95 research outputs found
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
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&
Local measurements of the mean interstellar polarization at high Galactic latitudes
Very little information exists concerning the properties of the interstellar medium (ISM)-induced starlight polarization at high Galactic latitudes. Future optopolarimetric surveys promise to fill this gap. We conduct a small-scale pathfinding survey designed to identify the average polarization properties of the diffuse ISM locally, at regions with the lowest dust content. We perform deep optopolarimetric surveys within three ~15′× 15′ regions located at b > 48° using the RoboPol polarimeter. The observed samples of stars are photometrically complete to ~16 mag in the R-band. The selected regions exhibit low total reddening compared to the majority of high-latitude sightlines. We measure the level of systematic uncertainty for all observing epochs and find it to be 0.1% in fractional linear polarization, p. The majority of individual stellar measurements have low signal-to-noise ratios. However, our survey strategy enables us to locate the mean fractional linear polarization p_(mean) in each of the three regions. The region with lowest dust content yields p_(mean) = (0.054 ± 0.038)%, not significantly different from zero. We find significant detections for the remaining two regions of: p_(mean) = (0.113 ± 0.036)% and p_(mean) = (0.208 ± 0.044)%. Using a Bayesian approach, we provide upper limits on the intrinsic spread of the small-scale distributions of q and u. At the detected p_(mean) levels, the determination of the systematic uncertainty is critical for the reliability of the measurements. We verify the significance of our detections with statistical tests, accounting for all sources of uncertainty. Using publicly available HI emission data, we identify the velocity components that most likely account for the observed p_(mean) and find their morphologies to be misaligned with the orientation of the mean polarization at a spatial resolution of 10′. We find indications that the standard upper envelope of p with reddening underestimates the maximum p at very low E(B–V) (≤0.01 mag)
Starlight-polarization-based tomography of the magnetized ISM: Pasiphae's line-of-sight inversion method
We present the first Bayesian method for tomographic decomposition of the
plane-of-sky orientation of the magnetic field with the use of stellar
polarimetry and distance. This standalone tomographic inversion method presents
an important step forward in reconstructing the magnetized interstellar medium
(ISM) in 3D within dusty regions. We develop a model in which the polarization
signal from the magnetized and dusty ISM is described by thin layers at various
distances. Our modeling makes it possible to infer the mean polarization
(amplitude and orientation) induced by individual dusty clouds and to account
for the turbulence-induced scatter in a generic way. We present a likelihood
function that explicitly accounts for uncertainties in polarization and
parallax. We develop a framework for reconstructing the magnetized ISM through
the maximization of the log-likelihood using a nested sampling method. We test
our Bayesian inversion method on mock data taking into account realistic
uncertainties from and as expected for the optical polarization survey
PASIPHAE according to the currently planned observing strategy. We demonstrate
that our method is effective in recovering the cloud properties as soon as the
polarization induced by a cloud to its background stars is higher than , for the adopted survey exposure time and level of systematic
uncertainty. Our method makes it possible to recover not only the mean
polarization properties but also to characterize the intrinsic scatter, thus
opening ways to characterize ISM turbulence and the magnetic field strength.
Finally, we apply our method to an existing dataset of starlight polarization
with known line-of-sight decomposition, demonstrating agreement with previous
results and an improved quantification of uncertainties in cloud properties.Comment: 28 pages, including 2 appendices, submitted to A&
Bright-Moon Sky as a Wide-Field Linear Polarimetric Flat Source for Calibration
Next-generation wide-field optical polarimeters like the Wide-Area Linear
Optical Polarimeters (WALOPs) have a field of view (FoV) of tens of arcminutes.
For efficient and accurate calibration of these instruments, wide-field
polarimetric flat sources will be essential. Currently, no established
wide-field polarimetric standard or flat sources exist. This paper tests the
feasibility of using the polarized sky patches of the size of around ten-by-ten
arcminutes, at a distance of up to 20 degrees from the Moon, on bright-Moon
nights as a wide-field linear polarimetric flat source. We observed 19 patches
of the sky adjacent to the bright-Moon with the RoboPol instrument in the
SDSS-r broadband filter. These were observed on five nights within two days of
the full-Moon across two RoboPol observing seasons. We find that for 18 of the
19 patches, the uniformity in the measured normalized Stokes parameters and
is within 0.2 %, with 12 patches exhibiting uniformity within 0.07 % or
better for both and simultaneously, making them reliable and stable
wide-field linear polarization flats. We demonstrate that the sky on
bright-Moon nights is an excellent wide-field linear polarization flat source.
Various combinations of the normalized Stokes parameters and can be
obtained by choosing suitable locations of the sky patch with respect to the
MoonComment: 8 pages including appendix, 6 figures and 3 tables. Submitted to
Astronomy and Astrophysics for review. Comments are welcom
Search for AGN counterparts of unidentified Fermi-LAT sources with optical polarimetry Demonstration of the technique
Context. 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.Aims. We demonstrate that optical polarimetry can be an advantageous and practical tool in the hunt for counterparts of the unidentified gamma-ray sources (UGSs).Methods. 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 3 sigma uncertainties of four unidentified 3FGL sources.Results. 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 z = 0.0609 +/- 0.0004. 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.Conclusions. 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
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