3,295 research outputs found
Constraints on cosmic-ray efficiency in the supernova remnant RCW 86 using multi-wavelength observations
Several young supernova remnants (SNRs) have recently been detected in the
high-energy and very-high-energy gamma-ray domains. As exemplified by RX
J1713.7-3946, the nature of this emission has been hotly debated, and direct
evidence for the efficient acceleration of cosmic-ray protons at the SNR shocks
still remains elusive. We analyzed more than 40 months of data acquired by the
Large Area Telescope (LAT) on-board the Fermi Gamma-Ray Space Telescope in the
HE domain, and gathered all of the relevant multi-wavelength (from radio to VHE
gamma-rays) information about the broadband nonthermal emission from RCW 86.
For this purpose, we re-analyzed the archival X-ray data from the ASCA/Gas
Imaging Spectrometer (GIS), the XMM-Newton/EPIC-MOS, and the RXTE/Proportional
Counter Array (PCA). Beyond the expected Galactic diffuse background, no
significant gamma-ray emission in the direction of RCW 86 is detected in any of
the 0.1-1, 1-10 and 10-100 GeV Fermi-LAT maps. In the hadronic scenario, the
derived HE upper limits together with the HESS measurements in the VHE domain
can only be accommodated by a spectral index Gamma <= 1.8, i.e. a value
in-between the standard (test-particle) index and the asymptotic limit of
theoretical particle spectra in the case of strongly modified shocks. The
interpretation of the gamma-ray emission by inverse Compton scattering of high
energy electrons reproduces the multi-wavelength data using a reasonable value
for the average magnetic field of 15-25 muG. For these two scenarios, we
assessed the level of acceleration efficiency. We discuss these results in the
light of existing estimates of the magnetic field strength, the effective
density and the acceleration efficiency in RCW 86.Comment: Accepted for publication in A&A; 10 pages and 4 figure
The moduli problem at the perturbative level
Moduli fields generically produce strong dark matter -- radiation and baryon
-- radiation isocurvature perturbations through their decay if they remain
light during inflation. We show that existing upper bounds on the magnitude of
such fluctuations can thus be translated into stringent constraints on the
moduli parameter space m_\sigma (modulus mass) -- \sigma_{inf} (modulus vacuum
expectation value at the end of inflation). These constraints are complementary
to previously existing bounds so that the moduli problem becomes worse at the
perturbative level. In particular, if the inflationary scale H_{inf}~10^{13}
GeV, particle physics scenarios which predict high moduli masses m_\sigma >
10-100 TeV are plagued by the perturbative moduli problem, even though they
evade big-bang nucleosynthesis constraints.Comment: 4 pages, 3 figures (revtex) -- v2: an important correction on the
amplitude/transfer of isocurvature modes at the end of inflation, typos
corrected, references added, basic result unchange
Trans-Planckian Dark Energy?
It has recently been proposed by Mersini et al. 01, Bastero-Gil and Mersini
02 that the dark energy could be attributed to the cosmological properties of a
scalar field with a non-standard dispersion relation that decreases
exponentially at wave-numbers larger than Planck scale (k_phys > M_Planck). In
this scenario, the energy density stored in the modes of trans-Planckian
wave-numbers but sub-Hubble frequencies produced by amplification of the vacuum
quantum fluctuations would account naturally for the dark energy. The present
article examines this model in detail and shows step by step that it does not
work. In particular, we show that this model cannot make definite predictions
since there is no well-defined vacuum state in the region of wave-numbers
considered, hence the initial data cannot be specified unambiguously. We also
show that for most choices of initial data this scenario implies the production
of a large amount of energy density (of order M_Planck^4) for modes with
momenta of order M_Planck, far in excess of the background energy density. We
evaluate the amount of fine-tuning in the initial data necessary to avoid this
back-reaction problem and find it is of order H/M_Planck. We also argue that
the equation of state of the trans-Planckian modes is not vacuum-like.
Therefore this model does not provide a suitable explanation for the dark
energy.Comment: RevTeX - 15 pages, 7 figures: final version to appear in PRD, minor
changes, 1 figure adde
Fermi Detection of the Pulsar Wind Nebula HESS J1640-465
We present observations of HESS J1640-465 with the Fermi-LAT. The source is
detected with high confidence as an emitter of high-energy gamma-rays. The
spectrum lacks any evidence for the characteristic cutoff associated with
emission from pulsars, indicating that the emission arises primarily from the
pulsar wind nebula. Broadband modeling implies an evolved nebula with a low
magnetic field resulting in a high gamma-ray to X-ray flux ratio. The Fermi
emission exceeds predictions of the broadband model, and has a steeper
spectrum, possibly resulting from a distinct excess of low energy electrons
similar to what is inferred for both the Vela X and Crab pulsar wind nebulae.Comment: 6 pages, 5 figures, accepted for publication in Ap
Living on the Edge: Inughuit Women and Geography of Contact
The women of northwestern Greenland experienced contact with Euro-American men in multiple ways and in a variety of geographical contexts. Together the archaeological record and unpublished historical documents reveal the complexities of these situations and women’s responses to them. Archaeological and documentary data from two early 20th-century contact situations at Iita in northwestern Greenland and Floeberg Beach on Ellesmere Island, Nunavut, highlight the varied nature of women’s responses to contact and the ways in which interaction with newcomers affected them individually and collectively. The women’s responses varied over time and across space. They responded to both the stresses and the opportunities of contact with resilience and resistance, acceptance and rejection, depending on circumstances.Les femmes du nord-ouest du Groenland sont entrées en contact avec les hommes euroaméricains de plusieurs manières et dans divers contextes géographiques. Ensemble, des dossiers archéologiques et des documents historiques inédits révèlent les complexités de ces situations et la réaction des femmes vis-à -vis de celles-ci. Les données archéologiques et documentaires de deux situations de contact datant du début du XXe siècle à Iita, dans le nord-ouest du Groenland et à Floeberg Beach sur l’île d’Ellesmere, au Nunavut jettent la lumière sur la nature variée des réactions des femmes face au contact et sur la façon dont l’interaction avec les nouveaux venus les ont touchées, individuellement et collectivement. La réaction des femmes variait en fonction du temps et de l’espace. Elles ont réagi au stress et aux opportunités avec résilience et résistance, acceptation et rejet, selon les circonstances
Belinfante Tensors Induced by Matter-Gravity Couplings
We show that any generally covariant coupling of matter fields to gravity
gives rise to a conserved, on-shell symmetric energy-momentum tensor equivalent
to the canonical energy-momentum tensor of the flat-space theory. For matter
fields minimally coupled to gravity our algorithm gives the conventional
Belinfante tensor. We establish that different matter-gravity couplings give
metric energy-momentum tensors differing by identically conserved tensors. We
prove that the metric energy-momentum tensor obtained from an arbitrary gravity
theory is on-shell equivalent to the canonical energy-momentum tensor of the
flat-space theory.Comment: 10 pages, LaTex; misprints corrected, references added; to appear in
Physical Review
Etude Des Contraintes Residuelles Engendrees Par le Procede de Soudage Vertical Montant. EUR 2801. = Study of Residual Constraints Caused by the Vertical Welding Process. EUR 2801.
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