1,473 research outputs found
Evidence for strong extragalactic magnetic fields from Fermi observations of TeV blazars
Magnetic fields in galaxies are produced via the amplification of seed
magnetic fields of unknown nature. The seed fields, which might exist in their
initial form in the intergalactic medium, were never detected. We report a
lower bound ~gauss on the strength of intergalactic
magnetic fields, which stems from the nonobservation of GeV gamma-ray emission
from electromagnetic cascade initiated by tera-electron volt gamma-ray in
intergalactic medium. The bound improves as if magnetic
field correlation length, , is much smaller than a megaparsec. This
lower bound constrains models for the origin of cosmic magnetic fields.Comment: 14 pages, 3 figure
Ladder operators for isospectral oscillators
We present, for the isospectral family of oscillator Hamiltonians, a
systematic procedure for constructing raising and lowering operators satisfying
any prescribed `distorted' Heisenberg algebra (including the
-generalization). This is done by means of an operator transformation
implemented by a shift operator. The latter is obtained by solving an
appropriate partial isometry condition in the Hilbert space. Formal
representations of the non-local operators concerned are given in terms of
pseudo-differential operators. Using the new annihilation operators, new
classes of coherent states are constructed for isospectral oscillator
Hamiltonians. The corresponding Fock-Bargmann representations are also
considered, with specific reference to the order of the entire function family
in each case.Comment: 13 page
Campus Mobility for the Future: The Electric Bicycle
Sustainable and practical personal mobility solutions for campus environments have traditionally revolved around the use of bicycles, or provision of pedestrian facilities. However many campus environments also experience traffic congestion, parking difficulties and pollution from fossil-fuelled vehicles. It appears that pedal power alone has not been sufficient to supplant the use of petrol and diesel vehicles to date, and therefore it is opportune to investigate both the reasons behind the continual use of environmentally unfriendly transport, and consider potential solutions. This paper presents the results from a year-long study into electric bicycle effectiveness for a large tropical campus, identifying barriers to bicycle use that can be overcome through the availability of public use electric bicycles
Moduli spaces of vector bundles over a Klein surface
A compact topological surface S, possibly non-orientable and with non-empty
boundary, always admits a Klein surface structure (an atlas whose transition
maps are dianalytic). Its complex cover is, by definition, a compact Riemann
surface M endowed with an anti-holomorphic involution which determines
topologically the original surface S. In this paper, we compare dianalytic
vector bundles over S and holomorphic vector bundles over M, devoting special
attention to the implications that this has for moduli varieties of semistable
vector bundles over M. We construct, starting from S, totally real, totally
geodesic, Lagrangian submanifolds of moduli varieties of semistable vector
bundles of fixed rank and degree over M. This relates the present work to the
constructions of Ho and Liu over non-orientable compact surfaces with empty
boundary (arXiv:math/0605587) .Comment: 19 pages, 1 figur
Primordial Magnetic Field Limits from Cosmic Microwave Background Bispectrum of Magnetic Passive Scalar Modes
Primordial magnetic fields lead to non-Gaussian signals in the cosmic
microwave background (CMB) even at the lowest order, as magnetic stresses and
the temperature anisotropy they induce depend quadratically on the magnetic
field. In contrast, CMB non-Gaussianity due to inflationary scalar
perturbations arises only as a higher order effect. Apart from a compensated
scalar mode, stochastic primordial magnetic fields also produce scalar
anisotropic stress that remains uncompensated till neutrino decoupling. This
gives rise to an adiabatic-like scalar perturbation mode that evolves passively
thereafter (called the passive mode). We compute the CMB reduced bispectrum
() induced by this passive mode, sourced via the
Sachs-Wolfe effect, on large angular scales. For any configuration of
bispectrum, taking a partial sum over mode-coupling terms, we find a typical
value of , for a magnetic field of nG, assuming a nearly
scale-invariant magnetic spectrum . We also evaluate, in full, the bispectrum
for the squeezed collinear configuration over all angular mode-coupling terms
and find . These values are more than times larger than the
previously calculated magnetic compensated scalar mode CMB bispectrum.
Observational limits on the bispectrum from WMAP7 data allow us to set upper
limits of nG on the present value of the cosmic magnetic field of
primordial origin. This is over 10 times more stringent than earlier limits on
based on the compensated mode bispectrum.Comment: 9 page
HandsOff: Labeled Dataset Generation With No Additional Human Annotations
Recent work leverages the expressive power of generative adversarial networks
(GANs) to generate labeled synthetic datasets. These dataset generation methods
often require new annotations of synthetic images, which forces practitioners
to seek out annotators, curate a set of synthetic images, and ensure the
quality of generated labels. We introduce the HandsOff framework, a technique
capable of producing an unlimited number of synthetic images and corresponding
labels after being trained on less than 50 pre-existing labeled images. Our
framework avoids the practical drawbacks of prior work by unifying the field of
GAN inversion with dataset generation. We generate datasets with rich
pixel-wise labels in multiple challenging domains such as faces, cars,
full-body human poses, and urban driving scenes. Our method achieves
state-of-the-art performance in semantic segmentation, keypoint detection, and
depth estimation compared to prior dataset generation approaches and transfer
learning baselines. We additionally showcase its ability to address broad
challenges in model development which stem from fixed, hand-annotated datasets,
such as the long-tail problem in semantic segmentation.Comment: 22 pages, 20 figure
Moduli Spaces for Principal Bundles in Arbitrary Characteristic
In this article, we solve the problem of constructing moduli spaces of
semistable principal bundles (and singular versions of them) over smooth
projective varieties over algebraically closed ground fields of positive
characteristic.Comment: V4: Final version, to appear in Advances in Mathematics, 69p
The curvaton scenario in the MSSM and predictions for non-Gaussianity
We provide a model in which both the inflaton and the curvaton are obtained
from within the minimal supersymmetric Standard Model, with known gauge and
Yukawa interactions. Since now both the inflaton and curvaton fields are
successfully embedded within the same sector, their decay products thermalize
very quickly before the electroweak scale. This results in two important
features of the model: firstly, there will be no residual isocurvature
perturbations, and secondly, observable non-Gaussianities can be generated with
the non-Gaussianity parameter being
determined solely by the combination of weak-scale physics and the Standard
Model Yukawas.Comment: 4 pages, no figure
Absence of ferromagnetism in Co and Mn substituted polycrystalline ZnO
We discuss the properties of semiconducting bulk ZnO when substituted with
the magnetic transition metal ions Mn and Co, with substituent fraction ranging
from = 0.02 to = 0.15. The magnetic properties were measured as a
function of magnetic field and temperature and we find no evidence for magnetic
ordering in these systems down to = 2 K. The magnetization can be fit by
the sum of a Curie-Weiss term with a Weiss temperature of 100 K and
a Curie term. We attribute this behavior to contributions from both \textit{t}M
ions with \textit{t}M nearest neighbors and from isolated spins. This
particular functional form for the susceptibility is used to explain why no
ordering is observed in \textit{t}M substituted ZnO samples despite the large
values of the Weiss temperature. We also discuss in detail the methods we used
to minimize any impurity contributions to the magnetic signal.Comment: 6 pages, 4 figures (revised
Secure Code Update for Embedded Devices via Proofs of Secure Erasure
Abstract. Remote attestation is the process of verifying internal state of a remote embedded device. It is an important component of many security protocols and applications. Although previously proposed re-mote attestation techniques assisted by specialized secure hardware are effective, they not yet viable for low-cost embedded devices. One no-table alternative is software-based attestation, that is both less costly and more efficient. However, recent results identified weaknesses in some proposed software-based methods, thus showing that security of remote software attestation remains a challenge. Inspired by these developments, this paper explores an approach that relies neither on secure hardware nor on tight timing constraints typi-cal of software-based technqiques. By taking advantage of the bounded memory/storage model of low-cost embedded devices and assuming a small amount of read-only memory (ROM), our approach involves a new primitive â Proofs of Secure Erasure (PoSE-s). We also show that, even though it is effective and provably secure, PoSE-based attestation is not cheap. However, it is particularly well-suited and practical for two other related tasks: secure code update and secure memory/storage erasure. We consider several flavors of PoSE-based protocols and demonstrate their feasibility in the context of existing commodity embedded devices.
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