We present new moderate-resolution, far-ultraviolet spectra from the Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) of the BL Lac object 1ES 1553+113 covering the wavelength range 1135Å < λ < 1795Å. The data show a smooth continuum with a wealth of narrow (b < 100 km s −1 ) absorption features arising in the interstellar medium (ISM) and intergalactic medium (IGM). These features include 41 Lyα absorbers at 0 < z abs < 0.43, fourteen of which are detected in multiple Lyman lines and six of which show absorption in one or more metal lines. We analyze a metal-rich triplet (∆cz ∼ 1000 km s −1 ) of Lyα absorbers at z abs ≈ 0.188 in which O VI, N V, and C III absorption is detected. Silicon ions (Si III, Si IV) are not detected to fairly strong upper limits, and we use the measured Si III/C III upper limit to derive an abundance limit (C/Si) ≥ 4 (C/Si) ⊙ for the strongest component of the absorber complex. Galaxy redshift surveys show a number of massive galaxies at approximately the same redshift as this absorption complex, suggesting that it arises in a large-scale galaxy filament. As one of the brightest extragalactic X-ray and γ-ray sources, 1ES 1553+113 is of great interest to the high-energy astrophysics community. With no intrinsic emission or absorption features, 1ES 1553+113 has no direct redshift determination. We use intervening Lyα absorbers to place a direct limit on the redshift: z em > 0.395 based on a confirmed Lyα+O VI absorber and z em > 0.433 based on a single-line detection of Lyα. The current COS data are only sensitive to Lyα absorbers at z < 0.47, but we present statistical arguments that z em 0.58 (at a 1σ confidence limit) based on the non-detection of any Lyβ absorbers at z > 0.4.

The soft diffuse X-ray emission of twelve fields observed with Suzaku are presented together with two additional fields from previous analyses. All have galactic longitudes 65$^\circ $$\lt$$\ell$$\lt$ 295$^\circ $ to avoid contributions from the very bright diffuse source that extends at least 30$^\circ $ from the Galactic center. The surface brightnesses of the Suzaku nine fields for which apparently uncontaminated ROSAT All Sky Survey (RASS) were available were statistically consistent with the RASS values, with an upper limit for differences of 17 $\times$ 10$^{-6}$cs$^{-1}$arcmin$^{-2}$ in R45-band. The OVII and OVIII intensities are well correlated to each other, and OVII emission shows an intensity floor at $\sim$2 photonss$^{-1}$cm$^{-2}$str$^{-1}$ (LU). The high-latitude OVIII emission shows a tight correlation with excess of OVII emission above the floor, with (OVIII intensity) $=$ 0.5 $\times$ [(OVII intensity) $-$ 2LU], suggesting that temperatures averaged over different line-of-sight show a narrow distribution around $\sim$0.2 keV. We consider that the offset intensity of OVII arises from the Heliospheric solar wind charge exchange and perhaps from the local hot bubble, and that the excess OVII (2–7LU) is emission from more distant parts of the Galaxy. The total bolometric luminosity of this galactic emission is estimated to be 4 $\times$ 10$^{39}$ergs$^{-1}$, and its characteristic temperature may be related to the virial temperature of the Galaxy.

We report the detection of highly ionized gas at z $ 0 seen in resonant UV and X-ray absorption lines toward the z ¼ 0:03 blazar Mrk 421. A total of 13 X-ray and three UV lines were measured (or upper limits derived), including three lines in the O vii K series and K transitions from neon, carbon, and nitrogen. From the three O vii lines we derive a 2 Doppler parameter constraint of 24 km s À1 < b < 55 km s À1 . The FUSE spectrum shows strong Galactic low-velocity O vi k1032 absorption and a possible weak O vi high-velocity component (HVC). The Doppler parameter of the low-velocity O vi measured with FUSE is $3 higher than that derived from the O vii line ratios, indicating that the O vii and Galactic O vi arise in different phases. This velocity dispersion along with limits on the gas temperature and density from the X-ray line ratios (assuming a single phase with collisional ionization equilibrium plus photoionization) are all consistent with an extragalactic absorber. However, the O vii Doppler parameter is inconsistent with the high temperature required to produce the observed O vi HVC /O vii ratio, implying that the HVC is probably not related to the O vii. In addition, the O vi K line detected by Chandra implies a column density $4 times higher than the k1032 absorption. Although an extragalactic absorber is fully consistent with the measured column density ratios, a Galactic origin cannot be ruled out given the uncertainties in the available data.

We present an X-ray absorption line spectroscopic study of the large-scale hot interstellar medium (HISM ) in the Galaxy. We detect Ne ix K absorption lines in the Chandra grating spectra of seven Galactic low-mass X-ray binaries. Three of these sources also show absorption of O vii K, O vii K, and/or O viii K . Both the centroid and width of the lines are consistent with a Galactic HISM origin of the absorption. By jointly fitting the multiple lines, accounting for line saturation, and assuming the collisional ionization equilibrium, we estimate the average absorbing gas temperature as $(2:4 AE 0:3) ; 10 6 K (90% confidence errors). We further characterize the spatial density distribution of the gas as 6:4 þ2:4 À1:7 exp (Àjzj/1:2 þ1:0 À0:5 kpc) ; 10 À3 cm À3 (a disk morphology) or 6:2 þ3:8 À2:1 ½1 þ (R /2:3 þ1:6 À1:1 kpc) 2 À1 ; 10 À3 cm À3 (a sphere morphology), where z and R are the distances from the Galactic plane and Galactic center (GC ) respectively. Since nearly all the sight lines with significant absorption lines detected are somewhat toward GC and at low Galactic latitudes, these results could be severely biased. More observations toward off-GC sight lines and at high latitudes are urgently needed to further the study. Nevertheless, the results demonstrate the excellent potential of X-ray absorption line spectroscopy in the study of the HISM.

We present Suzaku spectra of X-ray emission in the fields just off the LMC X-3 sight line. O VII, O VIII, and Ne IX emission lines are clearly detected, suggesting the presence of an optically thin thermal plasma with an average temperature of 2.4 ×10 6 K. This temperature is significantly higher than that inferred from existing Xray absorption line data obtained with Chandra grating observations of LMC X-3, strongly suggesting that the gas is not isothermal. We then jointly analyze these data to characterize the spatial and temperature distributions of the gas. Assuming a vertical exponential Galactic disk model, we estimate the gas temperature and density at the Galactic plane and their scale heights as 3.6(2.9, 4.7) × 10 6 K and 1.4(0.3, 3.4) × 10 −3 cm −3 and 1.4(0.2, 5.2) kpc and 2.8(1.0, 6.4) kpc, respectively. This characterization can account for all the O VI line absorption, as observed in a FUSE spectrum of LMC X-3, but only predicts less than one tenth of the O VI line emission intensity typically detected at high Galactic latitudes. The bulk of the O VI emission most likely arises at interfaces between cool and hot gases.

Diffuse hot gas can be traced effectively by its X-ray absorption and emission. We present a joint-analysis of these tracers to characterize the spatial and temperature distributions of the Galactic hot gas along the sightline toward the nearby bright active galactic nucleus Mrk 421. We also complement this analysis with far-UV O VI absorption observations. We find that the observed absorption line strengths of O VII and O VIII are inconsistent with the diffuse background emission line ratio of the same ions, if the gas is assumed to be isothermal in a collisional ionization equilibrium state. But all these lines as well as the diffuse 3/4-keV broadband background intensity in the field can be fitted with a plasma with a power law temperature distribution. We show that this distribution can be derived from a hot gaseous disk model with the gas temperature and density decreasing exponentially with the vertical distance from the Galactic plane. The joint fit gives the exponential scale heights as ∼ 1.0 kpc and 1.6 kpc and the middle plane values as 2.8 × 10 6 K and 2.4 × 10 −3 cm −3 for the temperature and density, respectively. These values are consistent with those inferred from X-ray observations of nearby edge-on galaxies similar to our own.

We present simultaneous Chandra High-Energy Transmission Gratings (HETG) and Rossi X-ray Timing Explorer (RXTE ) observations of a ''soft state'' of the black hole candidate 4U 1957+11. These spectra, having limited hard X-ray excess, are an excellent test of disk atmosphere models that include effects of black hole spin. The HETG data show, by modeling the broadband continuum and direct fitting of absorption edges, that the disk spectrum is only very mildly absorbed, with N H ¼ (1Y2) ; 10 21 cm À2 . These data additionally reveal kk13.449 Ne ix absorption consistent with the warm/hot phase of the interstellar medium. The fitted disk model implies an inclined disk around a low-mass black hole rotating with normalized spin a à % 1. We show, however, that pure Schwarzschild models describe the data extremely well, albeit with large disk atmosphere ''color-correction'' factors. Standard correction factors can be attained if one incorporates mild Comptonization. We find that the Chandra observations do not uniquely determine spin, even with this otherwise extremely well-measured, nearly pure disk spectrum. XMM-Newton RXTE observations, taken only six weeks later, are equally unconstraining. This lack of constraint is partly driven by the unknown mass and distance of 4U 1957+11; however, it is also driven by the limited Chandra and XMM-Newton bandpasses. We therefore present a series of 48 RXTE observations taken at different brightness/hardness levels. These data prefer a spin of a à % 1, even when including a mild Comptonization component; however, they also show evolution of the color-correction factors. If the rapid-spin models with standard correction factors are to be believed, then the RXTE observations predict that 4U 1957+11 can range from a 3 M black hole at 10 kpc with a à % 0:83 to a 16 M black hole at 22 kpc with a à % 1.

X-ray absorption lines of highly-ionized species such as O VII at about zero redshift have been firmly detected in the spectra of several active galactic nuclei. However, the location of the absorbing gas remains a subject of debate. To separate the Galactic and extragalactic contributions to the absorption, we have obtained Chandra LETG-HRC and Far Ultraviolet Spectroscopic Explorer observations of the black hole X-ray binary LMC X-3. We clearly detect the O VII Kα absorption line with an equivalent width of 20(14, 26) mÅ (90% confidence range). The Ne IX Kα absorption line is also detected, albeit marginally. A joint analysis of these lines, together with the non-detection of the O VII Kβ and O VIII Kα lines, gives the temperature, velocity dispersion, and hot oxygen column density as 1.3(0.7, 1.8) × 10 6 K, 79(62, 132) km s −1 , and 1.9(1.2, 3.2)×10 16 cm −2 , assuming a collisional ionization equilibrium of the X-ray-absorbing gas and a Galactic interstellar Ne/O number ratio of 0.18. The X-ray data allow us to place a 95% confidence lower limit to the Ne/O ratio as 0.14, but the upper limit is not meaningfully constrained. The O VII line centroid and its relative shift from the Galactic O I Kα absorption line, detected in the same observations, are inconsistent with the systemic velocity of LMC X-3 (+310 km s −1 ). The far-UV spectrum shows O VI absorption at Galactic velocities, but no O VI absorption is detected at the LMC velocity at > 3σ significance. The measured Galactic O VI column density is higher than the value predicted from the O VIIbearing gas, indicating multi-phase absorption. Both the nonthermal broadening and the decreasing scale height with the increasing ionization state further suggest an origin of the highly-ionized gas in a supernova-driven galactic fountain. In addition, we estimate the warm and hot electron column densities from our detected O II Kα line in the LMC X-3 X-ray spectra and from the dispersion measure of a pulsar in the LMC vicinity. We then infer the O/H ratio of the gas to be 8 × 10 −5 , consistent with the chemically-enriched galactic fountain scenario. We conclude that the Galactic hot interstellar medium should in general substantially contribute to zero-redshift X-ray absorption lines in extragalactic sources.