44,867 research outputs found
Bringing self assessment home: repository profiling and key lines of enquiry within DRAMBORA
Digital repositories are a manifestation of complex organizational, financial, legal, technological, procedural, and political interrelationships. Accompanying each of these are innate uncertainties, exacerbated by the relative immaturity of understanding prevalent within the digital preservation domain. Recent efforts have sought to identify core characteristics that must be demonstrable by successful digital repositories, expressed in the form of check-list documents, intended to support the processes of repository accreditation and certification. In isolation though, the available guidelines lack practical applicability; confusion over evidential requirements and difficulties associated with the diversity that exists among repositories (in terms of mandate, available resources, supported content and legal context) are particularly problematic. A gap exists between the available criteria and the ways and extent to which conformity can be demonstrated. The Digital Repository Audit Method Based on Risk Assessment (DRAMBORA) is a methodology for undertaking repository self assessment, developed jointly by the Digital Curation Centre (DCC) and DigitalPreservationEurope (DPE). DRAMBORA requires repositories to expose their organization, policies and infrastructures to rigorous scrutiny through a series of highly structured exercises, enabling them to build a comprehensive registry of their most pertinent risks, arranged into a structure that facilitates effective management. It draws on experiences accumulated throughout 18 evaluative pilot assessments undertaken in an internationally diverse selection of repositories, digital libraries and data centres (including institutions and services such as the UK National Digital Archive of Datasets, the National Archives of Scotland, Gallica at the National Library of France and the CERN Document Server). Other organizations, such as the British Library, have been using sections of DRAMBORA within their own risk assessment procedures.
Despite the attractive benefits of a bottom up approach, there are implicit challenges posed by neglecting a more objective perspective. Following a sustained period of pilot audits undertaken by DPE, DCC and the DELOS Digital Preservation Cluster aimed at evaluating DRAMBORA, it was stated that had respective project members not been present to facilitate each assessment, and contribute their objective, external perspectives, the results may have been less useful. Consequently, DRAMBORA has developed in a number of ways, to enable knowledge transfer from the responses of comparable repositories, and incorporate more opportunities for structured question sets, or key lines of enquiry, that provoke more comprehensive awareness of the applicability of particular threats and opportunities
The Design of Pumpjets for Hydrodynamic Propulsion
A procedure for use in the design of a wake adapted pumpjet mounted on the aft end of a body of revolution is presented. To this end, a pumpjet is designed for the Akron airship. The propulsor mass flow is selected to minimize kinetic energy losses through the duct and in the discharge jet. The shaft speed and disk size are selected to satisfy specified limits of cavitation performance and to provide acceptable blade loading. The streamtubes which pass through a propulsor mounted on a tapered afterbody follow essentially conical surfaces. A method is provided for defining these surfaces as a function of shroud geometry, rotor head distribution, and the energy distribution of the ingested mass flow. The three-dimensional effects to which the conical flow subjects the cylindrical blade design sections are described and a technique is presented which permits incorporation of these effects in the blade design procedure
Iron fluorescence from within the innermost stable orbit of black hole accretion disks
The fluorescent iron Ka line is a powerful observational probe of the inner
regions of black holes accretion disks. Previous studies have assumed that only
material outside the radius of marginal stability can contribute to the
observed line emission. Here, we show that fluorescence by material inside the
radius of marginal stability, which is in the process of spiralling towards the
event horizon, can have a observable influence on the iron line profile and
equivalent width. For concreteness, we consider the case of a geometrically
thin accretion disk, around a Schwarzschild black hole, in which fluorescence
is excited by an X-ray source placed at some height above the disk and on the
axis of the disk. Fully relativistic line profiles are presented for various
source heights and efficiencies. It is found that the extra line flux generally
emerges in the extreme red wing of the iron line, due to the large
gravitational redshift experienced by photons from the region within the radius
of marginal stability. We apply our models to the variable iron line seen in
the ASCA spectrum of the Seyfert nucleus MCG-6-30-15. It is found that the
change in the line profile, equivalent width, and continuum normalization, can
be well explained as being due to a change in the height of the source above
the disk. We discuss the implications of these results for distinguishing
rapidly-rotating black holes from slowly rotating holes using iron line
diagnostics.Comment: 20 pages, LaTeX. Accepted for publication in Astrophysical Journal.
Figures 3 to 7 replaced with corrected versions (previous figures affected by
calculational error). Some changes in the best fitting parameter
The Ionized Stellar Wind in Vela X-1 During Eclipse
We present a first analysis of a high resolution X-ray spectrum of the
ionized stellar wind of Vela X-1 during eclipse. The data were obtained with
the High Energy Transmission Grating Spectrometer onboard the Chandra X-ray
Observatory. The spectrum is resolved into emission lines with fluxes between
0.02 and 1.04x10^4 ph/cm^2/s. We identify lines from a variety of charge
states, including fluorescence lines from cold material, a warm photoionized
wind. We can exclude signatures from collisionally ionized plasmas. For the
first time we identify fluorescence lines from L-shell ions from lower Z
elements. We also detect radiative recombination continua from a kT = 10 eV
(1.2 x 10^5 K) photoionized optically thin gas. The fluorescence line fluxes
infer the existence of optically thick and clumped matter within or outside the
warm photoionized plasma.Comment: 4 pages, 2 figures, accepted by ApJ letter
Sequential Flavour Symmetry Breaking
The gauge sector of the Standard Model (SM) exhibits a flavour symmetry which
allows for independent unitary transformations of the fermion multiplets. In
the SM the flavour symmetry is broken by the Yukawa couplings to the Higgs
boson, and the resulting fermion masses and mixing angles show a pronounced
hierarchy. In this work we connect the observed hierarchy to a sequence of
intermediate effective theories, where the flavour symmetries are broken in a
step-wise fashion by vacuum expectation values of suitably constructed spurion
fields. We identify the possible scenarios in the quark sector and discuss some
implications of this approach.Comment: 22 pages latex, no figure
Cascades: A view from Audience
Cascades on online networks have been a popular subject of study in the past
decade, and there is a considerable literature on phenomena such as diffusion
mechanisms, virality, cascade prediction, and peer network effects. However, a
basic question has received comparatively little attention: how desirable are
cascades on a social media platform from the point of view of users? While
versions of this question have been considered from the perspective of the
producers of cascades, any answer to this question must also take into account
the effect of cascades on their audience. In this work, we seek to fill this
gap by providing a consumer perspective of cascade.
Users on online networks play the dual role of producers and consumers.
First, we perform an empirical study of the interaction of Twitter users with
retweet cascades. We measure how often users observe retweets in their home
timeline, and observe a phenomenon that we term the "Impressions Paradox": the
share of impressions for cascades of size k decays much slower than frequency
of cascades of size k. Thus, the audience for cascades can be quite large even
for rare large cascades. We also measure audience engagement with retweet
cascades in comparison to non-retweeted content. Our results show that cascades
often rival or exceed organic content in engagement received per impression.
This result is perhaps surprising in that consumers didn't opt in to see tweets
from these authors. Furthermore, although cascading content is widely popular,
one would expect it to eventually reach parts of the audience that may not be
interested in the content. Motivated by our findings, we posit a theoretical
model that focuses on the effect of cascades on the audience. Our results on
this model highlight the balance between retweeting as a high-quality content
selection mechanism and the role of network users in filtering irrelevant
content
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