1,391 research outputs found
Radio polarization maps of shell-type SNRs II. Sedov models with evolution of turbulent magnetic field
Polarized radio emission has been mapped with great detail in several
Galactic supernova remnants (SNRs), but has not yet been exploited to the
extent it deserves. We have developed a method to model maps of the Stokes
parameters for shell-like SNRs during their Sedov evolution phase. At first,
3-dimensional structure of a SNR has been computed, by modeling the
distribution of the magnetohydrodynamic parameters and of the accelerated
particles. The generation and dissipation of the turbulent component of
magnetic field everywhere in SNR are also considered taking into account its
interaction with accelerated particles. Then, in order to model the emission,
we have used a generalization of the classical synchrotron theory, valid for
the case in which the magnetic field has ordered and disordered components.
Finally, 2-dimensional projected maps have been derived, for different
orientations of SNR and of interstellar magnetic field with respect to the
observer. An important effect to consider is the Faraday rotation of the
polarization planes inside the SNR interior. In this paper we present details
of the model, and describe general properties of the images.Comment: accepted in MNRA
The Pulsar B2224+65 and Its Jets: A Two Epoch X-ray Analysis
We present an X-ray morphological and spectroscopic study of the pulsar
B2224+65 and its apparent jet-like X-ray features based on two epoch Chandra
observations. The main X-ray feature, which shows a large directional offset
from the ram-pressure confined pulsar wind nebula (Guitar Nebula), is broader
in apparent width and shows evidence for spectral hardening (at 95 percent
confidence) in the second epoch compared to the first. Furthermore, the sharp
leading edge of the feature is found to have a proper motion consistent with
that of the pulsar (~180 mas yr-1). The combined data set also provides
evidence for the presence of a counter feature, albeit substantially fainter
and shorter than the main one. Additional spectral trends along the major and
minor axes of the feature are only marginally detected in the two epoch data,
including softening counter to the direction of proper motion. Possible
explanations for the X-ray features include diffuse energetic particles being
confined by an organized ambient magnetic field as well as a simple ballistic
jet interpretation; however, the former may have difficulty in explaining
observed spectral trends between epochs and along the feature's major axis
whereas the latter may struggle to elucidate its linearity. Given the low
counting statistics available in the two epoch observations, it remains
difficult to determine a physical production scenario for these enigmatic X-ray
emitting features with any certainty.Comment: 9 pages, 6 figures, submitted to MNRAS; updated as per reviewer
comment
A Comprehensive Statistical Analysis of the Gas Distribution in Lyman-limit and Damped Lyman-alpha Absorption Systems
In this paper we show how to use data on Lyman-limit and Damped Lyman-alpha
absorption systems to derive the hydrogen ionization fractions and the
distribution of the face-on total gas column density. We consider axially
symmetric, randomly oriented absorbers, ionized by an external background
radiation field in order to relate the face-on total gas distribution to that
of the neutral hydrogen observed along the line of sight. We devise a
statistical procedure based on the Maximum Likelihood criterion, that is able
to treat simultaneously data coming from different surveys and statistically
recovers the "true" column densities in the presence of large uncertainties:
this is especially important for Lyman-limit systems which leave an
unmeasurable residual flux at wavelengths shorter than the Lyman break. We make
use of simulated data to look for possible observational biases and extensively
test our procedure. For a large statistical sample of real data in the redshift
range [1.75,3.25] (collected from all published surveys) our Maximum Likelihood
procedure gives a power-law slope for the total hydrogen distribution of -2.7.
All together Lyman-limit systems therefore contain more gas than Damped
Lyman-alpha systems. Analysis of data at other redshifts shows that more
observations are needed to reach a compelling evidence for a cosmological
evolution of the slope of the gas distribution.Comment: 30 pages with 7 eps figures, LaTeX accepted for publication in ApJ
main journa
The Cone-Like H-alpha Nebula in NGC 4945: A Galactic Superwind Bow Shock ?
We find that a non-axisymmetric bow shock model, with an appropriate choice
of parameters, could fit the line splitting velocity field of the cone-like
H nebula in NGC 4945 better than a canonical cone model. Meanwhile, the
bow shock model could also reproduce the morphology of the H nebula.
The bow shock results from the interaction of the galactic superwinds with a
giant HII region. It is implied that the starburst ring or disk around the
galactic nucleus should be generating strong winds, and the bright H
knot northwest of the nucleus be suffering an anisotropic mass loss process.Comment: 14 pages, aasms4.sty, 3 figures not included (available upon request)
To appear in ApJ Letters. email chy, [email protected]
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