{We present the high-resolution X-ray spectrum of the low-redshift quasar H 1821+643 and its surrounding hot cluster observed with the Chandra High Energy Transmission Grating Spectrometer (HETGS). An iron emission line attributed to the quasar at \~{}6.43 keV (rest frame) is clearly resolved, with an equivalent width of \~{}100 eV. Although we cannot rule out contributions to the line from a putative torus, the diskline model provides an acceptable fit to this iron line. We also detect a weak emission feature at \~{}6.9 keV (rest frame). We suggest that both lines could originate in an accretion disk comprised of a highly ionized optically thin atmosphere sitting atop a mostly neutral disk. We search for absorption features from a warm/hot component of the intergalactic medium along the \~{}1.5Gpc/h line of sight to the quasar. No absorption features are detected at or above the 3 sigma level while a total of six OVI intervening absorption systems have been detected with HST and FUSE. Based on the lack of OVII and OVIII absorption lines and by assuming collisionally ionization, we constrain the gas temperature of a typical OVI absorber to 10\^{}5 łt} T łt} 10\^{}6 K, which is consistent with the results from hydrodynamic simulations of the intergalactic medium. The zeroth order image reveals the extended emission from the surrounding cluster. We have been able to separate the moderate CCD X-ray spectrum of the surrounding cluster from the central quasar and find that this is a hot cluster with a temperature of \~{}10 keV and a metal abundance of \~{}0.3 Zo. We also independently obtain the redshift of the cluster, which is consistent with the optical results. We estimate that the cluster makes negligible contributions to the 6.9 keV iron K line flux. }

{We present detailed evidence for a warm absorber in the Seyfert 1 galaxy MCG -6-30-15 and dispute earlier claims for relativistic O line emission. The High-Energy Transmission Grating spectra show numerous narrow, unresolved (FWHMłt}\~{}200 km s$^{-1}$) absorption lines from a wide range of ionization states of N, O, Mg, Ne, Si, S, Ar, and Fe. The O VII edge and the 1s$^{2}$-1snp resonance line series to n=9 are clearly detected at rest in the active galactic nucleus frame. We attribute previous reports of an apparently highly redshifted O VII edge to the 1s$^{2}$-1snp (n{\gt}5) O VII resonance lines and a neutral Fe L absorption complex. The shape of the Fe L feature is nearly identical to that seen in the spectra of several X-ray binaries and in laboratory data. The implied dust column density agrees with that obtained from reddening studies and gives the first direct X-ray evidence for dust embedded in a warm absorber. The O VIII resonance lines and the weak edge are also detected, and the spectral rollover below \~{}2 keV is explained by the superposition of numerous absorption lines and edges. We identify, for the first time, a KLL resonance in the O VI photoabsorption cross section, giving a measure of the O VI column density. The O VII (f) emission detected at the systemic velocity implies a covering fraction of \~{}5% (depending on the observed vs. time-averaged ionizing flux). Our observations show that a dusty warm absorber model is not only adequate to explain all the spectral features {\gt}\~{}0.48 keV (łt}\~{}26 {\AA}) but that the data require it. This contradicts the interpretation of Branduardi-Raymont and coworkers that this spectral region is dominated by highly relativistic line emission from the vicinity of the black hole. }

{We report on a 39 ks observation of the 7.7 s low-mass X-ray binary pulsar 4U 1626-67 with the High Energy Transmission Grating Spectrometer (HETGS) on the Chandra X-Ray Observatory. This ultracompact system consists of a disk-accreting magnetic neutron star and a very low mass, hydrogen-depleted companion in a 42 minute binary. We have resolved the previously reported Ne/O emission line complex near 1 keV into Doppler pairs of broadened (\~{}2500 km s$^{-1}$ FWHM) lines from highly ionized Ne and O. In most cases, the blue and red line components are of comparable strength, with blueshifts of 1550-2610 km s$^{-1}$ and redshifts of 770-1900 km s$^{-1}$. The lines appear to originate in hot (\~{}10⁶ K), dense material just below the X-ray-heated skin of the outer Keplerian accretion disk, or else possibly in a disk wind driven from the pulsar's magnetopause. The observed photoelectric absorption edges of Ne and O appear nearly an order of magnitude stronger than expected from interstellar material and are likely formed in cool, metal-rich material local to the source. Based on the inferred local abundance ratios, we argue that the mass donor in this binary is probably the 0.02 M$_{solar}$ chemically fractionated core of a C-O-Ne or O-Ne-Mg white dwarf which has previously crystallized. }

{We present an analysis of the long Rossi X-ray Timing Explorer (RXTE) observation of the Seyfert 1 galaxy MCG-6-30-15, taken in 1997 July. We have previously used the data to place constraints for the first time on the iron abundance-reflection fraction relationship, and now expand the analysis to investigate in detail the spectral X-ray variability of the object. Our results show that the behaviour is complicated. We find clear evidence from colour ratios and direct spectral fitting that changes to the intrinsic photon index are taking place. In general, spectral hardening is evident during periods of diminished intensity, and in particular, a general trend for harder spectra is seen in the period following the hardest RXTE flare. Flux-correlated studies further show that the 3-10keV photon index {$\Gamma$}$_{3-10}$ steepens, while that in the 10-20keV band, {$\Gamma$}$_{10-20}$, flattens with flux. The largest changes come from the spectral index below 10keV however, changes in the intrinsic power-law slope (shown by changes in {$\Gamma$}$_{3-10}$), and reflection (shown by changes in {$\Gamma$}$_{10-20}$) both contribute in varying degrees to the overall spectral variability. We find that the iron-line flux F$_{Kα}$ is consistent with being constant over large time intervals on the order of days (although tentative evidence exists which show that F$_{Kα}$ changes on shorter time intervals of order łt}\~{}10ks during time periods surrounding flare events), and has an equivalent width which anticorrelates with the continuum flux and reflection fraction. A possible interpretation for the iron-line flux constancy and the relative Compton reflection increase with flux from the flux-correlated data is an increasing ionization of the emitting disc surface, while spectral analysis of short time intervals surrounding flare events hints tentatively at observed spectral responses to the flare. We present a simple model for partial ionization where the bulk of the variability comes from within 6r$_{g}$. Temporal analysis further provides evidence for possible time (łt}\~{}1000s) and phase ({\phis}\~{}0.6rad) lags. Finally, we report an apparent break in the power density spectrum (\~{}4-5{\times}10$^{-6}$Hz) and a possible 33-hour period. Estimates for the mass of the black hole in MCG-6-30-15 are discussed in the context of spectral and temporal findings. }

{We present the first high-resolution X-ray spectrum of the Seyfert 1.5 galaxy NGC 4151. Observations with the Chandra High-Energy Transmission Grating Spectrometer reveal a spectrum dominated by narrow emission lines from a spatially resolved (1.6 kpc), highly ionized nebula. The X-ray narrow-line region is composite, consisting of both photoionized and collisionally ionized components. The X-ray emission lines have similar velocities, widths, and spatial extent to the optical emission lines, showing that they arise in the same region. The clouds in the narrow-line region must contain a large range of ionization states in order to explain both the optical and X-ray photoionized emission. Chandra data give the first direct evidence of X-ray line emission from a hot plasma (T\~{}10⁷ K) that may provide pressure confinement for the cooler (T=3{\times}10⁴ K) photoionized clouds. }

{We present a first emission-line analysis of a high-resolution X-ray spectrum of the stellar wind of þetas}$^{1}$ Ori C obtained with the High-Energy Transmission Grating Spectrometer on board the Chandra X-Ray Observatory. The spectra are resolved into a large number of emission lines from H- and He-like O, Ne, Mg, Si, S, Ar, and Fe ions. The He-like Fe XXV and Li-like Fe XXIV appear quite strong, indicating very hot emitting regions. From H/He flux ratios as well as from Fe He/Li emission measure ratios, we deduce temperatures ranging from 0.5 to 6.1{\times}10⁷ K. The He triplets are very sensitive to density as well. At these temperatures the relative strengths of the intercombination and forbidden lines indicate electron densities well above 10$^{12}$ cm$^{-3}$. The lines appear significantly broadened, from which we deduce a mean velocity of 770 km s$^{-1}$ with a spread between 400 and 2000 km s$^{-1}$. Along with results of the deduced emission measure, we conclude that the X-ray emission could originate in dense and hot regions with a characteristic size of less then 4{\times}10$^{10}$ cm. }

{We report measurements of the mass density, Omega\_M, and cosmological-constant energy density, Omega\_Lambda, of the universe based on the analysis of 42 type Ia supernovae discovered by the Supernova Cosmology Project. The magnitude-redshift data for these supernovae, at redshifts between 0.18 and 0.83, are fitted jointly with a set of supernovae from the Calán/Tololo Supernova Survey, at redshifts below 0.1, to yield values for the cosmological parameters. All supernova peak magnitudes are standardized using a SN Ia light-curve width-luminosity relation. The measurement yields a joint probability distribution of the cosmological parameters that is approximated by the relation 0.8Omega\_M-0.6Omega\_Lambda\~{}-0.2+/-0.1 in the region of interest (Omega\_Młt}\~{}1.5). For a flat (Omega\_M+Omega\_Lambda=1) cosmology we find Omega\^{}flat\_M=0.28\^{}+0.09\_-0.08 (1 sigma statistical) \^{}+0.05\_-0.04 (identified systematics). The data are strongly inconsistent with a Lambda=0 flat cosmology, the simplest inflationary universe model. An ope

{We report on a joint ASCA and RXTE observation spanning an \~{}400ks time interval of the bright Seyfert 1 galaxy MCG-6-30-15. The data clearly confirm the presence of a broad skewed iron line (W$_{Kα}$\~{}266eV) and Compton reflection continuum at higher energies reported in our previous paper. We also investigate whether the gravitational and Doppler effects, which affect the iron line, may also be manifest in the reflected continuum. The uniqueness of this data set is underlined by the extremely good statistics that we obtain from the approximately four million photons that make up the 2-20keV RXTE PCA spectrum alone. This, coupled with the high energy coverage of HEXTE and the spectral resolution of ASCA in the iron line regime, has allowed us to constrain the relationship between abundance and reflection fraction for the first time at the 99per cent confidence level. The reflection fraction is entirely consistent with a flat disc, i.e. the cold material subtends 2{$π$} sr at the source, to an accuracy of 20per cent. Monte Carlo simulations show that the observed strong iron line intensity is explained by an overabundance of iron by a factor of \~{}2 and an underabundance of the lower-Z elements by a similar factor. By considering non-standard abundances, a clear and consistent picture can be made in which both the iron line and reflection continuum come from the same material such as, e.g., an accretion disc. }

{We report on a 50-ks observation of the bright Seyfert 1 galaxy MCG-6-30-15 with the Rossi X-ray Timing Explorer. The data clearly show the broad fluorescent iron line (equivalent width \~{} 250 eV) and the Compton reflection continuum at higher energies. A comparison of the iron line and the reflection continuum has enabled us to constrain the reflective fraction and the elemental abundances in the accretion disc. Temporal studies provide evidence that spectral variability is a result of changes in both the amount of reflection seen and the properties of the primary X-ray source itself. }

{The Supernova Cosmology Project has discovered over 28 supernovae (SNs) at 0.35 łt} z łt} 0.65 in an ongoing program that uses Type Ia SNs (SN Ia's) as high-redshift distance indicators. Here we present measurements of the ratio between the locally observed and global Hubble constants, H$^{L}$₀/H$^{G}$₀ , based on the first seven SNs of this high-redshift data set compared with 18 SNs at z łt}= 0.1 from the Calan/Tololo survey. If Omega M łt}= 1, then light-curve width corrected SN magnitudes yield H$^{L}$₀/H$^{G}$₀ łt} 1.10 (95% confidence level) in both a Lamda = 0 and a flat universe. The analysis using the SN Ia's as standard candles without a light-curve width correction yields similar results. These results rule out the hypothesis that the discrepant ages of the Universe derived from globular clusters and recent measurements of the Hubble constant are attributable to a locally underdense bubble. Using the Cepheid-distance–calibrated absolute magnitudes for SN Ia's of Sandage et al., we can also measure the global Hubble constant, H$^{G}$₀ . If Omega M {\gt}= 0.2, we find that H$^{G}$₀ łt} 70 km s-1 Mpc-1 in a Lamda = 0 universe and H$^{G}$₀ łt} 78 km s-1 Mpc-1 in a flat universe, correcting the distant and local SN apparent magnitudes for light-curve width. Lower results for H$^{G}$₀ are obtained if the magnitudes are not width-corrected. }

{We have developed a technique to systematically discover and study high-redshift supernovae that can be used to measure the cosmological parameters. We report here results based on the initial seven of more than 28 supernovae discovered to date in the high-redshift supernova search of the Supernova Cosmology Project. We find an observational dispersion in peak magnitudes of sigma \_$\{$M$_{B}$$\}$=0.27 ; this dispersion narrows to sigma \_$\{$M$_{B}$,$\{$corr$\}$$\}$=0.19 after ''correcting'' the magnitudes using the light-curve ''width-luminosity'' relation found for nearby (z łt}= 0.1) Type Ia supernovae from the Calan/Tololo survey (Hamuy et al.). Comparing light-curve width–corrected magnitudes as a function of redshift of our distant (z = 0.35–0.46) supernovae to those of nearby Type Ia supernovae yields a global measurement of the mass density, Omega \_$\{$$\{$M$\}$$\}$=0.88\^{}$\{$+0.69$\}$\_$\{$-0.60$\}$ for a Lamda = 0 cosmology. For a spatially flat universe (i.e., Omega M + Omega Lamda = 1), we find Omega \_$\{$$\{$M$\}$$\}$=0.94\^{}$\{$+0.34$\}$\_$\{$-0.28$\}$ or, equivalently, a measurement of the cosmological constant, Omega \_$\{$ Lamda $\}$=0.06\^{}$\{$+0.28$\}$\_$\{$-0.34$\}$ (łt}0.51 at the 95% confidence level). For the more general Friedmann-Lemaitre cosmologies with independent Omega M and Omega Lamda , the results are presented as a confidence region on the Omega M- Omega Lamda plane. This region does not correspond to a unique value of the deceleration parameter q0. We present analyses and checks for statistical and systematic errors and also show that our results do not depend on the specifics of the width-luminosity correction. The results for Omega Lamda -versus- Omega M are inconsistent with Lamda -dominated, low-density, flat cosmologies that have been proposed to reconcile the ages of globular cluster stars with higher Hubble constant values. }