Balloon-borne polarimetry

For about two weeks in 1995, the balloon-borne Flare Genesis Experiment will continuously observe the Sun well above the turbulent, image-blurring layers of the Earth's atmosphere. The polarization-free 80 cm telescope will supply images to a liquid-crystal based vector magnetograph, which will measure magnetic features at a resolution of 0.2 arcsec. An electrically tunable lithium-niobate Fabry-Perot provides a spectral resolution of about 0.015 nm. In a follow-up series of Antarctic balloon flights, the Flare Genesis Experiment (FGE) will provide unprecedented details about sunspots, flares, magnetic elements, filaments, and the quiet solar atmosphere.     

High sensitivity polarimetry

Over the last decade spectro‐polarimetry evolved to ever higher sensitivity levels. New techniques and instruments allow us to address weak polarization signals, which are caused by scattering in the solar atmosphere. In this paper a review on the development of spectro‐polarimetric investigations of scattering physics and its coupling to the solar magnetic field will be given. Starting from a technical point of view it will be demonstrated how our understanding of scattering phenomena and their role in solar physics in general has reached its current state. An outlook on future spectro‐polarimetry with new large solar telescopes concludes this review.     

Optical polarimetry of planet mercury

Polarimetric measurements were collected at different areas of the surface of Mercury, and for the whole disk in six wavelengths. The curves of polarization are compared with telescopic observations of the Moon and laboratory studies of minerals and returned lunar samples. The negative branch of polarization proves that Mercury's surface is almost everywhere covered by a regolith layer of fines of the lunar type, also made of dark and adsorbing material, and most probably of the same impact generated origin. The polarization maximum of Mercury is reproduced by lunar samples of fines of intermediate albedo corresponding to the lightest regolith found in the Apollo explored maria.The albedo of Mercury at phase angle 5° deduced from telescopic photometry is to be corrected by a factor of 1.20 and the best “polarimetric” values of albedos are 0.130 at λ = 0.585μm, 0.119 at λ = 0.520 μm, 0.093 at λ = 0.379μm and 0.087 at λ = 0.354μm. The contrast between light and dark-lined regions at the surface of Mercury is most probably much fainter than between the maria and continents on the Moon.The molecular atmosphere of Mercury, if any, has a surface pressure probably smaller than 2 × 10^?4 bars.     

Imaging polarimetry of proto-planetary nebulae

Polarization maps are presented for the three proto-planetary nebulae, Frosty Leo, MZ3 and M2-9. The structure of each object is discussed and geometries derived from these data are compared with those predicted by the currently fashionable two wind hydrodynamical models which account for many of the features of such transient nebulae.     

Infrared speckle interferometry of eleven binaries using a bispectral analysis

Infrared speckle-masking observations of eleven binary systems with the 6-m Special Astrophysical Observatory telescope are presented. A resolution of 43 mas in J (1.25 µm) and 76 mas in K (2.2 µm) has been achieved in reconstructed images. Accurate magnitude differences, separations, and position angles have been determined for all the resolved binaries. The pair HR 1071 with an abnormally low lithium abundance is considered in more detail.     

10 μm polarimetry of ceres

Linear polarimetry of Ceres at 10 μm is presented. These data represent the first published polarization measurements of an asteroid in the thermal infrared. It is found that Ceres is polarized at the 0.2-0.6% level. This data set is compared with theoretical models of the linear polarization of emitted radiation from a spherical plane. These models are used to derive the pole position and thermal inertia of Ceres. Ceres is best fit with a thermal inertia of $\text{0.0010±0.0003}\text{cal}\text{cm}{}^{\mathrm{?2}}\text{°}\text{K}{}^{\mathrm{?1}}\text{sec}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and a pole orientation of β_p = 36° ± 5°, λ_p = 270° ± 3°. It is concluded that 10μm polarimetry is a potentially powerful technique for remotely sensing the pole orientation and thermal inertia of asteroids.     

Near-infrared polarimetry of the Red Rectangle

Imaging polarimetry through J and H broad-band filters and a 3.4 μm narrow-band filter is used to highlight the regions of scattered light in the Red Rectangle. We find that the scattered light identifies the circumbinary dust component of the molecular disc seen in CO emission. This region also appears to be the origin of the recently discovered Blue Luminescence. We find that the degrees of polarization are consistent with the amorphous carbon dust model invoked by Men'shchikov. Spectropolarimetry from 1.4 to 2.5 μm confirms that the degree of polarization in the central arcsecond region is very low. This suggests that the central bicone seen in the near-infrared is predominantly due to emission from hot dust and/or from stochastically heated nanoparticles, rather than due to scattering by large grains.     

Recent progress in imaging polarimetry

Recent instrumental developments in imaging polarimetry allow array detectors to reach a polarimetric sensitivity of 1 × 10^–4 of the intensity. New instrumental effects appear at these levels of sensitivity and generate spurious polarization signals with amplitudes of up to 5 × 10^–4. Here I discuss these effects and present methods to avoid them. Polarized spectra with an rms noise of 6 × 10^–6 may then be obtained. Furthermore a method is brought to the reader's attention that allows polarization measurements at the 1 × 10^–4 level with regular array detectors, e.g. in the near-infrared.     

Multifrequency polarimetry of 300 radio pulsars

Polarimetric observations of 300 pulsars have been conducted with the 76-m Lovell telescope at Jodrell Bank at radio frequencies centred around 230, 400, 600, 920, 1400 and 1600 MHz. More than half of the pulsars have no previously published polarization profiles and this compilation represents about three times the sum of all previously published pulsar polarization data. A selection of integrated polarization profiles is provided. Tables of pulse widths and the degree of both linear and circular polarization are given for all pulsars, and these act as an index for all the data, which are available by anonymous ftp in numerical and graphical form.     

On polarimetry in solar active regions

A new modulation procedure for Zeeman polarimeters is described and tested. The azimuth rotation by means of two steady /4-plates, combined with the common EOLM, has several advantages as compared to two-EOLM-polarimeters. The new polarimeter operates with two /4-plates which are alternately passed through the beam in front of the EOLM by means of an electro-mechanical chopper. The exact time of the /4-plate change is monitored by a photoelectric sensor. The obtained signals drive a number of relays by use of an intervening bistable electronic device. These relays allow to cut-off the erroneous Doppler signal mode and they furthermore distribute the U and Q signals into the corresponding lock-in amplifiers. As a first application of the new polarimeter, the linear polarization is measured in a sunspot penumbra. The telescope was first compensated for instrumental linear polarization down to 5 × 10^-4 by means of a tilted glass plate and well as for phase retardation down to 1° by means of a Bowen compensator.     

On polarimetry in solar active regions

The usefulness of magnetically sensitive iron lines Fe 5250.2, 6173.3 and 6302.3 for solar polarimetry is investigated. The line-to-continuum absorption coefficient ₀ for Fe 5250.2 depends strongly on temperature variations. Thus a photospheric calibration of polarimeter signals cannot be used for the different parts of an active region. This is also true for the Doppler calibration of longitudinal magnetographs.Fe 6302.5 is shown to be useful for polarimetry in active regions. A Milne-Eddington approximation is possible so that Unno's formulae are sufficient for the interpretation of polarimetric data.The ambiguity of the azimuth of the linear polarization prevents the determination of the true field structure with respect to the solar surface. The magnetic flux cannot be determined outside the disc centre without making assumptions for the unknown field structure. This determination is possible only for single stable sunspots; for irregular active regions the field configuration remains ambiguous even at the disc centre.     

Search for extrasolar planets by polarimetry

Optical polarimetry is suggested as a new method for detecting “hot Jupiter” planets around stars. The polarimetric search method has been tested experimentally; for this purpose, the necessary astronomical observations and their processing have been performed. The results obtained allow us to assert with caution that the suggested method yields positive results and can be of use both in searching for exoplanets and in refining their masses. According to our results, a tangential transit of the planet 51 Peg b may be observed. The angle between the orbital plane of 51 Peg b and the observer’s direction must then be small, sin i ≈ 1, and the mass of 51 Peg b must be close to 0.46 M _J (Jupiter mass).     

K-band polarimetry of seven high-redshift radio galaxies

We present the results of K -band imaging polarimetry of seven 3CR radio galaxies with 0.7 <  z  < 1.2. We find strong evidence for polarization in three sources: 3C 22, 3C 41 and 3C 114. Of these, 3C 41 shows strong evidence of having a quasar core the infrared light of which is scattered by dust. We also find some evidence for polarization in 3C 54 and in 3C 356. The two point-like sources (3C 22 and 3C 41) and the barely-elongated 3C 54 appear to have of order 10 per cent of their K -band flux contributed by scattered light from the active nucleus. We conclude that scattered nuclear light can form a significant component of the near-infrared light emitted by high-redshift radio galaxies, and discuss models in which the scattering particles are electrons and dust grains.     

Prospects of hard X-ray polarimetry with Astrosat-CZTI

Astrosat is the first Indian satellite mission dedicated for astronomical studies. It is planned for launch during 2014 and will have five instruments for multi-wavelength observations from optical to hard X-rays. Cadmium Zing Telluride Imager (CZTI) is one of the five instruments aiming for simultaneous X-ray spectroscopy and imaging in the energy range of 10 keV to 100 keV (along with all sky photometric capability unto 250 keV). It is based on pixilated CZT detector array with total geometric area of 1024 cm². It will have two-dimensional coded mask for medium resolution X-ray imaging. The CZT detector plane will be realized using CZT detector modules having integrated readout electronics. Each CZT detector module consists of 4 cm × 4 cm CZT with thickness of 5 mm which is further pixilated into 16 × 16 array of pixels. Thus each pixel has size of 2.5 mm × 2.5 mm and thickness of 5 mm. Such pixilated detector plane can in principle be used for hard X-ray polarization measurements based on the principle of Compton scattering by measuring azimuthal distribution of simultaneous events in two adjacent pixels. We have carried out detailed Geant4 simulations for estimating polarimetric capabilities of CZTI detector plane. The results indicate that events in the energy range of 100 keV to 250 keV, where the 5 mm thick CZT detector has significant detection efficiency, can be used for polarimetric studies. Our simulation results indicate the minimum detectable polarization (MDP) at the level of ～ 10% can be achieved for bright Crab like X-ray sources with exposure time of ～500 ks. We also carried out preliminary experiments to verify the results from our simulations. Here we present detailed method and results of our simulations as well as preliminary results from the experimental verification of polarimetric capabilities of CZT detector modules used in Astrosat CZTI.     

On polarimetry in solar active regions

Measurements of the circular polarization V in different lines show that the deduced magnetic field strength and flux are systematically influenced by variations of the line absorption coefficient from photosphere to spot and faculae.Disbalances between preceding and following flux seem to be due mainly to such variations rather than to real physical conditions in active regions.The spatial distribution of the normal component of the magnetic field in an active region near the disc center have been observed during two days using the temperature insensitive line Fe 6302.5. The initial field structure seems to become more and more bipolar. The increase of the flux exceeds that of the area thus suggesting the appearance of new magnetic fields. Backward extrapolation in time leads to a date of first appearance of the magnetic field which agrees with the appearance of first H anomalies.     

Prospects for solar flare X-ray polarimetry

The history of solar flare X-ray polarimetry is reviewed and it is shown that as yet, there is no experimental evidence for such polarization. The present experimental limits are at the level of a few percent but these results may be biased by a large thermal component at low energies which may decrease the apparent polarization. To avoid this difficulty it will be necessary to make observations at higher energies where thermal emission is less important.The theoretical estimates of the polarization expected in the solar flare are also reviewed. The best present theoretical estimates are in the range of a few percent and are consistent with the present experimental limits.In this paper we discuss a new satellite instrument that has sufficient sensitivity at high energies to detect the polarization that is predicted by the present theories. The instrument sensitivity for a moderate (M class) event approaches polarization levels of 1% in each of 7 energy bins spanning the 10 to 100 keV range for integration times as short as 10 s. Comparable results can be obtained for an X class flare in 1 s.Presidential Young Investigator.     

On polarimetry in solar active regions

Phase retardation originating in a telescope is measured by means of polarizer and analyzer. The amount of this retardation depends only on the declination of the telescope. The retardation axes rotate with the Coudé image.The influence of this retardation on a Zeeman triplet, is measured photoelectrically. Consequences for solar polarimetry are discussed: Strong effects originate from the miscentering of the asymmetric Zeeman triplet introduced by the Doppler compensator. This yields a non-vanishing V-Stokes parameter in the central exit slit and thus the often observed crosstalk between the U and V-Stokes parameters.     

Near-infrared imaging polarimetry of Cygnus A

We present a 2.2-μm polarization image of the nuclear regions of Cygnus A. The degree of polarization in the central 1 arcsec is (4.1±0.50) per cent, at a position angle of (23.6±3.6)°, approximately perpendicular to the axis of the radio jet.
Modelling of the results suggests that at this wavelength the polarization along the line of sight to the central source is most likely produced by dichroism, through an A _v∼40 mag, with the polarization in surrounding regions produced by scattering. For this model, the K -band luminosity of the central source is calculated to be ∼2×10⁴⁴ erg s⁻¹.     

On polarimetry in solar active regions

The miscentering by the Doppler compensator of the Locarno polarimeter is investigated in detail. It is shown that the linear polarization is strongly falsified by this effect which also occurs at the Crimean and Izmiran polarimeters.The new design for the exit slits of the Locarno polarimeter is described. It avoids the ambiguities in the determination of the magnetic field vector that always occur when using two exit slits.A new simple electronic setup avoids most of the difficulties which are usually involved in eliminating instrumental polarization and compensating intensity fluctuations.The observational techniques for solar polarimetry at the Locarno observatory are described.To avoid mutual influences of V and U, the line centre ₀ (corresponding to V = 0 and U = max.) must coincide with the centre of slit II. Only in this case we have 234-01     

Optical polarimetry of Comet West 1976 VI

The polarization of the continuum of Comet West 1976 VI was measured in four narrowband filters spanning the wavelengths 440–850 nm. The postperihelion observations indicated wavelength independent linear polarization on each of the three occasions on which it was measured. The wavelength independence is in agreement with other polarization measurements of this comet from the visible to the near-infrared, but it counters the general tendency in comets for the polarization to increase with wavelength. The magnitude of the polarization as a function of scattering angle, the wavelength independence, and the infrared and optical photometric properties suggests that dirty silicates (n₁≈0.05) with radii smaller than 5 μm but approaching this size may be responsible. No circular polarization was detected.