A multidimensional abstract approach to data analysis has allowed new capabilities to be added while retaining existing interfaces. A key requirement for our community was interoperability with other data analysis systems, leading us to adopt standard file formats and an architecture which was as robust as possible to the input of foreign data files, as well as re-using a number of external libraries. We support users who are comfortable with coding themselves via a flexible user scripting paradigm, while the availability of tightly constrained pipeline programs are of benefit to less computationally-advanced users. As with other analysis systems, we have found that infrastructure maintenance and re-engineering is a necessary and significant ongoing effort and needs to be planned in to any long-lived astronomy software.

We present the first X-ray detection of resonant absorption from warm/hot local gas either in our Galaxy, or in the intergalactic space surrounding our Galaxy, along the line of sight toward the blazar PKS 2155À304. The Chandra HRCS/LETG spectrum of this z À330 km s À1 in the rest frame, from the O vii K line). O viii K and O vii K from the same system are also detected at a lower significance level (i.e., $3 ), while upper limits are set on O viii K, Ne x K, and Ne ix K. The Far Ultraviolet Spectroscopic Explorer spectrum of this source shows complex O vi 2s!2p absorption at the same redshift as the X-ray system, made by at least two components: one relatively narrow (FWHM ¼ 106 AE 9 km s À1 ) and slightly redshifted (cz ¼ 36 AE 6 km s À1 ), and one broader (FWHM ¼ 158 AE 26 km s À1 ) and blueshifted (cz ¼ À135 AE 14 km s À1 ). We demonstrate that the physical states of the UV and X-ray absorbers are hard to reconcile with a single, purely collisionally ionized, equilibrium plasma. We propose instead that the X-ray and at least the broader and blueshifted UV absorber are produced in a low-density intergalactic plasma, collapsing toward our Galaxy, consistent with the predictions of a warm-hot intergalactic medium from numerical simulations. We find that any reasonable solution requires overabundance of Ne compared to O by a factor of $2, with respect to the solar value. We propose several scenarios to account for this observation.

We present a detailed model for the ionized absorbing gas evident in the 900 ks Chandra HETGS spectrum of NGC 3783. The analysis was carried out with PHASE, a new tool designed to model X-ray and UV absorption features in ionized plasmas. The 0.5-10 keV intrinsic continuum of the source is well represented by a single power law (À ¼ 1:53) and a soft blackbody component (kT $ 0:1 keV). The spectrum contains over 100 features, which are well fitted by PHASE with just six free parameters. The model consists of a simple two-phase absorber with a difference of %35 in the ionization parameter and a difference of %4 in the column density of the phases. The two absorption components turned out to be in pressure equilibrium and are consistent with a single outflow (%750 km s À1 ), a single turbulent velocity (300 km s À1 ), and solar elemental abundances. The main features of the low-ionization phase are an Fe M-shell unresolved transition array (UTA) and the O vii lines. The O vii features, usually identified with the O viii and a warm absorber, are instead produced in a cooler medium that also produces O vi lines. The UTA sets tight constraints on the ionization degree of the absorbers, making the model more reliable. The high-ionization phase is required by the O viii and the Fe L-shell lines, and there is evidence for an even more ionized component in the spectrum. A continuous range of ionization parameters is disfavored by the fits, particularly to the UTA. Our model indicates a severe blending of the absorption and emission lines, as well as strong saturation of the most intense O absorption lines. This is in agreement with the O vii ( ¼ 0:33) and O viii ( ¼ 0:13) absorption edges required to fit the spectrum. The low-ionization phase can be decomposed into three subcomponents on the basis of the outflow velocity, FWHM, and H column densities found for three of the four UV absorbers detected in NGC 3783. However, the ionization parameters are systematically smaller in our model than those derived from UV data, indicating a lower degree of ionization. Finally, our model predicts a Ca xvi line for the feature observed at around 21.6 Å (a feature formerly identified as O vii), constraining the contribution from a zero-redshift absorber.

Using a 100 ks XMM-Newton exposure of NGC 4051, we show that the time evolution of the ionization state of the X-ray absorbers in response to the rapid and highly variable X-ray continuum constrains all the main physical and geometrical properties of an AGN ''warm absorber'' wind. The absorber consists of two different ionization components, with a difference of %100 in ionization parameter and %5 in column density. By tracking the response in the opacity of the gas to changes in the ionizing continuum, we were able to constrain the electron density of the system. We find n e ¼ (5:8Y21:0) ; 10 6 cm À3 for the high-ionization absorber and n e > 8:1 ; 10 7 cm À3 for the low-ionization absorber. These densities require that the high-and low-ionization absorbing components of NGC 4051 must be compact, at distances 0.5Y1.0 lt-days (2200R S Y4400R S ) and <3.5 lt-days (<15;800R S ) from the continuum source, respectively. This rules out an origin in the dusty obscuring torus, as the dust sublimation radius is at least an order of magnitude larger ($12 lt-days). An accretion-disk origin for the warm absorber wind is strongly suggested, and an association with the high-ionization, He ii emitting, broad emission line region (radius <2 lt-days) is possible. The two detected phases are consistent with pressure equilibrium, which suggests that the absorber consists of a two-phase medium. A radial flow in a spherical geometry is unlikely, and a conical wind geometry is preferred. The implied mass outflow rate from this wind can be well constrained and is 2%Y5% of the mass accretion rate. If the mass outflow rate scaling with accretion rate is representative of all quasars, our results imply that warm absorbers in powerful quasars are unlikely to produce important evolutionary effects on their larger environment, unless we are observing the winds before they get fully accelerated. Only in such a scenario can AGN winds be important for cosmic feedback.

It has been known for decades that the observed number of baryons in the local Universe falls about 30-40 per cent short of the total number of baryons predicted by Big Bang nucleosynthesis, as inferred from density fluctuations of the cosmic microwave background and seen during the first 2-3 billion years of the Universe in the so-called 'Lyman α forest' (a dense series of intervening H I Lyman α absorption lines in the optical spectra of background quasars). A theoretical solution to this paradox locates the missing baryons in the hot and tenuous filamentary gas between galaxies, known as the warm-hot intergalactic medium. However, it is difficult to detect them there because the largest by far constituent of this gas-hydrogen-is mostly ionized and therefore almost invisible in far-ultraviolet spectra with typical signal-to-noise ratios. Indeed, despite large observational efforts, only a few marginal claims of detection have been made so far. Here we report observations of two absorbers of highly ionized oxygen (O VII) in the high-signal-to-noise-ratio X-ray spectrum of a quasar at a redshift higher than 0.4. These absorbers show no variability over a two-year timescale and have no associated cold absorption, making the assumption that they originate from the quasar's intrinsic outflow or the host galaxy's interstellar medium implausible. The O VII systems lie in regions characterized by large (four times larger than average ) galaxy overdensities and their number (down to the sensitivity threshold of our data) agrees well with numerical simulation predictions for the long-sought warm-hot intergalactic medium. We conclude that the missing baryons have been found.