8,761 research outputs found
Indicator systems - resource use in organic systems
A balanced use of resources within organic farming systems is required to maintain sustainable systems. Hence, it is essential to have tools that can assess the use of resources within the farming system and their impact on the environment. The range of tools that have been developed include those assessing local farm-scale issues together with those that assess impacts at the global scale. At the global scale assessments are usually made on the basis of a unit of product whereas at the local scale assessments can also be made on an area basis. In addition, the tools also assess a variety of issues, e.g. biodiversity, pollution potential, energy and water use. The level of detail required for the different assessment tools differs substantially; nevertheless it is essential that the indicator systems developed are based on sound knowledge, are acceptable to the farmers and can guide their future actions
TeV Gamma Rays from Geminga and the Origin of the GeV Positron Excess
The Geminga pulsar has long been one of the most intriguing MeV-GeV gamma-ray
point sources. We examine the implications of the recent Milagro detection of
extended, multi-TeV gamma-ray emission from Geminga, finding that this reveals
the existence of an ancient, powerful cosmic-ray accelerator that can plausibly
account for the multi-GeV positron excess that has evaded explanation. We
explore a number of testable predictions for gamma-ray and electron/positron
experiments (up to ~100 TeV) that can confirm the first "direct" detection of a
cosmic-ray source.Comment: 4 pages and 3 figures; Minor revisions, accepted for publication in
Physical Review Letter
Physical parameters of GRB 970508 and GRB 971214 from their afterglow synchrotron emission
We have calculated synchrotron spectra of relativistic blast waves, and find
predicted characteristic frequencies that are more than an order of magnitude
different from previous calculations. For the case of an adiabatically
expanding blast wave, which is applicable to observed gamma-ray burst (GRB)
afterglows at late times, we give expressions to infer the physical properties
of the afterglow from the measured spectral features.
We show that enough data exist for GRB970508 to compute unambiguously the
ambient density, n=0.03/cm**3, and the blast wave energy per unit solid angle,
E=3E52 erg/4pi sr. We also compute the energy density in electrons and magnetic
field. We find that they are 12% and 9%, respectively, of the nucleon energy
density and thus confirm for the first time that both are close to but below
equipartition.
For GRB971214, we discuss the break found in its spectrum by Ramaprakash et
al. (1998). It can be interpreted either as the peak frequency or as the
cooling frequency; both interpretations have some problems, but on balance the
break is more likely to be the cooling frequency. Even when we assume this, our
ignorance of the self-absorption frequency and presence or absence of beaming
make it impossible to constrain the physical parameters of GRB971214 very well.Comment: very strongly revised analysis of GRB971214 and discussion, submitted
to ApJ, 11 pages LaTeX, 4 figures, uses emulateapj.sty (included
Degeneracy in exotic gravitational lensing
We present three different theoretically foreseen, but unusual, astrophysical
situations where the gravitational lens equation ends up being the same, thus
producing a degeneracy problem. These situations are (a) the case of
gravitational lensing by exotic stresses (matter violating the weak energy
condition and thus having a negative mass, particular cases of wormholes
solutions can be used as an example), (b) scalar field gravitational lensing
(i.e. when considering the appearance of a scalar charge in the lensing
scenario), and (c) gravitational lensing in closed universes (with
antipodes).The reasons that lead to this degeneracy in the lens equations, the
possibility of actually encountering it in the real universe, and eventually
the ways to break it, are discussed.Comment: Accepted for publication in Modern Physics Letters
Primordial magnetic fields and nonlinear electrodynamics
The creation of large scale magnetic fields is studied in an inflationary
universe where electrodynamics is assumed to be nonlinear. After inflation ends
electrodynamics becomes linear and thus the description of reheating and the
subsequent radiation dominated stage are unaltered. The nonlinear regime of
electrodynamics is described by lagrangians having a power law dependence on
one of the invariants of the electromagnetic field. It is found that there is a
range of parameters for which primordial magnetic fields of cosmologically
interesting strengths can be created.Comment: 21 pages, 3 figure
The HST Large Program on Omega Centauri. I. Multiple stellar populations at the bottom of the main sequence probed in NIR-Optical
As part of a large investigation with Hubble Space Telescope to study the
faintest stars within the globular cluster Omega Centauri, in this work we
present early results on the multiplicity of its main sequence (MS) stars,
based on deep optical and near-infrared observations. By using appropriate
color-magnitude diagrams we have identified, for the first time, the two main
stellar populations I, and II along the entire MS, from the turn-off towards
the hydrogen-burning limit. We have compared the observations with suitable
synthetic spectra of MS stars and conclude that the two MSs are consistent with
stellar populations with different metallicity, helium, and light-element
abundance. Specifically, MS-I corresponds to a metal-poor stellar population
([Fe/H]~-1.7) with Y~ 0.25 and [O/Fe]~0.30. The MS-II hosts helium-rich
(Y~0.37-0.40) stars with metallicity ranging from [Fe/H]~-1.7 to -1.4. Below
the MS knee (mF160W~19.5, our photometry reveals that each of the two main MSs
hosts stellar subpopulations with different oxygen abundances, with very O-poor
stars ([O/Fe]~-0.5) populating the MS-II. Such a complexity has never been
observed in previous studies of M-dwarfs in globular clusters. A few months
before the lunch of the James Webb Space Telescope, these results demonstrate
the power of optical and near-infrared photometry in the study of multiple
stellar populations in globular clusters.Comment: 13 pages, 9 figures, accepted for publication in MNRA
Spectropolarimetric observations of the Ca II 8498 A and 8542 A lines in the quiet Sun
The Ca II infrared triplet is one of the few magnetically sensitive
chromospheric lines available for ground-based observations. We present
spectropolarimetric observations of the 8498 A and 8542 A lines in a quiet Sun
region near a decaying active region and compare the results with a simulation
of the lines in a high plasma-beta regime. Cluster analysis of Stokes V profile
pairs shows that the two lines, despite arguably being formed fairly close,
often do not have similar shapes. In the network, the local magnetic topology
is more important in determining the shapes of the Stokes V profiles than the
phase of the wave, contrary to what our simulations show. We also find that
Stokes V asymmetries are very common in the network, and the histograms of the
observed amplitude and area asymmetries differ significantly from the
simulation. Both the network and internetwork show oscillatory behavior in the
Ca II lines. It is stronger in the network, where shocking waves, similar to
those in the high-beta simulation, are seen and large self-reversals in the
intensity profiles are common.Comment: 23 pages, 17 figures, accepted to ApJ some figures are low-res, for
high-res email [email protected]
Expanding Space: the Root of all Evil?
While it remains the staple of virtually all cosmological teaching, the
concept of expanding space in explaining the increasing separation of galaxies
has recently come under fire as a dangerous idea whose application leads to the
development of confusion and the establishment of misconceptions. In this
paper, we develop a notion of expanding space that is completely valid as a
framework for the description of the evolution of the universe and whose
application allows an intuitive understanding of the influence of universal
expansion. We also demonstrate how arguments against the concept in general
have failed thus far, as they imbue expanding space with physical properties
not consistent with the expectations of general relativity.Comment: 8 pages, accepted for publication in PAS
Gamma-Ray Burst Afterglows from Realistic Fireballs
A GRB afterglow has been commonly thought to be due to continuous
deceleration of a postburst fireball. Many analytical models have made
simplifications for deceleration dynamics of the fireball and its radiation
property, although they are successful at explaining the overall features of
the observed afterglows. We here propose a model for a GRB afterglow in which
the evolution of a postburst fireball is in an intermediate case between the
adiabatic and highly radiative expansion. In our model, the afterglow is both
due to the contribution of the adiabatic electrons behind the external
blastwave of the fireball and due to the contribution of the radiative
electrons. In addition, this model can describe evolution of the fireball from
the extremely relativistic phase to the non-relativistic phase. Our
calculations show that the fireball will go to the adiabatic expansion phase
after about a day if the accelerated electrons are assumed to occupy the total
internal energy. In all cases considered, the fireball will go to the mildly
relativistic phase about seconds later, and to the non-relativistic
phase after several days. These results imply that the relativistic adiabatic
model cannot describe the deceleration dynamics of the several-days-later
fireball. The comparison of the calculated light curves with the observed
results at late times may imply the presence of impulsive events or energy
injection with much longer durations.Comment: 18 pages, 10 figures, plain latex file, submitted to Ap
Pulsars versus Dark Matter Interpretation of ATIC/PAMELA
In this paper, we study the flux of electrons and positrons injected by
pulsars and by annihilating or decaying dark matter in the context of recent
ATIC, PAMELA, Fermi, and HESS data. We review the flux from a single pulsar and
derive the flux from a distribution of pulsars. We point out that the particle
acceleration in the pulsar magnetosphere is insufficient to explain the
observed excess of electrons and positrons with energy E ~ 1 TeV and one has to
take into account an additional acceleration of electrons at the termination
shock between the pulsar and its wind nebula. We show that at energies less
than a few hundred GeV, the flux from a continuous distribution of pulsars
provides a good approximation to the expected flux from pulsars in the
Australia Telescope National Facility (ATNF) catalog. At higher energies, we
demonstrate that the electron/positron flux measured at the Earth will be
dominated by a few young nearby pulsars, and therefore the spectrum would
contain bumplike features. We argue that the presence of such features at high
energies would strongly suggest a pulsar origin of the anomalous contribution
to electron and positron fluxes. The absence of features either points to a
dark matter origin or constrains pulsar models in such a way that the
fluctuations are suppressed. Also we derive that the features can be partially
smeared due to spatial variation of the energy losses during propagation.Comment: 23 pages, 15 figures, 1 table; v2: minor corrections, references
added; v3: 20 pages, 10 figures, 1 table, major changes in presentation, main
conclusions unchanged; v4: minor correction
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