5,259 research outputs found
Constraints on the average magnetic field strength of relic radio sources 0917+75 and 1401-33 from XMM-Newton observations
We observed two relic radio sources, 0917+75 and 1401-33, with the XMM-Newton
X-ray observatory. We did not detect any X-ray emission, thermal or
non-thermal, in excess of the local background level from either target. This
imposes new upper limits on the X-ray flux due to inverse Compton scattering of
photons from the cosmic microwave background by relativistic electrons in the
relic sources, and new lower limits on the magnetic field strength from the
relative strength of the radio and X-ray emission. The combination of radio and
X-ray observations provides a measure of the magnetic field independent of
equipartition or minimum energy assumptions. Due to increasing sensitivity of
radio observations, the known population of cluster relics has been growing;
however, studies of non-thermal X-ray emission from relics remain scarce. Our
study adds to the small sample of relics studied in X-rays. In both relics, our
field strength lower limits are slightly larger than estimates of the
equipartition magnetic field.Comment: 11 pages, 5 figures. Accepted by MNRA
An Improved Private Mechanism for Small Databases
We study the problem of answering a workload of linear queries ,
on a database of size at most drawn from a universe
under the constraint of (approximate) differential privacy.
Nikolov, Talwar, and Zhang~\cite{NTZ} proposed an efficient mechanism that, for
any given and , answers the queries with average error that is
at most a factor polynomial in and
worse than the best possible. Here we improve on this guarantee and give a
mechanism whose competitiveness ratio is at most polynomial in and
, and has no dependence on . Our mechanism
is based on the projection mechanism of Nikolov, Talwar, and Zhang, but in
place of an ad-hoc noise distribution, we use a distribution which is in a
sense optimal for the projection mechanism, and analyze it using convex duality
and the restricted invertibility principle.Comment: To appear in ICALP 2015, Track
Humphrey Center News: Spring/Summer 1986 v. 2, no. 2
Newsletter of the Hubert H. Humphrey Cancer Research Center at Boston University School of Medicine
Field Tuning of Ferromagnetic Domain Walls on Elastically Coupled Ferroelectric Domain Boundaries
We report on the evolution of ferromagnetic domain walls during magnetization
reversal in elastically coupled ferromagnetic-ferroelectric heterostructures.
Using optical polarization microscopy and micromagnetic simulations, we
demonstrate that the spin rotation and width of ferromagnetic domain walls can
be accurately controlled by the strength of the applied magnetic field if the
ferromagnetic walls are pinned onto 90 degrees ferroelectric domain boundaries.
Moreover, reversible switching between magnetically charged and uncharged
domain walls is initiated by magnetic field rotation. Switching between both
wall types reverses the wall chirality and abruptly changes the width of the
ferromagnetic domain walls by up to 1000%.Comment: 5 pages, 5 figure
Nearly Optimal Private Convolution
We study computing the convolution of a private input with a public input
, while satisfying the guarantees of -differential
privacy. Convolution is a fundamental operation, intimately related to Fourier
Transforms. In our setting, the private input may represent a time series of
sensitive events or a histogram of a database of confidential personal
information. Convolution then captures important primitives including linear
filtering, which is an essential tool in time series analysis, and aggregation
queries on projections of the data.
We give a nearly optimal algorithm for computing convolutions while
satisfying -differential privacy. Surprisingly, we follow
the simple strategy of adding independent Laplacian noise to each Fourier
coefficient and bounding the privacy loss using the composition theorem of
Dwork, Rothblum, and Vadhan. We derive a closed form expression for the optimal
noise to add to each Fourier coefficient using convex programming duality. Our
algorithm is very efficient -- it is essentially no more computationally
expensive than a Fast Fourier Transform.
To prove near optimality, we use the recent discrepancy lowerbounds of
Muthukrishnan and Nikolov and derive a spectral lower bound using a
characterization of discrepancy in terms of determinants
Exact solution of the anisotropic special transition in the O(n) model in 2D
The effect of surface exchange anisotropies is known to play a important role
in magnetic critical and multicritical behavior at surfaces. We give an exact
analysis of this problem in d=2 for the O(n) model by using Coulomb gas,
conformal invariance and integrability techniques. We obtain the full set of
critical exponents at the anisotropic special transition--where the symmetry on
the boundary is broken down to O(n_1)xO(n-n_1)--as a function of n_1. We also
obtain the full phase diagram and crossover exponents. Crucial in this analysis
is a new solution of the boundary Yang-Baxter equations for loop models. The
appearance of the generalization of Schramm-Loewner Evolution SLE_{\kappa,\rho}
is also discussed.Comment: 4 pages, 2 figure
A Unified PTAS for Prize Collecting TSP and Steiner Tree Problem in Doubling Metrics
We present a unified (randomized) polynomial-time approximation scheme (PTAS) for the prize collecting traveling salesman problem (PCTSP) and the prize collecting Steiner tree problem (PCSTP) in doubling metrics. Given a metric space and a penalty function on a subset of points known as terminals, a solution is a subgraph on points in the metric space, whose cost is the weight of its edges plus the penalty due to terminals not covered by the subgraph. Under our unified framework, the solution subgraph needs to be Eulerian for PCTSP, while it needs to be a tree for PCSTP. Before our work, even a QPTAS for the problems in doubling metrics is not known.
Our unified PTAS is based on the previous dynamic programming frameworks proposed in [Talwar STOC 2004] and [Bartal, Gottlieb, Krauthgamer STOC 2012]. However, since it is unknown which part of the optimal cost is due to edge lengths and which part is due to penalties of uncovered terminals, we need to develop new techniques to apply previous divide-and-conquer strategies and sparse instance decompositions
Privacy-preserving stream aggregation with fault tolerance
LNCS v. 7397 entitled: Financial cryptography and data security : 16th International Conference, FC 2012 ... Revised selected papersWe consider applications where an untrusted aggregator would like to collect privacy sensitive data from users, and compute aggregate statistics periodically. For example, imagine a smart grid operator who wishes to aggregate the total power consumption of a neighborhood every ten minutes; or a market researcher who wishes to track the fraction of population watching ESPN on an hourly basis. We design novel mechanisms that allow an aggregator to accurately estimate such statistics, while offering provable guarantees of user privacy against the untrusted aggregator. Our constructions are resilient to user failure and compromise, and can efficiently support dynamic joins and leaves. Our constructions also exemplify the clear advantage of combining applied cryptography and differential privacy techniques. © 2012 Springer-Verlag.postprin
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