266 research outputs found
High-accuracy estimation of magnetic field strength in the interstellar medium from dust polarization
Dust polarization is a powerful tool for studying the magnetic field
properties in the interstellar medium (ISM). However, it does not provide a
direct measurement of its strength. Different methods havebeen developed which
employ both polarization and spectroscopic data in order to infer the field
strength. The most widely applied methods have been developed by Davis (1951),
Chandrasekhar & Fermi (1953) (DCF), Hildebrand et al. (2009) and Houde et
al.(2009) (HH09). They rely on the assumption that isotropic turbulent motions
initiate the propagation of Alvf\'en waves. Observations,however, indicate that
turbulence in the ISM is anisotropic and non-Alfv\'enic (compressible) modes
may be important. Our goal is to develop a new method for estimating the field
strength in the ISM, which includes the compressible modes and does not
contradict the anisotropic properties of turbulence. We use simple energetics
arguments that take into account the compressible modes to estimate the
strength of the magnetic field. We derive the following equation:
, where is the
gas density, is the rms velocity as derived from the spread of
emission lines, and is the dispersion of polarization angles.
We produce synthetic observations from 3D MHD simulationsand we assess the
accuracy of our method by comparing the true field strength with the estimates
derived from our equation. We find a mean relative deviation of . The
accuracy of our method does not depend on the turbulence properties of the
simulated model. In contrast DCF and HH09 systematically overestimate the field
strength. HH09 produces accurate results only for simulations with high sonic
Mach numbers.Comment: Accepted for publication to Astronomy & Astrophysic
Extreme Starlight Polarization in a Region with Highly Polarized Dust Emission
Galactic dust emission is polarized at unexpectedly high levels, as revealed
by Planck. The origin of the observed polarization fractions can
be identified by characterizing the properties of optical starlight
polarization in a region with maximally polarized dust emission. We measure the
R-band linear polarization of 22 stars in a region with a submillimeter
polarization fraction of . A subset of 6 stars is also measured in
the B, V and I bands to investigate the wavelength dependence of polarization.
We find that starlight is polarized at correspondingly high levels. Through
multiband polarimetry we find that the high polarization fractions are unlikely
to arise from unusual dust properties, such as enhanced grain alignment.
Instead, a favorable magnetic field geometry is the most likely explanation,
and is supported by observational probes of the magnetic field morphology. The
observed starlight polarization exceeds the classical upper limit of
%mag and is at least
as high as 13%mag that was inferred from a joint analysis of Planck
data, starlight polarization and reddening measurements. Thus, we confirm that
the intrinsic polarizing ability of dust grains at optical wavelengths has long
been underestimated.Comment: Accepted by A&AL, data to appear on CDS after publication. 6 page
Lagrangian characterization of sub-Alfv\'enic turbulence energetics
The energetics of strongly magnetized turbulence has so far resisted all
attempts to understand them. Numerical simulations of compressible turbulence
reveal that kinetic energy can be orders of magnitude larger than fluctuating
magnetic energy. We solve this lack-of-balance puzzle by calculating the
energetics of compressible and sub-Alfv\'enic turbulence based on the dynamics
of coherent cylindrical fluid parcels. Using a Lagrangian formulation, we prove
analytically that the bulk of the magnetic energy transferred to kinetic is the
energy stored in the coupling between the initial and fluctuating magnetic
field. The analytical relations are in striking agreement with numerical data,
up to second order terms.Comment: 16 pages, 1 figure, submitted, comments welcom
Demonstration of magnetic field tomography with starlight polarization towards a diffuse sightline of the ISM
The availability of large datasets with stellar distance and polarization
information will enable a tomographic reconstruction of the
(plane-of-the-sky-projected) interstellar magnetic field in the near future. We
demonstrate the feasibility of such a decomposition within a small region of
the diffuse ISM. We combine measurements of starlight (R-band) linear
polarization obtained using the RoboPol polarimeter with stellar distances from
the second Gaia data release. The stellar sample is brighter than 17 mag in the
R band and reaches out to several kpc from the Sun. HI emission spectra reveal
the existence of two distinct clouds along the line of sight. We decompose the
line-of-sight-integrated stellar polarizations to obtain the mean polarization
properties of the two clouds. The two clouds exhibit significant differences in
terms of column density and polarization properties. Their mean
plane-of-the-sky magnetic field orientation differs by 60 degrees. We show how
our tomographic decomposition can be used to constrain our estimates of the
polarizing efficiency of the clouds as well as the frequency dependence of the
polarization angle of polarized dust emission. We also demonstrate a new method
to constrain cloud distances based on this decomposition. Our results represent
a preview of the wealth of information that can be obtained from a tomographic
map of the ISM magnetic field.Comment: 25 pages, 14 figures, published in ApJ, data appear in journa
Search for AGN counterparts of unidentified Fermi-LAT sources with optical polarimetry: Demonstration of the technique
The third Fermi-LAT catalog (3FGL) presented the data of the first four years
of observations from the Fermi Gamma-ray Space Telescope mission. There are
3034 sources, 1010 of which still remain unidentified. Identifying and
classifying gamma-ray emitters is of high significance with regard to studying
high-energy astrophysics. We demonstrate that optical polarimetry can be an
advantageous and practical tool in the hunt for counterparts of the
unidentified gamma-ray sources (UGSs). Using data from the RoboPol project, we
validated that a significant fraction of active galactic nuclei (AGN)
associated with 3FGL sources can be identified due to their high optical
polarization exceeding that of the field stars. We performed an optical
polarimetric survey within uncertainties of four unidentified 3FGL
sources. We discovered a previously unknown extragalactic object within the
positional uncertainty of 3FGL J0221.2+2518. We obtained its spectrum and
measured a redshift of . Using these measurements and
archival data we demonstrate that this source is a candidate counterpart for
3FGL J0221.2+2518 and most probably is a composite object: a star-forming
galaxy accompanied by AGN. We conclude that polarimetry can be a powerful asset
in the search for AGN candidate counterparts for unidentified Fermi sources.
Future extensive polarimetric surveys at high galactic latitudes (e.g.,
PASIPHAE) will allow the association of a significant fraction of currently
unidentified gamma-ray sources.Comment: accepted to A&
Ability of FFR-CT to detect the absence of hemodynamically significant lesions in patients with high-risk NSTE-ACS admitted in the emergency department with chest pain, study design and rationale.
In the era of High-sensitive troponin (hs-Tn), up to 50% of patients with a mild increase of hs-Tn will finally have a normal invasive coronary angiogram. Fractional Flow Reserve (FFR) derived from coronary computed tomographic angiography (FFR-CT) has never been used as a non-invasive tool for the diagnosis of coronary artery disease in patients with high-risk acute coronary syndrome without ST segment elevation (NSTE-ACS).
The study aims to determine the role of coronary CT angiography and FFR-CT in the setting of high-risk NSTE-ACS.
We will conduct a prospective trial, enrolling 250 patients admitted with high-risk NSTE-ACS who will rapidly undergo a coronary CT angiography and then a coronary angiography with FFR measurements. Results of coronary CT, FFR-CT and coronary angiography (± FFR) will be compared.
In conclusion, non-invasive identification of patients with high-risk NSTE-ACS who could avoid coronary angiography would reduce procedure related risks and medical costs
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