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Author

Steven L. Snowden

Bio: Steven L. Snowden is an academic researcher from Goddard Space Flight Center. The author has contributed to research in topics: ROSAT & Galaxy. The author has an hindex of 31, co-authored 68 publications receiving 5712 citations. Previous affiliations of Steven L. Snowden include Universities Space Research Association & University of Wisconsin-Madison.
Topics: ROSAT, Galaxy, Solar wind, Galactic plane, Luminosity


Papers
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Journal ArticleDOI
TL;DR: In this paper, the authors presented new maps of the soft X-ray background from the ROSAT all-sky survey, which represent a significant improvement over the previous version in that position resolution of the PSPC has been used to improve the angular resolution from 2? to 12' and the contributions of point sources have been removed to a uniform source flux level over most of the sky.
Abstract: This paper presents new maps of the soft X-ray background from the ROSAT all-sky survey. These maps represent a significant improvement over the previous version in that (1) the position resolution of the PSPC has been used to improve the angular resolution from ~2? to 12', (2) there are six energy bands that divide each of the previous three into two parts, and (3) the contributions of point sources have been removed to a uniform source flux level over most of the sky. These new maps will be available in electronic format later in 1997. In this paper we also consider the bright emission in the general direction of the Galactic center in the 0.5-2.0 keV band, and the apparent absorption trough that runs through it along the Galactic plane. We find that while the northern hemisphere data are confused by emission from Loop I, the emission seen south of the plane is consistent with a bulge of hot gas surrounding the Galactic center (in our simple model, a cylinder with an exponential fall-off of density with height above the plane). The cylinder has a radial extent of ~5.6 kpc. The X-ray emitting gas has a scale height of 1.9 kpc, an in-plane electron density of ~0.0035 cm-3, a temperature of ~106.6 K, a thermal pressure of ~28,000 cm-3 K, and a total luminosity of ~2 ? 1039 ergs s-1 using a collisional ionization equilibrium (CIE) plasma emission model.

645 citations

Journal ArticleDOI
TL;DR: In this paper, Parkes telescope observations of neutral hydrogen (Hi) in the Small Magellanic Cloud (SMC) were combined with an Australia Telescope Compact Array (ATCA) aperture synthesis mosaic to obtain a set of images sensitive to all angular (spatial) scales between 98 arcsec (30 pc) and 4° (4 kpc).
Abstract: We combine new Parkes telescope observations of neutral hydrogen (Hi) in the Small Magellanic Cloud (SMC) with an Australia Telescope Compact Array (ATCA) aperture synthesis mosaic to obtain a set of images sensitive to all angular (spatial) scales between 98 arcsec (30 pc) and 4° (4 kpc). The new data are used to study the HI spatial power spectrum over a range of contiguous scale sizes wider than those previously achieved in any other galaxy, including our own. The spatial power spectrum closely obeys the relation P(k) ∝ kγ, with γ =-3.04 ± 0.02, similar to values obtained by other authors for our own Galaxy which are in the range γ =-3.0 to -2.8. This is surprising given the very different morphology, gas-richness, star-formation rate and evolution of the two systems, and may imply similar mechanisms for structure formation. One interpretation of the P(k) power-law is that the interstellar medium (ISM) of the SMC is fractal in nature, consisting of a hierarchy of HI cloud structures created, for example, by homogeneous turbulence. The projected fractal dimension of Dp=1.5 is similar to values obtained by other authors for molecular clouds in the Galaxy in the size range ∼ 0.05 to 100 pc. Such a model is consistent with a low space-filling factor for the neutral gas. A kinematic study of the HI data reveals the existence of three supergiant shells which were previously undetectable in the ATCA data alone. These shells have diameters up to 1.8 kpc and require energies (in the standard supernova-driven models) up to 2×1054 erg. The structure and evolution of the ISM in the SMC are heavily influenced by the formation of these supergiant shells.

534 citations

Journal ArticleDOI
TL;DR: In this article, images and the radial profiles of the temperature, abundance, and brightness for 70 clusters of galaxies observed by XMM-Newton are presented along with a detailed discussion of the data reduction and analysis methods, including background modeling, which were used in the processing.
Abstract: Images and the radial profiles of the temperature, abundance, and brightness for 70 clusters of galaxies observed by XMM-Newton are presented along with a detailed discussion of the data reduction and analysis methods, including background modeling, which were used in the processing. Proper consideration of the various background components is vital to extend the reliable determination of cluster parameters to the largest possible cluster radii. The various components of the background including the quiescent particle background, cosmic diffuse emission, soft proton contamination, and solar wind charge exchange emission are discussed along with suggested means of their identification, filtering, and/or their modeling and subtraction. Every component is spectrally variable, sometimes significantly so, and all components except the cosmic background are temporally variable as well. The distributions of the events over the FOV vary between the components, and some distributions vary with energy. The scientific results from observations of low surface brightness objects and the diffuse background itself can be strongly affected by these background components and therefore great care should be taken in their consideration.

426 citations

Journal ArticleDOI
TL;DR: In this paper, a set of pulse-height bands are recommended for analyzing extended structure and the use of detector efficiency maps for proper flat-fielding of images are discussed and demonstrated using IDL routines for implementing these data reduction procedures.
Abstract: Proper reduction of ROSAT observations of extended sources or the diffuse background requires techniques that are quite different from the standard point-source analysis. We describe in detail an appropriate set of procedures for this purpose for the X-ray Telescope (XRT), Position Sensitive Proportional Counter (PSPC). We define a standard set of pulse-height bands recommended for analyzing extended structure and describe the use of detector efficiency maps for proper flat-fielding of images. Sources of noncosmic background contamination and their modeling and subtraction are discussed and demonstrated. IDL routines for implementing these data reduction procedures are being made available to the community through the ROSAT IDL library maintained at Goddard Space Flight Center (GSFC).

364 citations

Journal ArticleDOI
TL;DR: In this article, a new look at the distribution of emission responsible for the soft X-ray diffuse background (SXRB) using ROSAT all-sky survey data was presented.
Abstract: In this paper we present a new look at the distribution of emission responsible for the soft X-ray diffuse background (SXRB) using ROSAT all-sky survey data. This is prompted by the demonstration of the existence of extensive 1/4 keV emission in the Galactic halo that was not considered in the most successful previous model, in which the bulk of the observed 1/4 keV X-rays originated in a Local Hot Bubble (LHB) that surrounds the Sun. The basic distribution can be represented by two angularly varying Galactic components (the LHB and an X-ray halo) and an isotropic extragalactic component. In addition, there are the distinct enhancements of supernova remnants, superbubbles, and clusters of galaxies. Using the negative correlation between the SXRB and DIRBE-corrected IRAS 100 μm intensity, we find a LHB that is similar to previous models, although it is possibly more limited in extent. The emission of the LHB varies by a factor of ~3.3 with direction and produces ROSAT 1/4 keV intensities between ~250 and ~820 × 10-6 counts s-1 arcmin-2 (typically brighter at higher Galactic latitudes), with inferred emission measures of ~0.0018 to 0.0058 cm-6 pc (assuming thermal ionization equilibrium). The distribution of the emission temperature of the LHB is peaked at 106.07 K with a range of ±12% at FWHM. While this variation is small and is perhaps due to systematic uncertainties in the analysis, there is a suggestion of a cooler region in the anticenter direction consistent with previous studies. We derive halo plus extragalactic intensities that vary from ~400 to 3000 × 10-6 counts s-1 arcmin-2 outside the absorbing gas of the Galactic disk. As the low end of the range is comparable to what is expected for the extragalactic background, this implies that the halo emission varies considerably over the sky, with inferred emission measures ranging from near zero to >0.02 cm-6 pc. The distribution of emission temperatures in the halo, again derived from thermal equilibrium emission models, peaks near 106.02 K and varies over ±20% at FWHM.

318 citations


Cited by
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Journal ArticleDOI
TL;DR: In this paper, an improved model for the absorption of X-rays in the interstellar medium (ISM) is presented for use with data from future X-ray missions with larger effective areas and increased energy resolution such as Chandra and the X-Ray Multiple Mirror mission.
Abstract: We present an improved model for the absorption of X-rays in the interstellar medium (ISM) intended for use with data from future X-ray missions with larger effective areas and increased energy resolution such as Chandra and the X-Ray Multiple Mirror mission, in the energy range 100 eV. Compared with previous work, our formalism includes recent updates to the photoionization cross section and revised abundances of the interstellar medium, as well as a treatment of interstellar grains and the H2 molecule. We review the theoretical and observational motivations behind these updates and provide a subroutine for the X-ray spectral analysis program XSPEC that incorporates our model.

3,239 citations

Journal ArticleDOI
02 Nov 2007-Science
TL;DR: A 30-jansky dispersed burst, less than 5 milliseconds in duration, located 3° from the Small Magellanic Cloud is found, which implies that it was a singular event such as a supernova or coalescence of relativistic objects.
Abstract: Pulsar surveys offer a rare opportunity to monitor the radio sky for impulsive burst-like events with millisecond durations. We analyzed archival survey data and found a 30-jansky dispersed burst, less than 5 milliseconds in duration, located 3 degrees from the Small Magellanic Cloud. The burst properties argue against a physical association with our Galaxy or the Small Magellanic Cloud. Current models for the free electron content in the universe imply that the burst is less than 1 gigaparsec distant. No further bursts were seen in 90 hours of additional observations, which implies that it was a singular event such as a supernova or coalescence of relativistic objects. Hundreds of similar events could occur every day and, if detected, could serve as cosmological probes.

1,644 citations

Journal ArticleDOI
25 Nov 1999-Nature
TL;DR: It is shown that single quantum bit operations, Bell-basis measurements and certain entangled quantum states such as Greenberger–Horne–Zeilinger (GHZ) states are sufficient to construct a universal quantum computer.
Abstract: Algorithms such as quantum factoring1 and quantum search2 illustrate the great theoretical promise of quantum computers; but the practical implementation of such devices will require careful consideration of the minimum resource requirements, together with the development of procedures to overcome inevitable residual imperfections in physical systems3,4,5 Many designs have been proposed, but none allow a large quantum computer to be built in the near future6 Moreover, the known protocols for constructing reliable quantum computers from unreliable components can be complicated, often requiring many operations to produce a desired transformation3,4,5,7,8 Here we show how a single technique—a generalization of quantum teleportation9—reduces resource requirements for quantum computers and unifies known protocols for fault-tolerant quantum computation We show that single quantum bit (qubit) operations, Bell-basis measurements and certain entangled quantum states such as Greenberger–Horne–Zeilinger (GHZ) states10—all of which are within the reach of current technology—are sufficient to construct a universal quantum computer We also present systematic constructions for an infinite class of reliable quantum gates that make the design of fault-tolerant quantum computers much more straightforward and methodical

1,604 citations

Journal ArticleDOI
TL;DR: In this paper, gas and total mass profiles for 13 low-redshift, relaxed clusters spanning a temperature range 0.7-9 keV were derived from all available Chandra data of sufficient quality.
Abstract: We present gas and total mass profiles for 13 low-redshift, relaxed clusters spanning a temperature range 0.7-9 keV, derived from all available Chandra data of sufficient quality. In all clusters, gas-temperature profiles are measured to large radii (Vikhlinin et al.) so that direct hydrostatic mass estimates are possible to nearly r500 or beyond. The gas density was accurately traced to larger radii; its profile is not described well by a beta model, showing continuous steepening with radius. The derived ρtot profiles and their scaling with mass generally follow the Navarro-Frenk-White model with concentration expected for dark matter halos in ΛCDM cosmology. However, in three cool clusters, we detect a central mass component in excess of the Navarro-Frenk-White profile, apparently associated with their cD galaxies. In the inner region (r < 0.1r500), the gas density and temperature profiles exhibit significant scatter and trends with mass, but they become nearly self-similar at larger radii. Correspondingly, we find that the slope of the mass-temperature relation for these relaxed clusters is in good agreement with the simple self-similar behavior, M500 Tα, where α = (1.5-1.6) ± 0.1, if the gas temperatures are measured excluding the central cool cores. The normalization of this M-T relation is significantly, by ≈30%, higher than most previous X-ray determinations. We derive accurate gas mass fraction profiles, which show an increase with both radius and cluster mass. The enclosed fgas profiles within r2500 0.4r500 have not yet reached any asymptotic value and are still far (by a factor of 1.5-2) from the universal baryon fraction according to the cosmic microwave background (CMB) observations. The fgas trends become weaker and its values closer to universal at larger radii, in particular, in spherical shells r2500 < r < r500.

1,448 citations

Journal ArticleDOI
TL;DR: In this article, a two-part review summarizes the observations, theory, and simulations of interstellar turbulence and their implications for many fields of astrophysics, including basic fluid equations, solenoidal and compressible modes, global inviscid quadratic invariants, scaling arguments for the power spectrum, phenomenological models for the scaling of higher-order structu...
Abstract: ▪ Abstract Turbulence affects the structure and motions of nearly all temperature and density regimes in the interstellar gas. This two-part review summarizes the observations, theory, and simulations of interstellar turbulence and their implications for many fields of astrophysics. The first part begins with diagnostics for turbulence that have been applied to the cool interstellar medium and highlights their main results. The energy sources for interstellar turbulence are then summarized along with numerical estimates for their power input. Supernovae and superbubbles dominate the total power, but many other sources spanning a large range of scales, from swing-amplified gravitational instabilities to cosmic ray streaming, all contribute in some way. Turbulence theory is considered in detail, including the basic fluid equations, solenoidal and compressible modes, global inviscid quadratic invariants, scaling arguments for the power spectrum, phenomenological models for the scaling of higher-order structu...

1,195 citations