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The Vera C. Rubin Observatory is a unique facility for survey astronomy that will soon be commissioned and begin operations. Crucial to many of its scientific goals is the achievement of sustained high image quality, limited only by the seeing at the site. This will be maintained through an Active Optics System (AOS) that controls optical element misalignments and corrects mirror figure error to minimize aberrations caused by both thermal and gravitational distortions. However, the large number of adjustment degrees of freedom available on the Rubin Observatory introduces a range of degeneracies, including many that are \textit{noise-induced} due to imperfect measurement of the wavefront errors. We present a structured methodology for identifying these degeneracies through an analysis of image noise level. We also present a novel scaling strategy based on Truncated Singular Value Decomposition (TSVD) that mitigates the degeneracy, and optimally distributes the adjustment over the available degrees of freedom. Our approach ensures the attainment of optimal image quality, while avoiding excursions around the noise-induced subspace of degeneracies, marking a significant improvement over the previous techniques adopted for Rubin, which were based on an Optimal Integral Controller (OIC). This new approach is likely to also yield significant benefits for all telescopes that incorporate large numbers of degrees of freedom of adjustment., Comment: 19 pages, 8 figures, 4 tables. Submitted to ApJ

The transient optical sky has remained largely unexplored on very short timescales. While there have been some experiments searching for optical transients from minutes to years, none have had the capability to distinguish millisecond Fast Optical Bursts (FOB). Such very fast transients could be the optical counterparts of Fast Radio Bursts (FRB), the prompt emission from $\gamma$-Ray Bursts (GRB), or other previously unknown phenomena. Here, we investigate a novel approach to the serendipitous detection of FOBs, which relies on searching for anomalous spatial images. In particular, due to their short duration, the seeing distorted images of FOBs should look characteristically different than those of steady sources in a standard optical exposure of finite duration. We apply this idea to simulated observations with the Vera C. Rubin Observatory, produced by tracing individual photons through a turbulent atmosphere, and down through the optics and camera of the Rubin telescope. We compare these simulated images to steady-source star simulations in 15 s integrations, the nominal Rubin exposure time. We report the classification accuracy results of a Neural Network classifier for distinguishing FOBs from steady sources. From this classifier, we derive constraints in duration-intensity parameter space for unambiguously identifying FOBs in Rubin observations. We conclude with estimates of the total number of detections of FOB counterparts to FRBs expected during the 10-year Rubin Legacy Survey of Space and Time (LSST)., Comment: 7 pages, 4 figures, submitted to the Astrophysical Journal

The Vera C. Rubin Observatory will begin the Legacy Survey of Space and Time (LSST) in 2024, spanning an area of 18,000 square degrees in six bands, with more than 800 observations of each field over ten years. The unprecedented data set will enable great advances in the study of the formation and evolution of structure and exploration of physics of the dark universe. The observations will hold clues about the cause for the accelerated expansion of the universe and possibly the nature of dark matter. During the next decade, LSST will be able to confirm or dispute if tensions seen today in cosmological data are due to new physics. New and unexpected phenomena could confirm or disrupt our current understanding of the universe. Findings from LSST will guide the path forward post-LSST. The Rubin Observatory will still be a uniquely powerful facility even then, capable of revealing further insights into the physics of the dark universe. These could be obtained via innovative observing strategies, e.g., targeting new probes at shorter timescales than with LSST, or via modest instrumental changes, e.g., new filters, or through an entirely new instrument for the focal plane. This White Paper highlights some of the opportunities in each scenario from Rubin observations after LSST., Comment: Contribution to Snowmass 2021

We describe the simulated sky survey underlying the second data challenge (DC2) carried out in preparation for analysis of the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) by the LSST Dark Energy Science Collaboration (LSST DESC). Significant connections across multiple science domains will be a hallmark of LSST; the DC2 program represents a unique modeling effort that stresses this interconnectivity in a way that has not been attempted before. This effort encompasses a full end-to-end approach: starting from a large N-body simulation, through setting up LSST-like observations including realistic cadences, through image simulations, and finally processing with Rubin's LSST Science Pipelines. This last step ensures that we generate data products resembling those to be delivered by the Rubin Observatory as closely as is currently possible. The simulated DC2 sky survey covers six optical bands in a wide-fast-deep (WFD) area of approximately 300 deg^2 as well as a deep drilling field (DDF) of approximately 1 deg^2. We simulate 5 years of the planned 10-year survey. The DC2 sky survey has multiple purposes. First, the LSST DESC working groups can use the dataset to develop a range of DESC analysis pipelines to prepare for the advent of actual data. Second, it serves as a realistic testbed for the image processing software under development for LSST by the Rubin Observatory. In particular, simulated data provide a controlled way to investigate certain image-level systematic effects. Finally, the DC2 sky survey enables the exploration of new scientific ideas in both static and time-domain cosmology., Comment: 39 pages, 19 figures, version accepted for publication in ApJS

We present a unique method that allows the LSST to scan the sky for stellar variability on short timescales. The operational component of the strategy requires LSST to take star trail images. The image processing component uses deep learning to sift for transient events on timescales down to 10 ms. We advocate for enabling this observing mode with LSST, as coupling this capability with the LSST's tremendous 319.5 m$^2$deg$^2$ etendue will produce the first wide area optical survey of the universe on these timescales. We explain how these data will advance both planned lines of investigation and enable new research in the areas of stellar flares, cataclysmic variables, active galactic nuclei, Kuiper Belt objects, gamma-ray bursts, and fast radio bursts.

The Large Synoptic Survey Telescope (LSST) Dark Energy Science Collaboration (DESC) will use five cosmological probes: galaxy clusters, large scale structure, supernovae, strong lensing, and weak lensing. This Science Requirements Document (SRD) quantifies the expected dark energy constraining power of these probes individually and together, with conservative assumptions about analysis methodology and follow-up observational resources based on our current understanding and the expected evolution within the field in the coming years. We then define requirements on analysis pipelines that will enable us to achieve our goal of carrying out a dark energy analysis consistent with the Dark Energy Task Force definition of a Stage IV dark energy experiment. This is achieved through a forecasting process that incorporates the flowdown to detailed requirements on multiple sources of systematic uncertainty. Future versions of this document will include evolution in our software capabilities and analysis plans along with updates to the LSST survey strategy., Comment: 32 pages + 60 pages of appendices. This is v1.0.2 of the DESC SRD, an internal collaboration document that is being made public and is not planned for submission to a journal. Data products for reproducing key plots are available on Zenodo, https://doi.org/10.5281/zenodo.1409815 ; see "Executive Summary and User Guide" for how to use and cite those products

We present a method that enables wide field ground-based telescopes to scan the sky for sub-second stellar variability. The method has operational and image processing components. The operational component is to take star trail images. Each trail serves as a light curve for its corresponding source and facilitates sub-exposure photometry. We train a deep neural network to identify stellar variability in wide-field star trail images. We use the Large Synoptic Survey Telescope (LSST) Photon Simulator to generate simulated star trail images and include transient bursts as a proxy for variability. The network identifies transient bursts on timescales down to 10 milliseconds. We argue that there are multiple fields of astrophysics that can be advanced by the unique combination of time resolution and observing throughput that our method offers.

We discuss the synergy of Gaia and the Large Synoptic Survey Telescope (LSST) in the context of Milky Way studies. LSST can be thought of as Gaia's deep complement because the two surveys will deliver trigonometric parallax, proper-motion, and photometric measurements with similar uncertainties at Gaia's faint end at $r=20$, and LSST will extend these measurements to a limit about five magnitudes fainter. We also point out that users of Gaia data will have developed data analysis skills required to benefit from LSST data, and provide detailed information about how international participants can join LSST., Comment: Presented at "The Milky Way Unravelled by Gaia", Barcelona, Dec 1-5, 2014; 7 pages, 1 color figure

A survey that can cover the sky in optical bands over wide fields to faint magnitudes with a fast cadence will enable many of the exciting science opportunities of the next decade. The Large Synoptic Survey Telescope (LSST) will have an effective aperture of 6.7 meters and an imaging camera with field of view of 9.6 deg^2, and will be devoted to a ten-year imaging survey over 20,000 deg^2 south of +15 deg. Each pointing will be imaged 2000 times with fifteen second exposures in six broad bands from 0.35 to 1.1 microns, to a total point-source depth of r~27.5. The LSST Science Book describes the basic parameters of the LSST hardware, software, and observing plans. The book discusses educational and outreach opportunities, then goes on to describe a broad range of science that LSST will revolutionize: mapping the inner and outer Solar System, stellar populations in the Milky Way and nearby galaxies, the structure of the Milky Way disk and halo and other objects in the Local Volume, transient and variable objects both at low and high redshift, and the properties of normal and active galaxies at low and high redshift. It then turns to far-field cosmological topics, exploring properties of supernovae to z~1, strong and weak lensing, the large-scale distribution of galaxies and baryon oscillations, and how these different probes may be combined to constrain cosmological models and the physics of dark energy., Comment: 596 pages. Also available at full resolution at http://www.lsst.org/lsst/scibook

(Abridged) We describe here the most ambitious survey currently planned in the optical, the Large Synoptic Survey Telescope (LSST). A vast array of science will be enabled by a single wide-deep-fast sky survey, and LSST will have unique survey capability in the faint time domain. The LSST design is driven by four main science themes: probing dark energy and dark matter, taking an inventory of the Solar System, exploring the transient optical sky, and mapping the Milky Way. LSST will be a wide-field ground-based system sited at Cerro Pach\'{o}n in northern Chile. The telescope will have an 8.4 m (6.5 m effective) primary mirror, a 9.6 deg$^2$ field of view, and a 3.2 Gigapixel camera. The standard observing sequence will consist of pairs of 15-second exposures in a given field, with two such visits in each pointing in a given night. With these repeats, the LSST system is capable of imaging about 10,000 square degrees of sky in a single filter in three nights. The typical 5$\sigma$ point-source depth in a single visit in $r$ will be $\sim 24.5$ (AB). The project is in the construction phase and will begin regular survey operations by 2022. The survey area will be contained within 30,000 deg$^2$ with $\delta<+34.5^\circ$, and will be imaged multiple times in six bands, $ugrizy$, covering the wavelength range 320--1050 nm. About 90\% of the observing time will be devoted to a deep-wide-fast survey mode which will uniformly observe a 18,000 deg$^2$ region about 800 times (summed over all six bands) during the anticipated 10 years of operations, and yield a coadded map to $r\sim27.5$. The remaining 10\% of the observing time will be allocated to projects such as a Very Deep and Fast time domain survey. The goal is to make LSST data products, including a relational database of about 32 trillion observations of 40 billion objects, available to the public and scientists around the world., Comment: 57 pages, 32 color figures, version with high-resolution figures available from https://www.lsst.org/overview

We fit the Doppler profiles of the He-like triplet complexes of \ion{O}{7} and \ion{N}{6} in the X-ray spectrum of the O star $\zeta$ Puppis, using XMM-Newton RGS data collected over $\sim 400$ ks of exposure. We find that they cannot be well fit if the resonance and intercombination lines are constrained to have the same profile shape. However, a significantly better fit is achieved with a model incorporating the effects of resonance scattering, which causes the resonance line to become more symmetric than the intercombination line for a given characteristic continuum optical depth $\tau_*$. We discuss the plausibility of this hypothesis, as well as its significance for our understanding of Doppler profiles of X-ray emission lines in O stars., Comment: 29 pages, 8 figures, revised version accepted by ApJ

We present results from quantitative modeling and spectral analysis of the high mass X-ray binary Vela X-1 obtained with the Chandra HETGS. The spectra exhibit emission lines from H-like and He-like ions driven by photoionization, as well as fluorescent emission lines from several elements in lower charge states. In order to interpret and make full use of the high-quality data, we have developed a simulator, which calculates the ionization and thermal structure of a stellar wind photoionized by an X-ray source, and performs Monte Carlo simulations of X-ray photons propagating through the wind. The emergent spectra are then computed as a function of the viewing angle accurately accounting for photon transport in three dimensions including dynamics. From comparisons of the observed spectra with the simulation results, we are able to find the ionization structure and the geometrical distribution of material in Vela X-1 that can reproduce the observed spectral line intensities and continuum shapes at different orbital phases remarkably well. It is found that a large fraction of X-ray emission lines from highly ionized ions are formed in the region between the neutron star and the companion star. We also find that the fluorescent X-ray lines must be produced in at least three distinct regions --(1)the extended stellar wind, (2)reflection off the stellar photosphere, and (3)in a distribution of dense material partially covering and possibly trailing the neutron star, which may be associated with an accretion wake. Finally, from detailed analysis of the emission lines, we demonstrate that the stellar wind is affected by X-ray photoionization., Comment: 22 pages, 7 tables, 24 figures, accepted for publication in The Astrophysical Journal

We discuss new methods of measuring and interpreting the forbidden-to-intercombination line ratios of helium-like triplets in the X-ray spectra of O-type stars, including accounting for the spatial distribution of the X-ray emitting plasma and using the detailed photospheric UV spectrum. Measurements are made for four O stars using archival Chandra HETGS data. We assume an X-ray emitting plasma spatially distributed in the wind above some minimum radius R_0. We find minimum radii of formation typically in the range of 1.25 < R_0 / R_* < 1.67, which is consistent with results obtained independently from line profile fits. We find no evidence for anomalously low f/i ratios and we do not require the existence of X-ray emitting plasmas at radii that are too small to generate sufficiently strong shocks., Comment: 52 pages, 24 figures. Accepted in ApJ

In a series of papers, Nicastro et al. have reported the detection of z>0 OVII absorption features in the spectrum of Mrk421 obtained with the Chandra Low Energy Transmission Grating Spectrometer (LETGS). We evaluate this result in the context of a high quality spectrum of the same source obtained with the Reflection Grating Spectrometer (RGS) on XMM-Newton. The data comprise over 955ks of usable exposure time and more than 26000 counts per 50 milliAngstrom at 21.6 Angstroms. We concentrate on the spectrally clean region (21.3 < lambda < 22.5 Angstroms) where sharp features due to the astrophysically abundant OVII may reveal an intervening, warm--hot intergalactic medium (WHIM). We do not confirm detection of any of the intervening systems claimed to date. Rather, we detect only three unsurprising, astrophysically expected features down to the Log(N_i)~14.6 (3 sigma) sensitivity level. Each of the two purported WHIM features is rejected with a statistical confidence that exceeds that reported for its initial detection. While we can not rule out the existence of fainter, WHIM related features in these spectra, we suggest that previous discovery claims were premature. A more recent paper by Williams et al. claims to have demonstrated that the RGS data we analyze here do not have the resolution or statistical quality required to confirm or deny the LETGS detections. We show that our careful analysis resolves the issues encountered by Williams et al. and recovers the full resolution and statistical quality of the RGS data. We highlight the differences between our analysis and those published by Williams et al. as this may explain our disparate conclusions., Comment: 19 pages/7 figures/4 tables. 060424 submitted to ApJ 060522 re-submitted following ApJ request

We provide improved atomic calculation of wavelengths, oscillator strengths, and autoionization rates relevant to the $2\to 3$ inner-shell transitions of Fe~VI--XVI, the so-called Fe~M-shell unresolved transition array (UTA). A second order many-body perturbation theory is employed to obtain accurate transition wavelengths, which are systematically larger than previous theoretical results by 15--45~m{\AA}. For a few transitions of Fe~XVI and Fe~XV where laboratory measurements exist, our new wavelengths are accurate to within a few m{\AA}. Using these new calculations, the apparent discrepancy in the velocities between the Fe~M-shell UTA and other highly ionized absorption lines in the outflow of NGC 3783 disappears. The oscillator strengths in our new calculation agree well with the previous theoretical data, while the new autoionization rates are significantly larger, especially for lower charge states. We attribute this discrepancy to the missing autoionization channels in the previous calculation. The increased autoionization rates may slightly affect the column density analysis of the Fe~M-shell UTA for sources with high column density and very low turbulent broadening. The complete set of atomic data is provided as an electronic table., Comment: Accepted for publication in ApJ

We present high resolution laboratory spectra of K-shell X-ray lines from inner-shell excited and ionized ions of oxygen, obtained with a reflection grating spectrometer on the electron beam ion trap (EBIT-I) at the Lawrence Livermore National Laboratory. Only with a multi-ion model including all major atomic collisional and radiative processes, are we able to identify the observed K-shell transitions of oxygen ions from \ion{O}{3} to \ion{O}{6}. The wavelengths and associated errors for some of the strongest transitions are given, taking into account both the experimental and modeling uncertainties. The present data should be useful in identifying the absorption features present in astrophysical sources, such as active galactic nuclei and X-ray binaries. They are also useful in providing benchmarks for the testing of theoretical atomic structure calculations., Comment: 17 pages, 2 figures, to appear in ApJ

We present high-resolution X-ray observations of the chemically peculiar late-type B-star: mu Lep. However, we find spectroscopic and astrometric evidence, which show that the X-rays are not traced back to the B-star itself, but rather to a previously unresolved companion: mu Lep-B, whose X-ray spectrum resembles that of a coronally active source. We discuss the possibility that mu Lep-B is a pre-main-sequence companion, most likely of the non-accreting magnetically-active type., Comment: To be published in ApJ

We present XMM-Newton Reflection Grating Spectrometer observations of X-ray clusters and groups of galaxies. We demonstrate the failure of the standard cooling-flow model to describe the soft X-ray spectrum of clusters of galaxies. We also emphasize several new developments in the study of the soft X-ray spectrum of cooling flows. Although there is some uncertainty in the expected mass deposition rate for any individual cluster, we show that high resolution RGS spectra robustly demonstrate that the expected line emission from the isobaric cooling-flow model is absent below 1/3 of the background temperature rather than below a fixed temperature in all clusters. Furthermore, we demonstrate that the best-resolved cluster spectra are inconsistent with the predicted shape of the differential luminosity distribution and the measured distribution is tilted to higher temperatures. These observations create several fine-tuning challenges for current theoretical explanations for the soft X-ray cooling-flow problem. Comparisons between these observations and other X-ray measurements are discussed., Comment: 6 pages, 8 figures; Contribution for "The Riddle of Cooling Flows in Galaxies and Clusters of Galaxies", eds. Reiprich, T. H., Kempner, J. C., & Soker, N., Charlottesville, VA, USA, May 31-June 4, 2003, proceedings at http://www.astro.virginia.edu/coolflow

A long 280 ks observation of the Seyfert 1 galaxy NGC 3783 with XMM-Newton is reported. We focus on the oxygen line complex between 17 and 24 A as measured with the RGS spectrometers. Accurate absorption column densities and emission line fluxes are obtained. We explore several options for the geometry and physical form of the emitting and absorbing gas. The lack of change in ionization in the absorber despite an increase in continuum flux during the observation restricts the high-ionization (O-K) and the low-ionization (Fe-M) gas to distances of at least 0.5 pc and 2.8 pc, respectively, away from the central source. Given the P-Cygni type profiles in the resonance spectral lines and the similar velocity widths, column densities, and ionization structure inferred separately from the emission and absorption lines, it is tempting to relate the X-ray narrow-line emitting plasma with the X-ray absorbing gas. Under this assumption, the scenario of dense clumped clouds can be ruled out. Conversely, extended ionization cones (r > 10 pc) are consistent with the observation independent of this assumption. These findings are in stark contrast with the picture of numerous clumpy (n_e > 10^9 cm^-3) clouds drawn recently from UV spectra, but it is consistent with the extended X-ray emission cones observed directly in Seyfert 2 galaxies., Comment: To be published in ApJ

Recent observations with the dispersive X-ray spectrometers aboard Chandra and Newton Observatory have begun to probe the properties of the X-ray intergalactic medium (IGM) at small redshifts. Using large quantities (~950 ksec) of spectroscopic data acquired using the RGS aboard Newton Observatory, we investigated the intervening material toward three low redshift, high Galactic latitude AGNs with nominally featureless spectra: Mrk421, PKS2155-304 and 3C273. Each spectrum provides clear evidence for what appears to be a local (z~0), highly ionized absorbing medium betrayed by the OVII 1s-2p resonance transition feature seen at 21.6A (N[OVII] ~ 1E16 cm-2). Measurements are also made for the Lyman alpha transition of the adjacent ionization state, (OVIII; 18.97A), which potentially constrains the absorber's temperature. Finally, in a collisional equilibrium approximation, upper limits to diffuse emission intensities place upper limits on the electron density (ne < 2E-4 cm-3), lower limits on the scale length of the absorber (L > 140 kpc) and lower limits on its mass (M > 5E10 M[sun]). Limits on the absorber's scale length and its velocity distribution lead us to identify it with the Local Group., Comment: 6 pages, 1 table and 2 figs. Latex. To appear in "The IGM/Galaxy Connection: The Distribution of Baryons at z=0" to be published by Kluwer Academic Publishing. Resubmitted with 2 typos corrected, page 5 and figure 2. Thanks to Masao Sako for pointing these out

The Laboratory Astrophysics program employing the Lawrence Livermore National Laboratory (LLNL) Electron Beam Ion Trap (EBIT) has been providing useful atomic data in support of the x-ray missions Chandra and XMM-Newton. Major achievements have been made for Fe L-shell ions in hot, collisional plasmas, relevant to stellar coronae, supernova remnants, elliptical galaxies, and galaxy clusters. Measurements for L-shell ions of other cosmiscally important elements are also required, some of which are in the LLNL EBIT pipeline. On the other hand, data for inner-shelll excited lines relevant to photoionized plasmas near accretion sources are largely lacking. Even the wavelengths of these lines are only poorly known, which severely limits their use for diagnostics, despite the great potential., Comment: Review for proccedings of NASA Laboratory Astrophysics Workshop, May 1-3 2002, Farid Salama, Editor

We present XMM-Newton observations of the luminous star Eta Carinae, including a high resolution soft X-ray spectrum of the surrounding nebula obtained with the Reflection Grating Spectrometer. The EPIC image of the field around Eta Car shows many early-type stars and diffuse emission from hot, shocked gas. The EPIC spectrum of the star is similar to that observed in previous X-ray observations, and requires two temperature components. The RGS spectrum of the surrounding nebula shows K-shell emission lines from hydrogen- and helium-like nitrogen and neon and L-shell lines from iron, but little or no emission from oxygen. The observed emission lines are not consistent with a single temperature, but the range of temperatures observed is not large, spanning ~0.15-0.6 keV. We obtain upper limits for oxygen line emission and derive a lower limit of N/O > 9. This is consistent with previous abundance determinations for the ejecta of Eta Car, and with theoretical models for the evolution of massive, rotating stars., Comment: 20 pages, 8 figures, accepted by ApJ

High resolution spectroscopic observations of Seyfert galaxies with Chandra and XMM-Newton allows us to study the detailed ionization and thermal structures of the X-ray absorbing/emitting material in the circumnuclear environment. The vast improvement in the spectral resolving power by more than an order of magnitude enables us, for the first time, to unambiguously distinguish the dominant line emission mechanisms and to measure its dynamical properties as well. The X-ray band harbors spectral transitions from a wide range of ionization states, including valence-shell transitions in K-shell and L-shell ions from most cosmically abundant elements, as well as inner-shell transitions of iron and other mid-Z elements, which can be probed through absorption measurements. The X-ray spectrum, therefore, provides simultaneous velocity and column density constraints of highly ionized to only slightly ionized gas harbored in many of these systems. We summarize recent results that have emerged from observations of Seyfert 2 galaxies with the grating spectrometers onboard Chandra and XMM-Newton. We give particular emphasis to an empirical physical model that we have developed based on the observed spectra, and how it can be used for comparative studies with Seyfert 1 galaxies to test the AGN unification scenarios., Comment: 8 pages, 7 figures. Invited review. To appear in the proceedings of the workshop "X-ray spectroscopy of AGN with Chandra and XMM-Newton", eds., Th. Boller, S. Komossa, S. Kahn, and H. Kunieda

The massive X-ray binary Cen X-3 was observed over approximately one quarter of the system's 2.08 day orbit, beginning before eclipse and ending slightly after eclipse center with the Chandra X-ray Observatory using its High-Energy Transmission Grating Spectrometer. The spectra show K shell emission lines from hydrogen- and helium-like ions of magnesium, silicon, sulfur, and iron as well as a K-alpha fluorescence emission feature from near-neutral iron. The helium-like n=2->1 triplet of silicon is fully resolved and the analogous triplet of iron is partially resolved. The helium-like triplet component flux ratios outside of eclipse are consistent with emission from recombination and subsequent cascades (recombination radiation) from a photoionized plasma. In eclipse, however, the w (resonance) lines of silicon and iron are stronger than that expected for recombination radiation, and are consistent with emission from a collisionally ionized plasma. The triplet line flux ratios at both phases can be explained more naturally, however, as emission from a photoionized plasma if the effects of resonant line scattering are included in addition to recombination radiation. We show that the emissivity due to resonant scattering depends sensitively on the line optical depth and, in the case of winds in X-ray binaries, this allows constraints on the wind velocity even when Doppler shifts cannot be resolved., Comment: 31 pages, 8 figures, ApJ accepted, abridged in accord with referee's Comments

A full set of calculations is presented for inner-shell n = 2 to 3 photoexcitation of the 16 iron charge states: Fe I through Fe XVI. The blend of the numerous absorption lines arising from these excitations (mainly 2p - 3d) forms an unresolved transition array (UTA), which has been recently identified as a prominent feature between 16 - 17 \AA in the soft X-ray spectra of active galactic nuclei (AGN). Despite the blending within charge-states, the ample separation between the individual-ion features enables precise diagnostics of the ionization range in the absorbing medium. Column density and turbulent velocity diagnostics are also possible, albeit to a lesser accuracy. An abbreviated set of atomic parameters useful for modeling the Fe 2p - 3d UTA is given. It is shown that the effects of accompanying photoexcitation to higher levels ($n \ge$ 4), as well as the associated photoionization edges, may also be relevant to AGN spectra., Comment: Accepted for publication in The Astrophysical Journal

We present a long-term multi-wavelength light curve of Galactic black hole candidate GRS 1758-258 by combining previously published and archival data from GRANAT, ROSAT, CGRO, RXTE, SAX, ASCA, EXOSAT, and the VLA. In addition we include first spectral results from the balloon-borne Gamma-ray Arcminute Telescope Imaging System (GRATIS). In light of divergent analyses of the 1991-1993 ROSAT observations, we have re-analyzed these data; we find the soft X-rays track the hard X-rays, and that the fits require no black-body component-- indicating that GRS 1758-258 did not go to the high state in 1993. We offer an interpretation based on the ADAF model for a system with mdot \ltsim mdot_crit. We find the 1990-1993 coeval hard and soft X-ray observations support the ADAF predictions. We discuss a new way to constrain black-hole mass with spectral data and the ADAF theory, and apply this technique to GRS 1758-258 to find M_1 \gtsim 8--9 M_sol at an assumed distance of 8.5 kpc. Further investigations of the ADAF model allow us to evaluate the model critically against the data and flux-flux diagram of Barret, McClintock & Grindlay (1996) and to understand the limits of the latter's ``X-ray burster box.'', Comment: 32 pp., AASTEX, 8 ps figures, accepted to ApJ

We present a preliminary analysis of the X-ray spectrum of the accretion disk corona source, 4U 1822-37, obtained with the High Energy Transmission Grating Spectrometer onboard the Chandra X-ray Observatory. We detect discrete emission lines from photoionized iron, silicon, magnesium, neon, and oxygen, as well as a bright iron fluorescence line. Phase-resolved spectroscopy suggests that the recombination emission comes from an X-ray illuminated bulge located at the predicted point of impact between the disk and the accretion stream. The fluorescence emission originates in an extended region on the disk that is illuminated by light scattered from the corona., Comment: 12 pages, 6 figures; Accepted for publication in ApJ Letters

We derive a semi-analytic solution for the structure of conduction-mediated transition layers above an X-ray illuminated accretion disk, and calculate explicitly the X-ray line radiation resulting from both resonance line scattering and radiative recombination in these layers. The vertical thermal structure of the illuminated disk is found to depend on the illuminating continuum: for a hard power law continuum, there are two stable phases connected by a single transition layer; while for a softer continuum, there may exist three stable phases connected by two separate transition layers, with an intermediate stable layer in between. We show that the structure can be written as a function of the electron scattering optical depth through these layers, which leads to unique predictions of the equivalent width of the resulting line radiation from both recombination cascades and resonance line scattering. We find that resonance line scattering plays an important role, especially for the case where there is no intermediate stable layer., Comment: 27 pages, 6 figures, submitted to ApJ

We present a preliminary analysis of the first high-resolution X-ray spectrum of a Seyfert 2 galaxy, Mkn 3, obtained with the High Energy Transmission Grating Spectrometer onboard the Chandra X-ray Observatory. The high-energy spectrum (lambda < 4 Ang) is dominated by reflection of the AGN continuum radiation in a cold optically thick medium and contains bright K-alpha fluorescent lines from iron and silicon, as well as weak, blended lines from sulfur and magnesium. The soft X-ray emission (4 < lambda < 23 Ang) is spatially extended along the [O III] ionization cone and shows discrete signatures of emission following recombination and photoexcitation produced in a warm photoionized region. The measured iron L line fluxes indicate that emission from collisionally ionized plasma is almost completely negligible, and does not contribute significantly to the total energy budget of the X-ray emission. We find that significant fractions of the H- and He-like resonance lines, as well as the observed iron L lines are produced through re-emission from the warm absorbing medium observed in Seyfert 1 galaxies. Its X-ray spectral properties are qualitatively consistent with those of a typical Seyfert 1 galaxy viewed at a different orientation, and provide further convincing evidence for the existence of an obscured Seyfert 1 nucleus in Mkn 3., Comment: 16 pages, including 1 table and 2 figures

We find resolved interstellar O K, Ne K, and Fe L absorption spectra in the Chandra Low Energy Transmission Grating Spectrometer spectrum of the low mass X-ray binary X0614+091. We measure the column densities in O and Ne, and find direct spectroscopic constraints on the chemical state of the interstellar O. These measurements probably probe a low-density line of sight through the Galaxy and we discuss the results in the context of our knowledge of the properties of interstellar matter in regions between the spiral arms., Comment: 16 pages, 4 figures; ApJ, accepted

We present a detailed X-ray spectral analysis of the Circinus Galaxy using archival data obtained with the ASCA satellite. The spectrum shows numerous emission lines in the soft X-ray band from highly ionized ions, as well as Compton reflection and fluorescent lines from neutral or near-neutral matter. We analyze the spectrum in the context of a self-consistent recombination cascade model and find that a nearly flat differential emission measure (DEM) distribution in ionization parameter fits the data. For a fixed solid angle distribution of matter surrounding a point source, this corresponds to a run of electron density of the form, n(r) ~ r^(-3/2), which is suggestive of Bondi accretion onto a central compact mass. Using this density profile and comparing the resulting emission spectra with the ASCA data, the size of the X-ray emission line region is estimated to be < 1 kpc. We also show that the derived density as a function of radius is compatible with the X-ray recombination line emission region being the confining medium of the coronal line regions., Comment: 21 pages including 8 figures and 4 tables; accepted for publication to the Astrophysical Journal

An analysis of the iron L-shell emission in the publicly available spectrum of the Capella binary system, as obtained by the High Energy Transmission Grating Spectrometer on board the Chandra X-ray Observatory, is presented. The atomic-state model, based on the HULLAC code, is shown to be especially adequate for analyzing high-resolution x-ray spectra of this sort. Almost all of the spectral lines in the 10 - 18 Angstrom wavelength range are identified. It is shown that, for the most part, these lines can be attributed to emission from L-shell iron ions in the Capella coronae. Possibilities for electron temperature diagnostics using line ratios of Fe16+ are demonstrated. It is shown that the observed iron-L spectrum can be reproduced almost entirely by assuming a single electron temperature of kTe= 600 eV. This temperature is consistent with both the measured fractional ion abundances of iron and with the temperature derived from ratios of Fe16+ lines. A volume emission measure of 1053 cm-3 is calculated for the iron L-shell emitting regions of the Capella coronae indicating a rather small volume of 1029 cm3 for the emitting plasma if an electron density of 1012 cm-3 is assumed., Comment: Accepted to ApJ

We present a quantitative analysis of the X-ray spectrum of the eclipsing high mass X-ray binary Vela X-1 (4U 0900-40) using archival data from ASCA. The observation covers a time interval centered on eclipse of the X-ray pulsar by the companion. The spectrum exhibits two distinct sets of discrete features: (1) recombination lines and radiative recombination continua from mostly H- and He-like species produced by photoionization in an extended stellar wind; and (2) fluorescent K-shell lines associated with near-neutral species also present in the circumsource medium. Using a detailed spectral model that explicitly accounts for the recombination cascade kinetics for each of the constituent charge states, we are able to obtain a statistically acceptable (chi_r^2=0.88) fit to the observed spectrum and to derive emission measures associated with the individual K-shell ions of several elements. We find a best-fit mass loss rate of ~2.7 x 10^-7 M-solar/yr, which is approximately a factor of 10 lower than previous estimates of the mass loss rate for the Vela X-1 companion star, which have primarily been determined from P Cygni profiles, and X-ray absorption measurements. We argue that this discrepancy can be reconciled if the X-ray irradiated portion of the wind in Vela X-1 is structurally inhomogeneous, consisting of hundreds of cool, dense clumps embedded in a hotter, more ionized gas. Most of the mass is contained in the clumps, while most of the wind volume (>95%) is occupied by the highly ionized component. We show quantitatively, that this interpretation is also consistent with the presence of the X-ray fluorescent lines in the ASCA spectrum, which are produced in the cooler, clumped component., Comment: 45 pages, LaTeX, 9 PostScript figures. To be published in ApJ, November 20, 1999, Volume 526

The University of California, Berkeley (UCB) served as the Principal Investigator institution for the United States participation in the development of the Reflection Grating Spectrometer (RGS) which included the design, development, fabrication, and testing of the Reflection Grating Assembly (RGA). UCB was assisted in this role by the Lawrence Livermore National Laboratory and Columbia University who provided the primary facilities, materials, services and personnel necessary to complete the development. UC Berkeley's Dr. Steven Kahn provided the technical and scientific oversight for the design. development and testing of the RGA units by monitoring the performance of the units at various stages in their development. Dr. Kahn was also the primary contact with the Space Research Organization of the Netherlands (SRON) and represented the RGA development at all SRON and European Space Agency (ESA) reviews of the RGA status. In accordance with the contract, the team designed and developed novel optical technology to meet the unique requirements of the RGS. The ESA XMM-Newton Mission carries two identical Reflection Grating Spectrometers (RGS) behind two of its three nested sets of Wolter I type mirrors. The instrument allows high-resolution measurements in the soft X-ray range (6 to 38 angstroms or 2.1 to 0.3 keV) with a maximum effective area of about 140 sq cm at 15 angstroms. Its design is optimized for the detection of the K-shell transitions of carbon, nitrogen, oxygen, neon, magnesium, and silicon. as well as the L shell transitions of iron. The RGA itself consists of two units. A structure for each unit was designed to hold up to 220 gratings. In its final configuration, one unit holds 182 gratings and the second hold 181 gratings.