670 research outputs found
Particle-in-cell simulations of shock-driven reconnection in relativistic striped winds
By means of two- and three-dimensional particle-in-cell simulations, we
investigate the process of driven magnetic reconnection at the termination
shock of relativistic striped flows. In pulsar winds and in magnetar-powered
relativistic jets, the flow consists of stripes of alternating magnetic field
polarity, separated by current sheets of hot plasma. At the wind termination
shock, the flow compresses and the alternating fields annihilate by driven
magnetic reconnection. Irrespective of the stripe wavelength "lambda" or the
wind magnetization "sigma" (in the regime sigma>>1 of magnetically-dominated
flows), shock-driven reconnection transfers all the magnetic energy of
alternating fields to the particles, whose average Lorentz factor increases by
a factor of sigma with respect to the pre-shock value. In the limit
lambda/(r_L*sigma)>>1, where r_L is the relativistic Larmor radius in the wind,
the post-shock particle spectrum approaches a flat power-law tail with slope
around -1.5, populated by particles accelerated by the reconnection electric
field. The presence of a current-aligned "guide" magnetic field suppresses the
acceleration of particles only when the guide field is stronger than the
alternating component. Our findings place important constraints on the models
of non-thermal radiation from Pulsar Wind Nebulae and relativistic jets.Comment: 25 pages, 14 figures, movies available at
https://www.cfa.harvard.edu/~lsironi/sironi_movies.tar ; in press, special
issue of Computational Science and Discovery on selected research from the
22nd International Conference on Numerical Simulation of Plasma
TRIS II: search for CMB spectral distortions at 0.60, 0.82 and 2.5 GHz
With the TRIS experiment we have performed absolute measurements of the sky
brightness in a sky circle at at the frequencies
0.60, 0.82 and 2.5 GHz. In this paper we discuss the techniques used to
separate the different contributions to the sky emission and give an evaluation
of the absolute temperature of the Cosmic Microwave Background. For the
black-body temperature of the CMB we get: at GHz; at GHz; at
GHz. The first error bar is statistic (1) while the second
one is systematic. These results represent a significant improvement with
respect to the previous measurements. We have also set new limits to the
free-free distortions, ,
and slightly improved the Bose-Einstein upper limit, , both at 95% confidence level.Comment: accepted for publication in The Astrophysical Journa
Three-dimensional dynamics of strongly twisted magnetar magnetospheres: Kinking flux tubes and global eruptions
The origin of the various outbursts of hard X-rays from magnetars, highly
magnetized neutron stars, is still unknown. We identify instabilities in
relativistic magnetospheres that can explain a range of X-ray flare
luminosities. Crustal surface motions can twist the magnetar magnetosphere by
shifting the frozen-in footpoints of magnetic field lines in current-carrying
flux bundles. Axisymmetric (2D) magnetospheres exhibit strong eruptive
dynamics, as to say, catastrophic lateral instabilities triggered by a critical
footpoint displacement of . In contrast, our new
three-dimensional (3D) twist models with finite surface extension capture
important non-axisymmetric dynamics of twisted force-free flux bundles in
dipolar magnetospheres. Besides the well-established global eruption resulting
(as in 2D) from lateral instabilities, such 3D structures can develop helical,
kink-like dynamics, and dissipate energy locally (confined eruptions). Up to
of the induced twist energy is dissipated and available to power X-ray
flares in powerful global eruptions, with most of our models showing an energy
release in the range of the most common X-ray outbursts, erg.
Such events occur when significant energy builds up deeply buried in the dipole
magnetosphere. Less energetic outbursts likely precede powerful flares due to
intermittent instabilities and confined eruptions of a continuously twisting
flux tube. Upon reaching a critical state, global eruptions produce the
necessary Poynting-flux-dominated outflows required by models prescribing the
fast radio burst production in the magnetar wind, for example, via relativistic
magnetic reconnection or shocks.Comment: 21 pages, 11 figures, submitted to ApJ
Twinkling pulsar wind nebulae in the synchrotron cut-off regime and the gamma-ray flares in the Crab Nebula
Synchrotron radiation of ultra-relativistic particles accelerated in a pulsar
wind nebula may dominate its spectrum up to gamma-ray energies. Because of the
short cooling time of the gamma-ray emitting electrons, the gamma-ray emission
zone is in the immediate vicinity of the acceleration site. The particle
acceleration likely occurs at the termination shock of the relativistic striped
wind, where multiple forced magnetic field reconnections provide strong
magnetic fluctuations facilitating Fermi acceleration processes. The
acceleration mechanisms imply the presence of stochastic magnetic fields in the
particle acceleration region, which cause stochastic variability of the
synchrotron emission. This variability is particularly strong in the steep
gamma-ray tail of the spectrum, where modest fluctuations of the magnetic field
lead to strong flares of spectral flux. In particular, stochastic variations of
magnetic field, which may lead to quasi-cyclic gamma-ray flares, can be
produced by the relativistic cyclotron ion instability at the termination
shock. Our model calculations of the spectral and temporal evolution of
synchrotron emission in the spectral cut-off regime demonstrate that the
intermittent magnetic field concentrations dominate the gamma-ray emission from
highest energy electrons and provide fast, strong variability even for a
quasi-steady distribution of radiating particles. The simulated light curves
and spectra can explain the very strong gamma-ray flares observed in the Crab
nebula and the lack of strong variations at other wavelengths. The model
predicts high polarization in the flare phase, which can be tested with future
polarimetry observations.Comment: 5 pages, 3 figures, MNRAS in pres
The SPOrt Project: Cosmological and Astrophysical Goals
We present the cosmological and astrophysical objectives of the SPOrt
mission, which is scheduled for flying on the International Space Station (ISS)
in the year 2002 with the purpose of measuring the diffuse sky polarized
radiation in the microwave region. We discuss the problem of disentangling the
cosmic background polarized signal from the Galactic foregrounds.Comment: 10 pages; 5 PS figures; requires aipproc2.cls, aipproc2.sty,
epsfc.tex; to appear in Proc. of ``3K Cosmology'', Rome 5-10 Oct. 199
A Decline in the X-ray through Radio Emission from GW170817 Continues to Support an Off-Axis Structured Jet
We present new observations of the binary neutron star merger GW170817 at
days post-merger, at radio (Karl G. Jansky Very Large
Array; VLA), X-ray (Chandra X-ray Observatory) and optical (Hubble Space
Telescope; HST) wavelengths. These observations provide the first evidence for
a turnover in the X-ray light curve, mirroring a decline in the radio emission
at significance. The radio-to-X-ray spectral energy
distribution exhibits no evolution into the declining phase. Our full
multi-wavelength dataset is consistent with the predicted behavior of our
previously published models of a successful structured jet expanding into a
low-density circumbinary medium, but pure cocoon models with a choked jet
cannot be ruled out. If future observations continue to track our predictions,
we expect that the radio and X-ray emission will remain detectable until days post-merger.Comment: Accepted to ApJL. Updated version includes new VLA observations
extending through 2018 June
The Binary Neutron Star event LIGO/VIRGO GW170817 a hundred and sixty days after merger: synchrotron emission across the electromagnetic spectrum
We report deep Chandra, HST and VLA observations of the binary neutron star
event GW170817 at d after merger. These observations show that GW170817
has been steadily brightening with time and might have now reached its peak,
and constrain the emission process as non-thermal synchrotron emission where
the cooling frequency is above the X-ray band and the synchrotron
frequency is below the radio band. The very simple power-law spectrum
extending for eight orders of magnitude in frequency enables the most precise
measurement of the index of the distribution of non-thermal relativistic
electrons accelerated by a shock launched by a
NS-NS merger to date. We find , which indicates that radiation
from ejecta with dominates the observed emission. While
constraining the nature of the emission process, these observations do
\emph{not} constrain the nature of the relativistic ejecta. We employ
simulations of explosive outflows launched in NS ejecta clouds to show that the
spectral and temporal evolution of the non-thermal emission from GW170817 is
consistent with both emission from radially stratified quasi-spherical ejecta
traveling at mildly relativistic speeds, \emph{and} emission from off-axis
collimated ejecta characterized by a narrow cone of ultra-relativistic material
with slower wings extending to larger angles. In the latter scenario, GW170817
harbored a normal SGRB directed away from our line of sight. Observations at
days are unlikely to settle the debate as in both scenarios the
observed emission is effectively dominated by radiation from mildly
relativistic material.Comment: Updated with the latest VLA and Chandra dat
Interpretation of the flares of M87 at TeV energies in the cloud-jet interaction scenario
Active galactic nuclei with misaligned jets have been recently established as
a class of high-energy gamma-ray sources. M87, a nearby representative of this
class, shows fast TeV variability on timescales less than one day. We present
calculations performed in the framework of the scenario in which gamma-ray
flares in non-blazar active galactic nuclei are produced by a red giant or a
gas cloud interacting with the jet. We show that both the light curve and
energy spectrum of the spectacular April 2010 flare can be reproduced by this
model, assuming that a relatively massive cloud of approx 1.e29 g penetrates
into the jet at few tens of Schwarzschild radii from the super-massive black
hole.Comment: 8 pages, 8 figures, accepted by Ap
Importance of early nutritional screening in patients with gastric cancer
In the present study, we evaluated the relationship between nutritional status, disease stage and quality of life (QoL) in 100 patients recently diagnosed with gastric carcinoma. The patients' nutritional status was investigated with anthropometric, biochemical, inflammatory and functional variables; and we also evaluated the nutritional risk with the Nutritional Risk Screening 2002. Oncological staging was standard. QoL was evaluated using the Functional Assessment of Anorexia/Cachexia Therapy questionnaire. The statistical correlation between nutritional risk score (NRS) and oncological characteristics or QoL was evaluated using both univariable and multivariable analyses. Weight loss and reduction of food intake were the most frequent pathological nutritional indicators, while biochemical, inflammatory and functional variables were in the normal range. According to NRS, thirty-six patients were malnourished or at risk for malnutrition. Patients with NRS3 presented a significantly greater percentage of stage IV gastric cancer and pathological values of C-reactive protein, while no correlation was found with the site of tumour. NRS was negatively associated with QoL (P<0.001) and this relation was independent from oncological and inflammatory variables as confirmed by multivariable analysis. In the present study, we found that in patients with gastric cancer malnutrition is frequent at diagnosis and this is likely due to reduction in food intake. Moreover, NRS is directly correlated with tumour stage and inversely correlated with QoL, which makes it a useful tool to identify patients in need of an early nutritional intervention during oncological treatments
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