928 research outputs found
Viscous Torque and Dissipation in the Inner Region of a Thin Accretion Disk: Implications for Measuring Black Hole Spin
We consider a simple Newtonian model of a steady accretion disk around a
black hole. The model is based on height-integrated hydrodynamic equations,
alpha-viscosity, and a pseudo-Newtonian potential that results in an innermost
stable circular orbit (ISCO) that closely approximates the one predicted by GR.
We find that the hydrodynamic models exhibit increasing deviations from the
standard disk model of Shakura & Sunyaev as disk thickness H/R or the value of
alpha increases. The latter is an analytical model in which the viscous torque
is assumed to vanish at the ISCO. We consider the implications of the results
for attempts to estimate black hole spin by using the standard disk model to
fit continuum spectra of black hole accretion disks. We find that the error in
the spin estimate is quite modest so long as H/R < 0.1 and alpha < 0.2. At
worst the error in the estimated value of the spin parameter is 0.1 for a
non-spinning black hole; the error is much less for a rapidly spinning hole. We
also consider the density and disk thickness contrast between the gas in the
disk and that inside the ISCO. The contrast needs to be large if black hole
spin is to be successfully estimated by fitting the relativistically-broadened
X-ray line profile of fluorescent iron emission from reflection off an
accretion disk. In our hydrodynamic models, the contrast in density and
thickness is low when H/R>0.1, sugesting that the iron line technique may be
most reliable in extemely thin disks. We caution that these results have been
obtained with a viscous hydrodynamic model and need to be confirmed with MHD
simulations of radiatively cooled thin disks.Comment: 32 pages, 10 figures; accepted by Ap
The Risks of Key Recovery, Key Escrow, and Trusted Third-Party Encryption
A variety of "key recovery," "key escrow," and "trusted third-party" encryption requirements have been suggested in recent years by government agencies seeking to conduct covert surveillance within the changing environments brought about by new technologies. This report examines the fundamental properties of these requirements and attempts to outline the technical risks, costs, and implications of deploying systems that provide government access to encryption keys
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The Risks of Key Recovery, Key Escrow, and Trusted Third-Party Encryption
A variety of "key recovery," "key escrow," and "trusted third-party" encryption requirements have been suggested in recent years by government agencies seeking to conduct covert surveillance within the changing environments brought about by new technologies. This report examines the fundamental properties of these requirements and attempts to outline the technical risks, costs, and implications of deploying systems that provide government access to encryption keys
Bugs in our Pockets: The Risks of Client-Side Scanning
Our increasing reliance on digital technology for personal, economic, and
government affairs has made it essential to secure the communications and
devices of private citizens, businesses, and governments. This has led to
pervasive use of cryptography across society. Despite its evident advantages,
law enforcement and national security agencies have argued that the spread of
cryptography has hindered access to evidence and intelligence. Some in industry
and government now advocate a new technology to access targeted data:
client-side scanning (CSS). Instead of weakening encryption or providing law
enforcement with backdoor keys to decrypt communications, CSS would enable
on-device analysis of data in the clear. If targeted information were detected,
its existence and, potentially, its source, would be revealed to the agencies;
otherwise, little or no information would leave the client device. Its
proponents claim that CSS is a solution to the encryption versus public safety
debate: it offers privacy -- in the sense of unimpeded end-to-end encryption --
and the ability to successfully investigate serious crime. In this report, we
argue that CSS neither guarantees efficacious crime prevention nor prevents
surveillance. Indeed, the effect is the opposite. CSS by its nature creates
serious security and privacy risks for all society while the assistance it can
provide for law enforcement is at best problematic. There are multiple ways in
which client-side scanning can fail, can be evaded, and can be abused.Comment: 46 pages, 3 figure
Contributions to Auditory System Conduction Velocity: Insights With Multi-Modal Neuroimaging and Machine Learning in Children With ASD and XYY Syndrome
Introduction: The M50 electrophysiological auditory evoked response time can be measured at the superior temporal gyrus with magnetoencephalography (MEG) and its latency is related to the conduction velocity of auditory input passing from ear to auditory cortex. In children with autism spectrum disorder (ASD) and certain genetic disorders such as XYY syndrome, the auditory M50 latency has been observed to be elongated (slowed).
Methods: The goal of this study is to use neuroimaging (diffusion MR and GABA MRS) measures to predict auditory conduction velocity in typically developing (TD) children and children with autism ASD and XYY syndrome.
Results: Non-linear TD support vector regression modeling methods accounted for considerably more M50 latency variance than linear models, likely due to the non-linear dependence on neuroimaging factors such as GABA MRS. While SVR models accounted for ~80% of the M50 latency variance in TD and the genetically homogenous XYY syndrome, a similar approach only accounted for ~20% of the M50 latency variance in ASD, implicating the insufficiency of diffusion MR, GABA MRS, and age factors alone. Biologically based stratification of ASD was performed by assessing the conformance of the ASD population to the TD SVR model and identifying a sub-population of children with unexpectedly long M50 latency.
Discussion: Multimodal integration of neuroimaging data can help build a mechanistic understanding of brain connectivity. The unexplained M50 latency variance in ASD motivates future hypothesis generation and testing of other contributing biological factors
Major flaws in conflict prevention policies towards Africa : the conceptual deficits of international actorsâ approaches and how to overcome them
Current thinking on African conflicts suffers from misinterpretations oversimplification, lack of focus, lack of conceptual clarity, state-centrism and lack of vision). The paper analyses a variety of the dominant explanations of major international actors and donors, showing how these frequently do not distinguish with sufficient clarity between the âroot causesâ of a conflict, its aggravating factors and its triggers. Specifically, a correct assessment of conflict prolonging (or sustaining) factors is of vital importance in Africaâs lingering confrontations. Broader approaches (e.g. âstructural stabilityâ) offer a better analytical framework than familiar one-dimensional explanations. Moreover, for explaining and dealing with violent conflicts a shift of attention from the nation-state towards the local and sub-regional level is needed.Aktuelle Analysen afrikanischer Gewaltkonflikte sind hĂ€ufig voller Fehlinterpretationen (Mangel an Differenzierung, Genauigkeit und konzeptioneller Klarheit, Staatszentriertheit, fehlende mittelfristige Zielvorstellungen). Breitere AnsĂ€tze (z. B. das Modell der Strukturellen StabilitĂ€t) könnten die Grundlage fĂŒr bessere Analyseraster und Politiken sein als eindimensionale ErklĂ€rungen. hĂ€ufig differenzieren ErklĂ€rungsansĂ€tze nicht mit ausreichender Klarheit zwischen Ursachen, verschĂ€rfenden und auslösenden Faktoren. Insbesondere die richtige Einordnung konfliktverlĂ€ngernder Faktoren ist in den jahrzehntelangen gewaltsamen Auseinandersetzungen in Afrika von zentraler Bedeutung. Das Diskussionspapier stellt die groĂe Variationsbreite dominanter ErklĂ€rungsmuster der wichtigsten internationalen Geber und Akteure gegenĂŒber und fordert einen Perspektivenwechsel zum Einbezug der lokalen und der subregionalen Ebene fĂŒr die ErklĂ€rung und Bearbeitung gewaltsamer Konflikte
Genome Size and Transposable Element Content as Determined by High-Throughput Sequencing in Maize and Zea luxurians
The genome of maize (Zea mays ssp. mays) consists mostly of transposable elements (TEs) and varies in size among lines. This variation extends to other species in the genus Zea: although maize and Zea luxurians diverged only âŒ140,000 years ago, their genomes differ in size by âŒ50%. We used paired-end Illumina sequencing to evaluate the potential contribution of TEs to the genome size difference between these two species. We aligned the reads both to a filtered gene set and to an exemplar database of unique repeats representing 1,514 TE families; âŒ85% of reads mapped against TE repeats in both species. The relative contribution of TE families to the B73 genome was highly correlated with previous estimates, suggesting that reliable estimates of TE content can be obtained from short high-throughput sequencing reads, even at low coverage. Because we used paired-end reads, we could assess whether a TE was near a gene by determining if one paired read mapped to a TE and the second read mapped to a gene. Using this method, Class 2 DNA elements were found significantly more often in genic regions than Class 1 RNA elements, but Class 1 elements were found more often near other TEs. Overall, we found that both Class 1 and 2 TE families account for âŒ70% of the genome size difference between B73 and luxurians. Interestingly, the relative abundance of TE families was conserved between species (r = 0.97), suggesting genome-wide control of TE content rather than family-specific effects
Multi-Temperature Blackbody Spectrum of a Thin Accretion Disk around a Kerr Black Hole: Model Computations and Comparison with Observations
We use a ray-tracing technique to compute the observed spectrum of a thin
accretion disk around a Kerr black hole. We include all relativistic effects
such as frame-dragging, Doppler boost, gravitational redshift, and bending of
light by the gravity of the black hole. We also include self-irradiation of the
disk as a result of light deflection. Assuming that the disk emission is
locally blackbody, we show how the observed spectrum depends on the spin of the
black hole, the inclination of the disk, and the torque at the inner edge of
the disk. We find that the effect of a nonzero torque on the spectrum can, to a
good approximation, be absorbed into a zero-torque model by adjusting the mass
accretion rate and the normalization. We describe a computer model, called
KERRBB, which we have developed for fitting the spectra of black hole X-ray
binaries. Using KERRBB within the X-ray data reduction package XSPEC, and
assuming a spectral hardening factor f_col = 1.7, we analyze the spectra of
three black hole X-ray binaries: 4U1543-47, XTE J1550-564, and GRO J1655-40. We
estimate the spin parameters of the black holes in 4U1543-47 and GRO J1655-40
to be a/M ~ 0.6 and ~ 0.6-0.7, respectively. If f_col ~ 1.5-1.6, as in a recent
study, then we find a/M ~ 0.7-0.8 and ~ 0.8-0.9, respectively. These estimates
are subject to additional uncertainties in the assumed black hole masses,
distances and disk inclinations.Comment: 75 pages, including 14 figures; accepted for publication in ApJS on
December 14, 200
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