VI Emission Lines Research Articles

Absolute wavelength calibration of a Doppler spectrometer with a custom Fabry-Perot optical system.

An Ion Doppler Spectrometer (IDS) is used for fast measurements of C VI line emission (343.4 nm) in the Madison Symmetric Torus. Absolutely calibrated flow measurements are difficult because the IDS records data within 0.25 nm of the line. Commercial calibration lamps do not produce lines in this narrow range. A light source using an ultraviolet LED and etalon was designed to provide a fiducial marker 0.08 nm wide. The light is coupled into the IDS at f/4, and a holographic diffuser increases homogeneity of the final image. Random and systematic errors in data analysis were assessed. The calibration is accurate to 0.003 nm, allowing for flow measurements accurate to 3 km/s. This calibration is superior to the previous method which used a time-averaged measurement along a chord believed to have zero net Doppler shift.

Estimation of physical conditions in the cold phase of the interstellar medium in the sub-DLA system at z = 2.06 in the spectrum of the quasar J 2123–0050

An independent analysis of the molecular hydrogen absorption system at z = 2.059 in the spectrum of the quasar J2123-0050 is presented. The H_2 system consists of two components (A and B) with column densities log N^A(H_2) = 17.94+/-0.01 and log N^B(H_2) = 15.16+/-0.02. The spectrum exhibits the lines of HD molecules (log N^A(HD) = 13.87+/-0.06) and the neutral species C I and Cl I associated with the H_2 absorption system. For the molecular hydrogen lines near the quasar's Ly_beta and O VI emission lines, we detect a nonzero residual flux, ~3% of the total flux, caused by the effect of partial coverage of the quasar's broad-line region by an H_2 cloud. The uniqueness of the system being investigated is manifested in a high abundance of the neutral species H_2 and C I at the lowest H I column density, log N(H I) = 19.18+/-0.15, among the high redshift systems. The N(HD)/2N(H_2) ratio for component A has turned out to be also unusually high, (4.26+/-0.60)x10^{-5}. We have investigated the physical conditions in components A and B. Component A represents the optically thick case; the gas has a low number density (n~30 cm^{-3}) and a temperature T~140 K. In component B, the medium is optically thin with n<100 cm^{-3} and T<100 K. The ultraviolet (UV) background intensity in the clouds exceeds the mean intensity in our Galaxy by almost an order of magnitude. A high gas ionization fraction, n(H+)/n(H)~10^{-2}, which can be the result of partial shielding of the system from hard UV radiation, is needed to describe the high HD and C I column densities. Using our simulations with the PDR Meudon code, we can reconstruct the observed column densities of the species within the model with a constant density n(H)=40 cm^{-3}. A high H_2 formation rate (higher than the mean Galactic value by a factor of 10-40) and high gas ionization fraction and UV background intensity are needed in this case.

THE BORN-AGAIN PLANETARY NEBULA A78: AN X-RAY TWIN OF A30

We present the XMM-Newton discovery of X-ray emission from the planetary nebula (PN) A78, the second born-again PN detected in X-rays apart from A30. These two PNe share similar spectral and morphological characteristics: They harbor diffuse soft X-ray emission associated with the interaction between the H-poor ejecta and the current fast stellar wind, and a point-like source at the position of the central star (CSPN). We present the spectral analysis of the CSPN, using for the first time a NLTE code for expanding atmospheres which takes line blanketing into account for the UV and optical spectra. The wind abundances are used for the X-ray spectral analysis of the CSPN and the diffuse emission. The X-ray emission from the CSPN in A78 can be modeled by a single C VI emission line, while the X-ray emission from its diffuse component is better described by an optically thin plasma emission model with temperature $kT$=0.088 keV ($T\approx$1.0$\times$10${^6}$ K). We estimate X-ray luminosities in the 0.2--2.0 keV energy band of $L_{\mathrm{X,CSPN}}$=(1.2$\pm$0.3)$\times$10$^{31}$ erg~s$^{-1}$ and $L_{\mathrm{X,DIFF}}$=(9.2$\pm$2.3)$\times$10$^{30}$ erg~s$^{-1}$ for the CSPN and diffuse components, respectively.

On the Raman O VI and related lines in classical novae

We critically examine the recent claimed detection of Raman scattered O VI at around 6830\AA\ in the iron curtain stage spectra of the classical CO nova V339 Del. The observed line variations are compatible in profile and timing of emission line strength with an excited state transition of neutral carbon. Line formation in classical nova ejecta is physically very different from that in symbiotic binaries, in which the O VI emission line is formed within the wind of the companion red giant at low differential velocity. The ejecta velocity and density structure prevent the scattering from producing analogous features. There might , however, be a broadband spectropolarimetric signature of the Raman process and also Rayleigh scattering at some stage in the expansion. We show that the neutral carbon spectrum, hitherto under-exploited for novae, is especially useful as a probe of the structure of the ejecta during the early, optically thick stages of the expansion

FAR ULTRAVIOLET SPECTROSCOPIC EXPLOREROBSERVATIONS OF KPD 2055+3111, A STAR BEHIND THE CYGNUS LOOP

We have observed a star behind the Cygnus Loop supernova remnant using the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite to study the line-of-sight interstellar medium structures toward and through this prototypical remnant. An sdOB star, KPD 2055+3111, was identified from Ultraviolet Imaging Telescope UV images and lies in projection within the bright northeast Cygnus Loop filaments (NGC 6992). This is the first known UV background source for the Cygnus Loop. We have observed this star as well as the directly adjacent emission-line filaments. Although the intrinsic spectrum of the star is complex, a broad O VI λ1032 absorption line due the Cygnus Loop is present in the stellar spectrum, confirming that the star lies beyond the Cygnus Loop. Optical spectroscopy of the star and model fits permits a distance estimate to the star of 576 ± 61 pc, thus providing an independent upper limit on the distance to the Cygnus Loop. Numerous absorption transitions of molecular hydrogen are present in the FUSE spectrum of KPD 2055+3111. Assessment of the properties of the H2 indicates a column density of (3.3 ± 0.6) × 1016 cm–2 and a two-temperature J-level population T(J = 0-1) = 106 ± 40 K and T(J = 2-5) = 850 ± 230 K. There is no direct evidence from line widths, component structure, or velocity displacements that the detected H2 is associated with the Cygnus Loop as opposed to the interstellar gas along the sight line, so either source remains viable. The O VI emission line profiles directly adjacent to line of sight to the star show dramatic variability on small (20'') spatial scales, highlighting how differently the UV-emitting gas can be distributed compared with optical and other wave bands. This impacts the ability to directly compare the emission and absorption components along the sight line.

X-RAY AND ULTRAVIOLET SPECTROSCOPY OF GALACTIC DIFFUSE HOT GAS ALONG THE LARGE MAGELLANIC CLOUD X-3 SIGHT LINE

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 ? 106 K. This temperature is significantly higher than that inferred from existing X-ray 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) ? 106 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 Far Ultraviolet Spectroscopy Explorer 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.

Density Dependence of Ion Temperature Measured by Active Charge-Exchange Spectroscopy in ECRH Plasmas of the TJ-II Stellarator

First impurity ion temperature profiles obtained using an active diagnostic system, recently installed on the TJ-II stellarator, are presented. This diagnostic consists of a multichannel spectrometer and a compact diagnostic neutral beam injector system optimized for performing charge-exchange recombination spectroscopy. Here, after summarizing the experimental setup, details of the system alignment and calibration, as well as the data analysis method adopted, are presented. Next, impurity ion temperature profiles, determined from C VI emission line widths (at 529.06 nm), are presented for a range of plasma conditions (different densities plus two injected electron cyclotron resonance heating powers) in order to highlight the system capabilities. Then, the comportment of core impurity ion temperature for an electron density scan (4 × 1018 to 9 × 1018 m−3) is examined. It reveals a clear minimum between 〈ne 〉 = 6 × 1018 and 8 × 1018 m−3 that coincides with the values for the transition from the electron-to-ion root of the radial electric field. Finally, these results are compared with ion temperatures determined by passive methods to evaluate the system performance, and the physics behind the observed impurity ion temperature behavior is examined.

OviObservations of the Onset of Convection Zones in Main‐Sequence A Stars

If magnetic activity in outer stellar atmospheres is due to an interplay between rotation and subsurface convection, as is generally presumed, then one would not expect to observe indicators of activity in stars with Teff 8300 K. Any X-ray or ultraviolet line emission from hotter stars must be due either to a different mechanism or to an unresolved, active, binary companion. Due to their poor spatial resolution, X-ray instruments have been especially susceptible to source confusion. At wavelengths longward of Lyα, the near-ultraviolet spectra of stars hotter than this putative dividing line are dominated by the photospheric continuum. We have used the Far-Ultraviolet Spectroscopic Explorer (FUSE) to obtain spectra of the subcoronal O VI emission lines, which lie at a wavelength where the photospheric continuum of the middle and early A stars is relatively weak. We observed 14 stars spanning a range in Teff from 7720 to 10,000 K. Eleven of the 14 stars showed O VI emission lines, including 6 of the 8 targets with Teff > 8300 K. At face value, this suggests that activity does not fall off with increasing temperature. However, the emission lines are narrower than expected from the projected rotational velocities of these rapidly rotating stars, suggesting that the emission could come from unresolved late-type companions. Furthermore, the strength of the O VI emission is consistent with that expected from an unseen active K or M dwarf binary companion, and the high LX/L(O VI) ratios observed indicate that this must be the case. Our results are therefore consistent with earlier studies that have shown a rapid drop-off in activity at the radiative/convective boundary expected at Teff ~ 8300 K, in agreement with conventional stellar structure models.

Electron capture in collisions betweenO6+ions andH2Omolecules

By means of photon emission spectroscopy, state selective electron capture cross section for low energy (0.1-7.5 keV/amu) collisions of O6+ on H2O molecules have been measured. Over the range of interaction energies the state selective cross sections change strongly, i.e., by factors up to 5, while the total one-electron capture cross section turns out to increase by approximately 40%. Possibilities of using O VI line emission ratios as a remote diagnostics of astrophysical plasma are indicated.

Quasar Spectrum Classification with Principal Component Analysis (PCA): Emission Lines in the Lyα Forest

We investigate the variety in quasar UV spectra (??1020-1600), with emphasis on the weak emission lines in the Ly? forest region, using principal component analysis (PCA). We use 50 smooth continuum-fitted quasar spectra (0.14 < z < 1.04) taken by the Hubble Space Telescope (HST) Faint Object Spectrograph. The first, second, and third principal component spectra (PCS) account for 63.4%, 14.5%, and 6.2% of the variance, respectively, and the first seven PCS account for 96.1% of the total variance. Three weak emission lines in the Ly? forest are identified as Fe II ?1070.95, Fe II + Fe III ?1123.17, and C III* ?1175.88. Using the first two standardized PCS coefficients, we introduce five classifications. By actively using PCS, we can generate artificial quasar spectra that are useful in testing the detection of quasars, DLAs, and the continuum calibration. We show that the power-law-extrapolated continuum is inadequate to perform precise measurements of the mean flux in the Ly? forest because of the weak emission lines and the extended tails of Ly? and Ly?/O VI emission lines. We show that we miss 5.3% of the flux on average, and there are cases where we would miss 14% of the flux. These corrections are essential in the study of the intergalactic medium at high redshift in order to achieve precise measurements of physical properties, cosmological parameters, and the reionization epoch.

Oviin Elliptical Galaxies: Indicators of Cooling Flows

Early-type galaxies often contain a hot X-ray-emitting interstellar medium [(3-8) × 106 K] with an apparent radiative cooling time much less than a Hubble time. If unopposed by a heating mechanism, the gas will radiatively cool to temperatures 104 K at a rate proportional to LX/TX, typically 0.03-1 M☉ yr-1. We can test whether gas is cooling through the 3 × 105 K range by observing the O VI doublet, whose luminosity is proportional to the cooling rate. Here we report on a study of an unbiased sample of 24 galaxies, obtaining Far Ultraviolet Spectroscopic Explorer spectra to complement the X-ray data of ROSAT and Chandra. The O VI line emission was detected in about 40% of the galaxies and at a luminosity level similar to the prediction from the cooling flow model. There is a correlation between and X, although there is significant dispersion about the relationship, where the O VI is brighter or dimmer than expected by a factor of 3 or more. If the cooling flow picture is to be retained, then this dispersion requires that cooling flows be time-dependent, as might occur by the activity of an AGN. However, of detected objects, those with the highest or lowest values of /X are not systematically hot or cool, as one might predict from AGN heating.

14.5% near-normal incidence reflectance of Cr/Sc x-ray multilayer mirrors for the water window.

Cr/Sc multilayer mirrors, synthesized by ion-assisted magnetron sputter deposition, are proved to have a high near-normal reflectivity of R = 14.5% at a grazing angle of 87.5 degrees measured at the wavelength lambda = 3.11 nm, which is an improvement of more than 31% compared with previously published results. Elastic recoil detection analyses show that the mirrors contained as much as 15 at. % of N and traces of C and O. Soft x-ray reflectivity simulations reveal interface widths of sigma = 0.34 nm and an exceptionally small layer thickness drift of approximately 1.6 x 10(-5) nm/multilayer period throughout the stack. Simulations show that a reflectivity of R = 25.6% is attainable if impurities and layer thickness drift can be eliminated. The abrupt interfaces are achieved with ion assistance with a low ion energy of 24 eV and high ion-to-metal flux ratios of 7.1 and 23.1 during Cr and Sc sputter deposition, respectively. In addition, a near-normal incidence reflectivity of 5.5% for the C VI emission line (lambda = 3.374 nm) from a laser plasma source was verified.

Improved liquid jet laser-plasma source for X-ray microscopy

We increase the x-ray flux from a liquid-jet laser-plasma x-ray source by optimizing the target geometry. A new nozzle fabrication method allows us to produce stable microscopic liquid jets with a wide range of diameters. The improved x-ray flux is demonstrated by optimizing the diameter of an ethanol liquid-jet for our 3 ns, □ =532 nm Nd:YAG laser and measuring the flux at the □=3.37 nm C VI emission line. Preliminary data suggest that the x-ray flux can be increased by more than a factor of 4 compared to previous experiments. The goal is to significantly reduce the exposure time of our laser-plasma-based compact x-ray microscope by improving the source.

Far Ultraviolet Spectroscopic Explorer ($\it FUSE$) observations of emitting and absorbing gas in the Local Interstellar Chimney

We present Far Ultraviolet Spectroscopic Explorer () satellite measurements of the absorption and emission characteristics of interstellar gas associated with the Local Interstellar Chimney, which is an extension of the rarefied Local Bubble cavity that extends outward from the galactic disk towards the lower galactic halo. Far ultraviolet (FUV) diffuse background emission has been detected in the high ionization line of O VI (λ1032 Å) for two lines-of-sight (, ) and (, ) at emission levels of photons cm-2 s-1 sr-1 (LU) and LU respectively. These levels of O VI emission are very similar to those found for four other lines-of-sight sampled thus far by the satellite, implying a fairly constant level of average O VI surface brightness emission at high galactic latitudes of about 2700 LU with a standard deviation of 450 LU. These emission-line data are supplemented by FUV interstellar absorption line measurements taken towards the hot DA white dwarf star, REJ 1032+532 (, ), whose distance of 116 pc places it within the Local Bubble region. No high ionization interstellar O VI λ1032 Å absorption has been detected cm-2), which is consistent with the non-detections of interstellar C IV and Si IV absorption reported towards this star by Holberg et al. ([CITE]). Taken together, our FUV absorption and emission data may be explained by a scenario in which the O VI emission and absorption lines are formed at the conductive interface of the neutral boundary to the Local Bubble. For the presently sampled sight-lines we have found no correlation between the OVI emission line intensity and the associated 0.25 keV soft X-ray background flux as measured in the R1 and R2 bands by the satellite. The OVI line intensities also show no correlation with the soft X-ray background flux attributable to emission from the million degree K gas of the Local Hot Bubble as modeled by Kuntz & Snowden ([CITE]). Any (new) model of the Local Bubble must now be able to explain (i) the low levels of variability in both the O VI emission-line intensity and the associated soft X-ray background flux for galactic sight-lines > °, (ii) the observed pressure of cm-3 K for the local hot interstellar gas, and (iii) the paucity of high ionization absorption lines observed within the local ISM and the sudden increase in their measured column density for distances beyond the Local Bubble neutral boundary.

Far Ultraviolet Spectroscopic ExplorerObservations of the Seyfert 1.5 Galaxy NGC 5548 in a Low State

We present far-ultraviolet spectra of the Seyfert 1.5 galaxy NGC 5548 obtained in 2000 June with the Far Ultraviolet Spectroscopic Explorer (FUSE). Our data span the observed wavelength range 915-1185 A at a resolution of approximately 20 km s(exp -1). The spectrum shows a weak continuum and emission from O VI (lambda)(lambda)1032, 1038, C III (lambda)977, and He II (lambda)1085. The FUSE data were obtained when the AGN (Active Galactic Nuclei) was in a low state, which has revealed strong, narrow O VI emission lines. We also resolve intrinsic, associated absorption lines of O VI and the Lyman series. Several distinct kinematic components are present, spanning a velocity range of approximately 0 to -1300 km s(exp -1) relative to systemic, with kinematic structure similar to that seen in previous observations of longer wavelength ultraviolet (UV) lines. We explore the relationships between the far-UV (ultraviolet) absorbers and those seen previously in the UV and X-rays. We find that the high-velocity UV absorption component is consistent with being low-ionization, contrary to some previous claims, and is consistent with its non-detection in high-resolution X-ray spectra. The intermediate velocity absorbers, at -300 to -400 km s(exp -1), show H I and O VI column densities consistent with having contributions from both a high-ionization X-ray absorber and a low-ionization UV absorber. No single far-UV absorbing component can be solely identified with the X-ray absorber.

Contemporaneous Ultraviolet and Optical Observations of Direct and Raman‐scattered OviLines in Symbiotic Stars

Symbiotic stars are binary systems consisting of a hot star, typically a white dwarf, and a cool giant companion. The wind from the cool star is ionized by the radiation from the hot star, resulting in the characteristic combination of sharp nebular emission lines and stellar molecular absorption bands in the optical spectrum. Most of the emission lines are readily identifiable with common ions. However, two strong, broad emission lines at 6825 and 7082 A defied identification with known atoms and ions. In 1989 Schmid made the case that these long unidentified emission lines resulted from the Raman scattering of the O VI resonance photons at 1032, 1038 A by neutral hydrogen. We present contemporaneous far-UV and optical observations of direct and Raman-scattered O VI lines for nine symbiotic stars obtained with the Hopkins Ultraviolet Telescope (Astro-2) and various ground-based optical telescopes. The O VI emission lines are present in every instance in which the λλ6825, 7082 lines are present, in support of the Schmid Raman-scattering model. We calculate the scattering efficiencies and discuss the results in terms of the Raman-scattering model. Additionally, we measure the flux of the Fe II fluorescence line at 1776 A, which is excited by the O VI line at 1032 A, and calculate the first estimates of the conversion efficiencies for this process.

Study of a fast ablative capillary discharge dedicated to soft x-ray production

A capillary discharge has been developed to produce pulses of intense soft x-ray radiation of tens of nanoseconds duration. The soft x-ray photons were emitted in a plasma column resulting from polyethylene in capillary wall ablation. The spectrum was dominated by the C IV, C V, and C VI emission lines in the soft x-ray spectral range. The experimental value of the electrical circuit inductance has been measured and compared to the calculated one. The electron temperature was estimated to be higher than 50 eV from the plasma resistivity measurement. The time dependence of the electron density outside the capillary channel has been determined using spectroscopic measurement in the visible range. The time dependence of the electron temperature has been determined from the intensity ratio of C V and C VI emission lines, using a collisional radiative equilibrium code.

ORFEUS IIFar‐Ultraviolet Spectroscopy of AM Herculis

Six high-resolution (λ/Δλ ≈ 3000) far-UV (λλ = 910-1210 A) spectra of the magnetic cataclysmic variable AM Herculis were acquired in 1996 November during the flight of the ORFEUS-SPAS II mission. AM Her was in a high optical state at the time of the observations, and the spectra reveal emission lines of O VI λλ1032, 1038, C III λλ977, 1176, and He II λ1085 superposed on a nearly flat continuum. Continuum flux variations can be described as per Gansicke et al. by a ≈ 20 kK white dwarf with a ≈ 37 kK hot spot covering a fraction f ~ 0.15 of the surface of the white dwarf, but we caution that the expected Lyman absorption lines are not detected. The O VI emission lines have a broad and narrow component structure similar to that of the optical emission lines, the C III and He II emission lines are dominated by the broad component, and the radial velocities of these lines are consistent with an origin of the narrow and broad components in the irradiated face of the secondary and the accretion funnel, respectively. The density of the narrow- and broad-line regions is nnlr ~ 3 × 1010 cm-3 and nblr ~ 1 × 1012 cm-3, respectively, yet the narrow-line region is optically thick in the O VI line and the broad-line region is optically thin; apparently, the velocity shear in the broad-line region allows the O VI photons to escape, rendering the gas effectively optically thin. The orbital phase variations of the emission-line fluxes are unexplained.

Ultraviolet observations of a dynamic event in the solar corona

The Solar Ultraviolet Measurements of Emitted Radiation instrument (SUMER) observations show high Doppler shifts and temporal variations in profiles of ultraviolet lines from low temperature gas in the corona above the active region NOAA 7974. The profiles indicate ≈ 100 km s-1 flows coming from an almost stationary source that appears bright in the lines of N III and Si III. The variations in line-of-sight velocities and intensities suggest small knots of cooling plasma emanating from a small region high in the corona. A few arc sec sunward of the region where the cool flows are seen is an elongated region of enhanced higher temperature, low velocity Ne VI and Mg VI line emission.

Doppler Shifts and Nonthermal Broadening in the Quiet Solar Transition Region: O [CSC]vi[/CSC] [CLC][/CLC

Using observations from the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) experiment flown on the Solar and Heliospheric Observatory (SOHO) spacecraft, we have measured Doppler wavelength shifts and nonthermal broadening in the 1032 and 1038 A emission lines of O VI and the 1036 and 1037 A emission lines of C II. Near Sun center the C II lines exhibit an average redshift of 2 ± 3 km s-1, consistent with earlier observations in this temperature range. The O VI emission lines exhibit average Doppler velocities of 5 ± 3 km s-1, suggesting the presence of redshifted material at 2.9 × 105 K. For the O VI lines, the average nonthermal component of the observed line width is 34 ± 3 km s-1.