56,101 research outputs found
Stimulated Raman backscattering of laser radiation in deep plasma channels
Stimulated Raman backscattering (RBS) of intense laser radiation confined by
a single-mode plasma channel with a radial variation of plasma frequency
greater than a homogeneous-plasma RBS bandwidth is characterized by a strong
transverse localization of resonantly-driven electron plasma waves (EPW). The
EPW localization reduces the peak growth rate of RBS and increases the
amplification bandwidth. The continuum of non-bound modes of backscattered
radiation shrinks the transverse field profile in a channel and increases the
RBS growth rate. Solution of the initial-value problem shows that an
electromagnetic pulse amplified by the RBS in the single-mode deep plasma
channel has a group velocity higher than in the case of homogeneous-plasma
Raman amplification. Implications to the design of an RBS pulse compressor in a
plasma channel are discussed.Comment: 11 pages, 3 figures; submitted to Physics of Plasma
Finding role communities in directed networks using Role-Based Similarity, Markov Stability and the Relaxed Minimum Spanning Tree
We present a framework to cluster nodes in directed networks according to
their roles by combining Role-Based Similarity (RBS) and Markov Stability, two
techniques based on flows. First we compute the RBS matrix, which contains the
pairwise similarities between nodes according to the scaled number of in- and
out-directed paths of different lengths. The weighted RBS similarity matrix is
then transformed into an undirected similarity network using the Relaxed
Minimum-Spanning Tree (RMST) algorithm, which uses the geometric structure of
the RBS matrix to unblur the network, such that edges between nodes with high,
direct RBS are preserved. Finally, we partition the RMST similarity network
into role-communities of nodes at all scales using Markov Stability to find a
robust set of roles in the network. We showcase our framework through a
biological and a man-made network.Comment: 4 pages, 2 figure
Densities for random balanced sampling
A random balanced sample (RBS) is a multivariate distribution with n
components X_1,...,X_n, each uniformly distributed on [-1, 1], such that the
sum of these components is precisely 0. The corresponding vectors X lie in an
(n-1)-dimensional polytope M(n). We present new methods for the construction of
such RBS via densities over M(n) and these apply for arbitrary n. While simple
densities had been known previously for small values of n (namely 2,3 and 4),
for larger n the known distributions with large support were fractal
distributions (with fractal dimension asymptotic to n as n approaches
infinity). Applications of RBS distributions include sampling with antithetic
coupling to reduce variance, and the isolation of nonlinearities. We also show
that the previously known densities (for n<5) are in fact the only solutions in
a natural and very large class of potential RBS densities. This finding
clarifies the need for new methods, such as those presented here.Comment: 20 pages, 6 figures, to appear in Journal of Multivariate Analysi
Combined grazing incidence RBS and TEM analysis of luminescent nano-SiGe/SiO2 multilayers.
Multilayer structures with five periods of amorphous SiGe nanoparticles/SiO2 layers with different thickness were deposited by Low Pressure Chemical Vapor Deposition and annealed to crystallize the SiGe nanoparticles. The use of grazing incidence RBS was necessary to obtain sufficient depth resolution to separate the signals arising from the individual layers only a few nm thick. The average size and areal density of the embedded SiGe nanoparticles as well as the oxide interlayer thickness were determined from the RBS spectra. Details of eventual composition changes and diffusion processes caused by the annealing processes were also studied. Transmission Electron Microscopy was used to obtain complementary information on the structural parameters of the samples in order to check the information yielded by RBS. The study revealed that annealing at 900 °C for 60 s, enough to crystallize the SiGe nanoparticles, leaves the structure unaltered if the interlayer thickness is around 15 nm or higher
A candidate supermassive binary black hole system in the brightest cluster galaxy of RBS 797
The radio source at the center of the cool core galaxy cluster RBS 797
(z=0.35) is known to exhibit a misalignment of its radio jets and lobes
observed at different VLA-scale, with the innermost kpc-scale jets being almost
orthogonal to the radio emission which extends for tens of kpc filling the
X-ray cavities. Gitti et al. suggested that this peculiar radio morphology may
indicate a recurrent activity of the central radio source, where the jet
orientation is changing between the different outbursts due to the effects of
supermassive binary black holes (SMBBHs). We aim at unveiling the nuclear radio
properties of the brightest cluster galaxy (BCG) in RBS 797 and at
investigating the presence of a SMBBH system in its center. We have performed
new high-resolution observations at 5 GHz with the European VLBI Network (EVN),
reaching an angular resolution of 9x5 mas^2 and a sensitivity of 36
microJy/beam. We report the EVN detection of two compact components in the BCG
of RBS 797, with a projected separation of ~77 pc. We can envisage two possible
scenarios: the two components are two different nuclei in a close binary
system, or they are the core and a knot of its jet. Both interpretations are
consistent with the presence of SMBBHs. Our re-analysis of VLA archival data
seems to favor the first scenario, as we detect two pairs of radio jets
misaligned by ~90 degrees on the same kpc scale emanating from the central
radio core. If the two outbursts are almost contemporaneous, this is clear
evidence of the presence of two active SMBHs, whose radio nuclei are unresolved
at VLA resolution. The nature of the double source detected by our EVN
observations in the BCG of RBS 797 can be established only by future sensitive,
multi-frequency VLBI observations. If confirmed, RBS 797 would be the first
SMBBH system observed at medium-high redshift at VLBI resolution. (abridged)Comment: 4 pages, 2 figures, A&A Letter in pres
Accurate X-ray position and multiwavelength observations of the isolated neutron star RBS 1774
We report on X-ray, optical, infrared and radio observations of the X-ray dim
isolated neutron star (XDINS) 1RXS J214303.7+065419 (also known as RBS 1774).
The X-ray observation was performed with the High Resolution Camera on board of
the Chandra X-ray Observatory, allowing us to derive the most accurate position
for this source (alpha = 21h43m3.38s, delta= +6deg54'17".53; 90% uncertainty of
0."6). Furthermore, we confirmed with a higher spatial accuracy the point-like
nature of this X-ray source. Optical and infrared observations were taken in B,
V, r', i', J, H and Ks filters using the Keck, VLT, Blanco and Magellan
telescopes, while radio observations were obtained from the ATNF Parkes single
dish at 2.9GHz and 708MHz. No plausible optical and/or infrared counterpart for
RBS 1774 was detected within the refined sub-arsecond Chandra X-ray error
circle. Present upper limits to the optical and infrared magnitudes are r'>25.7
and J>22.6 (5 sigma confidence level). Radio observations did not show evidence
for radio pulsations down to a luminosity at 1.4 GHz of L < 0.02 mJy kpc^2, the
deepest limit up to date for any XDINS, and lower than what expected for the
majority of radio pulsars. We can hence conclude that, if RBS 1774 is active as
radio pulsar, its non detection is more probably due to a geometrical bias
rather than to a luminosity bias. Furthermore, no convincing evidence for
RRAT-like radio bursts have been found. Our results on RBS 1774 are discussed
and compared with the known properties of other thermally emitting neutron
stars and of the radio pulsar population.Comment: 8 pages, 9 figures, accepted for publication on MNRA
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