50,028 research outputs found
Mean field limit for bosons and propagation of Wigner measures
We consider the N-body Schr\"{o}dinger dynamics of bosons in the mean field
limit with a bounded pair-interaction potential. According to the previous work
\cite{AmNi}, the mean field limit is translated into a semiclassical problem
with a small parameter , after introducing an
-dependent bosonic quantization. The limit is expressed as a
push-forward by a nonlinear flow (e.g. Hartree) of the associated Wigner
measures. These object and their basic properties were introduced in
\cite{AmNi} in the infinite dimensional setting. The additional result
presented here states that the transport by the nonlinear flow holds for rather
general class of quantum states in their mean field limit.Comment: 10 page
Studying the Variation of the Fine Structure Constant Using Emission Line Multiplets
As an extension of the method by Bahcall et al. (2004) to investigate the
time dependence of the fine structure constant, we describe an approach based
on new observations of forbidden line multiplets from different ionic species.
We obtain optical spectra of fine structure transitions in [Ne III], [Ne V], [O
III], [OI], and [SII] multiplets from a sample of 14 Seyfert 1.5 galaxies in
the low-z range 0.035 < z < 0.281. Each source and each multiplet is
independently analyzed to ascertain possible errors. Averaging over our sample,
we obtain a conservative value alpha^2(t)/\alpha^2(0) = 1.0030+-0.0014.
However, our sample is limited in size and our fitting technique simplistic as
we primarily intend to illustrate the scope and strengths of emission line
studies of the time variation of the fine structure constant. The approach can
be further extended and generalized to a "many-multiplet emission line method"
analogous in principle to the corresponding method using absorption lines. With
that aim, we note that the theoretical limits on emission line ratios of
selected ions are precisely known, and provide well constrained selection
criteria. We also discuss several other forbidden and allowed lines that may
constitute the basis for a more rigorous study using high-resolution
instruments on the next generation of 8 m class telescopes.Comment: 20 pages, 4 figures, sumbitted to A
Drones for smart cities
Smart cities and unmanned aerial vehicles (UAVs) are two relatively recent concepts and also hot topics in research. The combination of these two technologies is expected to propel their capabilities even further for enabling revolutionary applications that will improve our quality of life. This Special Issue focuses on novel work done on the application of UAVs where state-of-the-art technologies in sensing, information dissemination, communications, and artificial intelligence (AI) are applied within the context of smart cities..
Pulsar J0453+1559: A Double Neutron Star System with a Large Mass Asymmetry
To understand the nature of supernovae and neutron star (NS) formation, as
well as binary stellar evolution and their interactions, it is important to
probe the distribution of NS masses. Until now, all double NS (DNS) systems
have been measured to have a mass ratio close to unity (q 0.91). Here we
report the measurement of the individual masses of the 4.07-day binary pulsar
J0453+1559 from measurements of the rate of advance of periastron and Shapiro
delay: The mass of the pulsar is 1.559(5) and that of its companion
is 1.174(4) ; q = 0.75. If this companion is also a neutron star
(NS), as indicated by the orbital eccentricity of the system (e=0.11), then its
mass is the smallest precisely measured for any such object. The pulsar has a
spin period of 45.7 ms and a spin derivative of 1.8616(7) x; from these
we derive a characteristic age of ~ 4.1 x years and a magnetic field of
~ 2.9 x G,i.e, this pulsar was mildly recycled by accretion of matter
from the progenitor of the companion star. This suggests that it was formed
with (very approximately) its current mass. Thus NSs form with a wide range of
masses, which is important for understanding their formation in supernovae. It
is also important for the search for gravitational waves released during a
NS-NS merger: it is now evident that we should not assume all DNS systems are
symmetric
Linear and nonlinear optical responses in the chiral multifold semimetal RhSi
Chiral topological semimetals are materials that break both inversion and
mirror symmetries. They host interesting phenomena such as the quantized
circular photogalvanic effect (CPGE) and the chiral magnetic effect. In this
work, we report a comprehensive theoretical and experimental analysis of the
linear and non-linear optical responses of the chiral topological semimetal
RhSi, which is known to host multifold fermions. We show that the
characteristic features of the optical conductivity, which display two distinct
quasi-linear regimes above and below 0.4 eV, can be linked to excitations of
different kinds of multifold fermions. The characteristic features of the CPGE,
which displays a sign change at 0.4 eV and a large non-quantized response peak
of around 160 at 0.7 eV, are explained by assuming that
the chemical potential crosses a flat hole band at the Brillouin zone center.
Our theory predicts that, in order to observe a quantized CPGE in RhSi, it is
necessary to increase the chemical potential as well as the quasiparticle
lifetime. More broadly our methodology, especially the development of the
broadband terahertz emission spectroscopy, could be widely applied to study
photo-galvanic effects in noncentrosymmetric materials and in topological
insulators in a contact-less way and accelerate the technological development
of efficient infrared detectors based on topological semimetals.Comment: Accepted in npj Quantum Materials; Abstract update
Pulsar J1411+2551: A Low Mass New Double Neutron Star System
In this work, we report the discovery and characterization of PSR J1411+2551,
a new binary pulsar discovered in the Arecibo 327 MHz Drift Pulsar Survey. Our
timing observations of the radio pulsar in the system span a period of about
2.5 years. This timing campaign allowed a precise measurement of its spin
period (62.4 ms) and its derivative (9.6 0.7) ; from these, we derive a characteristic age of Gyr and a
surface magnetic field strength of 2.5 G. These numbers
indicate that this pulsar was mildly recycled by accretion of matter from the
progenitor of the companion star. The system has an eccentric ()
2.61 day orbit. This eccentricity allows a highly significant measurement of
the rate of advance of periastron, . Assuming general relativity accurately models the
orbital motion, this implies a total system mass M = . The minimum companion mass is and the maximum
pulsar mass is . The large companion mass and the orbital
eccentricity suggest that PSR J1411+2551 is a double neutron star system; the
lightest known to date including the DNS merger GW 170817. Furthermore, the
relatively low orbital eccentricity and small proper motion limits suggest that
the second supernova had a relatively small associated kick; this and the low
system mass suggest that it was an ultra-stripped supernova.Comment: Accepted for publication in APJ letter
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