25,077 research outputs found
Cosmic variance of the galaxy cluster weak lensing signal
Intrinsic variations of the projected density profiles of clusters of
galaxies at fixed mass are a source of uncertainty for cluster weak lensing. We
present a semi-analytical model to account for this effect, based on a
combination of variations in halo concentration, ellipticity and orientation,
and the presence of correlated haloes. We calibrate the parameters of our model
at the 10 per cent level to match the empirical cosmic variance of cluster
profiles at M_200m=10^14...10^15 h^-1 M_sol, z=0.25...0.5 in a cosmological
simulation. We show that weak lensing measurements of clusters significantly
underestimate mass uncertainties if intrinsic profile variations are ignored,
and that our model can be used to provide correct mass likelihoods. Effects on
the achievable accuracy of weak lensing cluster mass measurements are
particularly strong for the most massive clusters and deep observations (with
~20 per cent uncertainty from cosmic variance alone at M_200m=10^15 h^-1 M_sol
and z=0.25), but significant also under typical ground-based conditions. We
show that neglecting intrinsic profile variations leads to biases in the
mass-observable relation constrained with weak lensing, both for intrinsic
scatter and overall scale (the latter at the 15 per cent level). These biases
are in excess of the statistical errors of upcoming surveys and can be avoided
if the cosmic variance of cluster profiles is accounted for.Comment: 14 pages, 6 figures; submitted to MNRA
Towards Quantitative Classification of Folded Proteins in Terms of Elementary Functions
A comparative classification scheme provides a good basis for several
approaches to understand proteins, including prediction of relations between
their structure and biological function. But it remains a challenge to combine
a classification scheme that describes a protein starting from its well
organized secondary structures and often involves direct human involvement,
with an atomary level Physics based approach where a protein is fundamentally
nothing more than an ensemble of mutually interacting carbon, hydrogen, oxygen
and nitrogen atoms. In order to bridge these two complementary approaches to
proteins, conceptually novel tools need to be introduced. Here we explain how
the geometrical shape of entire folded proteins can be described analytically
in terms of a single explicit elementary function that is familiar from
nonlinear physical systems where it is known as the kink-soliton. Our approach
enables the conversion of hierarchical structural information into a
quantitative form that allows for a folded protein to be characterized in terms
of a small number of global parameters that are in principle computable from
atomary level considerations. As an example we describe in detail how the
native fold of the myoglobin 1M6C emerges from a combination of kink-solitons
with a very high atomary level accuracy. We also verify that our approach
describes longer loops and loops connecting -helices with
-strands, with same overall accuracy.Comment: 3 figure
The long period eccentric orbit of the particle accelerator HD167971 revealed by long baseline interferometry
Using optical long baseline interferometry, we resolved for the first time
the two wide components of HD167971, a candidate hierarchical triple system
known to efficiently accelerate particles. Our multi-epoch VLTI observations
provide direct evidence for a gravitational link between the O8 supergiant and
the close eclipsing O + O binary. The separation varies from 8 to 15 mas over
the three-year baseline of our observations, suggesting that the components
evolve on a wide and very eccentric orbit (most probably e>0.5). These results
provide evidence that the wide orbit revealed by our study is not coplanar with
the orbit of the inner eclipsing binary. From our measurements of the
near-infrared luminosity ratio, we constrain the spectral classification of the
components in the close binary to be O6-O7, and confirm that these stars are
likely main-sequence objects. Our results are discussed in the context of the
bright non-thermal radio emission already reported for this system, and we
provide arguments in favour of a maximum radio emission coincident with
periastron passage. HD167971 turns out to be an efficient O-type particle
accelerator that constitutes a valuable target for future high angular
resolution radio imaging using VLBI facilities.Comment: 8 pages, including 4 figures, accepted by Monthly Notices of the
Royal Astronomical Societ
A Note on Flux Induced Superpotentials in String Theory
Non-vanishing fluxes in M-theory and string theory compactifications induce a
superpotential in the lower dimensional theory. Gukov has conjectured the
explicit form of this superpotential. We check this conjecture for the
heterotic string compactified on a Calabi-Yau three-fold as well as for warped
M-theory compactifications on Spin(7) holonomy manifolds, by performing a
Kaluza-Klein reduction.Comment: 19 pages, no figure
Fractal Geometry of Higher Derivative Gravity
We determine the scaling properties of geometric operators such as lengths, areas, and volumes in models of higher derivative quantum gravity by renormalizing appropriate composite operators. We use these results to deduce the fractal dimensions of such hypersurfaces embedded in a quantum spacetime at very small distances
Strong-field approximation for intense-laser atom processes: the choice of gauge
The strong-field approximation can be and has been applied in both length
gauge and velocity gauge with quantitatively conflicting answers. For
ionization of negative ions with a ground state of odd parity, the predictions
of the two gauges differ qualitatively: in the envelope of the angular-resolved
energy spectrum, dips in one gauge correspond to humps in the other. We show
that the length-gauge SFA matches the exact numerical solution of the
time-dependent Schr\"odinger equation.Comment: 5 pages, 3 figures, revtex
String vacua with flux from freely-acting obifolds
A precise correspondence between freely-acting orbifolds (Scherk-Schwarz
compactifications) and string vacua with NSNS flux turned on is established
using T-duality.
We focus our attention to a certain non-compact Z_2 heterotic freely-acting
orbifold with N=2 supersymmetry (SUSY). The geometric properties of the T-dual
background are studied. As expected, the space is non-Kahler with the most
generic torsion compatible with SUSY. All equations of motion are satisfied,
except the Bianchi identity for the NSNS field, that is satisfied only at
leading order in derivatives, i.e. without the curvature term. We point out
that this is due to unknown corrections to the standard heterotic T-duality
rules.Comment: 13 pages, no figures; v2: references added and rearranged, version to
appear in JHE
The 2.35 year itch of Cyg OB2 #9. II. Radio monitoring
Cyg OB2 #9 is one of a small set of non-thermal radio emitting massive O-star
binaries. The non-thermal radiation is due to synchrotron emission in the
colliding-wind region. Cyg OB2 #9 was only recently discovered to be a binary
system and a multi-wavelength campaign was organized to study its 2011
periastron passage. We report here on the results of the radio observations
obtained in this monitoring campaign. We used the Expanded Very Large Array
(EVLA) radio interferometer to obtain 6 and 20 cm continuum fluxes. The
observed radio light curve shows a steep drop in flux sometime before
periastron. The fluxes drop to a level that is comparable to the expected
free-free emission from the stellar winds, suggesting that the non-thermal
emitting region is completely hidden at that time. After periastron passage,
the fluxes slowly increase. We introduce a simple model to solve the radiative
transfer in the stellar winds and the colliding-wind region, and thus determine
the expected behaviour of the radio light curve. From the asymmetry of the
light curve, we show that the primary has the stronger wind. This is somewhat
unexpected if we use the astrophysical parameters based on theoretical
calibrations. But it becomes entirely feasible if we take into account that a
given spectral type - luminosity class combination covers a range of
astrophysical parameters. The colliding-wind region also contributes to the
free-free emission, which can help to explain the high values of the spectral
index seen after periastron passage. Combining our data with older Very Large
Array (VLA) data allows us to derive a period P = 860.0 +- 3.7 days for this
system. With this period, we update the orbital parameters that were derived in
the first paper of this series.Comment: 10 pages, 4 figures, accepted for publication in A&
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