340 research outputs found
Inferring telescope polarization properties through spectral lines without linear polarization
We present a technique to determine the polarization properties of a
telescope through observations of spectral lines that have no intrinsic linear
polarization signals. For such spectral lines, any observed linear polarization
must be induced by the telescope optics. We apply the technique to observations
taken with the SPINOR at the DST and demonstrate that we can retrieve the
characteristic polarization properties of the DST at three wavelengths of 459,
526, and 615 nm. We determine the amount of crosstalk between the intensity
Stokes I and the linear and circular polarization states Stokes Q, U, and V,
and between Stokes V and Stokes Q and U. We fit a set of parameters that
describe the polarization properties of the DST to the observed crosstalk
values. The values for the ratio of reflectivities X and the retardance tau
match those derived with the telescope calibration unit within the error bars.
Residual crosstalk after applying a correction for the telescope polarization
stays at a level of 3-10%. We find that it is possible to derive the parameters
that describe the polarization properties of a telescope from observations of
spectral lines without intrinsic linear polarization signal. Such spectral
lines have a dense coverage (about 50 nm separation) in the visible part of the
spectrum (400-615 nm), but none were found at longer wavelengths. Using
spectral lines without intrinsic linear polarization is a promising tool for
the polarimetric calibration of current or future solar telescopes such as
DKIST.Comment: 22 pages, 24 figures, accepted for publication in A&
Deviations From Axisymmetry Revealed by Line Polarization in the Normal Type Ia SN 2004S
We present a single epoch of high signal-to-noise ratio spectropolarimetry of
the Type Ia supernova (SN Ia) 2004S taken nine days after maximum light. The
flux spectrum is normal, but with the additional presence of high-velocity (HV)
line features in both Ca II and Fe II. The object shows continuum polarization
at the 0.4% level in the red, a value which appears to be typical of SNe Ia.
The continuum data are consistent with a ~10% global asphericity in an
axisymmetric geometry. Unlike previous observations of other SNe Ia with HV
features, the HV features in SN 2004S show no strong polarimetric signature,
though this may be due to the timing of our observations. Instead, the object
shows line polarization features (P < 0.5%) that are rotated with respect to
the axis of symmetry of the continuum. The line features are visible in Si II,
Fe II, and Ca II, and appear to be narrowly confined in velocity space just
above the photosphere. These polarization features are a result of
compositional inhomogeneities in the ejecta. They may represent newly
synthesized elements whose clumpy spatial distribution within the ejecta is
distinct from that of the globally aspherical ejecta as a whole.Comment: 25 pages, 6 figures, AJ, minor revisions to match accepted journal
versio
Hot DQ White Dwarfs: Something Different
We present a detailed analysis of all the known Hot DQ white dwarfs in the
Fourth Data Release of the Sloan Digital Sky Survey (SDSS) recently found to
have carbon dominated atmospheres. Our spectroscopic and photometric analysis
reveals that these objects all have effective temperatures between ~18,000 and
24,000 K. The surface composition is found to be completely dominated by
carbon, as revealed by the absence of Hbeta and HeI 4471 lines (or
determination of trace amount in a few cases). We find that the surface gravity
of all objects but one seems to be ''normal'' and around log g = 8.0 while one
is likely near log g = 9.0. The presence of a weak magnetic field is directly
detected by spectropolarimetry in one object and is suspected in two others. We
propose that these strange stars could be cooled down versions of the weird
PG1159 star H1504+65 and form a new family of hydrogen and helium deficient
objects following the post-AGB phase. Finally, we present the results of full
nonadiabatic calculations dedicated specifically to each of the Hot DQ that
show that only SDSS J142625.70+575218.4 is expected to exhibit luminosity
variations. This result is in excellent agreement with recent observations by
Montgomery et al. who find that J142625.70+575218.4 is the only pulsator among
6 Hot DQ white dwarfs surveyed in February 2008.Comment: 33 pages, 7 figures, accepted for publication in Ap
The HgMn Binary Star Phi Herculis: Detection and Properties of the Secondary and Revision of the Elemental Abundances of the Primary
Observations of the Mercury-Manganese star Phi Herculis with the Navy
Prototype Optical Interferometer (NPOI) conclusively reveal the previously
unseen companion in this single-lined binary system. The NPOI data were used to
predict a spectral type of A8V for the secondary star Phi Her B. This
prediction was subsequently confirmed by spectroscopic observations obtained at
the Dominion Astrophysical Observatory. Phi Her B is rotating at 50 +/-3
km/sec, in contrast to the 8 km/sec lines of Phi Her A. Recognizing the lines
from the secondary permits one to separate them from those of the primary. The
abundance analysis of Phi Her A shows an abundance pattern similar to those of
other HgMn stars with Al being very underabundant and Sc, Cr, Mn, Zn, Ga, Sr,
Y, Zr, Ba, Ce, and Hg being very overabundant.Comment: Accepted to ApJ, 45 pages, 11 figure
 Ocean Remote Sensing with Synthetic Aperture Radar
The ocean covers approximately 71% of the Earth’s surface, 90% of the biosphere and contains 97% of Earth’s water. The Synthetic Aperture Radar (SAR) can image the ocean surface in all weather conditions and day or night. SAR remote sensing on ocean and coastal monitoring has become a research hotspot in geoscience and remote sensing. This book—Progress in SAR Oceanography—provides an update of the current state of the science on ocean remote sensing with SAR. Overall, the book presents a variety of marine applications, such as, oceanic surface and internal waves, wind, bathymetry, oil spill, coastline and intertidal zone classification, ship and other man-made objects’ detection, as well as remotely sensed data assimilation. The book is aimed at a wide audience, ranging from graduate students, university teachers and working scientists to policy makers and managers. Efforts have been made to highlight general principles as well as the state-of-the-art technologies in the field of SAR Oceanography
Polarization leakage in epoch of reionization windows – I. Low Frequency Array observations of the 3C196 field
Detection of the 21-cm signal coming from the epoch of reionization (EoR) is challenging especially because, even after removing the foregrounds, the residual Stokes I maps contain leakage from polarized emission that can mimic the signal. Here, we discuss the instrumental polarization of LOFAR and present realistic simulations of the leakages between Stokes parameters. From the LOFAR observations of polarized emission in the 3C196 field, we have quantified the level of polarization leakage caused by the nominal model beam of LOFAR, and compared it with the EoR signal using power spectrum analysis. We found that at 134– 166 MHz, within the central 4◦ of the field the (Q,U)→I leakage power is lower than the EoR signal at k<0.3 Mpc−¹. The leakage was found to be localized around a Faraday depth of 0, and the rms of the leakage as a fraction of the rms of the polarized emission was shown to vary between 0.2–0.3%, both of which could be utilized in the removal of leakage. Moreover, we could define an ‘EoR window’ in terms of the polarization leakage in the cylindrical power spectrum above the PSF-induced wedge and below k∥∼0.5 Mpc−¹, and the window extended up to k∥∼1 Mpc−¹ at all k⊥ when 70% of the leakage had been removed. These LOFAR results show that even a modest polarimetric calibration over a field of view of ≲4∘ in the future arrays like SKA will ensure that the polarization leakage remains well below the expected EoR signal at the scales of 0.02–1 Mpc−¹
ALOS-2/PALSAR-2 Calibration, Validation, Science and Applications
Twelve edited original papers on the latest and state-of-art results of topics ranging from calibration, validation, and science to a wide range of applications using ALOS-2/PALSAR-2. We hope you will find them useful for your future research
Long-term studies with the Ariel 5 ASM. 2: The strong Cygnus sources
The three bright 3-6 keV X-ray sources in Cygnus are examined for regular temporal variability with a 1300-day record from the Ariel 5 All Sky Monitor. The only periods consistently observed are 5.6 days for Cyg X-1, 11.23 days for Cyg X-2, and 4.8 hours for Cyg X-3
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