2,075 research outputs found
How Do Tor Users Interact With Onion Services?
Onion services are anonymous network services that are exposed over the Tor
network. In contrast to conventional Internet services, onion services are
private, generally not indexed by search engines, and use self-certifying
domain names that are long and difficult for humans to read. In this paper, we
study how people perceive, understand, and use onion services based on data
from 17 semi-structured interviews and an online survey of 517 users. We find
that users have an incomplete mental model of onion services, use these
services for anonymity and have varying trust in onion services in general.
Users also have difficulty discovering and tracking onion sites and
authenticating them. Finally, users want technical improvements to onion
services and better information on how to use them. Our findings suggest
various improvements for the security and usability of Tor onion services,
including ways to automatically detect phishing of onion services, more clear
security indicators, and ways to manage onion domain names that are difficult
to remember.Comment: Appeared in USENIX Security Symposium 201
R-process Nucleosynthesis from Three-Dimensional Magnetorotational Core-Collapse Supernovae
We investigate r-process nucleosynthesis in three-dimensional (3D)
general-relativistic magnetohydrodynamic simulations of rapidly rotating
strongly magnetized core collapse. The simulations include a microphysical
finite-temperature equation of state and a leakage scheme that captures the
overall energetics and lepton number exchange due to postbounce neutrino
emission and absorption. We track the composition of the ejected material using
the nuclear reaction network SkyNet. Our results show that the 3D dynamics of
magnetorotational core-collapse supernovae (CCSN) are important for their
nucleosynthetic signature. We find that production of r-process material beyond
the second peak is reduced by a factor of 100 when the magnetorotational jets
produced by the rapidly rotating core undergo a kink instability. Our results
indicate that 3D magnetorotationally powered CCSNe are a robust r-process
source only if they are obtained by the collapse of cores with unrealistically
large precollapse magnetic fields of order G. Additionally, a
comparison simulation that we restrict to axisymmetry, results in overly
optimistic r-process production for lower magnetic field strengths.Comment: 10 pages, 9 figures, 2 tables. submitted to Ap
COS-Weak: Probing the CGM using analogs of weak Mg II absorbers at z < 0.3
We present a sample of 34 weak metal line absorbers at complied via
the simultaneous detections () of the SiII and
CII absorption lines, with (SiII) \AA\ and
(CII) \AA, in archival HST/COS spectra. Our sample increases the
number of known low- "weak absorbers" by a factor of . The column
densities of HI and low-ionization metal lines obtained from Voigt profile
fitting are used to build simple photoionization models using CLOUDY. The
inferred densities and total hydrogen column densities are in the ranges of
and , respectively. The line of sight thicknesses of the absorbers
have a wide range of 1 pc50 kpc with a median value of 500 pc.
The high-ionization OVI absorption, detected in 12/18 cases, always stems from
a different gas-phase. Most importantly, 85% (50%) of these absorbers show a
metallicity of (0.0). The fraction of systems showing high
metallicity (i.e., ) in our sample is significantly higher
than the HI-selected sample (Wotta et al. 2016) and the galaxy-selected sample
(Prochaska et al. 2017) of absorbers probing the circum-galactic medium (CGM)
at similar redshift. A search for galaxies has revealed a significant
galaxy-overdensity around these weak absorbers compared to random places with a
median impact parameter of 166 kpc to the nearest galaxy. Moreover, we find the
presence of multiple galaxies in % of the cases, suggesting group
environments. The observed of indicates that such
metal-enriched, compact, dense structures are ubiquitous in the halos of
low- galaxies that are in groups. We suggest that these are transient
structures that are related to outflows and/or stripping of metal-rich gas from
galaxies.Comment: Published (2018MNRAS.476.4965M) after minor revision. Appendix A is
newly added
Seismic anisotropy in deforming halite:Evidence from the Mahogany salt body
We present unambiguous evidence that the Mahogany salt body, located in the Northern part of the Gulf of Mexico, is seismically anisotropic. Evidence of anisotropy comes from shear wave splitting data obtained from a vertical seismic profile VSP. The data set consists of 48 vertically aligned receivers in a borehole drilled through the salt body. Splitting analysis is performed on shear wave phases that are converted from compressional waves at the top and bottom of the salt body. The phase converted at the top of the salt layer shows a clear signature of seismic anisotropy, while the phase at the base of the salt layer shows negligible splitting. We investigate the possibility of rock salt halite LPO as a cause of the observed anisotropy. A finite element geomechanical salt deformation model of the Mahogany salt body is developed, where deformation history is used as an input to the texture plasticity simulation program VPSC. Assuming a halite salt body, a full elasticity model is then calculated and used to create a synthetic VSP splitting data set. The comparison between the synthetic and real VSP data set shows that LPO of rock salt can explain the observed anisotropy remarkably well. This is the strongest evidence to date of seismic anisotropy in a deforming salt structure. Furthermore, for the first time, we are able to demonstrate clear evidence that deforming halite is the most likely cause of this anisotropy, combining data set analysis and synthetic full wave form modelling based on calculated rock salt elasticities. Neglecting anisotropy in seismic processing in salt settings could lead to potential imaging errors, for example the deformation models show an averaged delta parameter of δ=-0.06, which would lead in a zero offset reflection setting to a depth mismatch of 6.2 per cent. Our work also show how observations of salt anisotropy can be used to probe characteristics of salt deformation
Sampling for Inference in Probabilistic Models with Fast Bayesian Quadrature
We propose a novel sampling framework for inference in probabilistic models:
an active learning approach that converges more quickly (in wall-clock time)
than Markov chain Monte Carlo (MCMC) benchmarks. The central challenge in
probabilistic inference is numerical integration, to average over ensembles of
models or unknown (hyper-)parameters (for example to compute the marginal
likelihood or a partition function). MCMC has provided approaches to numerical
integration that deliver state-of-the-art inference, but can suffer from sample
inefficiency and poor convergence diagnostics. Bayesian quadrature techniques
offer a model-based solution to such problems, but their uptake has been
hindered by prohibitive computation costs. We introduce a warped model for
probabilistic integrands (likelihoods) that are known to be non-negative,
permitting a cheap active learning scheme to optimally select sample locations.
Our algorithm is demonstrated to offer faster convergence (in seconds) relative
to simple Monte Carlo and annealed importance sampling on both synthetic and
real-world examples
Deuterium fractionation in the Horsehead edge
Deuterium fractionation is known to enhance the [DCO+]/[HCO+] abundance ratio
over the D/H elemental ratio of about 1e-5 in the cold and dense gas typically
found in pre-stellar cores. We report the first detection and mapping of very
bright DCO+ J=3-2 and J=2-1 lines (3 and 4 K respectively) towards the
Horsehead photodissociation region (PDR) observed with the IRAM-30m telescope.
The DCO+ emission peaks close to the illuminated warm edge of the nebula (< 50"
or about 0.1 pc away). Detailed nonlocal, non-LTE excitation and radiative
transfer analyses have been used to determine the prevailing physical
conditions and to estimate the DCO+ and H13CO+ abundances from their line
intensities. A large [DCO+]/[HCO+] abundance ratio (>= 0.02) is inferred at the
DCO+ emission peak, a condensation shielded from the illuminating far-UV
radiation field where the gas must be cold (10-20 K) and dense (>= 2x10^5
cm-3). DCO+ is not detected in the warmer photodissociation front, implying a
lower [DCO+]/[HCO+] ratio (< 1e-3). According to our gas phase chemical
predictions, such a high deuterium fractionation of HCO+ can only be explained
if the gas temperature is below 20 K, in good agreement with DCO+ excitation
calculations.Comment: 4 pages, 3 PostScript figures. Accepted for publication in Astronomy
& Astrophysics in the letter section. Uses aa LaTeX macro
At Home and Abroad: The Use of Denial-of-service Attacks during Elections in Nondemocratic Regimes
In this article, we study the political use of denial-of-service (DoS) attacks, a particular form of cyberattack that disables web services by flooding them with high levels of data traffic. We argue that websites in nondemocratic regimes should be especially prone to this type of attack, particularly around political focal points such as elections. This is due to two mechanisms: governments employ DoS attacks to censor regime-threatening information, while at the same time, activists use DoS attacks as a tool to publicly undermine the governmentâs authority. We analyze these mechanisms by relying on measurements of DoS attacks based on large-scale Internet traffic data. Our results show that in authoritarian countries, elections indeed increase the number of DoS attacks. However, these attacks do not seem to be directed primarily against the country itself but rather against other states that serve as hosts for news websites from this country.publishe
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