5,031 research outputs found
Supertubes and special holonomy
We obtain a -supersymmetric 6-brane solution of IIA Supergravity by
T-dualizing the supertube recently found. The resulting electric
charge is related to the original -brane charge. The uplifted solution to
eleven dimensions results to be a purely geometrical configuration, which can
be interpreted as a bound state of a Taub-NUT space and a pp-wave. Being the
non trivial part of the metric pseudo-Riemannian, the resulting reduced
holonomy group is non-compact and locally isomorphic to a semidirect product of
an Abelian four dimensional group and SU(2).Comment: 11 pages, latex, no figure
Prediction of a Photon Peak in Heavy Ion Collisions
We show that if a flavour-less vector meson remains bound after
deconfinement, and if its limiting velocity in the quark-gluon plasma is
subluminal, then this meson produces a distinct peak in the spectrum of thermal
photons emitted by the plasma. We also demonstrate that this effect is a
universal property of all strongly coupled, large-Nc plasmas with a gravity
dual. For the J/psi the corresponding peak lies between 3 and 5 GeV and could
be observed at LHC.Comment: 4 pages, 5 figures. Minor corrections; references adde
Ecología de los colémbolos (Collembola, Insecta) edáficos del encinar montano del Montseny (Barcelona)
A New Mechanism of Quark Energy Loss
We show that a heavy quark moving sufficiently fast through a quark-gluon
plasma may lose energy by Cherenkov-radiating mesons. We demonstrate that this
takes place in all strongly coupled, large-Nc plasmas with a gravity dual. The
energy loss is exactly calculable in these models despite being an
O(1/Nc)-effect. We discuss phenomenological implications for heavy-ion
collision experiments.Comment: 4 pages, 4 figures; v2: plot modified, conclusions unchange
Noise characterization of an atomic magnetometer at sub-millihertz frequencies
Noise measurements have been carried out in the LISA bandwidth (0.1 mHz to
100 mHz) to characterize an all-optical atomic magnetometer based on nonlinear
magneto-optical rotation. This was done in order to assess if the technology
can be used for space missions with demanding low-frequency requirements like
the LISA concept. Magnetometry for low-frequency applications is usually
limited by noise and thermal drifts, which become the dominant
contributions at sub-millihertz frequencies. Magnetic field measurements with
atomic magnetometers are not immune to low-frequency fluctuations and
significant excess noise may arise due to external elements, such as
temperature fluctuations or intrinsic noise in the electronics. In addition,
low-frequency drifts in the applied magnetic field have been identified in
order to distinguish their noise contribution from that of the sensor. We have
found the technology suitable for LISA in terms of sensitivity, although
further work must be done to characterize the low-frequency noise in a
miniaturized setup suitable for space missions.Comment: 11 pages, 12 figure
Black-hole masses of type 1 AGN in the XMM-Newton bright serendipitous survey
We derive masses of the central super-massive black hole (SMBH) and accretion
rates for 154 type1 AGN belonging to a well-defined X-ray-selected sample, the
XMM-Newton Serendipitous Sample (XBS). To this end, we use the most recent
"single-epoch" relations, based on Hbeta and MgII2798A emission lines, to
derive the SMBH masses. We then use the bolometric luminosities, computed on
the basis of an SED-fitting procedure, to calculate the accretion rates, both
absolute and normalized to the Eddington luminosity (Eddington ratio). The
selected AGNs cover a range of masses from 10^7 to 10^10 Msun with a peak
around 8x10^8 Msun and a range of accretion rates from 0.01 to ~50 Msun/year
(assuming an efficiency of 0.1), with a peak at ~1 Msun/year. The values of
Eddington ratio range from 0.001 to ~0.5 and peak at 0.1.Comment: 14 pages, 11 figures. Accepted for publication in Astronomy &
Astrophysic
Bifurcation and Chaos in Coupled Ratchets exhibiting Synchronized Dynamics
The bifurcation and chaotic behaviour of unidirectionally coupled
deterministic ratchets is studied as a function of the driving force amplitude
() and frequency (). A classification of the various types of
bifurcations likely to be encountered in this system was done by examining the
stability of the steady state in linear response as well as constructing a
two-parameter phase diagram in the () plane. Numerical explorations
revealed varieties of bifurcation sequences including quasiperiodic route to
chaos. Besides, the familiar period-doubling and crises route to chaos
exhibited by the one-dimensional ratchet were also found. In addition, the
coupled ratchets display symmetry-breaking, saddle-nodes and bubbles of
bifurcations. Chaotic behaviour is characterized by using the sensitivity to
initial condition as well as the Lyapunov exponent spectrum; while a perusal of
the phase space projected in the Poincar cross-section confirms some
of the striking features.Comment: 7 pages; 8 figure
Detectability of low energy X-ray spectral components in type 1 AGN
In this paper we examine the percentage of type 1 AGN which require the
inclusion of a soft excess component and/or significant cold absorption in the
modelling of their X-ray spectra obtained by XMM-Newton. We do this by
simulating spectra which mimic typical spectral shapes in order to find the
maximum detectability expected at different count levels. We then apply a
correction to the observed percentages found for the Scott et al. (2011) sample
of 761 sources. We estimate the true percentage of AGN with a soft excess
component to be 75+/-23%, suggesting that soft excesses are ubiquitous in the
X-ray spectra of type 1 AGN. By carrying out joint fits on groups of low count
spectra in narrow z bins in which additional spectral components were not
originally detected, we show that the soft excess feature is recovered with a
mean temperature kT and blackbody to power-law normalisation ratio consistent
with those of components detected in individual high count spectra. Cold
absorption with nH values broadly consistent with those reported in individual
spectra are also recovered. We suggest such intrinsic cold absorption is found
in a minimum of ~5% of type 1 AGN and may be present in up to ~10%.Comment: 10 pages, 6 figures, 4 tables. Accepted for publication in MNRA
Poly(amido amine)s as Gene Delivery Vectors: Effects of Quaternary Nicotinamide Moieties in the Side Chains
To evaluate the effect of quaternary nicotinamide pendant groups on gene delivery properties, a series of poly(amido amine) (co)polymers were synthesized by Michael addition polymerization of N, N-cystaminebisacrylamide with variable ratios of 1-(4-aminobutyl)-3-carbamoylpyridinium (Nic-BuNH2), and tert-butyl-4-aminobutyl carbamate (BocNH-BuNH2), yielding poly(amido amine)s (NicX-NHBoc) with X=0, 10, 30, and 50 % of quaternary nicotinamide groups in the polymer side chains. Deprotection of the pendant Boc-NH groups afforded an analogous series of polymers (NicX-NH2) with higher charge density (due to the presence of protonated primary amino groups in the side chains) and subsequent acetylation yielded a series of polymers (NicX-NHAc) of lower hydrophobicity than the Boc-protected polymers. The polymers with the Boc-protected or the acetylated amino groups showed high buffer capacity in the range pH 5.1-7.4, which is a property that can contribute to endosomal escape of polyplexes. The presence of quaternary nicotinamide groups has distinct beneficial effects on the gene vector properties of these polymers. The polymers containing 30 % of quaternary nicotinamide groups in their side chains condense DNA into small, nanosized particles (200 nm) with positive surface charge (+15 mV). Fluorescence experiments using ethidium bromide as a competitor showed that the quaternary nicotinamide groups intercalate with DNA, contributing to a more intimate polymer-DNA binding and shielding. Polyplexes of nicotinamide-functionalized poly(amido amine)s NicX-NHBoc and NicX-NHAc, formed at 12/1 polymerDNA mass ratio, efficiently transfect COS-7 cells with efficacies up to four times higher than that of PEI (Exgen 500), and with essentially absence of cytotoxicity. NicX-NH2 polymers, possessing protonated primary amino groups in their side chains, have a higher cytotoxicity profile under these conditions, but at lower 3/1 polymer-DNA mass ratio also these polymers are capable of efficient transfection, while retaining full cell viability
Spherically Symmetric and Rotating Wormholes Produced by Lightlike Branes
Lightlike p-branes (LL-branes) with dynamical (variable) tension allow simple
and elegant Polyakov-type and dual to it Nambu-Goto-like world-volume action
formulations. Here we first briefly describe the dynamics of LL-branes as test
objects in various physically interesting gravitational backgrounds of black
hole type, including rotating ones. Next we show that LL-branes are the
appropriate gravitational sources that provide proper matter energy momentum
tensors in the Einstein equations of motion needed to generate traversable
wormhole solutions, in particular, self-consistent cylindrical rotating
wormholes, with the LL-branes occupying their throats. Here a major role is
being played by the dynamical LL-brane tension which turns out to be negative
but may be of arbitrary small magnitude. As a particular solution we obtain
traversable wormhole with Schwarzschild geometry generated by a LL-brane
positioned at the wormhole throat, which represents the correct consistent
realization of the original Einstein-Rosen "bridge" manifold.Comment: 27 pages; important clarifications regarding the meaning of the
original Einstein-Rosen "bridge" construction; an important addition to the
Appendix; acknowledgments adde
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