19,180 research outputs found
Possible Contribution to Electron and Positron Fluxes from Pulsars and their Nebulae
The AMS-02 experiment confirms the excess of positrons in cosmic rays (CRs)
for energy above 10 GeV with respect to the secondary production of positrons
in the interstellar medium. This is interpreted as evidence of the existence of
a primary source of these particles. Possible candidates are dark matter or
astrophysical sources. In this work we discuss the possible contribution due to
pulsars and their nebulae. Our key assumption is that the primary spectrum of
electrons and positrons at the source is the same of the well known photon
spectrum observed from gamma-rays telescopes. Using a diffusion model in the
Galaxy we propagate the source spectra up to the Solar System. We compare our
results with the recent experiments and with the LIS modelComment: To appear in the Proceedings of the 14th ICATPP Conference, Villa
Olmo 23-27 September 201
Pulsar Wind Nebulae as a source of the observed electron and positron excess at high energy: the case of Vela-X
We investigate, in terms of production from pulsars and their nebulae, the
cosmic ray positron and electron fluxes above GeV, observed by the
AMS-02 experiment up to 1 TeV. We concentrate on the Vela-X case. Starting from
the gamma-ray photon spectrum of the source, generated via synchrotron and
inverse Compton processes, we estimated the electron and positron injection
spectra. Several features are fixed from observations of Vela-X and unknown
parameters are borrowed from the Crab nebula. The particle spectra produced in
the pulsar wind nebula are then propagated up to the Solar System, using a
diffusion model. Differently from previous works, the omnidirectional intensity
excess for electrons and positrons is obtained as a difference between the
AMS-02 data and the corresponding local interstellar spectrum. An equal amount
of electron and positron excess is observed and we interpreted this excess
(above 100 GeV in the AMS-02 data) as a supply coming from Vela-X. The
particle contribution is consistent with models predicting the gamma-ray
emission at the source. The input of a few more young pulsars is also allowed,
while below 100 GeV more aged pulsars could be the main contributors.Comment: Accepted for publication in Journal of High Energy Astrophysics
(2015
Simulation of MeV/atom cluster correlations in matter
We present an efficient algorithm able to predict the trajectories of individual cluster constituents as they penetrate relatively thick amorphous targets. Our algorithm properly treats both the intracluster Coulomb repulsion and the collisions between cluster constituents and target atoms. We have compared our simulation predictions to experimental measurements of the distribution of lateral exit velocities, and demonstrated that the in-target Coulomb explosion of 2MeV/atom carbon clusters in carbon foils must be shielded with a screening length of less than 2.5 Å. We also present a simple phenomenological model for the suppression of the exit-side charge of ions in clusters which depends on the enhanced ionization potential that an electron near an ion feels due to the ion’s charged comoving neighbors. By using our simulation algorithm we have predicted the exit correlations of the cluster constituents and verified that the charge suppression model fits the observed charge suppression of ions in clusters to within the experimental uncertainties
Prediction of disease progression, treatment response and dropout in chronic obstructive pulmonary disease (COPD).
Drug development in chronic obstructive pulmonary disease (COPD) has been characterised by unacceptably high failure rates. In addition to the poor sensitivity in forced expiratory volume in one second (FEV1), numerous causes are known to contribute to this phenomenon, which can be clustered into drug-, disease- and design-related factors. Here we present a model-based approach to describe disease progression, treatment response and dropout in clinical trials with COPD patients
The cool core state of Planck SZ-selected clusters versus X-ray selected samples: evidence for cool core bias
We characterized the population of galaxy clusters detected with the SZ
effect with Planck, by measuring the cool core state of the objects in a
well-defined subsample of the Planck catalogue. We used as indicator the
concentration parameter Santos et al. (2008). The fraction of cool core
clusters is and does not show significant indications of
evolution in the redshift range covered by our sample. We compare the
distribution of the concentration parameter in the Planck sample with the one
of the X-ray selected sample MACS (Mann & Ebeling, 2011): the distributions are
significantly different and the cool core fraction in MACS is much higher (). Since X-ray selected samples are known to be biased towards cool
cores due to the presence of their prominent surface brightness peak, we
simulated the impact of the "cool core bias" following Eckert et al. (2011). We
found that it plays a large role in the difference between the fractions of
cool cores in the two samples. We examined other selection effects that could
in principle bias SZ-surveys against cool cores but we found that their impact
is not sufficient to explain the difference between Planck and MACS. The
population of X-ray under-luminous objects, which are found in SZ-surveys but
missing in X-ray samples (Planck Collaboration 2016), could possibly contribute
to the difference, as we found most of them to be non cool cores, but this
hypothesis deserves further investigation.Comment: Accepted for publication in MNRA
Measurement of energy and angular distributions of secondary ions in the sputtering of gold by swift Au-n clusters: Study of emission mechanisms
Energy and angular distributions of negative ions (Au–, Au2-, Au3-, and Au5-) emitted from gold target bombarded by Au, Au4, and Au9 projectiles at 200 keV/atom were measured with a multipixel position sensitive detector. The angular distributions are symmetrical with respect to the normal to the target surface and forward peaked. They depend on the type of emitted ions, on the emission energy, and on the projectile size. More forward directed emission is observed with Au9 projectiles. The secondary ion energy distributions obtained with Au and Au4 projectiles are well reproduced by a sum of linear collision cascades and thermal spike processes. However, in the case of Au9 projectiles the energy distributions are better described by using a simple spike model with two different average temperature regimes: the first one corresponds to high emission energy occurring in the early stage of the whole process, and the second to the low energy component
Pengaruh Prosentase Solvent Non Polar Dalam Campuran Pelarut Terhadap Pemisahan Senyawa Non Polar Dari Minyak Nyamplung
Minyak nyamplung (Calophyllum inophyllum oil) dikenal sebagai minyak yang tidak dapat dikonsumsi. Oleh sebab itu, penelitian tentang minyak ini kebanyakan hanya terfokus pada konversi minyak menjadi biodiesel. Pada penelitian ini, diharapkan agar trigliserida (senyawa non polar) terpisah dengan resin beracun yang ada di dalam minyak nyamplung itu sendiri, dengan tujuan agar minyak nyamplung bisa dikonsumsi oleh manusia. Minyak nyamplung sendiri disinyalir mengandung senyawa anti HIV dan anti tumor yang sangat berfungsi bagi manusia. Resin beracun yang terdapat dalam minyak ini diidentifikasi sebagai phthalic acid ester (PAE). Trigliserida dalam minyak nyamplung sendiri berkisar antara 70-80%, sehingga jika trigliserida ini dapat terpisah dengan baik dari PAE atau komponen lain yang berbahaya dalam minyak nyamplung, bukan tidak mungkin minyak nyamplung nantinya akan dapat dikonsumsi oleh manusia. Proses isolasi trigliserida dimulai dengan memisahkan senyawa yang diinginkan dari lipid menggunakan ekstraksi pelarut-pelarut dengan dua macam variable solvent yaitu : n-hexane-methanol serta petroleum eter-methanol. Pemilihan pelarut berdasarkan atas nilai kepolaran yang dimilikinya karena solvent yang saling larut tidak dapat digunakan dalam ekstraksi ini. Rasio jumlah solvent non polar dan polar ini juga divariasikan, yaitu : 100:0, 75:25, 50:50 dan 0:100
Calibrated cryo-cell UV-LA-ICPMS elemental concentrations from the NGRIP ice core reveal abrupt, sub-annual variability in dust across the GI-21.2 interstadial period
Several abrupt shifts from periods of extreme cold (Greenland
stadials, GS) to relatively warmer conditions (Greenland interstadials, GI)
called Dansgaard–Oeschger events are recorded in the Greenland ice cores.
Using cryo-cell UV-laser-ablation inductively coupled-plasma mass
spectrometry (UV-LA-ICPMS), we analysed a 2.85 m NGRIP ice core section
(2691.50–2688.65 m depth, age interval 84.86–85.09 ka b2k, thus covering
 ∼  230 years) across the transitions of GI-21.2, a short-lived
interstadial prior to interstadial GI-21.1. GI-21.2 is a  ∼  100-year
long period with δ18O values 3–4 ‰ higher than the
following  ∼  200 years of stadial conditions (GS-21.2), which precede
the major GI-21.1 warming. We report concentrations of major elements
indicative of dust and/or sea salt (Na, Fe, Al, Ca, Mg) at a spatial
resolution of  ∼  200 µm, while maintaining detection limits
in the low-ppb range, thereby achieving sub-annual time resolution even in
deep NGRIP ice. We present an improved external calibration and
quantification procedure using a set of five ice standards made from aqueous
(international) standard solutions. Our results show that element
concentrations decrease drastically (more than 10-fold) at the warming onset
of GI-21.2 at the scale of a single year, followed by relatively low
concentrations characterizing the interstadial part before gradually reaching
again typical stadial values
Classical realization of two-site Fermi-Hubbard systems
A classical wave optics realization of the two-site Hubbard model, describing
the dynamics of interacting fermions in a double-well potential, is proposed
based on light transport in evanescently-coupled optical waveguides.Comment: 4 page
- …