60 research outputs found
Polarization power spectra and dust cloud morphology
In the framework of studies of the CMB polarization and its Galactic
foregrounds, the angular power spectra of thermal dust polarization maps have
revealed an intriguing E/B asymmetry and a positive TE correlation. In
interpretation studies of these observations, magnetized ISM dust clouds have
been treated as filamentary structures only; however, sheet-like shapes are
also supported by observational and theoretical evidence. In this work, we
study the influence of cloud shape and its connection to the local magnetic
field on angular power spectra of thermal dust polarization maps. We simulate
realistic filament-like and sheet-like interstellar clouds, and generate
synthetic maps of their thermal dust polarized emission using the software
. We compute their polarization power spectra in multipole range
and quantify the E/B power asymmetry through the
ratio, and the correlation coefficient between T and E modes. We
quantify the dependence of and values on the offset angle
(between longest cloud axis and magnetic field) and inclination angle (between
line-of-sight and magnetic field) for both cloud shapes embedded either in a
regular or a turbulent magnetic field. We find that both cloud shapes cover the
same regions of the (, ) parameter space. The dependence on
inclination and offset angles are similar for both shapes although sheet-like
structures generally show larger scatter. In addition to the known dependence
on the offset angle, we find a strong dependence of and on
the inclination angle. The fact that filament-like and sheet-like structures
may lead to polarization power spectra with similar (, ) values
complicates their interpretation. In future analyses, this degeneracy should be
accounted for as well as the connection to the magnetic field geometry.Comment: 16 pages, 24 figures. Accepted for publication by A&
Local alignments of parsec-scale AGN radiojets
Context.Coherence in the characteristics of neighboring sources in 2D and 3D
space may suggest the existence of large-scale cosmic structures, which are
useful for cosmological studies. Numerous works have been conducted to detect
such features in global scalesas well as in confined areas of the sky. However,
results are often contradictory and their interpretation remains controversial.
Aims.We investigate the potential alignment of parsec-scale radio jets in
localized regions of the coordinates-redshift space. Methods.We use data from
the Astrogeo VLBI FITS image database to deduce jet directions of radio
sources. We perform the search for statistical alignments between nearby
sources and explore the impact of instrumental biases. Results.We unveil four
regions for which the alignment between jet directions deviates from randomness
at a significance level of more than 5 sigma and is unlikely due to
instrumental systematics. Intriguingly, their locations coincide with other
known large-scale cosmic structures and/or regions of alignments.
Conclusions.If the alignments found are the result of physical processes, the
discovered regions may designate some of the largest structures known to date.Comment: 11 pages, 9 figure
Pulsar Scintillation through Thick and Thin: Bow Shocks, Bubbles, and the Broader Interstellar Medium
Observations of pulsar scintillation are among the few astrophysical probes
of very small-scale ( au) phenomena in the interstellar medium (ISM).
In particular, characterization of scintillation arcs, including their
curvature and intensity distributions, can be related to interstellar
turbulence and potentially over-pressurized plasma in local ISM
inhomogeneities, such as supernova remnants, HII regions, and bow shocks. Here
we present a survey of eight pulsars conducted at the Five-hundred-meter
Aperture Spherical Telescope (FAST), revealing a diverse range of scintillation
arc characteristics at high sensitivity. These observations reveal more arcs
than measured previously for our sample. At least nine arcs are observed toward
B192910 at screen distances spanning of the pulsar's pc
path-length to the observer. Four arcs are observed toward B035554, with one
arc yielding a screen distance as close as au ( pc) from either
the pulsar or the observer. Several pulsars show highly truncated,
low-curvature arcs that may be attributable to scattering near the pulsar. The
scattering screen constraints are synthesized with continuum maps of the local
ISM and other well-characterized pulsar scintillation arcs, yielding a
three-dimensional view of the scattering media in context.Comment: 20 pages, 14 figures. Submitted to MNRAS and comments welcome.
Interactive version of Figure 12 available at
https://stella-ocker.github.io/scattering_ism3d_ocker202
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
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&
WALOP-South: A Four Camera One Shot Imaging Polarimeter for PASIPHAE Survey. Paper I -- Optical Design
The WALOP-South instrument will be mounted on the 1 m SAAO telescope in South
Africa as part of the PASIPHAE program to carry out a linear imaging
polarization survey of the Galactic polar regions in the optical band. Designed
to achieve polarimetric sensitivity of across a
arcminute field of view, it will be capable of measuring the Stokes parameters
I, q and u in a single exposure in the SDSS-r broadband and narrowband filters
between . For each measurement, four images of the
full field corresponding to linear polarization angles of 0 deg, 45 deg, 90 deg
and 135 deg in the instrument coordinate system will be created on four
detectors from which the Stokes parameters can be found using differential
photometry. In designing the optical system, major challenges included
correcting for the dispersion introduced by large split angle Wollaston Prisms
used as analysers as well as other aberrations from the entire field to obtain
imaging quality PSF at the detector. We present the optical design of the
WALOP-South instrument which overcomes these challenges and delivers near
seeing limited PSFs for the entire field of view.Comment: 31 pages, 18 Figures and 8 Tables. Accepted in the Journal of
Astronomical Telescopes, Instruments, and System
SMILE: Search for MIlli-LEnses
ABSTRACTDark matter (DM) haloes with masses below ∼108 M⊙, which would help to discriminate between DM models, may be detected through their gravitational effect on distant sources. The same applies to primordial black holes, considered as an alternative scenario to DM particle models. However, there is still no evidence for the existence of such objects. With the aim of finding compact objects in the mass range of ∼106–109 M⊙, we search for strong gravitational lenses on milliarcsec scales (</p
Extreme-scale alignments of quasar optical polarizations and Galactic dust contamination
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