Photopolarimetric Data Research Articles

Optical Polarimetric and Multiwavelength Flaring Activity of Blazar 3C 279

Abstract An exhaustive analysis of 9 yr optical R-band photopolarimetric data of the flat-spectrum radio quasar 3C 279 from 2008 February 27 to 2017 May 25 is presented, along with multiwavelength observing campaigns performed during the flaring activity exhibited in 2009 February/March, 2011 June, 2014 March/April, 2015 June, and 2017 February. In the R band, this source showed the maximum brightness state of 13.68 ± 0.11 mag (1.36 ± 0.20 mJy) on 2017 March 2 and the lowest brightness state ever recorded of 18.20 ± 0.87 mag (0.16 ± 0.03 mJy) on 2010 June 17. During the entire period of observations, the polarization degree varied between 0.48% ± 0.17% and 31.65% ± 0.77%, and the electric vector position angle (EVPA) exhibited large rotations between 82.°98 ± 0.°92 and 446.°32 ± 1.°95. Optical polarization data show that this source has a stable polarized component that varied from ∼6% (before the 2009 flare) to ∼13% after the flare. The overall behavior of our polarized variability data supports the scenario of jet precessions as responsible for the observed large rotations of the EVPA. Discrete correlation function analysis shows that the lags between gamma-rays and X-rays compared to the optical R-band fluxes are Δt ∼ 31 days and 1 day in 2009. Lags were also found among gamma-rays compared with X-rays and radio of Δt ∼ 30 and 43 days in 2011, and among radio and optical R band of Δt ∼ 10 days in 2014. A very intense flare in 2017 was observed in optical bands, with a dramatic variation in the polarization degree (from ∼6% to 20%) in 90 days without exhibiting flaring activity in other wavelengths.

Dual-beam optical linear polarimetry from Southern skies: characterization of CasPol for high-precision polarimetry

We present a characterization of CasPol, a dual-beam polarimeter mounted at the 2.15-m Jorge Sahade Telescope, located at the Complejo Astronomico El Leoncito, Argentina. The telescope is one of the few available meter-sized optical telescopes located in the southern hemisphere hosting a polarimeter. To carry out this work, we collected photopolarimetric data along five observing campaigns, the first one during January 2014, and the remaining ones spread between August 2017 and March 2018. The data were taken through the Johnson–Cousins V, R, and I filters. Along the campaigns, we observed eight unpolarized and four polarized standard stars. Our analysis began characterizing the impact of seeing and aperture into the polarimetric measurements, defining an optimum aperture extraction and setting a clear limit for seeing conditions. Then, we used the unpolarized standard stars to characterize the level of instrumental polarization and to assess the presence of polarization dependent on the position across the charge-coupled device. Polarized standard stars were investigated to quantify the stability of the instrument with wavelength. Specifically, we find that the overall instrumental polarization of CasPol is ∼0.2 % in the V, R, and I bands, with a negligible polarization dependence on the position of the stars on the detector. The stability of the half-wave plate retarder is about 0.35 deg, making CasPol comparable to already existing instruments. We also provide measurements in the three photometric bands for both the unpolarized and polarized standard stars. Finally, we show scientific results, illustrating the capabilities of CasPol for precision polarimetry of relatively faint objects.

Analysis of the Optical Cyclotron Emission of Polar CRTS CSS081231 J071126+440405

We have modeled the cyclotron spectra and photopolarimetric data of polar CRTS CSS081231 J071126+440405. A series of spectra of the polar were obtained in the intermediate brightness state spanning an orbital period. Modeling spectra allowed us to determine the magnetic field strength in the vicinity of both magnetic poles of the white dwarf: B1 = 38 MG and B2 = 51 MG. We obtained a temperature estimate for the accretion spots near both magnetic poles of the white dwarf. The estimate for the first spot is kT ~ 20–30 keV, whereas the second spot turned out to be cooler by about 10 keV. An analysis of the light curves and polarization of the polar obtained in the low state shows that the interaction of the accretion stream with the star’s atmosphere occurs close (within Δθ = 10◦) to the magnetic pole.

Long-term Optical Polarization Variability and Multiwavelength Analysis of Blazar Mrk 421

Abstract The results of 8 yr R-band photopolarimetric data of blazar Mrk 421 collected from 2008 February to 2016 May are presented, along with extensive multiwavelength observations covering radio to TeV γ-rays around the flares observed in 2008 May, 2010 March, and 2013 April. The most important results are found in 2013, when the source displayed in the R band a very high brightness state of 11.29 ± 0.03 mag (93.60 ± 1.53 mJy) on April 10 and a polarization degree of 11.00% ± 0.44% on May 13. The analysis of the optical data shows that the polarization variability is due to the superposition of two polarized components that might be produced in two distinct emitting regions. An intranight photopolarimetric variability study carried out over seven nights after the 2013 April maximum found flux and polarization variations on the nights of April 14, 15, 16, and 19. In addition, the flux shows a minimum variability timescale of Δt = 2.34 ± 0.12 hr, and the polarization degree presents variations of ∼1%–2% on a timescale of Δ t ∼ minutes. Also, a detailed analysis of the intranight data shows a coherence length of the large-scale magnetic field of pc, which is the same order of magnitude as the distance traveled by the relativistic shocks. This result suggests that there is a connection between the intranight polarimetric variations and spatial changes of the magnetic field. Analysis of the complete R-band data along with the historical optical light curve found for this object shows that Mrk 421 varies with a period of 16.26 ± 1.78 yr.

Multiwavelength Variability Study of the Classical BL Lac Object PKS 0735+178 on Timescales Ranging from Decades to Minutes

Abstract We present the results of our power spectral analysis for the BL Lac object PKS 0735+178, utilizing the Fermi-LAT survey at high-energy γ-rays, several ground-based optical telescopes, and single-dish radio telescopes operating at GHz frequencies. The novelty of our approach is that, by combining long-term and densely sampled intra-night light curves in the optical regime, we were able to construct for the first time the optical power spectrum of the blazar for a time domain extending from 23 years down to minutes. Our analysis reveals that: (1) the optical variability is consistent with a pure red noise, for which the power spectral density can be well approximated by a single power law throughout the entire time domain probed; (2) the slope of power spectral density at high-energy γ-rays (∼1) is significantly flatter than that found at radio and optical frequencies (∼2) within the corresponding time variability range; (3) for the derived power spectra, we did not detect any low-frequency flattening, nor do we see any evidence for cutoffs at the highest frequencies down to the noise floor levels due to measurement uncertainties. We interpret our findings in terms of a model where the blazar variability is generated by the underlying single stochastic process (at radio and optical frequencies), or a linear superposition of such processes (in the γ-ray regime). Along with the detailed PSD analysis, we also present the results of our extended (1998–2015) intra-night optical monitoring program and newly acquired optical photo-polarimetric data for the source.

Bayesian Approach to Find a Long-Term Trend in Erratic Polarization Variations Observed in Blazars

Abstract We developed a method of separating a long-term trend from observed temporal variations of polarization in blazars using a Bayesian approach. The temporal variation of the polarization vector is apparently erratic in most blazars, while several objects have occasionally exhibited systematic variations, for example an increase of the polarization degree associated with a flare of the total flux. We assume that the observed polarization vector is a superposition of two distinct components: a long-term trend and a short-term variation component responsible for short flares. Our Bayesian model estimates a long-term trend that satisfies the condition that the total flux correlates with the polarized flux of the short-term component. We demonstrate that assumed long-term polarization components are successfully separated by the Bayesian model for artificial data. We applied this method to the three photopolarimetric data of OJ 287, S5 0716$+$714, and S2 0109$+$224. Simple and systematic long-term trends were obtained in OJ 287 and S2 0109$+$224, while no such trend was confirmed in S5 0716$+$714. We propose that the apparently erratic variations of polarization in OJ 287 and S2 0109$+$224 are due to the presence of the long-term polarization component. The behavior of polarization in S5 0716$+$714 during the period of observation implies the presence of a number of polarization components showing variations on a quite short time-scale.

Reflection models for soil and vegetation surfaces from multiple-viewing angle photopolarimetric measurements

The reflection properties of soil and vegetation surfaces have been investigated using airborne photopolarimetric data from the research scanning polarimeter (RSP). For both surface types, it was found that the ratios of total reflectances taken at two different wavelengths from visible and short-wave infrared channels are the same for different illumination and scattering geometries, and in general independent of the scattering angle. From an analysis of the angular and spectral dependencies of the intensity and polarization, we show that the modeled total and polarized reflectances can be expressed in the same form both for soil and vegetation surfaces, namely, as the product of geometrical scattering term depending only on illumination and viewing angles, and a term that varies solely with wavelength and scattering angle.

Stereo Image Analysis of Non-Lambertian Surfaces

Stereo image analysis is based on establishing correspondences between a pair of images by determining similarity measures for potentially corresponding image parts. Such similarity criteria are only strictly valid for surfaces with Lambertian (diffuse) reflectance characteristics. Specular reflections are viewpoint dependent and may thus cause large intensity differences at corresponding image points. In the presence of specular reflections, traditional stereo approaches are often unable to establish correspondences at all, or the inferred disparity values tend to be inaccurate, or the established correspondences do not belong to the same physical surface point. The stereo image analysis framework for non-Lambertian surfaces presented in this contribution combines geometric cues with photometric and polarimetric information into an iterative scheme that allows to establish stereo correspondences in accordance with the specular reflectance behaviour and at the same time to determine the surface gradient field based on the known photometric and polarimetric reflectance properties. The described approach yields a dense 3D reconstruction of the surface which is consistent with all observed geometric and photopolarimetric data. Initially, a sparse 3D point cloud of the surface is computed by traditional blockmatching stereo. Subsequently, a dense 3D profile of the surface is determined in the coordinate system of camera 1 based on the shape from photopolarimetric reflectance and depth technique. A synthetic image of the surface is rendered in the coordinate system of camera 2 using the illumination direction and reflectance properties of the surface material. Point correspondences between the rendered image and the observed image of camera 2 are established with the blockmatching technique. This procedure yields an increased number of 3D points of higher accuracy, compared to the initial 3D point cloud. The improved 3D point cloud is used to compute a refined dense 3D surface profile. These steps are iterated until convergence of the 3D reconstruction. An experimental evaluation of our method is provided for areas of several square centimetres of forged and cast iron objects with rough surfaces displaying both diffuse and significant specular reflectance components, where traditional stereo image analysis largely fails. A comparison to independently measured ground truth data reveals that the root-mean-square error of the 3D reconstruction results is typically of the order 30---100 μm at a lateral pixel resolution of 86 μm. For two example surfaces, the number of stereo correspondences established by the specular stereo algorithm is several orders of magnitude higher than the initial number of 3D points. For one example surface, the number of stereo correspondences decreases by a factor of about two, but the 3D point cloud obtained with the specular stereo method is less noisy, contains a negligible number of outliers, and shows significantly more surface detail than the initial 3D point cloud. For poorly known reflectance parameters we observe a graceful degradation of the accuracy of 3D reconstruction.

The effect of aerosol shape in retrieving optical properties of cloud particles in the planetary atmospheres from the photopolarimetric data. Jupiter

The influence of the assumed shape of aerosols on the estimates of the refractive index and size of particles (based on the data of ground-based spectropolarimetric measurements) is investigated for Jupiter’s cloud layer. In the present analysis, we supposed the atmospheric particles to be chaotically oriented spheroids and cylinders with a gamma size distribution. Their single-scattering characteristics were calculated with the T-matrix method, and the intensity and degree of linear polarization of the radiation reflected by the center of the planetary disk were found by solving the vector radiative-transfer equation with consideration for multiple scattering in a plane-parallel atmosphere. We considered a spectral interval from 0.423 to 0.798 µm and phase angles α from 0° to 11°. It has been shown that, if we use the optical characteristics of aerosols found within the frames of a spherical model (Mishchenko, 1990a), the models with the nonspherical particles considered here cannot fit the observational data. The refractive index and the sizes of spheroidal and cylindrical particles were estimated from a comparison of the data of measurements and calculations, and the simplest models for the Jovian cloud layer structure have been considered. We have concluded that the optical parameters of cloud particles (specifically, the refractive index) cannot be reliably estimated only on the basis of measurements made in a narrow range of phase angles.

Case Studies of Aerosol Retrievals over the Ocean from Multiangle, Multispectral Photopolarimetric Remote Sensing Data

To evaluate the global effects of aerosols on the direct radiative balance, tropospheric chemistry, and cloud properties of the earth’s atmosphere requires high-precision remote sensing that is sensitive to the aerosol optical thickness, size distribution, refractive index, and number density. This study uses the multiangle 0.41-, 0.55-, 0.865-, and 2.25-mm channel data from the airborne Research Scanning Polarimeter to retrieve aerosol properties over the Pacific Ocean. It is shown that such photopolarimetric data are highly sensitive to the size distribution and refractive index of aerosol particles, which reduces the nonuniqueness in aerosol retrievals using such data as compared with less comprehensive datasets. Moreover, it is found that polarized reflectances obtained at the shorter wavelengths (0.41 and 0.55 mm) are significantly less sensitive to the contribution of the ocean’s upwelling light than total reflectance measurements, providing a natural tool for the separation between the estimation of oceanic and atmospheric scattering properties.

Deriving terrestrial cloud top pressure from photopolarimetry of reflected light

The linear polarization of sunlight reflected by cloudy areas on Earth is sensitive to the cloud top pressure as a result of molecular scattering above the clouds. We consider the derivation of cloud top pressures using polarimetric data from satellites or aircraft. The inversion method used is based on adding/doubling calculations and a Newton–Raphson iteration scheme developed earlier to analyze the polarization of the planet Venus. A modification of the adding/doubling scheme is presented and used. This approach reduces the execution time for multiple scattering calculations with about an order of magnitude. Two different atmospheric models were used. The first model includes multiple scattering by a cloud layer of spherical water drops and a higher layer of molecules and spherical aerosol particles, whereas in the second model the reflection by the cloud layer is approximated by that of a Lambertian surface and the aerosols in the higher layer are ignored, thereby reducing the necessary computer time by about two orders of magnitude. Using simulated observations, the errors in derived cloud top pressures due to the approximations made in the second model are compared with those due to measurement errors. For a first application of our method to real measurements we used some photopolarimetric data obtained over the Atlantic Ocean by the Global Ozone Monitoring Experiment (GOME). It is shown that for the data considered the second model leads to errors in derived cloud top pressures which are typically smaller than 80 mb. The measurement errors in the GOME polarization observations at about 355 and 490 nm lead to errors in derived cloud top pressures which are typically smaller than 100 and 200 mb, respectively, so that the second model is sufficiently accurate to derive cloud top pressures from these observations. It also appears that the measurements at about 355 nm are more suited to derive cloud top pressures than those at about 490 nm. If more precise measurements are made, a more realistic model, such as our first model, will be required.

Polarimetry of the eclipsing polar RX J0929.1 -- 2404

We report polarimetric, spectropolarimetric and photometric observations of the eclipsing ROSAT cataclysmic variable RX J0929.1−2404, which confirm that the system is a new polar (AM Herculis system). This brings the number of eclipsing polars to nine, with RX J0929.1−2404 being only the third such system above the period gap. Circular polarization variations from ∼−20 to 10 per cent are seen over the 3.39-h orbital period, with a minimum around the time of eclipse. The photopolarimetric data were modelled using arc-shaped cyclotron emission regions in a centred dipole geometry. Results imply that RX J0929.1−2404 is a ‘two-pole’ system, with one emission region partially visible at all orbital phases. Spectropolarimetry observations show some evidence for the presence of cyclotron humps in the continuum, with spacings consistent with a magnetic field strength of ∼20 MG. Photometry of the eclipses provides information on the size of the emission region, which is consistent with a hotspot on the surface of the white dwarf. The eclipse duration implies an inclination in the range 70°≲i≲78°.

Measurements of light scattering by rough surfaces

We present the laboratory measurements of light scattering by rough surfaces, i.e. alumina plate with different degree of roughness, two layers of alumina particles with diameters of 2mm and 500μm, and a layer of alumina powder. It is found that a specular reflection disappears as the roughness of the plate or the size of particle layer increases. In addition, the anisotropic scattering observed in our measurements can be described by bidirectional reflectance models. Our measurements will be used to analyse the in situ photo-polarimetric data of asteroid NEREUS obtained by an asteroid sample return mission (MUSES-C in ISAS) in 2002.

Analysis of photopolarimetric data of comets at small phase angles by rough surface scattering

Photopolarimetric data for comets in the backscattering region, including negative polarization and the opposition effect, are interpreted on the basis of rough scattering by large irregular particles combined with Mie scattering by submicron grains. Both types of grains are assumed to have the same average optical constants, i.e., n = 1.385 and k = 0.035 at wavelength λ = 0.62 μm, where n and k, respectively, denote a refractive index and absorption coefficient of grain material. Scattering from large, rough particles reduces the strong negative polarization in the backward direction contributed by Mie scattering from small grains and is adequate to model observed amplitudes of negative polarization in cometary comae. The introduction of rough scattering also leads to a somewhat steeper increase of intensity with decreasing phase angle, α, in the range 0° ⩽ α ⩽ 20°, and is more consistent with observations than that predicted from Mie theory alone and Halley's grain-size distribution.

A spectral photopolarimeter for Giotto: Halley optical probe experiment

The Halley Optical Probe Experiment (HOPE) is designed to provide in situ photopolarimetric data on both the dust cloud and the gaseous atmosphere in Halley's coma. The optical probe concept is presented here, together with a description of the instrumentation and with the possibilities for cross-checks between HOPE results and those of other space and ground-based experiments.

Discussion of the Pioneer 11 observations of the F ring of Saturn

The photopolarimetric and charged particle data from the Pioneer 11 Saturn encounter were used as diagnostic tools in a preliminary analysis of the physical nature of the F ring of Saturn. It is argued that the F ring may have two components, with the outer one composed of solid particles of centimetre-diameter and the inner one of micrometre-sized dust particles. Also, the outer component as scanned by the charged particle detectors may have two rings instead of one. The double-ring option, if verified, could have interesting implications for the dynamical origin of the planetary ring systems.