Wavelength Dependence Of Polarization Research Articles

The microvariability and wavelength dependence of polarization degree/angle of BL Lacertae in the outburst 2020 to 2021

Abstract We have obtained simultaneous and continuous photo-polarization observations of the blazar BL Lacertae in optical and near-infrared (NIR) bands during an historical outburst from 2020 to 2021. In total, 14 nights of observations were performed where 10 observations show microvariability on timescales of a few minutes to several hours. This suggests a compact emission region, and the timescales are difficult to explain by a one-zone shock-in-jet model. Moreover, we found significant differences in the polarization degree (PD) and angle between optical and NIR bands. Nine nights showed a PD in the optical band that is greater than or equal to that in the NIR band, which can be explained by either a shock-in-jet model or the Turbulent Extreme Multi-Zone (TEMZ) model. On the other hand, five nights showed higher PD in an NIR band than an optical band, which cannot be explained by simple shock-in-jet models nor the simple TEMZ model. The observed timescales and wavelength-dependency of the PD and polarization angle suggest the existence of complicated multiple emission regions such as an irregular TEMZ model.

Interstellar Extinction and Polarization of Stars in the Open Cluster NGC 6709

We present BVRI polarimetric measurements of 23 stars in the direction of the open cluster NGC 6709. Fifteen stars display wavelength-dependence of polarization which is typically due to selective extinction in the interstellar medium and could be fitted to the Serkwoski equation. Based on these stars, we obtained an average maximum degree of polarization of 0.722 ± 0.022% and an average wavelength at a maximum polarization of 0.615 ± 0.011 μm. The observed low polarization may be due to the alignment of grains along a more longitudinally-directed magnetic field in this region which would cause the aspect ratio of the grains to be reduced. This would have the observational consequence of decreased polarization as well as shifting toward longer wavelengths. Using the expression proposed by Whittet & van Breda we calculated the ratio of total-to-selective extinction as , higher than the nominal value of 3.1 for the general ISM. The remaining eight sample stars display an unusual wavelength-dependence that is similar to some Be stars. However, there are only two Be stars currently listed in this cluster, yet, based on our data, they both present reliable fits within the Serkowski equation. The overall alignment of the V-band polarization vectors implies polarization by selective extinction due to foreground dust. Using the Planck 353 GHz channel we find there is an agreement between polarization by emission and selective extinction. Since the optical polarization traces a magnetic field, this suggests that the magnetic field in this region is inclined to the Galactic plane, with an overall average Galactic position angle of 104 ± 12 (st. dev.) degrees.

Interstellar Extinction and Polarization of Stars in the Open Cluster NGC 7160

We present polarimetric observations of stars in the field of the open cluster NGC 7160. The multi-wavelength UBVRI polarization measurements of 21 stars that we performed indicate that some of the observed stars follow the typical wavelength dependence indicative of polarization by selective extinction in the interstellar medium, while others display multi-wavelength dependence strikingly different from nominal behavior. Considering only the former stars, we obtained a value of R V = 3.01 ± 0.03 for the total-to-selective extinction ratio toward the cluster, almost identical to the nominal expected value. For the remaining stars we suggest that their wavelength dependence of polarization is most likely altered by the presence of circumstellar material. Patterns in both the wavelength dependence of the degree of polarization and position angle show similarities found in the wavelength dependence of some carbon stars and PMS stars. PMS stars in NGC 7160 have been identified by Gray & Corbally and the multi-wavelength polarization obtained here suggests additional candidates. The rotation of the position vectors for a number of stars seems indicative for mixing of intrinsic and interstellar polarization components. It seems, however, that intra-cluster dust does not play a significant overall role in the polarization of these stars. The distribution of the interstellar extinction with distance as obtained from dust maps supports this finding. We also present V-band polarization measurements for 78 stars in and around the central region of cluster. The polarization ranges from 0.12% to 3.75% with a median value of 1.23%. While the polarization vectors display some scatter, an overall alignment exists, which would indicate polarization by foreground interstellar dust. Using the Planck 353 GHz channel we find agreement between polarization by emission and selective extinction and also that the optical polarization traces a magnetic field that is nearly parallel to the Galactic plane in this region.

On the Shapes of Interstellar Grains: Modeling Infrared Extinction and Polarization by Spheroids and Continuous Distributions of Ellipsoids

Abstract Although interstellar grains are known to be aspherical, their actual shapes remain poorly constrained. We assess whether three continuous distributions of ellipsoids (CDEs) from the literature are suitable for describing the shapes of interstellar grains. Randomly selected shapes from each distribution are shown as illustrations. The often-used Bohren–Huffman CDE includes a very large fraction of extreme shapes: fully 10% of random draws have axial ratio a 3/a 1 > 19.7, and 5% have a 3/a 1 > 33. The CDE2 distribution includes a much smaller fraction of extreme shapes, and appears to be the most realistic. For each of the three CDEs considered, we derive shape-averaged cross sections for extinction and polarization in the Rayleigh limit. Finally, we describe a method for “synthesizing” a dielectric function for an assumed shape or shape distribution if the actual absorption cross sections per grain volume in the Rayleigh limit are known from observations. This synthetic dielectric function predicts the wavelength dependence of polarization, which can then be compared to observations to constrain the grain shape.

Modelling heterogeneous dust particles: an application to cometary polarization

ABSTRACT In this work, we introduce a comet dust model that incorporates multiple dust morphologies along with inhomogeneous mixture of silicate minerals and carbonaceous materials under power-law size distribution, to replicate the standard polarization-phase curve observed in several comets in the narrow-band continuum. Following the results from Rosetta/midas and COSIMA, we create high porosity hierarchical aggregates (HA) and low porosity (<10 per cent) Solids in the form of agglomerated debris. We also introduce a moderate porosity structure with solids in the core, surrounded by fluffy aggregates called fluffy solids (FS). We study the mixing combinations, (HA and Solids), (HA and FS), and (HA, FS, and Solids) for a range of power-law index n= 2.0 to 3.0 for different sets of mixing percentage of silicate minerals and carbonaceous materials. Polarimetry of the short period comets 1P/Halley and 67P/Churyumov-Gerasimenko match best with the polarization resulting from the combination of HA and Solids while the combinations (HA and FS) and (HA, FS, and Solids) provide the best-fitting results for the long period comets C/1995 O1 (Hale-Bopp) and C/1996 B2 (Hyakutake). The best-fitting model results also recreate the observed wavelength dependence of polarization. Our dust model agree with the idea that the long period comets may have high percentage of loose particles (HA and FS) compared to those in the case of short period comets as the short period comets experience more frequent and/or higher magnitude of weathering.

An optical spectroscopic and polarimetric study of the microquasar binary system SS 433

Aims. Our aim is to study the mass transfer, accretion environment, and wind outflows in the SS 433 system, concentrating on the so-called stationary lines. Methods. We used archival high-resolution (X-shooter) and low-resolution (EMMI) optical spectra, new optical multi-filter polarimetry, and low-resolution optical spectra (Liverpool Telescope), spanning an interval of a decade and a broad range of precessional and orbital phases, to derive the dynamical properties of the system. Results. Using optical interstellar absorption lines and H I 21 cm profiles, we derive E(B − V) = 0.86 ± 0.10, with an upper limit of E(B − V) = 1.8 ± 0.1 based on optical Diffuse Interstellar Bands. We obtain revised values for the ultraviolet and U band polarizations and polarization angles (PA), based on a new calibrator star at nearly the same distance as SS 433 that corrects the published measurement and yields the same PA as the optical. The polarization wavelength dependence is consistent with optical-dominating electron scattering with a Rayleigh component in U and the UV filters. No significant phase modulation was found for PA while there is significant variability in the polarization level. We fortuitously caught a flare event; no polarization changes were observed but we confirm the previously reported associated emission line variations. Studying profile modulation of multiple lines of H I, He I, O I, Na I, Si II, Ca II, Fe II with precessional and orbital phase, we derive properties for the accretion disk and present evidence for a strong disk wind, extending published results. Using transition-dependent systemic velocities, we probe the velocity gradient of the wind, and demonstrate that it is also variable on timescales unrelated to the orbit. Using the rotational velocity, around 140 ± 20 km s−1, a redetermined mass ratio q = 0.37 ± 0.04, and masses MX = 4.2 ± 0.4 M⊙, MA = 11.3 ± 0.6 M⊙, the radius of the A star fills – or slightly overfills – its Roche surface. We devote particular attention to the O I 7772 Å and 8446 Å lines, finding that they show different but related orbital and precessional modulation and there is no evidence for a circumbinary component. The spectral line profile variability can, in general, be understood with an ionization stratified outflow predicted by thermal wind modeling, modulated by different lines of sight through the disk produced by its precession. The wind can also account for an extended equatorial structure detected at long wavelength.

Extreme starlight polarization in a region with highly polarized dust emission

Context. Galactic dust emission is polarized at unexpectedly high levels, as revealed by Planck. Aims. The origin of the observed ≃20% polarization fractions can be identified by characterizing the properties of optical starlight polarization in a region with maximally polarized dust emission. Methods. We measure the R-band linear polarization of 22 stars in a region with a submillimeter polarization fraction of ≃20%. A subset of 6 stars is also measured in the B, V, and I bands to investigate the wavelength dependence of polarization. Results. 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 [pV/E(B−V)]max = 9% mag−1 and is at least as high as 13% mag−1, as 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.

Linear spectropolarimetry of polarimetric standard stars with VLT/FORS2

We reduced ESO's archival linear spectropolarimetry data (4000-9000\AA) of 6 highly polarized and 8 unpolarized standard stars observed between 2010 and 2016, for a total of 70 epochs, with the FOcal Reducer and low dispersion Spectrograph (FORS2) mounted at the Very Large Telescope. We provide very accurate standard stars polarization measurements as a function of wavelength, and test the performance of the spectropolarimetric mode (PMOS) of FORS2. We used the unpolarized stars to test the time stability of the PMOS mode, and found a small ($\leq$0.1%), but statistically significant, on-axis instrumental polarization wavelength dependency, possibly caused by the tilted surfaces of the dispersive element. The polarization degree and angle are found to be stable at the level of $\leq$0.1% and $\leq$0.2 degrees, respectively. We derived the polarization wavelength dependence of the polarized standard stars and found that, in general, the results are consistent with those reported in the literature, e.g. Fossati et al. (2007) who performed a similar analysis using FORS1 data. The re-calibrated data provide a very accurate set of standards that can be very reliably used for technical and scientific purposes. The analysis of the Serkowski parameters revealed a systematic deviation from the width parameter $K$ reported by Whittet et al. (1992). This is most likely explained by incorrect effective wavelengths adopted in that study for the R and I bands.

Wavelength dependence of polarization and physical mechanisms of magnetic field generation in accretion disks around supermassive black holes in active galactic nuclei

Analysis of the wavelength dependence of the polarization of radiation from active galactic nuclei (AGNs) is shown to allow the main physical mechanisms of magnetic field generation in accretion disks around supermassive black holes in these objects to be determined. These main processes include the generation of magnetic fields as a result of the equality between the magnetic and radiation pressures or as a result of the equality between the magnetic and gas pressures. In several cases, the wavelength dependence of polarization is shown to be explained, provided that the Shakura-Sunyaev viscosity parameter depends on the accretion-disk radius.

THE EFFICIENCY AND WAVELENGTH DEPENDENCE OF NEAR-INFRARED INTERSTELLAR POLARIZATION TOWARD THE GALACTIC CENTER

Near-infrared polarimetric imaging observations toward the Galactic center have been carried out to examine the efficiency and wavelength dependence of interstellar polarization. A total area of about 5.7 deg$^2$ is covered in the $J$, $H$, and $K_S$ bands. We examined the polarization efficiency, defined as the ratio of degree of polarization to color excess. The interstellar medium between the Galactic center and us shows the polarization efficiency lower than that in the Galactic disk by a factor of three. Moreover we investigated the spatial variation of the polarization efficiency by comparing it with those of color excess, degree of polarization, and position angle. The spatial variations of color excess and degree of polarization depend on the Galactic latitude, while the polarization efficiency varies independently of the Galactic structure. Position angles are nearly parallel to the Galactic plane, indicating the longitudinal magnetic field configuration between the Galactic center and us. The polarization efficiency anticorrelates with dispersions of position angles. The low polarization efficiency and its spatial variation can be explained by the differences of the magnetic field directions along the line-of-sight. From the lower polarization efficiency, we suggest a higher strength of a random component relative to a uniform component of the magnetic field between the Galactic center and us. We also derived the ratios of degree of polarization $p_H/p_J$ = 0.581 $\pm$ 0.004 and $p_{K_S}/p_H$ = 0.620 $\pm$ 0.002. The power law indices of the wavelength dependence of polarization are $\beta_{JH}$ = 2.08 $\pm$ 0.02 and $\beta_{HK_S}$ = 1.76 $\pm$ 0.01. Therefore the wavelength dependence of interstellar polarization exhibits flattening toward longer wavelengths in the range of 1.25$-$2.14 $\micron$. The flattening would be caused by aligned large-size dust grains.

Topology of magnetic field and polarization in accretion discs of AGN

In this paper we demonstrate that the wavelength dependence of polarization degree and position angle allows us to derive the distribution of magnetic field in accretion disc. The polarized radiation arises due to scattering of emission light by electrons in a magnetized optically thick accretion disc. Faraday rotation of polarization plane is taken into consideration. Through wavelength dependence of polarization it is possible to derive the value of the magnetic Prandtl number in the accretion disc plasma. The power law index of the polarization wavelength dependence is related with the radial distribution of magnetic field in an accretion disc. This allows us to test the various models of an accretion disc around the central black hole.

Polarization of light scattered by large aggregates

Study of cosmic dust and planetary aerosols indicate that some of them contain a large number of aggregates of the size that significantly exceeds the wavelengths of the visible light. In some cases such large aggregates may dominate in formation of the light scattering characteristics of the dust. In this paper we present the results of computer modeling of light scattering by aggregates that contain more than 1000 monomers of submicron size and study how their light scattering characteristics, specifically polarization, change with phase angle and wavelength. Such a modeling became possible due to development of a new version of Multi Sphere T-Matrix (MSTM) code for parallel computing. The results of the modeling are applied to the results of comet polarimetric observations to check if large aggregates dominate in formation of light scattering by comet dust. We compare aggregates of different structure and porosity. We show that large aggregates of more than 98% porosity (e.g. ballistic cluster–cluster aggregates) have angular dependence of polarization almost identical to the Rayleigh one. Large compact aggregates (less than 80% porosity) demonstrate the curves typical for solid particles. This rules out too porous and too compact aggregates as typical comet dust particles. We show that large aggregates not only can explain phase angle dependence of comet polarization in the near infrared but also may be responsible for the wavelength dependence of polarization, which can be related to their porosity.

POLARIZED REFLECTED LIGHT FROM THE EXOPLANET HD189733b: FIRST MULTICOLOR OBSERVATIONS AND CONFIRMATION OF DETECTION

We report first multicolor polarimetric measurements (UBV bands) for the hot Jupiters HD189733b and confirm our previously reported detection of polarization in the B band (Berdyugina et al. 2008). The wavelength dependence of polarization indicates the dominance of Rayleigh scattering with a peak in the blue B and U bands of ~10^-4+/-10^-5 and at least a factor of two lower signal in the V band. The Rayleigh-like wavelength dependence, detected also in the transmitted light during transits, implies a rapid decrease of the polarization signal toward longer wavelengths. Therefore, the nondetection by Wiktorowicz (2009), based on a measurement integrated within a broad passband covering the V band and partly B and R bands, is inconclusive and consistent with our detection in B. We discuss possible sources of the polarization and demonstrate that effects of incomplete cancellation of stellar limb polarization due to starspots or tidal perturbations are negligible as compared to scattering polarization in the planetary atmosphere. We compare the observations with a Rayleigh-Lambert model and determine effective radii and geometrical albedos for different wavelengths. We find a close similarity of the wavelength dependent geometrical albedo with that of the Neptune atmosphere, which is known to be strongly influenced by Rayleigh and Raman scattering. Our result establishes polarimetry as a reliable means for directly studying exoplanetary atmospheres.

Polarimetry of main belt asteroids: Wavelength dependence

New UBVRI polarimetric observations of ten asteroids, including space mission targets 1 Ceres and 21 Lutetia, are presented. These observations were obtained with the 1.25-m telescope of the Crimean Astrophysical Observatory and have been used to study the wavelength dependence of polarization for a sample of asteroids belonging to the M and low albedo classes. A more general analysis including also a larger data set of UBVRI polarimetric observations available in the literature for more than 50 main belt asteroids belonging to different taxonomic classes shows that the variation of the polarization degree P r as a function of wavelength is generally well described by a linear trend. It typically does not exceed 0.2% in the studied spectral range 0.37–0.83 microns and tends to increase for increasing phase angle. Asteroids belonging to the S and M classes are found to exhibit a deeper negative branch and smaller positive polarization for increasing wavelength (negative sign of the slope of Δ P r / Δ λ ). Since the objects belonging to these classes are known to exhibit reddish reflectance spectra, the observed wavelength behavior of negative polarization contradicts the well-known inverse correlation of P min and albedo. Low albedo asteroids show larger dispersion of spectral slopes, but the overall trend is characterized by a shallower negative branch and a larger positive polarization for increasing wavelength (positive sign of the slope of Δ P r / Δ λ ). A few exceptions from this general trend are discussed. The observed variety in the wavelength dependence of asteroid polarization seems to be mainly attributed to surface composition.

Linear polarimetry of five comets

We made linear polarimetry of five comets, C/1989 X1 (Austin), 109P/Swift-Tuttle, C/1996 B2 (Hyakutake), C/1995 O1 (Hale-Bopp), and C/1996 Q1 (Tabur) simultaneously at eight wavelengths, in order to study phase angle and wavelength dependence of polarization in continuum light, and polarization behavior of molecular origin. Polarization in continuum of Swift-Tuttle and Hyakutake were rather high i.e. similar to that of 1P/Halley, while those of Austin and Tabur were low in the positive branch. Hale-Bopp showed the highest polarization ever observed at the corresponding phase angles in the positive branch. We also made observations scanning across the coma for Hyakutake and Hale-Bopp to investigate polarization structure. For Hale-Bopp, we found: (1) The position of maximum polarization was not at the visible center, and a red part was observed in the central region. (2) Optical properties and/or size spectrum of dust varied in a jet. On the contrary, any peculiar structure was not found for Hyakutake in spite of a much finer spatial resolution. The scanning observation of Hale-Bopp suggests that dust including its size spectrum evolves after the ejection from the nucleus, and that the optical depth in the central region plays an important role.

VW Cygni: new photometric and polarimetric observations

An analysis of linear polarization for an eclipsing variable star VW Cyg is performed. The dependence of polarization values on the phase of orbital period is derived. The wavelength dependence of polarization out-of-eclipse shows a superposition of two different mechanisms of polarization. We assume that one of them is interstellar polarization while the other one is scattering by electrons in the gas shell of the system. Assuming that the polarization caused by electron scattering does not depend on the wavelength and the wavelength dependence of the interstellar polarization fits the Serkowski law, one can derive the most probable parameters of both the components. It is shown that the proper polarization is 0.3±0.03%. The UBVRI photometry was also performed.

An Optical, Dual-Beam, Automated Medium Resolution Spectropolarimeter for the Vainu Bappu Telescope

Fabrication of an optical dual-beam spectropolarimeter as an add-on facility to an existing astronomical spectrograph at the Vainu Bappu Telescope is described. The polarimetric optics consists of a superachromatic Pancharatnam design half-wave plate and a modified Glan-Taylor polarizing beam splitter. Instrumental response, calibration and characterization of the system are presented. Performance of the spectropolarimeter has been assessed based on the results of observations of polarized and unpolarized standard stars. The attainable accuracy of the instrument is found to be dependent on the S/N of the data. The present data set yields an accuracy of ∼ ±0.5% at 4000 A and ∼ ±0.3% at 7500 A, at a spectral resolution of ∼7.2 A. Analysis of spectropolarimetric data is systematically covered for removal of instrumental errors. The spectropolarimetric reduction software (SPRS), extremely versatile, user friendly and compatible with the IRAF image processing package, was developed for reducing spectropolarimetric data. The empirical relation by Serkowski (IAU Symposium 52, Interstellar Dust and Related Topics, J. M. Greenberg and H. C. van de Hulst (eds.), Dordrecht, Reidel, 1975, p. 145) for wavelength dependence of polarization due to interstellar medium has been fitted to the data for the star HD 43384 (9 Gem). Our result shows a polarization larger by 0.49% from Hsu and Berger (ApJ 262, 1982, 732). We attribute this difference to the long term variation in P for this star.

Broadband polarimetry of novae in outburst

We present broadband optical polarimetry of the classical novae V705 Cas, V4362 Sgr, V2313 Oph and BY Cir in outburst. The data indicate that, in all cases, there is an intrinsic component of polarization and in some cases the variability is very rapid, on a time-scale 1 day. In the case of V705 Cas, we suggest that the origin of the intrinsic component may lie in the clumpiness and/or inhomogeneities of the ejecta, while electron scattering is the most likely cause in V2313 Oph. The wavelength-dependence of polarization in the case of V4362 Sgr suggests scattering by small dust grains, while polarization in resonance lines is the most probable cause of the observed polarization in BY Cir.

Multiband optical polarimetry of BL Lacertae objects with the Nordic Optical Telescope

Optical polarization of seven selected BL Lac objects in UBVRI bands was studied with the Nordic Optical Telescope from December 10{14, 1999. Two of them, 3C 66A and PKS 0735+178, were monitored for 4 nights for a total integration time of 4.75 and 5.5 hours, respectively. Other objects (1 Jy 0138 097, H 0414+009, PKS 0823 223, OJ287 and BL Lac) were observed sparsely during the run. Apart from PKS 0823 223 (more polarized than observed in the past), the sources show levels of flux and polarization consistent with results at previous epochs. 3C 66A and PKS 0735+178 were intensively observed during December 11 and 12 and exhibited variability of polarization, both on internight and intranight time scales. Wavelength dependence of polarization has been investigated, as well as circular polarization. The results are discussed within the standard model for BL Lacs.

The Spectral Signature of Dust Scattering and Polarization in the Near‐Infrared to Far‐Ultraviolet. I. Optical Depth and Geometry Effects

Spectropolarimetry from the near-IR to the far-UV of light scattered by dust provides a valuable diagnostic of the dust composition, grain size distribution and spatial distribution. To facilitate the use of this diagnostic, we present detailed calculations of the intensity and polarization spectral signature of light scattered by optically thin and optically thick dust in various geometries. The polarized light radiative transfer calculations are carried out using the adding-doubling method for a plane-parallel slab and are extended to an optically thick sphere by integrating over its surface. The calculations are for the Mathis, Rumpl, & Nordsieck Galactic dust model, and cover the range from 1 μm to 500 A. We find that the wavelength dependence of the scattered light intensity provides a sensitive probe of the optical depth of the scattering medium, while the polarization wavelength dependence provides a probe of the grain scattering properties, which is practically independent of optical depth. We provide a detailed set of predictions, including polarization maps, which can be used to probe the properties of dust through imaging spectropolarimetry in the near-IR to far-UV of various Galactic and extragalactic objects. In a following paper we use the codes developed here to provide predictions for the dependence of the intensity and polarization on grain size distribution and composition.