Low Energy Transmission Grating Spectrometer Research Articles

X-Ray Spectroscopy of Interstellar Carbon: Evidence for Scattering by Carbon-bearing Material in the Spectrum of 1ES 1553+113

Molecules and particles make up ∼40%–70% of carbon in the interstellar medium, yet the exact chemical structure of these constituents remains unknown. We present carbon K-shell absorption spectroscopy of the Galactic interstellar medium obtained with the Low Energy Transmission Grating Spectrometer on board the Chandra Observatory that directly addresses this question. We probe several lines of sight, using bright active galactic nuclei as backlighters. We make our measurements differentially with respect to the bright source Mrk 421, in order to take the significant carbon K absorption in the instrument into account. In the spectrum of the blazar 1ES 1553+113 we find evidence for a novel feature: strong extinction on the low-energy side of the neutral C 1s−2p resonance, which is indicative of scattering by graphite particles. We find evidence for characteristic particle radii of order 0.1–0.15 μm. If this explanation for the feature is correct, limits on the mass of the available carbon along the line of sight may imply that the grains are partially aligned, and the X-rays from the source may have intrinsic polarization.

The High-resolution Soft X-Ray Spectrum of Nova Delphini 2013

We present the high-resolution soft X-ray spectrum of Nova Delphini 2013. Two spectra were taken with the Low Energy Transmission Grating Spectrometer on the Chandra X-ray Observatory, on 2013 November 9 and 2013 December 6, 87 and 114 days after the nova eruption, respectively. The spectra are of very high statistical quality, and reveal clear spectral evolution between the two observations. The source is bright enough on the two occasions that the third spectral order, with resolving power up to ∼3000, can easily be seen. We observe the photospheric emission spectrum of the hot white dwarf, which exhibits a rich absorption line spectrum from an atmosphere of effective temperature likely near T eff ∼ 640,000 K, and complex chemical abundances. Superimposed on this photospheric spectrum, we detect the absorption spectrum of a shell of highly ionized gas, comprising absorption by the K-shell ions of C and N, blueshifted (outflowing) by ∼1400 km s−1, and with a velocity width of ∼1000 km s−1. The abundance ratio C/N is clearly very nonsolar, and indicative of thermonuclear fusion by the CNO cycle. We discuss the physical properties (kinematics, ionization balance, radiative transfer, mechanical energy balance, chemical abundances) of this hot shell, in the context of the physics of nova eruptions.

DENSITY DETERMINATIONS OF THE CORONAE OF COOL STARS USING A NEWLY ASSIGNED PAIR OF Fe xiv LINES IN THE SPECTRA OFαCANIS MINOR,αCENTAURI, AND THE SUN

We have identified a previously unassigned pair of lines between 169 and 170 A in the coronae of cool stars. We attribute these lines to Fe xiv and show that their intensity ratio is sensitive to the electron density. Using observations taken with the Low Energy Transmission Grating Spectrometer of the Chandra X-ray Observatory we infer a density of log(ne/cm−3) = 10.2 ± 0.7 and 10.3 ± 0.8 from the newly identified line pair in the coronae of Procyon and α Cen A, respectively.

THE UPS AND DOWNS OF α CENTAURI

The following is a progress report on the long-term coronal activity of Alpha Centauri A (HD128620: G2V) and B (HD128621: K1V). Since 2005, Chandra X-ray Observatory has carried out semiannual pointings on AB, mainly with the High Resolution Camera (HRC-I), but also on two occasions with the Low-Energy Transmission Grating Spectrometer (LETGS), fully resolving the close pair in all cases. During 2008-2013, Chandra captured the rise, peak, and initial decline of B's coronal luminosity. Together with previous high states documented by ROSAT and XMM-Newton, the long-term X-ray record suggests a period of 8.2+/-0.2 yr, compared to 11 yr for the Sun; with a minimum-to-peak contrast of 4.5, about half the typical solar cycle amplitude. Meanwhile, the A component has been mired in a Maunder-Minimum-like low state since 2005, initially recognized by XMM. But now, A finally appears to be climbing out of the extended lull. If interpreted simply as an over-long cycle, the period would be 19.1+/-0.7 yr, with a minimum-to-peak contrast of 3.4. The short X-ray cycle of B, and possibly long cycle of A, are not unusual compared with the diverse (albeit much lower amplitude) chromospheric variations recorded, for example, by the HK Project. Further, the deep low state of A also is not unusual, but instead is similar to the Sun during recent minima of the sunspot cycle.

NEW IDENTIFICATIONS OF Fe IX, Fe X, Fe XI, Fe XII, AND Fe XIII LINES IN THE SPECTRUM OF PROCYON OBSERVED WITH THE CHANDRA X-RAY OBSERVATORY

We have analyzed 280 ks of co-added observations performed with Chandra's Low Energy Transmission Grating Spectrometer using theoretical spectra of Fe VIII through Fe XVII. The model spectral data were produced by combining collisional excitation data generated with the Flexible Atomic Code and transition energies generated with a relativistic code based on the multi-reference Moller-Plesset perturbation theory. The spectroscopic accuracy of the theoretical Fe IX wavelengths was ascertained in a comparison with existing laboratory measurements. We find several new Fe IX lines in the 100-140 A region and confirm two previous identifications. We also have identified a new line from Fe X near 111 A; several weak features near 102 A may also be ascribed to Fe X. A line near 100.5 A is identified as originating from Fe XI; a neighboring feature near 101 A may also be from Fe XI. A cluster of three weak lines between 117 and 118 A may be ascribed to Fe XII. Two lines near 104 and 106 A, respectively, have been assigned to Fe XIII. In addition, we confirmed the presence of two out of four Fe VIII lines that were thought to exist in the spectrum. These two lines are located near 131 A. The Fe IX emission is weakly sensitive to the assumed electron density, while the Fe XIII is strongly dependent on density. We find that a density between 109 and 1010 cm–3 provides the best fit to the Procyon spectrum. We note that several of the new identifications have come at the expense of prior assignments to magnesium or calcium lines, removing evidence for the presence of these elements in this spectral region. No evidence for Fe XVIII, Fe XIX, or Fe XX was found.

ChandraLETGS observation of the variable NLS1 galaxy Ark 564

We present an analysis of the 100 ks X-ray spectrum of the narrow-line Seyfert 1 Galaxy Ark 564, taken with the Low Energy Transmission Grating Spectrometer (LETGS) on board Chandra .U singχ 2 statistics, several continuum models of the time-averaged spectrum of this object are compared, obtaining a semi-empirical solution for the description of the intrinsic emission continuum and a physical solution for the intrinsic absorption of the system. We find that the 0.1−10 keV spectrum can be well described by a power law plus two thermal components that account for the soft step. We are also able to detect and measure several narrow, unresolved absorption lines arising from highly ionized species of C, N, O, and Fe. The material seems to have a velocity consistent with the systemic velocity of the galaxy. This piece of evidence, in addition to the very low observed column density of NH ∼ 10 20 cm −2 ,i s in good agreement with the scenario of a transverse biconical outflow with a gas density of n 3 × 10 12 cm −3 at distances beyond the

TESTING EUV/X-RAY ATOMIC DATA FOR THESOLAR DYNAMICS OBSERVATORY

The Atmospheric Imaging Assembly (AIA) and the Exteme-ultraviolet Variability Experiment (EVE) onboard the Solar Dynamics Observatory include spectral windows in the X-ray/EUV band. Accuracy and completeness of the atomic data in this wavelength range is essential for interpretation of the spectrum and irradiance of the solar corona, and of SDO observations made with the AIA and EVE instruments. Here we test the X-ray/EUV data in the CHIANTI database to assess their completeness and accuracy in the SDO bands, with particular focus on the 94A and 131A AIA passbands. Given the paucity of solar observations adequate for this purpose, we use high-resolution X-ray spectra of the low-activity solar-like corona of Procyon obtained with the Chandra Low Energy Transmission Grating Spectrometer (LETGS). We find that while spectral models overall can reproduce quite well the observed spectra in the soft X-ray range ll < 50A, and at the EUV wavelengths ll >130A, they significantly underestimate the observed flux in the 50-130A wavelength range. The model underestimates the observed flux by a variable factor ranging from \approx 1.5, at short wavelengths below \sim50A, up to \approx5-7 in the \sim 70-125A range. In the AIA bands covered by LETGS, i.e. 94A and 131A, we find that the observed flux can be underestimated by large factors (\sim 3 and \sim 1.9 respectively, for the case of Procyon presented here). We discuss the consequences for analysis of AIA data and possible empirical corrections to the AIA responses to model more realistically the coronal emission in these passbands.

Multiwavelength campaign on Mrk 509

We present medium-resolution (λ/Δλ ~ 20 000) ultraviolet spectra covering the 1155−1760 Å spectral range of the Seyfert 1 galaxy Mrk 509 obtained using the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST). Our observations were obtained simultaneously with a Low Energy Transmission Grating Spectrometer observation using the Chandra X-ray Observatory, and they are part of a multiwavelength campaign in September through December 2009 which also included observations with XMM-Newton, Swift, and INTEGRAL. Our spectra are the highest signal-to-noise observations to date of the intrinsic absorption components seen in numerous prior ultraviolet observations. To take advantage of the high S/N, we describe special calibrations for wavelength, flat-field and line-spread function corrections that we applied to the COS data. We detect additional complexity in the absorption troughs compared to prior observations made with the Space Telescope Imaging Spectrograph (STIS) on HST. We attribute the UV absorption to a variety of sources in Mrk 509, including an outflow from the active nucleus, the interstellar medium and halo of the host galaxy, and possible infalling clouds or stripped gaseous material from a merger that are illuminated by the ionizing radiation of the active nucleus. Variability between the STIS and COS observation of the most blue-shifted component (#1) allows us to set an upper limit on its distance of <250 pc. Similarly, variability of component 6 between FUSE observations limits its distance to <1.5 kpc. The absorption lines in all components only partially cover the emission from the active nucleus with covering fractions that are lower than those seen in the prior STIS observations and are comparable to those seen in spectra from the Far Ultraviolet Spectroscopic Explorer (FUSE). Given the larger apertures of COS and FUSE compared to STIS, we favor scattered light from an extended region near the active nucleus as the explanation for the partial covering. As observed in prior X-ray and UV spectra, the UV absorption has velocities comparable to the X-ray absorption,but the bulk of the ultraviolet absorption is in a lower ionization state with lower total column density than the gas responsible for the X-ray absorption. We conclude that the outflow from the active nucleus is a multiphase wind.

Effective area calibration of the reflection grating spectrometers of XMM-Newton

<i>Context. <i/> White dwarf spectra have been widely used as a calibration source for X-ray and EUV instruments. The in-flight effective area calibration of the reflection grating spectrometers (RGS) of XMM-Newton depend upon the availability of reliable calibration sources. <i>Aims. <i/> We investigate how well these white dwarf spectra can be used as standard candles at the lowest X-ray energies in order to gauge the absolute effective area scale of X-ray instruments.<i>Methods. <i/> We calculate a grid of model atmospheres for Sirius B and HZ 43A, and adjust the parameters using several constraints until the ratio of the spectra of both stars agrees with the ratio as observed by the low energy transmission grating spectrometer (LETGS) of Chandra. This ratio is independent of any errors in the effective area of the LETGS. <i>Results. <i/> We find that we can constrain the absolute X-ray spectrum of both stars with better than 5% accuracy. The best-fit model for both stars is close to a pure hydrogen atmosphere, and we put tight limits to the amount of helium or the thickness of a hydrogen layer in both stars. Our upper limit to the helium abundance in Sirius B is 4 times below the previous detection based on EUVE data. We also find that our results are sensitive to the adopted cut-off in the Lyman pseudo-continuum opacity in Sirius B. We get best agreement with a long wavelength cut-off.<i>Conclusions. <i/> White dwarf model atmospheres can be used to derive the effective area of X-ray spectrometers in the lowest energy band. An accuracy of 3–4% in the absolute effective area can be achieved.

High-resolution X-ray spectroscopy of the low and high states of the Seyfert 1 galaxy NGC 4051 with Chandra LETGS

Context. With the new generation of high-resolution X-ray spectrometers the understanding of warm absorbers in active galactic nuclei has improved considerably. However, the important questions regarding the distance and structure of the photoionised wind remain unsolved. Aims. To constrain the distance of the photoionised wind, we study the variability of the continuum, absorption, and, emission in one of the brightest and most variable low-luminosity AGN: the narrow-line Seyfert galaxy NGC 4051. Methods. We analyse two observations taken with the Low Energy Transmission Grating Spectrometer of Chandra. We investigated the spectral response to a sudden flux decrease by a factor of 5, which occurred during the second observation. Results. We detect a highly ionised absorption component with an outflow velocity of −4670 km s −1 , one of the highest outflow velocity components observed in a Seyfert 1 galaxy. Furthermore, this is one of the only observations whereby the X-ray observed absorption component is unaccompanied by a corresponding UV absorption component with the same outflow velocity. The spectra contain a relativistic O viii Lyα line with properties similar to those determined for this source with XMM-Newton, and four absorption components spanning a range in ionisation parameter ξ between 0.07 and 3.19 (log values, and units of 10 −9 W m). An emission component producing radiative recombination continua of C vi and C v appears during the low state. The black body temperature decreases with the drop in flux observed in the second observation. Conclusions. For all absorber components we exclude that the ionisation parameter linearly responded to the decrease in flux by a factor of 5. The variability of the absorber suggest that at least three out of four detected components are located in the range 0.02−1 pc. For one component we only have a lower limit of 0.3 pc. These distances are different from earlier suggestions.

ChandraLETGS Spectroscopy of the Quasar MR 2251−178 and Its Warm Absorber

We present an analysis of our Chandra Low Energy Transmission Grating Spectrometer (LETGS) observation of the quasar MR 2251–178. The warm absorber of MR 2251–178 is well described by a hydrogen column density NH ≈ 2 × 1021 cm−2 and an ionization parameter log (ξ ) ≈ 0.6. We find in the spectrum weak evidence for narrow absorption lines from carbon and nitrogen which indicate that the ionized material is in outflow. We note changes (in time) of the absorption structure in the band 0.6-1 keV (around the unresolved transition arrays [UTAs] plus the O VII and O VIII K edges) at different periods of the observation. We measure a 0.1-2 keV flux of 2.58 × 10−11 ergs cm−2 s−1. This flux implies that the nuclear source of MR 2251–178 is in a relatively low state. No significant variability is seen in the light curve. We do not find evidence for extra cold material in the line of sight, and set an upper limit of NH ≈ 1.2 × 1020 cm−2. The X-ray spectrum does not appear to show evidence for dusty material, although an upper limit in the neutral carbon and oxygen column densities can only be set to cm−2 and cm−2, respectively.

High-Resolution X-Ray Spectroscopy of RX J0806.3+1527 with Chandra

RX J0806.3+1527 is a candidate double-degenerate binary with possibly the shortest known orbital period. The source shows an ≈100% X-ray intensity modulation at the putative orbital period of 321.5 s, but there still is no direct detection of orbital motion at this period. Here we report the results of high-resolution X-ray spectroscopic observations of the source, obtained with the Chandra Low Energy Transmission Grating (LETG) spectrometer. We obtained a total of ≈80 ks of exposure in two pointings separated by about 5 months. We detect the source in both the zeroth-order image and the dispersed spectrum. Detection of the 321.5 s pulsations in the zeroth-order image allows us to extract dispersed spectra as a function of the pulse phase of the X-ray modulation. The bulk of the source flux appears in the 20-60 A band. We find evidence for emission features in the 25-50 A range. The strongest feature (detected at better than 3 σ confidence) is an ≈0.6 A-wide (FWHM) emission line centered at 27 A. Weaker features at 31.6 and 46.4 A are also suggested, but they require confirmation with deeper exposures. The continuum is adequately described by an absorbed blackbody model with a temperature of 65 ± 5 eV and column depth of (2 ± 0.5) × 1020 cm−2. Interestingly, the spectrum does not show strong emission lines of typically abundant elements such as carbon, nitrogen, oxygen, and neon. We briefly discuss possible identifications for the emission features.

The corona and upper transition region of ε Eridani

We present analyses of observations of e Eridani (K2 V) made with the Low Energy Transmission Grating Spectrometer onboard Chandra and the Extreme Ultraviolet Explorer, supplemented by observations made with the Space Telescope Imaging Spectrograph, the Far Ultraviolet Spectroscopic Explorer and the Reflection Grating Spectrometer onboard XMM-Newton. The observed emission lines are used to find relative element abundances, to place limits on the electron densities and pressures and to determine the mean apparent emission measure distribution. As in a previous paper by Sim & Jordan, the mean emitting area as a function of the electron temperature is derived by comparisons with a theoretical emission measure distribution found from energy balance arguments. The final model has a coronal temperature of 3.4 x 10 6 K, an electron pressure of 1.3 x 101 6 cm -3 K at T e = 2 × 10 5 K and an area filling factor of 0.14 at 3.2 x 10 5 K. We discuss a number of issues concerning the atomic data currently available. Our analyses are based mainly on the latest version of CHIANTI (v5.2). We conclude that the Ne/O relative abundance is 0.30, larger than that recommended from solar studies, and that there is no convincing evidence for enhanced coronal abundances of elements with low first ionization potentials.

An investigation of Fe XVI emission lines in solar and stellar extreme-ultraviolet and soft X-ray spectra

New fully relativistic calculations of radiative rates and electron impact excitation cross-sections for Fe XVΙ are used to determine theoretical emission-line ratios applicable to the 251-361 and 32-77 A portions of the extreme-ultraviolet (EUV) and soft X-ray spectral regions, respectively. A comparison of the EUV results with observations from the Solar Extreme-Ultraviolet Research Telescope and Spectrograph (SERTS) reveals excellent agreement between theory and experiment. However, for emission lines in the 32A 9 A portion of the soft X-ray spectral region, there are large discrepancies between theory and measurement for both a solar flare spectrum obtained with the X-Ray Spectrometer/Spectrograph Telescope (XSST) and for observations of Capella from the Low-Energy Transmission Grating Spectrometer (LETGS) on the Chandra X-ray Observatory. These are probably due to blending in the solar flare and Capella data from both first-order lines and from shorter wavelength transitions detected in second and third order. By contrast, there is very good agreement between our theoretical results and the XSST and LETGS observations in the 50-77 A wavelength range, contrary to previous results. In particular, there is no evidence that the Fe XVI emission from the XSST flare arises from plasma at a much higher temperature than that expected for Fe xvi in ionization equilibrium, as suggested by earlier work.

Spectral and Rotational Changes in the Isolated Neutron Star RX J0720.4-3125

RX J0720.4-3125 is an isolated neutron star that, uniquely in its class, has shown changes in its thermal X-ray spectrum. We use new spectra taken with Chandra's Low Energy Transmission Grating Spectrometer, as well as archival observations, to try to understand the timescale and nature of these changes. We construct lightcurves, which show both small, slow variations on a timescale of years, and a larger event that occurred more quickly, within half a year. From timing, we find evidence for a `glitch' coincident with this larger event, with a fractional increase in spin frequency of 5x10^{-8}. We compare the `before' and `after' spectra with those from RX J1308.6+2127, an isolated neutron star with similar temperature and magnetic field strength, but with a much stronger absorption feature in its spectrum. We find that the `after' spectrum can be represented remarkably well by the superposition of the `before' spectrum, scaled by two thirds, and the spectrum of RX J1308.6+2127, thus suggesting that the event affected approximately one third of the surface. We speculate the event reflects a change in surface composition caused by, e.g., an accretion episode.

Coronal Density Diagnostics with Six:ChandraLETGS Observations of Procyon, α Centauri A and B, Capella, and ε Eridani

Electron density diagnostics based on the line intensity ratio of Si X are applied to the X-ray spectra of Procyon, α Cen A and B, Capella, and Eri measured with the Low Energy Transmission Grating Spectrometer combined with the High Resolution Camera on board the Chandra X-Ray Observatory. The ratio R1 of the intensities of the Si X lines at 50.524 and 50.691 Å is adopted. A certain temperature effect in R1 appears near the low-density limit region, which is due to the contamination of the Si X line at 50.703 Å. Using the emission measure distribution model derived by Audard and coworkers for Capella and emissivities calculated with the Astrophysical Plasma Emission Code model by Smith and coworkers, we successfully estimate the contributions of the Fe XVI lines at 50.367 and 50.576 Å (73% and 62%, respectively). A comparison between the observed ratios and theoretical predictions constrains the (logarithmic) electron densities for Procyon to be 8.61 cm-3, while for α Cen A and B, Capella, and Eri they are 8.81, 8.60, 9.30-0.48, and 9.11 cm-3, respectively. The comparison of our results with those constrained by the triplet of He-like carbon shows good agreement. For normal stars, our results display a narrow uncertainty, while for active stars, a relatively larger uncertainty due to contamination from Fe XVI lines is found. Another possible reason may be the uncertainty of the continuum level, since the emission lines of Si X become weak for active stars. For Eri, an electron density in the C V-forming region was first estimated through Si X emission.

Electron density diagnostic potential of the Si x ion and its application in Procyon

Theoretical electron density sensitive line ratios R 1 -R 6 of Si x soft X-ray emission lines are presented. We found that these line ratios are sensitive to electron density n e , and the ratio R 1 is insensitive to electron temperature T e . For reliable determination of the electron density of laboratory and astrophysical plasmas, atomic data, such as electron impact excitation rates, are very important. Our results reveal that the discrepancy of the line ratios from different atomic data calculated with the distorted wave (DW) approximation and the R-matrix method is up to 19 per cent at n e = 2 x 10 8 cm -3 . We applied the theoretical intensity ratio R 1 to the Low Energy Transmission Grating Spectrometer (LETGS) spectrum of the solar-like star Procyon. By comparing the observed value (1.29) with the theoretical calculation, the derived electron density n e is 2.6 x 10 8 cm -3 , which is consistent with that derived from (C v < 8.3 x 10 8 cm -3 ). When the temperature structure of the Procyon corona is taken into account, the derived electron density increases from n e = 2.6 × 10 8 to 2.8 × 10 8 cm -3 .

High‐Resolution Grating Spectroscopy of Gamma‐Ray Bursts 030328 and 041006 withChandraLETGS

We present high-resolution X-ray spectroscopy of two recent γ-ray burst (GRB) afterglows observed with the Low Energy Transmission Grating Spectrometer on Chandra. The afterglows to GRBs 030328 and 041006 are detected beginning 15.33 and 16.8 hr after each burst, respectively, and are observed to fade in time during each ~90 ks observation. We fit for the continuum emission in each full data set and for the data sliced into half and quarter time sections. For both afterglows, the continuum emission is well described by an absorbed power-law model, and the model parameters describing the absorption and spectral slope do not appear to evolve in time. We perform a careful search for deviations from the model continua for the full and time-sliced data and find no evidence for significant (3 σ) narrow emission/absorption lines or edges. The lack of detections implies that line emission—if it is a general feature in GRB X-ray afterglows—occurs early (t ~ 0.3 days in the source frame) and/or is short-lived (dt 10 ks). We also comment on synchrotron fireball models for the X-ray and optical data.

Non‐LTE Model Atmosphere Analysis of the Large Magellanic Cloud Supersoft X‐Ray Source CAL 83

We present a non-LTE (NLTE) model atmosphere analysis of Chandra High Resolution Camera (HRC-S) and Low Energy Transmission Grating (LETG) and XMM-Newton Reflection Grating Spectrometer (RGS) spectroscopy of the prototypical supersoft source CAL 83 in the Large Magellanic Cloud. Taken with a 16 month interval, the Chandra and XMM-Newton spectra are very similar. They reveal a very rich absorption-line spectrum from the hot white dwarf photosphere but no spectral signatures of a wind. We also report a third X-ray off-state during a later Chandra observation, demonstrating the recurrent nature of CAL 83. Moreover, we found evidence of short-timescale variability in the soft X-ray spectrum. We completed the analysis of the LETG and RGS spectra of CAL 83 with new NLTE line-blanketed model atmospheres that explicitly include 74 ions of the 11 most abundant species. We successfully matched the Chandra and XMM-Newton spectra assuming a model composition with LMC metallicity. We derived the basic stellar parameters of the hot white dwarf, but the current state of atomic data in the soft X-ray domain precludes a detailed chemical analysis. We have obtained the first direct spectroscopic evidence that the white dwarf is massive (MWD 1 M☉). The short timescale of the X-ray off-states is consistent with a high white dwarf mass. Our analysis thus provides direct support for supersoft sources as likely progenitors of Type Ia supernovae (SNe Ia).

The Total Gas and Dust Abundances in the Line of Sight to X Persei, Based onChandraSpectroscopy

We have observed the high-mass X-ray binary X Persei/4U 0352+30 with the Low Energy Transmission Grating Spectrometer (LETGS) aboard the Chandra X-Ray Observatory. We fit the spectrum near the oxygen K-shell absorption edge to determine the total (gas+dust) abundance of oxygen along this line of sight. Comparison with UV gas-phase measurements along this same line of sight gives an abundance of grain-bound oxygen which is in general agreement with published grain model fits to the observed extinction. No evidence is found for a significant reservoir of oxygen in large, previously undetected grains. We also attempt to extract and fit the radial profile of a halo of scattered X-rays in the zero-order image, but our efforts are hampered by grating artifacts and high detector background. We do not obtain useful results using standard models for dust scattering.