89 research outputs found
Fermi Large Area Telescope Observations of the Cygnus Loop Supernova Remnant
We present an analysis of the gamma-ray measurements by the Large Area
Telescope(LAT) onboard the \textit{Fermi Gamma-ray Space Telescope} in the
region of the supernova remnant(SNR) Cygnus Loop(G74.08.5). We detect
significant gamma-ray emission associated with the SNR in the energy band
0.2--100 GeV. The gamma-ray spectrum shows a break in the range 2--3 GeV. The
gamma-ray luminosity is erg s between 1--100
GeV, much lower than those of other GeV-emitting SNRs. The morphology is best
represented by a ring shape, with inner/outer radii 0.7
0.1 and 1.6 0.1. Given the association among
X-ray rims, \halpha filaments and gamma-ray emission, we argue that gamma rays
originate in interactions between particles accelerated in the SNR and
interstellar gas or radiation fields adjacent to the shock regions. The decay
of neutral pions produced in nucleon-nucleon interactions between accelerated
hadrons and interstellar gas provides a reasonable explanation for the
gamma-ray spectrum.Comment: accepted by ApJ, 34 pages, 6 figure
On the Momentum Diffusion of Radiating Ultrarelativistic Electrons in a Turbulent Magnetic Field
Here we investigate some aspects of stochastic acceleration of
ultrarelativistic electrons by magnetic turbulence. In particular, we discuss
the steady-state energy spectra of particles undergoing momentum diffusion due
to resonant interactions with turbulent MHD modes, taking rigorously into
account direct energy losses connected with different radiative cooling
processes. For the magnetic turbulence we assume a given power spectrum of the
type . In contrast to the previous approaches, however, we
assume a finite range of turbulent wavevectors , consider a variety of
turbulence spectral indexes , and concentrate on the case of a
very inefficient particle escape from the acceleration site. We find that for
different cooling and injection conditions, stochastic acceleration processes
tend to establish a modified ultrarelativistic Maxwellian distribution of
radiating particles, with the high-energy exponential cut-off shaped by the
interplay between cooling and acceleration rates. For example, if the timescale
for the dominant radiative process scales with the electron momentum as
, the resulting electron energy distribution is of the form
, where , and
is the equilibrium momentum defined by the balance between stochastic
acceleration and energy losses timescales. We also discuss in more detail the
synchrotron and inverse-Compton emission spectra produced by such an electron
energy distribution, taking into account Klein-Nishina effects. We point out
that the curvature of the high frequency segments of these spectra, even though
being produced by the same population of electrons, may be substantially
different between the synchrotron and inverse-Compton components.Comment: 42 pages, 14 figures included. Slightly modified version, accepted
for publication in Ap
Diversity of conidial fungi and some abiotic variables of the water after the reopening of the Pirarungaua stream in the Jardim Botânico, São Paulo, São Paulo State, Brazil
High brightness temperatures and circular polarisation in extra-galactic radio sources
Some rapidly variable extra-galactic radio sources show very high brightness
temperatures T_B>10^{12}K and high degrees of circular polarisation (1%).
Standard synchrotron models that assume a power-law electron distribution
cannot produce such high temperatures and have much lower degrees of intrinsic
circular polarisation. We examine the synchrotron and inverse Compton radiation
from a monoenergetic electron distribution using standard synchrotron theory.
Constraints on the source parameters are found by formulating the results as
functions of the source size, Doppler boosting factor, optical depth to
synchrotron self-absorption, maximum frequency of synchrotron emission, and the
strength of the inverse Compton radiation. The model gives brightness
temperatures T_B=10^{13}K to 10^{14}K for moderate (<10) Doppler boosting
factors and intrinsic degrees of circular polarisation at the percent level. It
predicts a spectrum I_\nu\propto\nu^{1/3} between the radio and the infra-red
as well as emission in the MeV to GeV range. We find the conditions under which
electrons do not cool within the source, enabling the GHz emission to emerge
without absorption and the potentially catastrophic energy losses by inverse
Compton scattering to be avoided. We suggest that sources such as PKS 1519
-273, PKS 0405 -385 and J 1819 +3845 can be understood within this scenario
without invoking high Doppler boosting factors, coherent emission mechanisms,
or the dominance of proton synchrotron radiation.Comment: 4 pages, to appear in A&A Letter
Pulsed radiation from neutron star winds
The radiation of a pulsar wind is computed assuming that at roughly 10 to 100
light cylinder radii from the star, magnetic energy is dissipated into particle
energy. The synchrotron emission of heated particles appears periodic, with, in
general, both a pulse and an interpulse. The predicted spacing agrees well with
the Crab and Vela pulse profiles.Using parameters appropriate for the Crab
pulsar (magnetisation parameter at the light cylinder ,
Lorentz factor ) agreement is found with the observed total pulsed
luminosity. This suggests that the high-energy pulses from young pulsars
originate not in the corotating magnetosphere within the light cylinder (as in
all other models) but from the radially directed wind well outside it.Comment: 4 pages, 2 figures, accepted for publication in A&A Letter
The inverse Compton catastrophe and high brightness temperature radio sources
The occurrence of the inverse Compton catastrophe when the synchrotron
brightness temperature exceeds a threshold value, usually estimated to be
10^{12} K, appears to be in contradiction with observation because: (i) the
threshold is substantially exceeded by several intra-day variable radio
sources, but the inverse Compton emission is not observed, (ii) powerful,
extra-galactic radio sources of known angular size do not appear to congregate
close to the predicted maximum brightness temperature. We re-examine the
parameter space available to synchrotron sources using a population of
monoenergetic electrons, in order to see whether the revised threshold
temperature is consistent with the data. The electron distribution and the
population of each generation of scattered photons are computed using spatially
averaged equations. We confirm our previous finding that intrinsic brightness
temperatures T_{\rm B}~10^{14} K can occur without catastrophic cooling. We
show that substantially higher temperatures cannot be achieved either in
transitory solutions or in solutions that balance losses with a powerful
acceleration mechanism. Depending on the observing frequency, we find strong
cooling can set in at a range of threshold temperatures and the imposition of
the additional constraint of equipartition between particle and magnetic field
energy is not warranted by the data. Postulating a monoenergetic electron
distribution, which approximates one that is truncated below a certain Lorentz
factor, \gamma_{min}, alleviates several theoretical difficulties associated
with the inverse Compton catastrophe, including those mentioned above.Comment: 11 pages, 3 figures, to appear in A&
Delimitation of Funga as a valid term for the diversity of fungal communities: the Fauna, Flora & Funga proposal (FF&F)
As public policies and conservation requirements for biodiversity evolve there is a need for a term for the kingdom Fungi equivalent to Fauna and Flora. Thisneed is considered to be urgent in order to simplify projects oriented toward implemention of educational and conservation goals. In an informal meeting held duringthe IX Congreso Latinoamericano de Micología by the authors, the idea of clarifying this matter initiated an extensive search of pertinent terminologies. As a result ofthese discussions and reviews, we propose that the word Funga be employed as an accurate and encompassing term for these purposes. This supports the proposal of thethree Fs, Fauna, Flora and Funga, to highlight parallel terminology referring to treatments of these macrorganism of particular geographical areas. Alternative terms andproposals are acknowledged and discussedFil: Kuhar, José Francisco. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto Multidisciplinario de Biología Vegetal. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Instituto Multidisciplinario de Biología Vegetal; ArgentinaFil: Furci, Giuliana. Fundación Fungi; ChileFil: Drechsler-Santos, Elisandro Ricardo. Universidade Federal de Santa Catarina; BrasilFil: Pfister, Donald H.. Harvard University; Estados Unido
Magnetic fields in cosmic particle acceleration sources
We review here some magnetic phenomena in astrophysical particle accelerators
associated with collisionless shocks in supernova remnants, radio galaxies and
clusters of galaxies. A specific feature is that the accelerated particles can
play an important role in magnetic field evolution in the objects. We discuss a
number of CR-driven, magnetic field amplification processes that are likely to
operate when diffusive shock acceleration (DSA) becomes efficient and
nonlinear. The turbulent magnetic fields produced by these processes determine
the maximum energies of accelerated particles and result in specific features
in the observed photon radiation of the sources. Equally important, magnetic
field amplification by the CR currents and pressure anisotropies may affect the
shocked gas temperatures and compression, both in the shock precursor and in
the downstream flow, if the shock is an efficient CR accelerator. Strong
fluctuations of the magnetic field on scales above the radiation formation
length in the shock vicinity result in intermittent structures observable in
synchrotron emission images. Resonant and non-resonant CR streaming
instabilities in the shock precursor can generate mesoscale magnetic fields
with scale-sizes comparable to supernova remnants and even superbubbles. This
opens the possibility that magnetic fields in the earliest galaxies were
produced by the first generation Population III supernova remnants and by
clustered supernovae in star forming regions.Comment: 30 pages, Space Science Review
Genome-Wide Association Study and Gene Expression Analysis Identifies CD84 as a Predictor of Response to Etanercept Therapy in Rheumatoid Arthritis
Anti-tumor necrosis factor alpha (anti-TNF) biologic therapy is a widely used treatment for rheumatoid arthritis (RA). It is unknown why some RA patients fail to respond adequately to anti-TNF therapy, which limits the development of clinical biomarkers to predict response or new drugs to target refractory cases. To understand the biological basis of response to anti-TNF therapy, we conducted a genome-wide association study (GWAS) meta-analysis of more than 2 million common variants in 2,706 RA patients from 13 different collections. Patients were treated with one of three anti-TNF medications: etanercept (n = 733), infliximab (n = 894), or adalimumab (n = 1,071). We identified a SNP (rs6427528) at the 1q23 locus that was associated with change in disease activity score (ΔDAS) in the etanercept subset of patients (P = 8×10-8), but not in the infliximab or adalimumab subsets (P>0.05). The SNP is predicted to disrupt transcription factor binding site motifs in the 3′ UTR of an immune-related gene, CD84, and the allele associated with better response to etanercept was associated with higher CD84 gene expression in peripheral blood mononuclear cells (P = 1×10-11 in 228 non-RA patients and P = 0.004 in 132 RA patients). Consistent with the genetic findings, higher CD84 gene expression correlated with lower cross-sectional DAS (P = 0.02, n = 210) and showed a non-significant trend for better ΔDAS in a subset of RA patients with gene expression data (n = 31, etanercept-treated). A small, multi-ethnic replication showed a non-significant trend towards an association among etanercept-treated RA patients of Portuguese ancestry (n = 139, P = 0.4), but no association among patients of Japanese ancestry (n = 151, P = 0.8). Our study demonstrates that an allele associated with response to etanercept therapy is also associated with CD84 gene expression, and further that CD84 expression correlates with disease activity. These findings support a model in which CD84 genotypes and/or expression may serve as a useful biomarker for response to etanercept treatment in RA patients of European ancestry. © 2013 Cui et al
Relativity in social cognition: basic processes and novel applications of social comparisons
A key challenge for social psychology is to identify unifying principles that account for the complex dynamics of social behaviour. We propose psychological relativity and its core mechanism of comparison as one such unifying principle. Social cognition is relative in that it is shaped by comparative thinking. If comparative thinking is indeed a central mechanism in social psychology, then it should be affected by, and affect itself, a wide variety of phenomena. To support our proposal, we review recent evidence investigating basic processes underlying and novel applications of social comparisons. Specifically, we clarify determinants of assimilation and contrast, evaluative consequences of comparing similarities vs. differences, attitudinal effects of spatial relativity, and how spatial arrangements determine perceived similarity, one of the antecedents of social comparisons. We then move to behavioural relativity effects on motivation and self-regulation, as well as imitation behaviour. Finally, we address relativity within the more applied areas of morality and political psychology. The reviewed research thereby illustrates how unifying principles of social cognition may be instrumental in answering old questions and discovering new phenomena and explanations
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