24,190 research outputs found
Modelling the Galactic Magnetic Field on the Plane in 2D
We present a method for parametric modelling of the physical components of
the Galaxy's magnetised interstellar medium, simulating the observables, and
mapping out the likelihood space using a Markov Chain Monte-Carlo analysis. We
then demonstrate it using total and polarised synchrotron emission data as well
as rotation measures of extragalactic sources. With these three datasets, we
define and study three components of the magnetic field: the large-scale
coherent field, the small-scale isotropic random field, and the ordered field.
In this first paper, we use only data along the Galactic plane and test a
simple 2D logarithmic spiral model for the magnetic field that includes a
compression and a shearing of the random component giving rise to an ordered
component. We demonstrate with simulations that the method can indeed constrain
multiple parameters yielding measures of, for example, the ratios of the
magnetic field components. Though subject to uncertainties in thermal and
cosmic ray electron densities and depending on our particular model
parametrisation, our preliminary analysis shows that the coherent component is
a small fraction of the total magnetic field and that an ordered component
comparable in strength to the isotropic random component is required to explain
the polarisation fraction of synchrotron emission. We outline further work to
extend this type of analysis to study the magnetic spiral arm structure, the
details of the turbulence as well as the 3D structure of the magnetic field.Comment: 18 pages, 11 figures, updated to published MNRAS versio
Invaded cluster algorithm for equilibrium critical points
A new cluster algorithm based on invasion percolation is described. The
algorithm samples the critical point of a spin system without a priori
knowledge of the critical temperature and provides an efficient way to
determine the critical temperature and other observables in the critical
region. The method is illustrated for the two- and three-dimensional Ising
models. The algorithm equilibrates spin configurations much faster than the
closely related Swendsen-Wang algorithm.Comment: 13 pages RevTex and 4 Postscript figures. Submitted to Phys. Rev.
Lett. Replacement corrects problem in printing figure
Nonexponential decay of an unstable quantum system: Small--value s-wave decay
We study the decay process of an unstable quantum system, especially the
deviation from the exponential decay law. We show that the exponential period
no longer exists in the case of the s-wave decay with small value, where
the value is the difference between the energy of the initially prepared
state and the minimum energy of the continuous eigenstates in the system. We
also derive the quantitative condition that this kind of decay process takes
place and discuss what kind of system is suitable to observe the decay.Comment: 17 pages, 6 figure
Lattice Model of Sweeping Interface for Drying Process in Water-Granule Mixture
Based on the invasion percolation model, a lattice model for the sweeping
interface dynamics is constructed to describe the pattern forming process by a
sweeping interface upon drying the water-granule mixture. The model is shown to
produce labyrinthine patterns similar to those found in the experiment[Yamazaki
and Mizuguchi, J. Phys. Soc. Jpn. \textbf{69} (2000) 2387]. Upon changing the
initial granular density, resulting patterns undergo the percolation
transition, but estimated critical exponents are different from those of the
conventional percolation. Loopless structure of clusters in the patterns
produced by the sweeping dynamics seems to influence the nature of the
transition.Comment: 6 pages, 7 figure
On the Enhanced Interstellar Scattering Toward B1849+005
(Abridged) This paper reports new Very Large Array (VLA) and Very Long
Baseline Array (VLBA) observations of the extragalactic source B1849+005 at
frequencies between 0.33 and 15 GHz and the re-analysis of archival VLA
observations at 0.33, 1.5, and 4.9 GHz. The structure of this source is complex
but interstellar scattering dominates the structure of the central component at
least to 15 GHz. An analysis of the phase structure functions of the
interferometric visibilities shows the density fluctuations along this line of
sight to be anisotropic (axial ratio = 1.3) with a frequency-independent
position angle, and having an inner scale of roughly a few hundred kilometers.
The anisotropies occur on length scales of order 10^{15} cm (D/5 kpc), which
within the context of certain magnetohydrodynamic turbulence theories indicates
the length scale on which the kinetic and magnetic energy densities are
comparable. A conservative upper limit on the velocity of the scattering
material is 1800 km/s. In the 0.33 GHz field of view, there are a number of
other sources that might also be heavily scattered. Both B1849+005 and PSR
B1849+00 are highly scattered, and they are separated by only 13'. If the lines
of sight are affected by the same ``clump'' of scattering material, it must be
at least 2.3 kpc distant. However, a detailed attempt to account for the
scattering observables toward these sources does not produce a self-consistent
set of parameters for such a clump. A clump of H\alpha emission, possibly
associated with the H II region G33.418-0.004, lies between these two lines of
sight, but it seems unable to account for all of the required excess
scattering.Comment: 23 pages, LaTeX2e AASTeX, 13 figures in 14 PostScript files, accepted
for publication in Ap
A survey of polarization in the JVAS/CLASS flat-spectrum radio source surveys: I. The data and catalogue production
We have used the very large JVAS/CLASS 8.4-GHz surveys of flat-spectrum radio
sources to obtain a large, uniformly observed and calibrated, sample of radio
source polarizations. These are useful for many investigations of the
properties of radio sources and the interstellar medium. We discuss comparisons
with polarization measurements from this survey and from other large-scale
surveys of polarization in flat-spectrum sources.Comment: Accepted by MNRAS. 8 pages, 5 figures. Full version of Table 2
available at http://www.jb.man.ac.uk/~njj/classqu_po
CBI limits on 31 GHz excess emission in southern HII regions
We have mapped four regions of the southern Galactic plane at 31 GHz with the
Cosmic Background Imager. From the maps, we have extracted the flux densities
for six of the brightest \hii regions in the southern sky and compared them
with multi-frequency data from the literature. The fitted spectral index for
each source was found to be close to the theoretical value expected for
optically thin free-free emission, thus confirming that the majority of flux at
31 GHz is due to free-free emission from ionised gas with an electron
temperature of K.
We also found that, for all six sources, the 31 GHz flux density was slightly
higher than the predicted value from data in the literature. This excess
emission could be due to spinning dust or another emission mechanism.
Comparisons with m data indicate an average dust emissivity of
K (MJy/sr), or a 95 per cent confidence limit of K (MJy/sr). This is lower than that found in diffuse clouds at high
Galactic latitudes by a factor of . The most significant detection
() was found in (RCW49) and may account for up to
per cent of the total flux density observed at 31 GHz. Here, the
dust emissivity of the excess emission is K (MJy/sr) and
is within the range observed at high Galactic latitudes.
Low level polarised emission was observed in all six sources with
polarisation fractions in the range per cent. This is likely to be
mainly due to instrumental leakage and is therefore upper an upper limit to the
free-free polarisation. It corresponds to an upper limit of per cent
for the polarisation of anomalous emission.Comment: Accepted in MNRAS. 12 pages, 10 figures, 5 table
Membrane Type 1 Matrix Metalloproteinase Regulates Monocyte Migration and Collagen Destruction in Tuberculosis
Tuberculosis (TB) remains a global pandemic and drug resistance is rising. Multicellular granuloma formation is the pathological hallmark of Mycobacterium tuberculosis infection. The membrane type 1 matrix metalloproteinase (MT1-MMP or MMP-14) is a collagenase that is key in leukocyte migration and collagen destruction. In patients with TB, induced sputum MT1-MMP mRNA levels were increased 5.1-fold compared with matched controls and correlated positively with extent of lung infiltration on chest radiographs (r = 0.483; p < 0.05). M. tuberculosis infection of primary human monocytes increased MT1-MMP surface expression 31.7-fold and gene expression 24.5-fold. M. tuberculosis-infected monocytes degraded collagen matrix in an MT1-MMP-dependent manner, and MT1-MMP neutralization decreased collagen degradation by 73%. In human TB granulomas, MT1-MMP immunoreactivity was observed in macrophages throughout the granuloma. Monocyte-monocyte networks caused a 17.5-fold increase in MT1-MMP surface expression dependent on p38 MAPK and G protein-coupled receptor-dependent signaling. Monocytes migrating toward agarose beads impregnated with conditioned media from M. tuberculosis-infected monocytes expressed MT1-MMP. Neutralization of MT1-MMP activity decreased this M. tuberculosis network-dependent monocyte migration by 44%. Taken together, we demonstrate that MT1-MMP is central to two key elements of TB pathogenesis, causing collagen degradation and regulating monocyte migration
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