A Revised View of the Linear Polarization in the Subparsec Core of M87 at 7 mm

The linear polarization images of the jet in the giant elliptical galaxy M87 have previously been observed with Very Long Baseline Array at 7 mm. They exhibit a complex polarization structure surrounding the optically thick and compact subparsec-scale core. However, given the low level of linear polarization in the core, it is required to verify that this complex structure does not originate from residual instrumental polarization signals in the data. We have performed a new analysis of the same data sets observed in four epochs by using the Generalized Polarization CALibration pipeline (GPCAL). This novel instrumental polarization calibration pipeline overcomes the limitations of LPCAL, a conventional calibration tool used in the previous M87 studies. The resulting images show a compact linear polarization structure with its peak nearly coincident with the total intensity peak, which is significantly different from the results of previous studies. The core linear polarization is characterized as fractional polarization of ∼0.2%–0.6% and polarization angles of ∼66°–92°, showing moderate variability. We demonstrate that, based on tests with synthetic data sets, LPCAL using calibrators having complex polarization structures cannot achieve sufficient calibration accuracy to obtain the true polarization image of M87 due to a breakdown of the “similarity approximation.” We find that GPCAL obtains more accurate D-terms than LPCAL by using observed closure traces of calibrators that are insensitive to both antenna gain and polarization leakage corruptions. This study suggests that polarization imaging of very weakly polarized sources has become possible with the advanced instrumental polarization calibration techniques.

Scan Mirror Assembly for the Multispectral Scanning Polarimeter of Aerosol-UA Space Mission

In this paper, the scan mirror assembly for the space experiment Aerosol-UA scanning polarimeter (ScanPol) is described. The aim of the Ukrainian space mission Aerosol-UA is to create a database of the optical characteristics of aerosol and cloud particles in the Earth’s atmosphere over a long period of time. The optical characteristics of aerosol and cloud particles are derived from multiangular measurements. Multiangular scanning in ScanPol is provided by scan mirror assembly, which contains a reactive torque compensator electric motor and two scan mirrors, mounted on the shaft of the motor. The control system of the scan mirror assembly enables continuous scanning with a constant speed of the space under investigation. This control system tolerates movements of the orbiting satellite and preserves invariability of its spatial position. The polarimeter ScanPol is designed to acquire spatial, temporal, and spectral-polarimetric measurements simultaneously to minimize instrumental effects and “false” polarizations due to scene movement. Instrumental polarization, introduced by the mirrors of scan assembly, is minimized through the polarization compensated two-mirror scheme which contains two mirrors with orthogonal planes of incidence. In this paper, the polarimetric model of the polarization compensated two scan mirrors is considered. Theoretical calculations are given that substantiate the maximum allowable error of the relative angular position of the mirrors is 15 arcmin (0.25°), and the method of adjustment and control of the angular position of the mirrors is proposed. The polarization properties of mirrors are modelled in the spectral range of 370–1680 nm for bulk oxide-free aluminum. It is obtained that the maximum instrumental polarization of the unadjusted mirror system should be observed at 865 nm, and thus, the polarization characteristics of the scanning system at a given wavelength could be considered as representative for ScanPol in general. The key steps for assembling the unit are illustrated.

Master equation theory applied to the redistribution of polarized radiation in the weak radiation field limit

Context. This paper presents a numerical application of a self-consistent theory of partial redistribution in nonlocal thermodynamical equilibrium conditions, developed in previous papers of the series. Aims. The code was described in IV of this series. However, in that previous paper, the numerical results were unrealistic. The present paper presents an approximation able to restore the reliability of the outgoing polarization profiles. Methods. The convergence of the results is also proved. It is demonstrated that the step increment decreases like 1/Nα, with α > 1. Results. Thanks to these additions, the results series behaves like a Riemann series, which is absolutely convergent. However, convergence is not fully reached in line wings within the allocated computing time. Development of efficient acceleration methods would be desirable for future work. Conclusions. Agreement between the computed and observed linear polarization profiles remains qualitative only. The discrepancy is assigned to the plane parallel atmosphere model, which is insufficient to describe the chromosphere, where these lines are formed. As all the integrals are numerical in the code, it could probably be adapted to more realistic and higher dimensional model atmospheres. However, this is time consuming for lines with a hyperfine structure, as in the Na I D lines. The net linear polarization observed in Na I D1 with the Zürich Imaging Polarimeter ZIMPOL mounted on the McMath-Pierce telescope at Kitt Peak is not confirmed by the present calculations and could be an artefact of instrumental polarization. The presence of instrumental polarization could be confirmed by the higher linear polarization degree observed by this instrument in the Na I D2 line center with respect to the present calculation result where the magnetic field is not accounted for. At this precise point, the Hanle effect acts as a depolarizing effect in the second solar spectrum. The observed linear polarization excess is found to be of the same order of magnitude in both line centers, namely 0.1%, which is also comparable to the instrumental polarization compensation level of this experiment.

Characterization of a dual-beam, dual-camera optical imaging polarimeter

ABSTRACT Polarization plays an important role in various time-domain astrophysics to understand the magnetic fields, geometry, and environments of spatially unresolved variable sources. In this paper we present the results of laboratory and on-sky testing of a novel dual-beam, dual-camera optical imaging polarimeter (MOPTOP) exploiting high sensitivity, low-noise CMOS technology, and designed to monitor variable and transient sources with low systematic errors and high sensitivity. We present a data reduction algorithm that corrects for sensitivity variations between the cameras on a source-by-source basis. Using our data reduction algorithm, we show that our dual-beam, dual-camera technique delivers the benefits of low and stable instrumental polarization (<0.05 per cent for lab data and <0.25 per cent for on sky data) and high throughput while avoiding the additional sky brightness and image overlap problems associated with dual-beam, single-camera polarimeters.

Tips and tricks in linear imaging polarimetry of extended sources with FORS2 at the VLT

Context. Polarimetry is a very powerful tool for uncovering various properties of astronomical objects that otherwise remain hidden in standard imaging or spectroscopic observations. While common observations only measure the intensity of light, polarimetric measurements allow us to distinguish and measure the two perpendicular components of the electric field associated with the incoming light. By using polarimetry it is possible to unveil asymmetries in supernova explosions, properties of intervening dust, characteristics of atmosphere of planets, among others. However, the reliable measurement of the low polarization signal from astronomical sources requires a good control of spurious instrumental polarization induced by the various components of the optical system and the detector. Aims. We perform a detailed multi-wavelength calibration study of the FORS2 instrument at the VLT operating in imaging polarimetric mode to characterize the spatial instrumental polarization that may affect the study of extended sources. Methods. We used imaging polarimetry of high signal-to-noise ratio blank field BVRI observations during the full moon, when the polarization is expected to be constant across the field of view and deviations originate from the instrument, and a crowded star cluster in broad-band RI and narrow-band Hα filters, where the individual polarization values of each star across the field can be measured. Results. We find an instrumental polarization pattern that increases radially outwards from the optical axis of the instrument reaching up to 1.4% at the edges, depending on the filter. Our results are closely approximated by an elliptical paraboloid down to less than ∼0.05% accuracy, and ∼0.02% when using non-analytic fits. We present 2D maps to correct for this spurious instrumental polarization. We also give several tips and tricks for analyzing polarimetric measurements of extended sources. Conclusions. FORS2 is a powerful instrument that allows the linear polarimetry of extended sources to be mapped. We present and discuss a methodology that can be used to measure the polarization of such sources, and to correct for the spatial polarization induced in the optical system. This methodology could be applied to polarimetric measurements using other dual-beam polarimeters.