67 research outputs found
Faraday Rotation Measure Synthesis
We extend the rotation measure work of Burn (1966) to the cases of limited
sampling of lambda squared space and non-constant emission spectra. We
introduce the rotation measure transfer function (RMTF), which is an excellent
predictor of n-pi ambiguity problems with the lambda squared coverage. Rotation
measure synthesis can be implemented very efficiently on modern computers.
Because the analysis is easily applied to wide fields, one can conduct very
fast RM surveys of weak spatially extended sources. Difficult situations, for
example multiple sources along the line of sight, are easily detected and
transparently handled. Under certain conditions, it is even possible to recover
the emission as a function of Faraday depth within a single cloud of ionized
gas. Rotation measure synthesis has already been successful in discovering
widespread, weak, polarized emission associated with the Perseus cluster (De
Bruyn and Brentjens, 2005). In simple, high signal to noise situations it is as
good as traditional linear fits to polarization angle versus lambda squared
plots. However, when the situation is more complex or very weak polarized
emission at high rotation measures is expected, it is the only viable option.Comment: 17 pages, 14 figures, accepted by A&A, added references, corrected
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A Model of Polarized X-ray Emission from Twinkling Synchrotron Supernova Shells
Synchrotron X-ray emission components were recently detected in many young
supernova remnants (SNRs). There is even an emerging class - SN1006,
RXJ1713.72-3946, Vela Jr, and others - that is dominated by non-thermal
emission in X-rays, also probably of synchrotron origin. Such emission results
from electrons/positrons accelerated well above TeV energies in the spectral
cut-off regime. In the case of diffusive shock acceleration, which is the most
promising acceleration mechanism in SNRs, very strong magnetic fluctuations
with amplitudes well above the mean magnetic field must be present. Starting
from such a fluctuating field, we have simulated images of polarized X-ray
emission of SNR shells and show that these are highly clumpy with high
polarizations up to 50%. Another distinct characteristic of this emission is
the strong intermittency, resulting from the fluctuating field amplifications.
The details of this "twinkling" polarized X-ray emission of SNRs depend
strongly on the magnetic-field fluctuation spectra, providing a potentially
sensitive diagnostic tool. We demonstrate that the predicted characteristics
can be studied with instruments that are currently being considered. These can
give unique information on magnetic-field characteristics and high-energy
particle acceleration in SNRs.Comment: 7 pages, 8 figures, MNRAS (in press
Measuring energy dependent polarization in soft gamma-rays using Compton scattering in PoGOLite
Linear polarization in X- and gamma-rays is an important diagnostic of many
astrophysical sources, foremost giving information about their geometry,
magnetic fields, and radiation mechanisms. However, very few X-ray polarization
measurements have been made, and then only mono-energetic detections, whilst
several objects are assumed to have energy dependent polarization signatures.
In this paper we investigate whether detection of energy dependent polarization
from cosmic sources is possible using the Compton technique, in particular with
the proposed PoGOLite balloon-experiment, in the 25-100 keV range. We use
Geant4 simulations of a PoGOLite model and input photon spectra based on Cygnus
X-1 and accreting magnetic pulsars (100 mCrab). Effective observing times of 6
and 35 hours were simulated, corresponding to a standard and a long duration
flight respectively. Both smooth and sharp energy variations of the
polarization are investigated and compared to constant polarization signals
using chi-square statistics. We can reject constant polarization, with energy,
for the Cygnus X-1 spectrum (in the hard state), if the reflected component is
assumed to be completely polarized, whereas the distinction cannot be made for
weaker polarization. For the accreting pulsar, constant polarization can be
rejected in the case of polarization in a narrow energy band with at least 50%
polarization, and similarly for a negative step distribution from 30% to 0%
polarization.Comment: 11 pages, 12 figures; updated to match version accepted for
publication in Astroparticle Physics (only minor changes
Magnetic fields in cosmic particle acceleration sources
We review here some magnetic phenomena in astrophysical particle accelerators
associated with collisionless shocks in supernova remnants, radio galaxies and
clusters of galaxies. A specific feature is that the accelerated particles can
play an important role in magnetic field evolution in the objects. We discuss a
number of CR-driven, magnetic field amplification processes that are likely to
operate when diffusive shock acceleration (DSA) becomes efficient and
nonlinear. The turbulent magnetic fields produced by these processes determine
the maximum energies of accelerated particles and result in specific features
in the observed photon radiation of the sources. Equally important, magnetic
field amplification by the CR currents and pressure anisotropies may affect the
shocked gas temperatures and compression, both in the shock precursor and in
the downstream flow, if the shock is an efficient CR accelerator. Strong
fluctuations of the magnetic field on scales above the radiation formation
length in the shock vicinity result in intermittent structures observable in
synchrotron emission images. Resonant and non-resonant CR streaming
instabilities in the shock precursor can generate mesoscale magnetic fields
with scale-sizes comparable to supernova remnants and even superbubbles. This
opens the possibility that magnetic fields in the earliest galaxies were
produced by the first generation Population III supernova remnants and by
clustered supernovae in star forming regions.Comment: 30 pages, Space Science Review
Simulating polarized Galactic synchrotron emission at all frequencies, the Hammurabi code
We present a publicly available code called Hammurabi for generating mock
polarized observations of Galactic synchrotron emission for telescopes like
LOFAR, SKA, Planck and WMAP, based on model inputs for the Galactic magnetic
field (GMF), the cosmic-ray density distribution and the thermal electron
density. We also present mock UHECR deflection measure (UDM) maps based on
model inputs for the GMF. In future, when UHECR sources are identified, this
will allow us to define UDM as a GMF probe in a similar way as polarized radio
sources permit us to define rotation measures. To demonstrate the code's
abilities mock observations are compared to real data as a means to constrain
the input parameters of our simulations with a focus on large-scale magnetic
field properties. As expected, attempts at trying to model the synchrotron,
UHECR deflection and RM input parameters, show that any additional
observational data set greatly increases the constraints on the models. The
hammurabi code addresses this by allowing to perform simulations of several
different data sets simultaneously, providing the means for a more reliable
constraint of the magnetized inter-stellar-medium.Comment: Submitted to A&A (Numerical methods and codes section
Polarimetry of binary systems: polars, magnetic CVs, XRBs
Polarimetry provides key physical information on the properties of
interacting binary systems, sometimes difficult to obtain by any other type of
observation. Indeed, radiation processes such as scattering by free electrons
in the hot plasma above accretion discs, cyclotron emission by mildly
relativistic electrons in the accretion shocks on the surface of highly
magnetic white dwarfs and the optically thin synchrotron emission from jets can
be observed. In this review, I will illustrate how optical/near-infrared
polarimetry allows one to estimate magnetic field strengths and map the
accretion zones in magnetic Cataclysmic Variables as well as determine the
location and nature of jets and ejection events in X-ray binaries.Comment: 26 pages, 16 figures; to be published in Astrophysics and Space
Science Library 460, Astronomical Polarisation from the Infrared to Gamma
Rays, Editors: Mignani, R., Shearer, A., S{\l}owikowska, A., Zane,
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