173 research outputs found
Fermi gamma-ray `bubbles' from stochastic acceleration of electrons
Gamma-ray data from Fermi-LAT reveal a bi-lobular structure extending up to
50 degrees above and below the galactic centre, which presumably originated in
some form of energy release there less than a few million years ago. It has
been argued that the gamma-rays arise from hadronic interactions of high energy
cosmic rays which are advected out by a strong wind, or from inverse-Compton
scattering of relativistic electrons accelerated at plasma shocks present in
the bubbles. We explore the alternative possibility that the relativistic
electrons are undergoing stochastic 2nd-order Fermi acceleration by plasma wave
turbulence through the entire volume of the bubbles. The observed gamma-ray
spectral shape is then explained naturally by the resulting hard electron
spectrum and inverse Compton losses. Rather than a constant volume emissivity
as in other models, we predict a nearly constant surface brightness, and
reproduce the observed sharp edges of the bubbles.Comment: 4 pages, 4 figures; REVTeX4-1; discussion amended and one figure
added; to appear in PR
Diffusive propagation of cosmic rays from supernova remnants in the Galaxy. I: spectrum and chemical composition
In this paper we investigate the effect of stochasticity in the spatial and
temporal distribution of supernova remnants on the spectrum and chemical
composition of cosmic rays observed at Earth. The calculations are carried out
for different choices of the diffusion coefficient D(E) experienced by cosmic
rays during propagation in the Galaxy. In particular, at high energies we
assume that D(E)\sim E^{\delta}, with and being the
reference scenarios. The large scale distribution of supernova remnants in the
Galaxy is modeled following the distribution of pulsars, with and without
accounting for the spiral structure of the Galaxy. We find that the stochastic
fluctuations induced by the spatial and temporal distribution of supernovae,
together with the effect of spallation of nuclei, lead to mild but sensible
violations of the simple, leaky-box-inspired rule that the spectrum observed at
Earth is with , where
is the slope of the cosmic ray injection spectrum at the sources. Spallation of
nuclei, even with the small rates appropriate for He, may account for slight
differences in spectral slopes between different nuclei, providing a possible
explanation for the recent CREAM observations. For we find that
the slope of the proton and helium spectra are and
respectively at energies above 1 TeV (to be compared with the measured values
of and ). For the hardening of the He
spectra is not observed. We also comment on the effect of time dependence of
the escape of cosmic rays from supernova remnants, and of a possible clustering
of the sources in superbubbles. In a second paper we will discuss the
implications of these different scenarios for the anisotropy of cosmic rays.Comment: 28 pages, To appear in JCA
Systematic effects in the extraction of the 'WMAP haze'
The extraction of a 'haze' from the WMAP microwave skymaps is based on
subtraction of known foregrounds, viz. free-free (bremsstrahlung), thermal dust
and synchrotron, each traced by other skymaps. While the 408 MHz all-sky survey
is used for the synchrotron template, the WMAP bands are at tens of GHz where
the spatial distribution of the radiating cosmic ray electrons ought to be
quite different because of the energy-dependence of their diffusion in the
Galaxy. The systematic uncertainty this introduces in the residual skymap is
comparable to the claimed haze and can, for certain source distributions, have
a very similar spectrum and latitudinal profile and even a somewhat similar
morphology. Hence caution must be exercised in interpreting the 'haze' as a
physical signature of, e.g., dark matter annihilation in the Galactic centre.Comment: 17 pages, 12 figures; improved diffusion model; extended discussion
of spectral index maps; clarifying comments, figures and references added; to
appear in JCA
Sommerfeld Enhancement from Multiple Mediators
We study the Sommerfeld enhancement experienced by a scattering object that
couples to a tower of mediators. This can occur in, e.g., models of secluded
dark matter when the mediator scale is generated naturally by hidden-sector
confinement. Specializing to the case of a confining CFT, we show that
off-resonant values of the enhancement can be increased by ~ 20% for cases of
interest when (i) the (strongly-coupled) CFT admits a weakly-coupled dual
description and (ii) the conformal symmetry holds up to the Planck scale.
Larger enhancements are possible for lower UV scales due to an increase in the
coupling strength of the tower.Comment: 17p, 2 figures; v2 JHEP version (inconsequential typo fixed,
references added
Secluded Dark Matter Coupled to a Hidden CFT
Models of secluded dark matter offer a variant on the standard WIMP picture
and can modify our expectations for hidden sector phenomenology and detection.
In this work we extend a minimal model of secluded dark matter, comprised of a
U(1)'-charged dark matter candidate, to include a confining hidden-sector CFT.
This provides a technically natural explanation for the hierarchically small
mediator-scale, with hidden-sector confinement generating m_{gamma'}>0.
Furthermore, the thermal history of the universe can differ markedly from the
WIMP picture due to (i) new annihilation channels, (ii) a (potentially) large
number of hidden-sector degrees of freedom, and (iii) a hidden-sector phase
transition at temperatures T << M_{dm} after freeze out. The mediator allows
both the dark matter and the Standard Model to communicate with the CFT, thus
modifying the low-energy phenomenology and cosmic-ray signals from the secluded
sector.Comment: ~50p, 8 figs; v2 JHEP versio
Planck Intermediate Results. IX. Detection of the Galactic haze with Planck
Using precise full-sky observations from Planck, and applying several methods
of component separation, we identify and characterize the emission from the
Galactic "haze" at microwave wavelengths. The haze is a distinct component of
diffuse Galactic emission, roughly centered on the Galactic centre, and extends
to |b| ~35 deg in Galactic latitude and |l| ~15 deg in longitude. By combining
the Planck data with observations from the WMAP we are able to determine the
spectrum of this emission to high accuracy, unhindered by the large systematic
biases present in previous analyses. The derived spectrum is consistent with
power-law emission with a spectral index of -2.55 +/- 0.05, thus excluding
free-free emission as the source and instead favouring hard-spectrum
synchrotron radiation from an electron population with a spectrum (number
density per energy) dN/dE ~ E^-2.1. At Galactic latitudes |b|<30 deg, the
microwave haze morphology is consistent with that of the Fermi gamma-ray "haze"
or "bubbles," indicating that we have a multi-wavelength view of a distinct
component of our Galaxy. Given both the very hard spectrum and the extended
nature of the emission, it is highly unlikely that the haze electrons result
from supernova shocks in the Galactic disk. Instead, a new mechanism for
cosmic-ray acceleration in the centre of our Galaxy is implied.Comment: 15 pages, 9 figures, submitted to Astronomy and Astrophysic
Galactic electrons and positrons at the Earth:new estimate of the primary and secondary fluxes
We analyse predictions of the CR lepton fluxes at the Earth of both secondary
and primary origins, evaluate the theoretical uncertainties, and determine
their level of consistency with respect to the available data. For propagation,
we use a relativistic treatment of the energy losses for which we provide
useful parameterizations. We compute the secondary components by improving on
the method that we derived earlier for positrons. For primaries, we estimate
the contributions from astrophysical sources (supernova remnants and pulsars)
by considering all known local objects within 2 kpc and a smooth distribution
beyond. We find that the electron flux in the energy range 5-30 GeV is well
reproduced by a smooth distant distribution of sources with index , while local sources dominate the flux at higher energy. For
positrons, local pulsars have an important effect above 5-10 GeV. Uncertainties
affecting the source modeling and propagation are degenerate and each
translates into about one order of magnitude error in terms of local flux. The
spectral shape at high energy is weakly correlated with the spectral indices of
local sources, but more strongly with the hierarchy in their distance, age and
power. Despite the large theoretical errors that we describe, our global and
self-consistent analysis can explain all available data without over-tuning the
parameters, and therefore without the need to consider any exotic physics.
Though a \emph{standard paradigm} of Galactic CRs is well established, our
results show that we can hardly talk about any \emph{standard model} of CR
leptons, because of the very large theoretical uncertainties. Our analysis
provides details about the impact of these uncertainties, thereby sketching a
roadmap for future improvements.Comment: 34 pages, 14 figures. V2: few changes, results unchanged; matches the
version accepted in Astron. Astrophy
The Pierre Auger Observatory III: Other Astrophysical Observations
Astrophysical observations of ultra-high-energy cosmic rays with the Pierre
Auger ObservatoryComment: Contributions to the 32nd International Cosmic Ray Conference,
Beijing, China, August 201
Analysis of WMAP 7-year Temperature Data: Astrophysics of the Galactic Haze
We analyse WMAP 7-year temperature data, jointly modeling the cosmic
microwave background (CMB) and Galactic foreground emission. We use the
Commander code based on Gibbs sampling. Thus, from the WMAP7 data, we derive
simultaneously the CMB and Galactic components on scales larger than 1deg with
sensitivity improved relative to previous work. We conduct a detailed study of
the low-frequency foreground with particular focus on the "microwave haze"
emission around the Galactic center. We demonstrate improved performance in
quantifying the diffuse galactic emission when Haslam 408MHz data are included
together with WMAP7, and the spinning and thermal dust emission is modeled
jointly. We also address the question of whether the hypothetical galactic haze
can be explained by a spatial variation of the synchrotron spectral index. The
excess of emission around the Galactic center appears stable with respect to
variations of the foreground model that we study. Our results demonstrate that
the new galactic foreground component - the microwave haze - is indeed present.Comment: 16 pages, 16 figures, Published on Ap
The high energy neutrino cross-section in the Standard Model and its uncertainty
Updated predictions are presented for high energy neutrino and antineutrino
charged and neutral current cross-sections within the conventional DGLAP
formalism of NLO QCD using modern PDF fits. PDF uncertainties from model
assumptions and parametrization bias are considered in addition to the
experimental uncertainties. Particular attention is paid to assumptions and
biases which could signal the need for extension of the conventional formalism
to include effects such as ln(1/x) resummation or non-linear effects of high
gluon density.Comment: 15 pages, 13 figures, 2 tables (REVTeX4); clarifying comments and
link to tabulated cross sections at
http://www-pnp.physics.ox.ac.uk/~cooper/neutrino/ added; to appear in JHE
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