273 research outputs found
Relativistic MHD with Adaptive Mesh Refinement
This paper presents a new computer code to solve the general relativistic
magnetohydrodynamics (GRMHD) equations using distributed parallel adaptive mesh
refinement (AMR). The fluid equations are solved using a finite difference
Convex ENO method (CENO) in 3+1 dimensions, and the AMR is Berger-Oliger.
Hyperbolic divergence cleaning is used to control the
constraint. We present results from three flat space tests, and examine the
accretion of a fluid onto a Schwarzschild black hole, reproducing the Michel
solution. The AMR simulations substantially improve performance while
reproducing the resolution equivalent unigrid simulation results. Finally, we
discuss strong scaling results for parallel unigrid and AMR runs.Comment: 24 pages, 14 figures, 3 table
Numerical Relativity: A review
Computer simulations are enabling researchers to investigate systems which
are extremely difficult to handle analytically. In the particular case of
General Relativity, numerical models have proved extremely valuable for
investigations of strong field scenarios and been crucial to reveal unexpected
phenomena. Considerable efforts are being spent to simulate astrophysically
relevant simulations, understand different aspects of the theory and even
provide insights in the search for a quantum theory of gravity. In the present
article I review the present status of the field of Numerical Relativity,
describe the techniques most commonly used and discuss open problems and (some)
future prospects.Comment: 2 References added; 1 corrected. 67 pages. To appear in Classical and
Quantum Gravity. (uses iopart.cls
Self-Similarity in General Relativity \endtitle
The different kinds of self-similarity in general relativity are discussed,
with special emphasis on similarity of the ``first'' kind, corresponding to
spacetimes admitting a homothetic vector. We then survey the various classes of
self-similar solutions to Einstein's field equations and the different
mathematical approaches used in studying them. We focus mainly on spatially
homogenous and spherically symmetric self-similar solutions, emphasizing their
possible roles as asymptotic states for more general models. Perfect fluid
spherically symmetric similarity solutions have recently been completely
classified, and we discuss various astrophysical and cosmological applications
of such solutions. Finally we consider more general types of self-similar
models.Comment: TeX document, 53 page
Crystal Structure of Escherichia coli CusC, the Outer Membrane Component of a Heavy Metal Efflux Pump
Background: While copper has essential functions as an enzymatic co-factor, excess copper ions are toxic for cells, necessitating mechanisms for regulating its levels. The cusCBFA operon of E. coli encodes a four-component efflux pump dedicated to the extrusion of Cu(I) and Ag(I) ions. Methodology/Principal Findings: We have solved the X-ray crystal structure of CusC, the outer membrane component of the Cus heavy metal efflux pump, to 2.3 A ˚ resolution. The structure has the largest extracellular opening of any outer membrane factor (OMF) protein and suggests, for the first time, the presence of a tri-acylated N-terminal lipid anchor. Conclusions/Significance: The CusC protein does not have any obvious features that would make it specific for metal ions, suggesting that the narrow substrate specificity of the pump is provided by other components of the pump, most likely by the inner membrane component CusA
Science-Driven Optimization of the LSST Observing Strategy
The Large Synoptic Survey Telescope is designed to provide an unprecedented optical imaging dataset that will support investigations of our Solar System, Galaxy and Universe, across half the sky and over ten years of repeated observation. However, exactly how the LSST observations will be taken (the observing strategy or "cadence") is not yet finalized. In this dynamically-evolving community white paper, we explore how the detailed performance of the anticipated science investigations is expected to depend on small changes to the LSST observing strategy. Using realistic simulations of the LSST schedule and observation properties, we design and compute diagnostic metrics and Figures of Merit that provide quantitative evaluations of different observing strategies, analyzing their impact on a wide range of proposed science projects. This is work in progress: we are using this white paper to communicate to each other the relative merits of the observing strategy choices that could be made, in an effort to maximize the scientific value of the survey. The investigation of some science cases leads to suggestions for new strategies that could be simulated and potentially adopted. Notably, we find motivation for exploring departures from a spatially uniform annual tiling of the sky: focusing instead on different parts of the survey area in different years in a "rolling cadence" is likely to have significant benefits for a number of time domain and moving object astronomy projects. The communal assembly of a suite of quantified and homogeneously coded metrics is the vital first step towards an automated, systematic, science-based assessment of any given cadence simulation, that will enable the scheduling of the LSST to be as well-informed as possible
The Ultra-Fast Outflow of the Quasar PG 1211+143 as Viewed by Time-Averaged Chandra Grating Spectroscopy
This is an author-created, un-copyedited version of an article published in The Astrophysical Journal. The Version of Record is available online at https://doi.org/10.3847/1538-4357/aaa427We present a detailed X-ray spectral study of the quasar PG 1211+143 based on Chandra High Energy Transmission Grating Spectrometer (HETGS) observations collected in a multi-wavelength campaign with UV data using the Hubble Space Telescope Cosmic Origins Spectrograph (HST-COS) and radio bands using the Jansky Very Large Array (VLA). We constructed a multi-wavelength ionizing spectral energy distribution using these observations and archival infrared data to create xstar photoionization models specific to the PG 1211+143 flux behavior during the epoch of our observations. Our analysis of the Chandra-HETGS spectra yields complex absorption lines from H-like and He-like ions of Ne, Mg, and Si, which confirm the presence of an ultra-fast outflow (UFO) with a velocity of approximately -17,300 km s -1 (outflow redshift z out ∼ -0.0561) in the rest frame of PG 1211+143. This absorber is well described by an ionization parameter and column density. This corresponds to a stable region of the absorber's thermal stability curve, and furthermore its implied neutral hydrogen column is broadly consistent with a broad Lyα absorption line at a mean outflow velocity of approximately -16,980 km s -1 detected by our HST-COS observations. Our findings represent the first simultaneous detection of a UFO in both X-ray and UV observations. Our VLA observations provide evidence for an active jet in PG 1211+143, which may be connected to the X-ray and UV outflows; this possibility can be evaluated using very-long-baseline interferometric observations.Peer reviewedFinal Accepted Versio
Digging deeper into the Southern skies: a compact Milky Way companion discovered in first-year Dark Energy Survey data
We use the first-year Dark Energy Survey (DES) data down to previously unprobed photometric depths to search for stellar systems in the Galactic halo, therefore complementing the previous analysis of the same data carried out by our group earlier this year. Our search is based on a matched filter algorithm that produces stellar density maps consistent with stellar population models of various ages, metallicities, and distances over the survey area. The most conspicuous density peaks in these maps have been identified automatically and ranked according to their significance and recurrence for different input models. We report the discovery of one additional stellar system besides those previously found by several authors using the same first-year DES data. The object is compact, and consistent with being dominated by an old and metal-poor population. DES 1 is found at high significance and appears in the DES images as a compact concentration of faint blue point sources. Assuming different spatial profile parameterizations, the best-fitting heliocentric distance and total absolute magnitude in the range of 77.6-87.1 kpc and -3.00 ≲ MV ≲ -2.21, respectively. The half-light radius of this object, rh ˜ 10 pc and total luminosity are consistent with it being a low-mass halo cluster. It is also found to have a very elongated shape (ε ˜ 0.57). In addition, our deeper probe of DES first-year data confirms the recently reported satellite galaxy candidate Horologium II as a significant stellar overdensity. We also infer its structural properties and compare them to those reported in the literature
Combining dark energy survey science verification data with near-infrared data from the ESO VISTA hemisphere survey
We present the combination of optical data from the Science Verification
phase of the Dark Energy Survey (DES) with near infrared data from the ESO
VISTA Hemisphere Survey (VHS). The deep optical detections from DES are used to
extract fluxes and associated errors from the shallower VHS data. Joint 7-band
() photometric catalogues are produced in a single 3 sq-deg DECam
field centred at 02h26m04d36m where the availability of ancillary
multi-wavelength photometry and spectroscopy allows us to test the data
quality. Dual photometry increases the number of DES galaxies with measured VHS
fluxes by a factor of 4.5 relative to a simple catalogue level matching
and results in a 1.5 mag increase in the 80\% completeness limit of the
NIR data. Almost 70\% of DES sources have useful NIR flux measurements in this
initial catalogue. Photometric redshifts are estimated for a subset of galaxies
with spectroscopic redshifts and initial results, although currently limited by
small number statistics, indicate that the VHS data can help reduce the
photometric redshift scatter at both . We present example
DES+VHS colour selection criteria for high redshift Luminous Red Galaxies
(LRGs) at as well as luminous quasars. Using spectroscopic
observations in this field we show that the additional VHS fluxes enable a
cleaner selection of both populations with 10\% contamination from galactic
stars in the case of spectroscopically confirmed quasars and
contamination from galactic stars in the case of spectroscopically confirmed
LRGs. The combined DES+VHS dataset, which will eventually cover almost 5000
sq-deg, will therefore enable a range of new science and be ideally suited for
target selection for future wide-field spectroscopic surveys.We thank the referee, Nicholas Cross, for a very useful report on
this manuscript. MB acknowledges a postdoctoral fellowship via
OL’s Advanced European Research Council Grant (TESTDE).
Funding for the DES Projects has been provided by the U.S.
Department of Energy, the U.S. National Science Foundation, the
Ministry of Science and Education of Spain, the Science and Technology
Facilities Council of the United Kingdom, the Higher Education
Funding Council for England, the National Center for Supercomputing
Applications at the University of Illinois at Urbana-
Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, Financiadora de Estudos e Projetos, Fundac¸ ˜ao
Carlos Chagas Filho de Amparo `a Pesquisa do Estado do Rio de
Janeiro, Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnol
´ogico and the Minist´erio da Ciˆencia e Tecnologia, the Deutsche
Forschungsgemeinschaft and the Collaborating Institutions in the
Dark Energy Survey.
The Collaborating Institutions are Argonne National Laboratories,
the University of California at Santa Cruz, the University of
Cambridge, Centro de Investigaciones Energeticas, Medioambientales
y Tecnologicas-Madrid, the University of Chicago, University
College London, the DES-Brazil Consortium, the Eidgen¨ossische
Technische Hochschule (ETH) Z¨urich, Fermi National Accelerator
Laboratory, the University of Edinburgh, the University of
Illinois at Urbana-Champaign, the Institut de Ciencies de l’Espai
(IEEC/CSIC), the Institut de Fisica d’Altes Energies, the Lawrence
Berkeley National Laboratory, the Ludwig-Maximilians Universit
¨at and the associated Excellence Cluster Universe, the University
of Michigan, the National Optical Astronomy Observatory,
the University of Nottingham, The Ohio State University, the University
of Pennsylvania, the University of Portsmouth, SLAC National
Laboratory, Stanford University, the University of Sussex,
and Texas A&M University.
The DES participants from Spanish institutions are partially
supported by MINECO under grants AYA2009-13936, AYA2012-
39559, AYA2012-39620, and FPA2012-39684, which include
FEDER funds from the European Union.
We are grateful for the extraordinary contributions of our
CTIO colleagues and the DES Camera, Commissioning and Science
Verification teams in achieving the excellent instrument and
telescope conditions that have made this work possible. The success
of this project also relies critically on the expertise and dedication
of the DES Data Management organisation.
The analysis presented here is based on observations obtained
as part of the VISTA Hemisphere Survey, ESO Progam, 179.A-
2010 (PI: McMahon) and data products from observations made
with ESO Telescopes at the La Silla Paranal Observatory under
programme ID 179.A-2006 (PI: Jarvis).
Data for the OzDES spectroscopic survey were obtained with
the Anglo-Australian Telescope (program numbers 12B/11 and
13B/12). Parts of this research were conducted by the Australian
Research Council Centre of Excellence for All-sky Astrophysics
(CAASTRO), through project number CE110001020. TMD acknowledges
the support of the Australian Research Council through
Future Fellowship, FT100100595.This is the final published version. It first appeared at http://mnras.oxfordjournals.org/content/446/3/2523.abstract
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