855 research outputs found
Do Android Taint Analysis Tools Keep Their Promises?
In recent years, researchers have developed a number of tools to conduct
taint analysis of Android applications. While all the respective papers aim at
providing a thorough empirical evaluation, comparability is hindered by varying
or unclear evaluation targets. Sometimes, the apps used for evaluation are not
precisely described. In other cases, authors use an established benchmark but
cover it only partially. In yet other cases, the evaluations differ in terms of
the data leaks searched for, or lack a ground truth to compare against. All
those limitations make it impossible to truly compare the tools based on those
published evaluations.
We thus present ReproDroid, a framework allowing the accurate comparison of
Android taint analysis tools. ReproDroid supports researchers in inferring the
ground truth for data leaks in apps, in automatically applying tools to
benchmarks, and in evaluating the obtained results. We use ReproDroid to
comparatively evaluate on equal grounds the six prominent taint analysis tools
Amandroid, DIALDroid, DidFail, DroidSafe, FlowDroid and IccTA. The results are
largely positive although four tools violate some promises concerning features
and accuracy. Finally, we contribute to the area of unbiased benchmarking with
a new and improved version of the open test suite DroidBench
The relativistic massless harmonic oscillator
A detailed study of the relativistic classical and quantum mechanics of the
massless harmonic oscillator is presented.Comment: 15 pages, 4 figure
Challenges for creating magnetic fields by cosmic defects
We analyse the possibility that topological defects can act as a source of
magnetic fields through the Harrison mechanism in the radiation era. We give a
detailed relativistic derivation of the Harrison mechanism at first order in
cosmological perturbations, and show that it is only efficient for temperatures
above T ~ 0.2 keV. Our main result is that the vector metric perturbations
generated by the defects cannot induce vorticity in the matter fluids at linear
order, thereby excluding the production of currents and magnetic fields. We
show that anisotropic stress in the matter fluids is required to source
vorticity and magnetic fields. Our analysis is relevant for any mechanism
whereby vorticity is meant to be transferred purely by gravitational
interactions, and thus would also apply to dark matter or neutrinos.Comment: 9 pages, 1 figure; minor corrections and additions; accepted for
publication in Physical Review
A Number-Theoretic Error-Correcting Code
In this paper we describe a new error-correcting code (ECC) inspired by the
Naccache-Stern cryptosystem. While by far less efficient than Turbo codes, the
proposed ECC happens to be more efficient than some established ECCs for
certain sets of parameters. The new ECC adds an appendix to the message. The
appendix is the modular product of small primes representing the message bits.
The receiver recomputes the product and detects transmission errors using
modular division and lattice reduction
Magnetic Helicity Generation from the Cosmic Axion Field
The coupling between a primordial magnetic field and the cosmic axion field
generates a helical component of the magnetic field around the time in which
the axion starts to oscillate. If the energy density of the seed magnetic field
is comparable to the energy density of the universe at that time, then the
resulting magnetic helicity is about |H_B| \simeq (10^{-20} G)^2 kpc and
remains constant after its generation. As a corollary, we find that the
standard properties of the oscillating axion remain unchanged even in the
presence of very strong magnetic fields.Comment: 6 pages, 2 figures. Accepted for publication in Phys. Rev. D. Minor
revisions and new references adde
CMB anisotropies due to cosmological magnetosonic waves
We study scalar mode perturbations (magnetosonic waves) induced by a helical
stochastic cosmological magnetic field and derive analytically the
corresponding cosmic microwave background (CMB) temperature and polarization
anisotropy angular power spectra. We show that the presence of a stochastic
magnetic field, or an homogeneous magnetic field, influences the acoustic
oscillation pattern of the CMB anisotropy power spectrum, effectively acting as
a reduction of the baryon fraction. We find that the scalar magnetic energy
density perturbation contribution to the CMB temperature anisotropy is small
compared to the contribution to the CMB -polarization anisotropy.Comment: 17 pages, references added, version accepted for publication in Phys.
Rev.
IGR J17544-2619: A new supergiant fast X-ray transient revealed by optical/infrared observations
One of the most recent discoveries of the INTEGRAL observatory is the
existence of a previously unknown population of X-ray sources in the inner arms
of the Galaxy. IGR J17544-2619, IGR J16465-4507 and XTE J1739-302 are among
these sources. Although the nature of these systems is still unexplained, the
investigations of the optical/NIR counterparts of the two last sources,
combined with high energy data, have provided evidence of them being highly
absorbed high mass X-ray binaries with blue supergiant secondaries and
displaying fast X-ray transient behaviour. In this work we present our
optical/NIR observations of IGR J17544-2619, aimed at identifying and
characterizing its counterpart. We show that the source is a high mass X-ray
binary at a distance of 2-4 kpc with a strongly absorbed O9Ib secondary, and
discuss the nature of the system.Comment: 7 pages, 4 figures. Accepted for publication in A&
Self-organization in a phonon laser
We make an adaptation of laser modelling equations to describe the behavior
of a phonon laser (saser). Our saser consists of an AlGaAs/GaAs double barrier
heterostructure designed to generate an intense beam of transversal acoustic
(TA) phonons. To study our system, we begin with a Hamiltonian that describes
the decay of primary longitudinal optical phonons (LO_1) into secondary (LO_2)
and TA (LO_1 -> LO_2 + TA) and its inverse process (recombination). Using this
Hamiltonian, a set of coupled equations of motion for the phonons is obtained.
We also consider the interaction between the phonons and its reservoirs. These
interactions are introduced in the equations of motion leading to a set of
coupled Langevin equations. In order to obtain an expression to describe our
saser we apply, in the Langevin equations, an adiabatic elimination of some
variables of the subsystem. Following the method above we obtain the value of
the injection threshold for the operation of our phonon laser. At this
threshold occurs a phase transition from a disordered to a coherent state. It
is shown that it is not necessary a big "optical" pumping to get a sasing
region.Comment: 4 figure
Stellar tracers of the Cygnus Arm. II: A young open cluster in Cam OB3
Cam OB3 is the only defined OB association believed to belong to the Outer
Galactic Arm or Cygnus Arm. Very few members have been observed and the
distance modulus to the association is not well known. We attempt a more
complete description of the population of Cam OB3 and a better determination of
its distance modulus. We present uvby photometry of the area surrounding the
O-type stars BD +56 864 and LS I +57 138, finding a clear sequence of
early-type stars that define an uncatalogued open cluster, which we call
Alicante 1. We also present spectroscopy of stars in this cluster and the
surrounding association. From the spectral types for 18 very likely members of
the association and UBV photometry found in the literature, we derive
individual reddenings, finding a extinction law close to standard and an
average distance modulus DM=13.0+-0.4. This value is in excellent agreement
with the distance modulus to the new cluster Alicante 1 found by fitting the
photometric sequence to the ZAMS. In spite of the presence of several O-type
stars, Alicante 1 is a very sparsely populated open cluster, with an almost
total absence of early B-type stars. Our results definitely confirm Cam OB3 to
be located on the Cygnus Arm and identify the first open cluster known to
belong to the association.Comment: Accepted for publication in Astronomy & Astrophysics. Tables 7 & 8 to
appear only in electronic forma
Nonequilibrium Electron Interactions in Metal Films
Ultrafast relaxation dynamics of an athermal electron distribution is
investigated in silver films using a femtosecond pump-probe technique with 18
fs pulses in off-resonant conditions. The results yield evidence for an
increase with time of the electron-gas energy loss rate to the lattice and of
the free electron damping during the early stages of the electron-gas
thermalization. These effects are attributed to transient alterations of the
electron average scattering processes due to the athermal nature of the
electron gas, in agreement with numerical simulations
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