234 research outputs found
Magnetic Flux Expulsion in the Powerful Superbubble Explosions and the Alpha-Omega Dynamo
The possibility of the magnetic flux expulsion from the Galaxy in the
superbubble (SB) explosions, important for the Alpha-Omega dynamo, is
considered. Special emphasis is put on the investigation of the downsliding of
the matter from the top of the shell formed by the SB explosion which is able
to influence the kinematics of the shell. It is shown that either Galactic
gravity or the development of the Rayleigh-Taylor instabilities in the shell
limit the SB expansion, thus, making impossible magnetic flux expulsion. The
effect of the cosmic rays in the shell on the sliding is considered and it is
shown that it is negligible compared to Galactic gravity. Thus, the question of
possible mechanism of flux expulsion in the Alpha-Omega dynamo remains open.Comment: MNRAS, in press, 11 pages, 9 figure
The 511 keV emission from positron annihilation in the Galaxy
The first gamma-ray line originating from outside the solar system that was
ever detected is the 511 keV emission from positron annihilation in the Galaxy.
Despite 30 years of intense theoretical and observational investigation, the
main sources of positrons have not been identified up to now. Observations in
the 1990's with OSSE/CGRO showed that the emission is strongly concentrated
towards the Galactic bulge. In the 2000's, the SPI instrument aboard ESA's
INTEGRAL gamma-ray observatory allowed scientists to measure that emission
across the entire Galaxy, revealing that the bulge/disk luminosity ratio is
larger than observed in any other wavelength. This mapping prompted a number of
novel explanations, including rather "exotic ones (e.g. dark matter
annihilation). However, conventional astrophysical sources, like type Ia
supernovae, microquasars or X-ray binaries, are still plausible candidates for
a large fraction of the observed total 511 keV emission of the bulge. A closer
study of the subject reveals new layers of complexity, since positrons may
propagate far away from their production sites, making it difficult to infer
the underlying source distribution from the observed map of 511 keV emission.
However, contrary to the rather well understood propagation of high energy
(>GeV) particles of Galactic cosmic rays, understanding the propagation of low
energy (~MeV) positrons in the turbulent, magnetized interstellar medium, still
remains a formidable challenge. We review the spectral and imaging properties
of the observed 511 keV emission and we critically discuss candidate positron
sources and models of positron propagation in the Galaxy.Comment: 62 pages, 35 figures. Review paper to appear in Reviews of Modern
Physic
In Situ Origin of Large Scale Galactic Magnetic Fields Without Kinetic Helicity?
The origin and sustenance of large scale galactic magnetic fields has been a
long standing and controversial astrophysical problem. Here an alternative to
the ``standard'' \a-\Omega mean field dynamo and primordial theories is
pursued. The steady supply of supernovae induced turbulence exponentiates the
total field energy, providing a significant seed mean field that can be
linearly stretched by shear. The observed micro-Gauss fields would be produced
primarily within one vertical diffusion time since it is only during this time
that linear stretching can compete with diffusion. This approach does not
invoke exponential mean field dynamo growth from the helicity \a-effect but
does employ turbulent diffusion, which limits the number of large scale
reversals. The approach could be of interest if the helicity effect is
suppressed independently of the turbulent diffusion. This is an important but
presently unresolved issue.Comment: 15 pages TeX, accepted, ApJ
The spatial energy spectrum of magnetic fields in our Galaxy
Interstellar magnetic fields exist over a broad range of spatial scales,
extending from the large Galactic scales ( kpc) down to the very small
dissipative scales ( pc). In this paper, we use a set of 490 pulsars
distributed over roughly one third of the Galactic disk out to a radius kpc (assuming kpc) and combine their observed
rotation and dispersion measures with their estimated distances to derive the
spatial energy spectrum of the Galactic interstellar magnetic field over the
scale range kpc. We obtain a nearly flat spectrum, with a 1D
power-law index for and an
rms field strength of approximately G over the relevant scales. Our
study complements the derivation of the magnetic energy spectrum over the scale
range pc by \citet{ms96b}, showing that the magnetic spectrum
becomes flatter at larger scales. This observational result is discussed in the
framework of current theoretical and numerical models.Comment: 7 pages, 6 figures, ApJ accepte
Spectral analysis of the Galactic e+e- annihilation emission
We present a spectral analysis of the e+e- annihilation emission from the
Galactic Centre region based on the first year of measurements made with the
spectrometer SPI of the INTEGRAL mission. We have found that the annihilation
spectrum can be modelled by the sum of a narrow and a broad 511 keV line plus
an ortho-Ps continuum. The broad line is detected with a flux of
(0.35+/-0.11)e-3 s-1 cm-2. The measured width of 5.4+/-1.2 keV FWHM is in
agreement with the expected broadening of 511 keV photons emitted in the
annihilation of Ps that are formed by the charge exchange process of slowing
down positrons with H atoms. The flux of the narrow line is (0.72+/-0.12)e-3
s-1 cm-2 and its width is 1.3+/-0.4 keV FWHM. The measured ortho-Ps continuum
flux yields a fraction of Ps of (96.7+/-2.2)%. To derive in what phase of the
interstellar medium positrons annihilate, we have fitted annihilation models
calculated for each phase to the data. We have found that 49(+2,-23)% of the
annihilation emission comes from the warm neutral phase and 51(+3,-2)% from the
warm ionized phase. While we may not exclude that less than 23% of the emission
might come from cold gas, we have constrained the fraction of annihilation
emission from molecular clouds and hot gas to be less than 8% and 0.5%,
respectively. We have compared our knowledge of the interstellar medium in the
bulge and the propagation of positrons with our results and found that they are
in good agreement if the sources are diffusively distributed and if the initial
kinetic energy of positrons is lower than a few MeV. Despite its large filling
factor, the lack of annihilation emission from the hot gas is due to its low
density, which allows positrons to escape this phase.Comment: 12 pages, 6 figures, accepted in A&
The Evolution of Adiabatic Supernova Remnants in a Turbulent, Magnetized Medium
(Abridged) We present the results of three dimensional calculations for the
MHD evolution of an adiabatic supernova remnant in both a uniform and turbulent
interstellar medium using the RIEMANN framework of Balsara. In the uniform
case, which contains an initially uniform magnetic field, the density structure
of the shell remains largely spherical, while the magnetic pressure and
synchrotron emissivity are enhanced along the plane perpendicular to the field
direction. This produces a bilateral or barrel-type morphology in synchrotron
emission for certain viewing angles. We then consider a case with a turbulent
external medium as in Balsara & Pouquet, characterized by .
Several important changes are found. First, despite the presence of a uniform
field, the overall synchrotron emissivity becomes approximately spherically
symmetric, on the whole, but is extremely patchy and time-variable, with
flickering on the order of a few computational time steps. We suggest that the
time and spatial variability of emission in early phase SNR evolution provides
information on the turbulent medium surrounding the remnant. The
shock-turbulence interaction is also shown to be a strong source of
helicity-generation and, therefore, has important consequences for magnetic
field generation. We compare our calculations to the Sedov-phase evolution, and
discuss how the emission characteristics of SNR may provide a diagnostic on the
nature of turbulence in the pre-supernova environment.Comment: ApJ, in press, 5 color figure
Evidence for dark matter in the inner Milky Way
The ubiquitous presence of dark matter in the universe is today a central
tenet in modern cosmology and astrophysics. Ranging from the smallest galaxies
to the observable universe, the evidence for dark matter is compelling in
dwarfs, spiral galaxies, galaxy clusters as well as at cosmological scales.
However, it has been historically difficult to pin down the dark matter
contribution to the total mass density in the Milky Way, particularly in the
innermost regions of the Galaxy and in the solar neighbourhood. Here we present
an up-to-date compilation of Milky Way rotation curve measurements, and compare
it with state-of-the-art baryonic mass distribution models. We show that
current data strongly disfavour baryons as the sole contribution to the
galactic mass budget, even inside the solar circle. Our findings demonstrate
the existence of dark matter in the inner Galaxy while making no assumptions on
its distribution. We anticipate that this result will compel new
model-independent constraints on the dark matter local density and profile,
thus reducing uncertainties on direct and indirect dark matter searches, and
will shed new light on the structure and evolution of the Galaxy.Comment: First submitted version of letter published in Nature Physics on
Febuary 9, 2015:
http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3237.htm
COrE (Cosmic Origins Explorer) A White Paper
COrE (Cosmic Origins Explorer) is a fourth-generation full-sky,
microwave-band satellite recently proposed to ESA within Cosmic Vision
2015-2025. COrE will provide maps of the microwave sky in polarization and
temperature in 15 frequency bands, ranging from 45 GHz to 795 GHz, with an
angular resolution ranging from 23 arcmin (45 GHz) and 1.3 arcmin (795 GHz) and
sensitivities roughly 10 to 30 times better than PLANCK (depending on the
frequency channel). The COrE mission will lead to breakthrough science in a
wide range of areas, ranging from primordial cosmology to galactic and
extragalactic science. COrE is designed to detect the primordial gravitational
waves generated during the epoch of cosmic inflation at more than
for . It will also measure the CMB gravitational lensing
deflection power spectrum to the cosmic variance limit on all linear scales,
allowing us to probe absolute neutrino masses better than laboratory
experiments and down to plausible values suggested by the neutrino oscillation
data. COrE will also search for primordial non-Gaussianity with significant
improvements over Planck in its ability to constrain the shape (and amplitude)
of non-Gaussianity. In the areas of galactic and extragalactic science, in its
highest frequency channels COrE will provide maps of the galactic polarized
dust emission allowing us to map the galactic magnetic field in areas of
diffuse emission not otherwise accessible to probe the initial conditions for
star formation. COrE will also map the galactic synchrotron emission thirty
times better than PLANCK. This White Paper reviews the COrE science program,
our simulations on foreground subtraction, and the proposed instrumental
configuration.Comment: 90 pages Latex 15 figures (revised 28 April 2011, references added,
minor errors corrected
A Broadband Study of Galactic Dust Emission
We have combined infrared data with HI, H2 and HII surveys in order to
spatially decompose the observed dust emission into components associated with
different phases of the gas. An inversion technique is applied. For the
decomposition, we use the IRAS 60 and 100 micron bands, the DIRBE 140 and 240
micron bands, as well as Archeops 850 and 2096 micron wavelengths. In addition,
we apply the decomposition to all five WMAP bands. We obtain longitude and
latitude profiles for each wavelength and for each gas component in carefully
selected Galactic radius bins.We also derive emissivity coefficients for dust
in atomic, molecular and ionized gas in each of the bins.The HI emissivity
appears to decrease with increasing Galactic radius indicating that dust
associated with atomic gas is heated by the ambient interstellar radiation
field (ISRF). By contrast, we find evidence that dust mixed with molecular
clouds is significantly heated by O/B stars still embedded in their progenitor
clouds. By assuming a modified black-body with emissivity law lambda^(-1.5), we
also derive the radial distribution of temperature for each phase of the gas.
All of the WMAP bands except W appear to be dominated by emission from
something other than normal dust, most likely a mixture of thermal
bremstrahlung from diffuse ionized gas, synchrotron emission and spinning dust.
Furthermore, we find indications of an emissivity excess at long wavelengths
(lambda > 850 micron) in the outer Galaxy (R > 8.9 kpc). This suggests either
the existence of a very cold dust component in the outer Galaxy or a
temperature dependence of the spectral emissivity index. Finally, it is shown
that ~ 80% of the total FIR luminosity is produced by dust associated with
atomic hydrogen, in agreement with earlier findings by Sodroski et al. (1997).Comment: accepted for publication by A&
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