4,725 research outputs found
Competition of fusion and quasi-fission in the reactions leading to production of the superheavy elements
The mechanism of fusion hindrance, an effect observed in the reactions of
cold, warm and hot fusion leading to production of the superheavy elements, is
investigated. A systematics of transfermium production cross sections is used
to determine fusion probabilities. Mechanism of fusion hindrance is described
as a competition of fusion and quasi-fission. Available evaporation residue
cross sections in the superheavy region are reproduced satisfactorily. Analysis
of the measured capture cross sections is performed and a sudden disappearance
of the capture cross sections is observed at low fusion probabilities. A
dependence of the fusion hindrance on the asymmetry of the projectile-target
system is investigated using the available data. The most promising pathways
for further experiments are suggested.Comment: 8 pages, 7 figures, talk presented at 7th International
School-Seminar on Heavy-Ion Physics, May 27 - June 1, 2002, Dubna, Russi
Identification of the Mass Donor Star's Spectrum in SS 433
We present spectroscopy of the microquasar SS 433 obtained near primary
eclipse and disk precessional phase Psi = 0.0, when the accretion disk is
expected to be most ``face-on''. The likelihood of observing the spectrum of
the mass donor is maximized at this combination of orbital and precessional
phases since the donor is in the foreground and above the extended disk
believed to be present in the system. The spectra were obtained over four
different runs centered on these special phases. The blue spectra show clear
evidence of absorption features consistent with a classification of A3-7 I. The
behavior of the observed lines indicates an origin in the mass donor. The
observed radial velocity variations are in anti-phase to the disk, the
absorption lines strengthen at mid-eclipse when the donor star is expected to
contribute its maximum percentage of the total flux, and the line widths are
consistent with lines created in an A supergiant photosphere. We discuss and
cast doubt on the possibility that these lines represent a shell spectrum
rather than the mass donor itself. We re-evaluate the mass ratio of the system
and derive masses of 10.9 +/- 3.1 Msun and 2.9 +/- 0.7 Msun for the mass donor
and compact object plus disk, respectively. We suggest that the compact object
is a low mass black hole.
In addition, we review the behavior of the observed emission lines from both
the disk/wind and high velocity jets.Comment: submitted to ApJ, 24 pages, 7 figure
LOFAR tied-array imaging and spectroscopy of solar S bursts
Context. The Sun is an active source of radio emission that is often associated with energetic phenomena ranging from nanoflares to coronal mass ejections (CMEs). At low radio frequencies (<100 MHz), numerous millisecond duration radio bursts have been reported, such as radio spikes or solar S bursts (where S stands for short). To date, these have neither been studied extensively nor imaged because of the instrumental limitations of previous radio telescopes.
Aims. Here, LOw Frequency ARray (LOFAR) observations were used to study the spectral and spatial characteristics of a multitude of S bursts, as well as their origin and possible emission mechanisms.
Methods. We used 170 simultaneous tied-array beams for spectroscopy and imaging of S bursts. Since S bursts have short timescales and fine frequency structures, high cadence (~50 ms) tied-array images were used instead of standard interferometric imaging, that is currently limited to one image per second.
Results. On 9 July 2013, over 3000 S bursts were observed over a time period of ~8 h. S bursts were found to appear as groups of short-lived (<1 s) and narrow-bandwidth (~2.5 MHz) features, the majority drifting at ~3.5 MHzâs-1 and a wide range of circular polarisation degrees (2â8 times more polarised than the accompanying Type III bursts). Extrapolation of the photospheric magnetic field using the potential field source surface (PFSS) model suggests that S bursts are associated with a trans-equatorial loop system that connects an active region in the southern hemisphere to a bipolar region of plage in the northern hemisphere.
Conclusions. We have identified polarised, short-lived solar radio bursts that have never been imaged before. They are observed at a height and frequency range where plasma emission is the dominant emission mechanism, however, they possess some of the characteristics of electron-cyclotron maser emission
Heating of gas inside radio sources to mildly relativistic temperatures via induced Compton scattering
Measured values of the brightness temperature of low-frequency synchrotron
radiation emitted by powerful extragalactic sources reach 10^11--10^12 K. If
some amount of nonrelativistic ionized gas is present within such sources, it
should be heated as a result of induced Compton scattering of the radiation. If
this heating is counteracted by cooling due to inverse Compton scattering of
the same radio radiation, then the plasma can be heated up to mildly
relativistic temperatures kT~10--100 keV. The stationary electron velocity
distribution can be either relativistic Maxwellian or quasi-Maxwellian (with
the high-velocity tail suppressed), depending on the efficiency of Coulomb
collisions and other relaxation processes. We derive several easy-to-use
approximate expressions for the induced Compton heating rate of mildly
relativistic electrons in an isotropic radiation field, as well as for the
stationary distribution function and temperature of electrons. We also give
analytic expressions for the kernel of the integral kinetic equation (one as a
function of the scattering angle and another for the case of an isotropic
radiation field), which describes the redistribution of photons in frequency
caused by induced Compton scattering in thermal plasma. These expressions can
be used in the parameter range hnu<< kT<~ 0.1mc^2 (the formulae earlier
published in Sazonov, Sunyaev, 2000 are less accurate).Comment: 22 pages, 7 figures, submitted to Astronomy Letter
Age-related gene and miRNA expression changes in airways of healthy individuals
© 2019, The Author(s). Knowledge on age-related miRNA changes in healthy individuals and their interaction with mRNAs is lacking. We studied age-related mRNA and miRNA expression changes and their interactions in normal airways. RNA and small RNA sequencing was performed on bronchial biopsies of 86 healthy individuals (age: 18â73) to determine age-related expression changes. Per age-related miRNA we determined the enrichment of age-related predicted targets and their correlation. We identified 285 age-related genes and 27 age-related miRNAs. Pathway enrichment showed that genes higher expressed with age were involved in synapse-related processes. Genes lower expressed with age were involved in cell cycle regulation, the immune system and DNA damage/repair. MiR-146a-5p, miR-146b-5p and miR-142-5p were lower expressed with increasing age and we found a significant enrichment for predicted targets of these miRNAs among genes that were higher expressed with age. The expression levels of the enriched predicted targets RIMS2 and IGSF1 were negatively correlated with both miR-146a-5p and miR-146b-5p. RIMS2 was present in the enriched process, i.e. positive regulation of synaptic transmission. In conclusion, genes decreased with ageing are involved in several of the ageing hallmarks. Genes higher expressed with ageing were involved in synapse-related processes, of which RIMS2 is potentially regulated by two age-related miRNAs
Characterisation of nanoparticles by means of high-resolution SEM/EDS in transmission mode
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd
Imaging Jupiter's radiation belts down to 127 MHz with LOFAR
Context. Observing Jupiter's synchrotron emission from the Earth remains
today the sole method to scrutinize the distribution and dynamical behavior of
the ultra energetic electrons magnetically trapped around the planet (because
in-situ particle data are limited in the inner magnetosphere). Aims. We perform
the first resolved and low-frequency imaging of the synchrotron emission with
LOFAR at 127 MHz. The radiation comes from low energy electrons (~1-30 MeV)
which map a broad region of Jupiter's inner magnetosphere. Methods (see article
for complete abstract) Results. The first resolved images of Jupiter's
radiation belts at 127-172 MHz are obtained along with total integrated flux
densities. They are compared with previous observations at higher frequencies
and show a larger extent of the synchrotron emission source (>=4 ). The
asymmetry and the dynamic of east-west emission peaks are measured and the
presence of a hot spot at lambda_III=230 {\deg} 25 {\deg}. Spectral flux
density measurements are on the low side of previous (unresolved) ones,
suggesting a low-frequency turnover and/or time variations of the emission
spectrum. Conclusions. LOFAR is a powerful and flexible planetary imager. The
observations at 127 MHz depict an extended emission up to ~4-5 planetary radii.
The similarities with high frequency results reinforce the conclusion that: i)
the magnetic field morphology primarily shapes the brightness distribution of
the emission and ii) the radiating electrons are likely radially and
latitudinally distributed inside about 2 . Nonetheless, the larger extent
of the brightness combined with the overall lower flux density, yields new
information on Jupiter's electron distribution, that may shed light on the
origin and mode of transport of these particles.Comment: 10 pages, 12 figures, accepted for publication in A&A (27/11/2015) -
abstract edited because of limited character
First LOFAR observations at very low frequencies of cluster-scale non-thermal emission: the case of Abell 2256
Abell 2256 is one of the best known examples of a galaxy cluster hosting
large-scale diffuse radio emission that is unrelated to individual galaxies. It
contains both a giant radio halo and a relic, as well as a number of head-tail
sources and smaller diffuse steep-spectrum radio sources. The origin of radio
halos and relics is still being debated, but over the last years it has become
clear that the presence of these radio sources is closely related to galaxy
cluster merger events. Here we present the results from the first LOFAR Low
band antenna (LBA) observations of Abell 2256 between 18 and 67 MHz. To our
knowledge, the image presented in this paper at 63 MHz is the deepest ever
obtained at frequencies below 100 MHz in general. Both the radio halo and the
giant relic are detected in the image at 63 MHz, and the diffuse radio emission
remains visible at frequencies as low as 20 MHz. The observations confirm the
presence of a previously claimed ultra-steep spectrum source to the west of the
cluster center with a spectral index of -2.3 \pm 0.4 between 63 and 153 MHz.
The steep spectrum suggests that this source is an old part of a head-tail
radio source in the cluster. For the radio relic we find an integrated spectral
index of -0.81 \pm 0.03, after removing the flux contribution from the other
sources. This is relatively flat which could indicate that the efficiency of
particle acceleration at the shock substantially changed in the last \sim 0.1
Gyr due to an increase of the shock Mach number. In an alternative scenario,
particles are re-accelerated by some mechanism in the downstream region of the
shock, resulting in the relatively flat integrated radio spectrum. In the radio
halo region we find indications of low-frequency spectral steepening which may
suggest that relativistic particles are accelerated in a rather inhomogeneous
turbulent region.Comment: 13 pages, 13 figures, accepted for publication in A\&A on April 12,
201
Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km
For more than 80 years, the counterintuitive predictions of quantum theory
have stimulated debate about the nature of reality. In his seminal work, John
Bell proved that no theory of nature that obeys locality and realism can
reproduce all the predictions of quantum theory. Bell showed that in any local
realist theory the correlations between distant measurements satisfy an
inequality and, moreover, that this inequality can be violated according to
quantum theory. This provided a recipe for experimental tests of the
fundamental principles underlying the laws of nature. In the past decades,
numerous ingenious Bell inequality tests have been reported. However, because
of experimental limitations, all experiments to date required additional
assumptions to obtain a contradiction with local realism, resulting in
loopholes. Here we report on a Bell experiment that is free of any such
additional assumption and thus directly tests the principles underlying Bell's
inequality. We employ an event-ready scheme that enables the generation of
high-fidelity entanglement between distant electron spins. Efficient spin
readout avoids the fair sampling assumption (detection loophole), while the use
of fast random basis selection and readout combined with a spatial separation
of 1.3 km ensure the required locality conditions. We perform 245 trials
testing the CHSH-Bell inequality and find . A
null hypothesis test yields a probability of that a local-realist
model for space-like separated sites produces data with a violation at least as
large as observed, even when allowing for memory in the devices. This result
rules out large classes of local realist theories, and paves the way for
implementing device-independent quantum-secure communication and randomness
certification.Comment: Raw data will be made available after publicatio
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