14,014 research outputs found
A multi-wavelength view of magnetic flaring from PMS stars
Flares from the Sun and other stars are most prominently observed in the soft
X-ray band. Most of the radiated energy, however, is released at optical/UV
wavelengths. In spite of decades of investigation, the physics of flares is not
fully understood. Even less is known about the powerful flares routinely
observed from pre-main sequence stars, which might significantly influence the
evolution of circumstellar disks. Observations of the NGC2264 star forming
region were obtained in Dec. 2011, simultaneously with three telescopes,
Chandra (X-rays), CoRoT (optical), and Spitzer (mIR), as part of the
"Coordinated Synoptic Investigation of NGC2264" (CSI-NGC2264). Shorter Chandra
and CoRoT observations were also obtained in March 2008. We analyzed the
lightcurves to detect X-ray flares with an optical and/or mIR counterpart.
Basic flare properties from the three datasets, such as emitted energies and
peak luminosities, were then compared to constrain the spectral energy
distribution of the flaring emission and the physical conditions of the
emitting regions. Flares from stars with and without circumstellar disks were
also compared to establish any difference that might be attributed to the
presence of disks. Seventy-eight X-ray flares with an optical and/or mIR
counterpart were detected. Their optical emission is found to correlate well
with, and to be significantly larger than, the X-ray emission. The slopes of
the correlations suggest that the difference becomes smaller for the most
powerful flares. The mIR flare emission seems to be strongly affected by the
presence of a circumstellar disk: flares from stars with disks have a stronger
mIR emission with respect to stars without disks. This might be attributed to
the reprocessing of the optical (and X-ray) flare emission by the inner
circumstellar disk, providing evidence for flare-induced disk heating.Comment: 16 pages (36 including appendixes), 8 figures (main text), accepted
for publication by Astronomy & Astrophysics (section 8
Deformations of quantum field theories on de Sitter spacetime
Quantum field theories on de Sitter spacetime with global U(1) gauge symmetry
are deformed using the joint action of the internal symmetry group and a
one-parameter group of boosts. The resulting theory turns out to be wedge-local
and non-isomorphic to the initial one for a class of theories, including the
free charged Dirac field. The properties of deformed models coming from
inclusions of CAR-algebras are studied in detail.Comment: 26 pages, no figure
Quantifying structural damage from self-irradiation in a plutonium superconductor
The 18.5 K superconductor PuCoGa5 has many unusual properties, including
those due to damage induced by self-irradiation. The superconducting transition
temperature decreases sharply with time, suggesting a radiation-induced Frenkel
defect concentration much larger than predicted by current radiation damage
theories. Extended x-ray absorption fine-structure measurements demonstrate
that while the local crystal structure in fresh material is well ordered, aged
material is disordered much more strongly than expected from simple defects,
consistent with strong disorder throughout the damage cascade region. These
data highlight the potential impact of local lattice distortions relative to
defects on the properties of irradiated materials and underscore the need for
more atomic-resolution structural comparisons between radiation damage
experiments and theory.Comment: 7 pages, 5 figures, to be published in PR
Comment on "Evidence for Neutrinoless Double Beta Decay"
We comment on the recent claim for the experimental observation of
neutrinoless double-beta decay. We discuss several limitations in the analysis
provided in that paper and conclude that there is no basis for the presented
claim.Comment: A comment written to Modern Physics Letters A. 4 pages, no figures.
Updated version, accepted for publicatio
On the equivalence of two deformation schemes in quantum field theory
Two recent deformation schemes for quantum field theories on the
two-dimensional Minkowski space, making use of deformed field operators and
Longo-Witten endomorphisms, respectively, are shown to be equivalent.Comment: 14 pages, no figure. The final version is available under Open
Access. CC-B
Casimir force in brane worlds: coinciding results from Green's and Zeta function approaches
Casimir force encodes the structure of the field modes as vacuum fluctuations
and so it is sensitive to the extra dimensions of brane worlds. Now, in flat
spacetimes of arbitrary dimension the two standard approaches to the Casimir
force, Green's function and zeta function, yield the same result, but for brane
world models this was only assumed. In this work we show both approaches yield
the same Casimir force in the case of Universal Extra Dimensions and
Randall-Sundrum scenarios with one and two branes added by p compact
dimensions. Essentially, the details of the mode eigenfunctions that enter the
Casimir force in the Green's function approach get removed due to their
orthogonality relations with a measure involving the right hyper-volume of the
plates and this leaves just the contribution coming from the Zeta function
approach. The present analysis corrects previous results showing a difference
between the two approaches for the single brane Randall-Sundrum; this was due
to an erroneous hyper-volume of the plates introduced by the authors when using
the Green's function. For all the models we discuss here, the resulting Casimir
force can be neatly expressed in terms of two four dimensional Casimir force
contributions: one for the massless mode and the other for a tower of massive
modes associated with the extra dimensions.Comment: 30 pages, title, abstract and discussion have change
Exchange Bias Induced by the Fe3O4 Verwey transition
We present a study of the exchange bias in different configurations of V2O3
thin films with ferromagnetic layers. The exchange bias is accompanied by a
large vertical shift in the magnetization. These effects are only observed when
V2O3 is grown on top of Ni80Fe20 permalloy. The magnitude of the vertical shift
is as large as 60% of the total magnetization which has never been reported in
any system. X-Ray diffraction studies show that the growth conditions promote
the formation of a ferrimagnetic Fe3O4 interlayer. The change in the easy
magnetization axis of Fe3O4 across the Verwey transition at 120 K is correlated
with the appearance of exchange bias and vertical shift in magnetization. Both
phenomena disappear above 120 K, indicating for the first time a direct
relationship between the magnetic signature of the Verwey transition and
exchange bias.Comment: Accepted for publication Physical Review
On the spectrum of AdS/CFT beyond supergravity
We test the spectrum of string theory on AdS_5 x S^5 derived in
hep-th/0305052 against that of single-trace gauge invariant operators in free
N=4 super Yang-Mills theory. Masses of string excitations at critical tension
are derived by extrapolating plane-wave frequencies at g_{YM}=0 down to finite
J. On the SYM side, we present a systematic description of the spectrum of
single-trace operators and its reduction to PSU(2,2|4) superconformal primaries
via a refined Eratostenes' supersieve. We perform the comparison of the
resulting SYM/string spectra of charges and multiplicities order by order in
the conformal dimension \Delta up to \Delta=10 and find perfect agreement.
Interestingly, the SYM/string massive spectrum exhibits a hidden symmetry
structure larger than expected, with bosonic subgroup SO(10,2) and thirty-two
supercharges.Comment: 28 pages, LaTeX2
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