635 research outputs found
Probing the internal magnetic field of slowly pulsating B-stars through g modes
We suggest that high-order g modes can be used as a probe of the internal
magnetic field of SPB (slowly pulsating B) stars. The idea is based on earlier
work by the authors which analytically investigated the effect of a vertical
magnetic field on p and g modes in a plane-parallel isothermal stratified
atmosphere. It was found that even a weak field can significantly shift the
g-mode frequencies -- the effect increases with mode order. In the present
study we adopt the classical perturbative approach to estimate the internal
field of a 4 solar mass SPB star by looking at its effect on a low-degree
() and high-order () g mode with a period of about 1.5 d. We find
that a polar field strength of about 110 kG on the edge of the convective core
is required to produce a frequency shift of 1%. Frequency splittings of that
order have been observed in several SPB variables, in some cases clearly too
small to be ascribed to rotation. We suggest that they may be due to a poloidal
field with a strength of order 100 kG, buried in the deep interior of the star.Comment: 4 pages, 2 figures (to appear in Astronomy & Astrophysics
Solar Magnetic Field Signatures in Helioseismic Splitting Coefficients
Normal modes of oscillation of the Sun are useful probes of the solar
interior. In this work, we use the even-order splitting coefficients to study
the evolution of magnetic fields in the convection zone over solar cycle 23,
assuming that the frequency splitting is only due to rotation and a large scale
magnetic field. We find that the data are best fit by a combination of a
poloidal field and a double-peaked near-surface toroidal field. The toroidal
fields are centered at r=0.999R_solar and r=0.996R_solar and are confined to
the near-surface layers. The poloidal field is a dipole field. The peak
strength of the poloidal field is 124 +/- 17G. The toroidal field peaks at 380
+/- 30G and 1.4 +/- 0.2kG for the shallower and deeper fields respectively. The
field strengths are highly correlated with surface activity. The toroidal field
strength shows a hysteresis-like effect when compared to the global 10.7 cm
radio flux. The poloidal field strength shows evidence of saturation at high
activity.Comment: 10 pages, accepted for publication in Ap
Asteroseismic Signatures of Stellar Magnetic Activity Cycles
Observations of stellar activity cycles provide an opportunity to study
magnetic dynamos under many different physical conditions. Space-based
asteroseismology missions will soon yield useful constraints on the interior
conditions that nurture such magnetic cycles, and will be sensitive enough to
detect shifts in the oscillation frequencies due to the magnetic variations. We
derive a method for predicting these shifts from changes in the Mg II activity
index by scaling from solar data. We demonstrate this technique on the
solar-type subgiant beta Hyi, using archival International Ultraviolet Explorer
spectra and two epochs of ground-based asteroseismic observations. We find
qualitative evidence of the expected frequency shifts and predict the optimal
timing for future asteroseismic observations of this star.Comment: 5 pages including 3 figures and 1 table, MNRAS Letters accepte
Leakage-Resilient Cryptography
We construct a stream-cipher SC whose \emph{implementation} is secure even if arbitrary (adversely chosen) information on the internal state of SC is leaked during computation. This captures \emph{all} possible side-channel attacks on SC where the amount of information leaked in a given period is bounded, but overall cankbe arbitrary large, in particular much larger than the internalkstate of SC. The only other assumption we make on the \emph{implementation} of SC is that only data that is accessedkduring computation leaks information. The construction can be based on any pseudorandom generator, and the only computational assumption we make is that this PRG is secure against non-uniform adversaries in the classical sense (i.e. when there are no side-channels).
The stream-cipher SC generates its output in chunks , and arbitrary but bounded information leakage is modeled by allowing the adversary to adaptively chose a function before is computed, she then gets where is the internal state of \SC that is accessed during the computation of . One notion of security we prove for \SC is that is indistinguishable from random when given , and also the complete internal state of SC after has been computed (i.e. our cipher is forward-secure).
The construction is based on alternating extraction (previously used in the intrusion-resilient secret-sharing scheme from FOCS'07). We move this concept to the computational setting by proving a lemma that states that the output of any PRG has high HILL pseudoentropy (i.e. is indistinguishable from some distribution with high min-entropy) even if arbitrary information about the seed is leaked. The amount of leakage \leak that we can tolerate in each step depends on the strength of the underlying PRG, it is at least logarithmic, but can be as large as a constant fraction of the internal state of SC if the PRG is exponentially hard
High Resolution Spectroscopy of the Pulsating White Dwarf G29-38
We present the analysis of time-resolved, high resolution spectra of the cool
white dwarf pulsator, G29-38. From measuring the Doppler shifts of the H-alpha
core, we detect velocity changes as large as 16.5 km/s and conclude that they
are due to the horizontal motions associated with the g-mode pulsations on the
star. We detect seven pulsation modes from the velocity time-series and
identify the same modes in the flux variations. We discuss the properties of
these modes and use the advantage of having both velocity and flux measurements
of the pulsations to test the convective driving theory proposed for DAV stars.
Our data show limited agreement with the expected relationships between the
amplitude and phases of the velocity and flux modes. Unexpectedly, the velocity
curve shows evidence for harmonic distortion, in the form of a peak in the
Fourier transform whose frequency is the exact sum of the two largest
frequencies. Combination frequencies are a characteristic feature of the
Fourier transforms of light curves of G29-38, but before now have not been
detected in the velocities, nor does published theory predict that they should
exist. We compare our velocity combination frequency to combination frequencies
found in the analysis of light curves of G29-38, and discuss what might account
for the existence of velocity combinations with the properties we observe.
We also use our high-resolution spectra to determine if either rotation or
pulsation can explain the truncated shape observed for the DAV star's line
core. We are able to eliminate both mechanisms: the average spectrum does not
fit the rotationally broadened model and the time-series of spectra provides
proof that the pulsations do not significantly truncate the line.Comment: 24 pages, 9 figures, Accepted for publication in ApJ (June
Non-malleable codes for space-bounded tampering
Non-malleable codesâintroduced by Dziembowski, Pietrzak and Wichs at ICS 2010âare key-less coding schemes in which mauling attempts to an encoding of a given message, w.r.t. some class of tampering adversaries, result in a decoded value that is either identical or unrelated to the original message. Such codes are very useful for protecting arbitrary cryptographic primitives against tampering attacks against the memory. Clearly, non-malleability is hopeless if the class of tampering adversaries includes the decoding and encoding algorithm. To circumvent this obstacle, the majority of past research focused on designing non-malleable codes for various tampering classes, albeit assuming that the adversary is unable to decode. Nonetheless, in many concrete settings, this assumption is not realistic
Nonlinear Couplings Between r-modes of Rotating Neutron Stars
The r-modes of neutron stars can be driven unstable by gravitational
radiation. While linear perturbation theory predicts the existence of this
instability, linear theory can't provide any information about the nonlinear
development of the instability. The subject of this paper is the weakly
nonlinear regime of fluid dynamics. In the weakly nonlinear regime, the
nonlinear fluid equations are approximated by an infinite set of oscillators
which are coupled together so that terms quadratic in the mode amplitudes are
kept in the equations of motion. In this paper, the coupling coefficients
between the r-modes are computed. The stellar model assumed is a polytropic
model where a source of buoyancy is included so that the Schwarzschild
discriminant is nonzero. The properties of these coupling coefficients and the
types of resonances possible are discussed in this paper. It is shown that no
exact resonance involving the unstable r-mode occur and that only a
small number of modes have a dimensionless coupling constant larger than unity.
However, an infinite number of resonant mode triplets exist which couple
indirectly to the unstable r-mode. All couplings in this paper involve the
l>|m| r-modes which only exist if the star is slowly rotating. This work is
complementary to that of Schenk et al (2002) who consider rapidly rotating
stars which are neutral to convection.Comment: 21 pages, 1 figure, to appear in Ap
Generalized parton distributions in a meson cloud model
We present a model calculation of the generalized parton distributions where
the nucleon is described by a quark core surrounded by a mesonic cloud. In the
one-meson approximation, we expand the Fock state of the physical nucleon in a
series involving a bare nucleon and two-particle, meson-baryon, states. We
discuss the role of the different Fock-state components of the nucleon by
deriving a convolution formalism for the unpolarized generalized parton
distributions, and showing predictions at different kinematics.Comment: 7 pages, 3 figures; Invited talk at the Fifth International
Conference on Perspectives in Hadronic Physics, Miramare - Trieste (Italy),
22-26 May 200
On the Orthogonal Vector Problem and the Feasibility of Unconditionally Secure Leakage-Resilient Computation
We consider unconditionally secure leakage resilient two-party
computation, where security means that the leakage obtained by an
adversary can be simulated using a similar amount of leakage from the
private inputs or outputs. A related problem is known as circuit
compilation, where there is only one device doing a computation on
public input and output. Here the goal is to ensure that the adversary
learns only the input/output behaviour of the computation, even given
leakage from the internal state of the device. We study these
problems in an enhanced version of the ``only computation leaks\u27\u27
model, where the adversary is additionally allowed a bounded amount of
{\em global} leakage from the state of the entity under attack. In
this model, we show the first unconditionally secure leakage resilient
two-party computation protocol. The protocol assumes access to
correlated randomness in the form of a functionality \fOrt that
outputs pairs of orthogonal vectors over some
finite field, where the adversary can leak independently from
and from . We also construct a general circuit
compiler secure in the same leakage model. Our constructions work,
even if the adversary is allowed to corrupt a constant fraction of the
calls to \fOrt and decide which vectors should be output. On the
negative side, we show that unconditionally secure two-party
computation and circuit compilation are in general impossible in the
plain version of our model. For circuit compilation we need a
computational assumption to exhibit a function that cannot be securely
computed, on the other hand impossibility holds even if global leakage
is not allowed. It follows that even a somewhat unreliable version of
\fOrt cannot be implemented with unconditional security in the plain
leakage model, using classical communication. However, we show that an
implementation using quantum communication does exist. In particular,
we propose a simple ``prepare-and-measure\u27\u27 type protocol which we
show secure using a new result on sampling from a quantum
population. Although the protocol may produce a small number of
incorrect pairs, this is sufficient for leakage resilient computation
by our other results
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