220 research outputs found
Transport of positrons in the interstellar medium
This work investigates some aspects of the transport of low-energy positrons
in the interstellar medium (ISM). We consider resonance interactions with
magnetohydrodynamic waves above the resonance threshold. Below the threshold,
collisions take over and deflect positrons in their motion parallel to
magnetic-field lines. Using Monte-Carlo simulations, we model the propagation
and energy losses of positrons in the different phases of the ISM until they
annihilate. We suggest that positrons produced in the disk by an old population
of stars, with initial kinetic energies below 1 MeV, and propagating in the
spiral magnetic field of the disk, can probably not penetrate the Galactic
bulge.Comment: 4 pages, 3 figures, accepted for publication in the proceeding of the
6th INTEGRAL worksho
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
Biermann Mechanism in Primordial Supernova Remnant and Seed Magnetic Fields
We study generation of magnetic fields by the Biermann mechanism in the
pair-instability supernovae explosions of first stars. The Biermann mechanism
produces magnetic fields in the shocked region between the bubble and
interstellar medium (ISM), even if magnetic fields are absent initially. We
perform a series of two-dimensional magnetohydrodynamic simulations with the
Biermann term and estimate the amplitude and total energy of the produced
magnetic fields. We find that magnetic fields with amplitude
G are generated inside the bubble, though the amount of
magnetic fields generated depend on specific values of initial conditions. This
corresponds to magnetic fields of erg per each supernova
remnant, which is strong enough to be the seed magnetic field for galactic
and/or interstellar dynamo.Comment: 12 pages, 3 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
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
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
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
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&
Magnetic field structure due to the global velocity field in spiral galaxies
We present a set of global, self-consistent N-body/SPH simulations of the
dynamic evolution of galactic discs with gas and including magnetic fields. We
have implemented a description to follow the evolution of magnetic fields with
the ideal induction equation in the SPH part of the Vine code. Results from a
direct implementation of the field equations are compared to a representation
by Euler potentials, which pose a div(B)-free description, an constraint not
fulfilled for the direct implementation. All simulations are compared to an
implementation of magnetic fields in the Gadget code which includes also
cleaning methods for div(B).
Starting with a homogeneous seed field we find that by differential rotation
and spiral structure formation of the disc the field is amplified by one order
of magnitude within five rotation periods of the disc. The amplification is
stronger for higher numerical resolution. Moreover, we find a tight connection
of the magnetic field structure to the density pattern of the galaxy in our
simulations, with the magnetic field lines being aligned with the developing
spiral pattern of the gas. Our simulations clearly show the importance of
non-axisymmetry for the evolution of the magnetic field.Comment: 17 pages, 18 figure
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