Polarimetric Map Research Articles

Forgotten treasures in the HST/FOC UV imaging polarimetric archives of active galactic nuclei

Context. Over its 13 yr of operation (1990–2002), the Faint Object Camera (FOC) on board the Hubble Space Telescope (HST) observed 26 individual active galactic nuclei (AGNs) in ultraviolet (UV) imaging polarimetry. However, not all of the observations have been reduced and analyzed or set within a standardized framework. Aims. We plan to reduce and analyze the AGN observations that have been neglected in the FOC archives using a consistent, novel, and open-access reduction pipeline of our own. We then extend the method to the full AGN sample, thus leading to potential discoveries in the near future. Methods. We developed a new pipeline in Python that will be able to reduce all the FOC observations in imaging polarimetry in a homogeneous way. Most of the previously published reduced observations are dispersed throughout the literature, with the range of different analyses and approaches making it difficult to fully interpret the FOC AGN sample. By standardizing the method, we have enabled a coherent comparison among the different observational sets. Results. In this first paper of a series exploring the full HST/FOC AGN sample, we present an exhaustively detailed account of how to properly reduce the observational data. Current progress in cross-correlation functions, convolution kernels, and a sophisticated merging and smoothing of the various polarization filter images, together with precise propagation of errors, has provided state-of-the-art UV polarimetric maps. We compare our new maps to the benchmark AGN case of NGC 1068 and successfully reproduce the main results previously published, while pushing the polarimetric exploration of this AGN futher, thanks to a finer resolution and a higher signal-to-noise ratio (S/N) than previously reported. We also present, for the first time, an optical polarimetric map of the radio-loud AGN IC 5063 and we examine the complex interactions between the AGN outflows and the surrounding interstellar medium (ISM). Conclusions. Thanks to our newly and standardized reduction pipeline, we were able to explore the full HST/FOC AGN sample, starting with observations that had not been previously published (e.g., IC 5063 here). This pipeline will allow us to make a complete atlas of UV polarimetric images of the 26 unique AGNs observed by the FOC, highlighting the importance and necessity of (imaging) polarimeters for the upcoming new generation of 30-m class telescopes.

Persymmetric Adaptive Target Detection With Dual-Polarization in Compound Gaussian Sea Clutter With Inverse Gamma Texture

This paper investigates the dual-polarimetric target detection problem in non-Gaussian sea clutter. The compound Gaussian model with inverse Gamma texture is adopted to describe the real sea clutter of which characteristics are time-varying and heterogeneous. Three novel polarimetric detectors are proposed based on the two-step maximum <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a posteriori</i> (MAP) generalized likelihood ratio test (GLRT), MAP Rao, and MAP Wald criteria. Specifically, we assume the polarimetric clutter covariance matrix (PCCM) and the inverse Gamma textures are known in the first step, and the test statistics of the proposed polarimetric detectors are derived. In the second step, we take advantage of the proposed polarimetric persymmetric property and MAP criterion to estimate the PCCM and inverse Gamma textures, respectively. The fully adaptive persymmetric polarimetric detectors are obtained by substituting the estimates into the test statistics of the proposed detectors. Then the proposed detectors are proved to have the constant false alarm rate (CFAR) properties with respect to the real PCCM. The performance assessments are evaluated by contrasting the proposed detectors with their counterparts on the simulated and measured sea clutter data. The numerical results verify the performance of the proposed persymmetric polarimetric detectors.

PolSAR Image Classification Based on Low-Frequency and Contour Subbands-Driven Polarimetric SENet

In order to more efficiently mine the features of PolSAR images and build a more suitable classification model that combines the features of the polarimetric domain and the spatial domain, this article proposes a PolSAR image classification method, called low-frequency and contour subbands-driven polarimetric squeeze-and-excitation network (LC-PSENet). First, the proposed LC-PSENet introduces the nonsubsampled Laplacian pyramid to decompose polarimetric feature maps, so as to construct a multichannel PolSAR image based on the low-frequency subband and contour subband of these maps. It guides the network to perform feature mining and selection in the subbands of each polarimetric map in a supervised way, automatically balancing the contributions of polarimetric features and their subbands and the influence of interference information such as noise, making the network learning more efficient. Second, the method introduces squeeze-and-excitation operation in the convolutional neural network (CNN) to perform channel modeling on the polarimetric feature subbands. It strengthens the learning of the contributions of local maps of the polarimetric features and subbands, thereby, effectively combining the features of the polarimetric domain and the spatial domain. Experiments on the datasets of Flevoland, The Netherlands, and Oberpfaffenhofen show that the proposed LC-PSENet achieves overall accuracies of 99.66%, 99.72%, and 95.89%, which are 0.87%, 0.27%, and 1.42% higher than the baseline CNN, respectively. The isolated points in the classification results are obviously reduced, and the distinction between boundary and nonboundary is more clear and delicate. Also, the method performs better than many current state-of-the-art methods in terms of classification accuracy.

Unsupervised polarimetric synthetic aperture radar image classification based on sketch map and adaptive Markov random field

Markov random field (MRF) model is an effective tool for polarimetric synthetic aperture radar (PolSAR) image classification. However, due to the lack of suitable contextual information in conventional MRF methods, there is usually a contradiction between edge preservation and region homogeneity in the classification result. To preserve edge details and obtain homogeneous regions simultaneously, an adaptive MRF framework is proposed based on a polarimetric sketch map. The polarimetric sketch map can provide the edge positions and edge directions in detail, which can guide the selection of neighborhood structures. Specifically, the polarimetric sketch map is extracted to partition a PolSAR image into structural and nonstructural parts, and then adaptive neighborhoods are learned for two parts. For structural areas, geometric weighted neighborhood structures are constructed to preserve image details. For nonstructural areas, the maximum homogeneous regions are obtained to improve the region homogeneity. Experiments are taken on both the simulated and real PolSAR data, and the experimental results illustrate that the proposed method can obtain better performance on both region homogeneity and edge preservation than the state-of-the-art methods.

MAGNETIC FIELD CONFIGURATION AT THE GALACTIC CENTER INVESTIGATED BY WIDE FIELD NEAR-INFRARED POLARIMETRY

We present a polarimetric map of a 20' × 20' area toward the Galactic center. The polarization of point sources has been measured in the J, H, and KS bands using the near-infrared polarimetric camera SIRPOL on the 1.4 m telescope IRSF. One percent or better accuracy of polarization degree is achieved for sources with J < 14.5, H < 13.5, and KS < 12.0. Comparing the Stokes parameters between high extinction stars and relatively low extinction ones, we have obtained a polarization originating from magnetically aligned dust grains at the central region of our Galaxy of at most 1–2 kpc. The distribution of the position angles shows a peak at ∼20°, nearly parallel to the Galactic plane, suggesting a toroidal magnetic configuration. The derived direction of the magnetic field is in good agreement with that obtained from far-infrared/submillimeter observations, which detect polarized thermal emission from dust in the molecular clouds at the Galactic center. Our results show that, by subtracting foreground components, near-infrared polarimetry allows investigation of the magnetic field structure at the Galactic center.

AO-assisted observations of G61.48+0.09

The characterisation of the stellar populations of ultra-compact Hii (UCHii) regions is one of the key means of understanding the formation and evolution of massive stars. Adaptive Optics (AO) assisted near-infrared (NIR) observations provide sufficient resolution and sensitivity to detect such populations at moderate extinction values. We present NIR high-resolution observations of G61.48+0.09, a morphologically complex UCHii region with two components, whose accessible stellar content in the NIR has been widely studied before. A polarimetric map in the K' band, as well as H 2 (1–0) S(1) and Br γ narrow-band images of the region have been obtained with the AO system ALFA at the Calar Alto Observatory's 3.5 m telescope. We also present high-resolution imaging of the same region in the L' band with NAOS+CONICA at the VLT (UT4). The study of the nebular scattered light points to an internal-illumination model for the eastern component (B2). This model is confirmed by the detection of a point-like source in the L' -band data. In the western component (B1) the observed NIR colours of star 82 are consistent with spectral types BIa or early OV, while star 83 is reproduced by a B0V. Therefore, we identify these objects as the main detected ionising sources. However, in terms of illumination, our polarimetric map indicates that these two stars are not the dominating illuminators of the reflection nebula. Illumination from multiple sources (possibly still undetected) seems to influence the polarisation pattern for this component. Geometric considerations in the calculation of the energetics of this region, combined with the knowledge of the detected stellar content, also points to the presence of other ionising sources. Our study reinforces the hypothesis that there is a champagne flow towards the south-west part of the region.

Polarimetric evolution of V838 Monocerotis

We present the results of our polarimetric and spectropolarimetric monitoring of V838 Monocerotis, performed at the Asiago and Crimean observatories during and after the multiple outbursts that occurred in January-March 2002. The polarization of the object is mainly due to interstellar polarization (P ∼ 2.48%). Intrinsic polarization up to ∼0.7% at 5000 A is present during the second maximum of the object (February 2002). This intrinsic component increases toward shorter wavelengths but our limited spectral coverage (5000-7500 A) does not allow conclusive inferences about its origin. A strong depolarization across the H α profile is observed. The interstellar polarization gives a lower limit to the reddening of E(B - V) > 0.28, with E(B - V) ∼ 0.5 being the most probable value. A normal ratio of total to selective absorption (R v = 3.22 ± 0.17) was derived from the wavelength of maximum interstellar polarization. This suggests a low (if any) contribution by circumstellar material with a peculiar dust to gas ratio. A polarimetric map of a portion of the light echo shows a complex polarization distribution reaching P max = 45%.

ITAL]Infrared Space Observatory[/ITAL] Polarimetric Imaging of the Egg Nebula (RAFGL 2688)

We present polarimetric imaging of the protoplanetary nebula RAFGL 2688 obtained at 4.5 microns with the Infrared Space Observatory (ISO). We have deconvolved the images to remove the signature of the point spread function of the ISO telescope, to the extent possible. The deconvolved 4.5 micron image and polarimetric map reveal a bright point source with faint, surrounding reflection nebulosity. The reflection nebula is brightest to the north-northeast, in agreement with previous ground- and space-based infrared imaging. Comparison with previous near-infrared polarimetric imaging suggests that the polarization of starlight induced by the dust grains in RAFGL 2688 is more or less independent of wavelength between 2 microns and 4.5 microns. This, in turn, indicates that scattering dominates over thermal emission at wavelengths as long as ~5 microns, and that the dust grains have characteristic radii < 1 micron.

Broken symmetry: The structure of the dust envelope of IRC +10216

view Abstract Citations (56) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Broken Symmetry: The Structure of the Dust Envelope of IRC +10216 Kastner, Joel H. ; Weintraub, David A. Abstract We present polarimetric, coronagraphic near-infrared images of the reflection nebula surrounding the carbon star IRC +10216. These images demonstrate convincingly that the dust envelope is not spherically symmetric. In unocculted images at J (1.25 micrometers) and in occulted images at H (1.65 micrometers) and K (2.2 micrometers), the nebula appears elongated, with major axis lying at position angle approximately 20 deg. The position angle of elongation is the same as that previously determined from near-infrared speckle interferometry for the inner approximately 1 sec of the envelope. The elliptical symmetry of the J polarimetric map offers additional evidence of the axial symmetry of the envelope and indicates that the equatorial plane of IRC +10216 lies perpendicular to the major axis of the nebula. The polarization map also indicates that the source of illumination -- presumably the stellar photosphere or hot dust located very near the photosphere -- is directly detected at wavelengths as short as approximately 1 micrometers. We find that the density distribution of grains in the polar regions is roughly n varies as r-2 and estimate that the scattering dust mass within 12 sec in radius of the central star is approximately 5 x 10-6 solar masses. These results support a model in which the envelope of IRC +10216 is weakly bipolar and is viewed at an intermediate inclination angle such that we have a direct line of sight to the central star. The axisymmetric near-infrared intensity and polarization morphologies are best understood in terms of enhanced mass loss in the equatorial plane. An examination of previous near-infrared and millimeter-wave mapping observations supports this hypothesis. These observations make clear that axisymmetric structure can be well established before an intermediate-mass star leaves the asymptotic giant branch. It is possible that the dust envelope of IRC +10216 presents an early manifestation of the more profound bipolar structure that characterizes highly evolved preplanetary nebulae. Publication: The Astrophysical Journal Pub Date: October 1994 DOI: 10.1086/174774 Bibcode: 1994ApJ...434..719K Keywords: Broken Symmetry; Carbon Stars; Cosmic Dust; Density Distribution; Infrared Astronomy; Optical Polarization; Reflection Nebulae; Stellar Envelopes; Astronomical Interferometry; Astronomical Polarimetry; Coronagraphs; Infrared Imagery; Stellar Evolution; Astronomy; ISM: DUST; EXTINCTION; ISM: STRUCTURE; POLARIZATION; ISM: REFLECTION NEBULAE; STARS: CARBON; STARS: INDIVIDUAL ALPHANUMERIC: IRC; 10216 full text sources ADS | data products SIMBAD (6)

A polarimeter for monochromatic studies of solar prominences

The photoelectric polarimeter which is described associates the three major discoveries of B. Lyot: the coronagraph, the birefringent filter and the polarimeter. However it differs completely from the coronameter. It is possible to study many emission lines by tuning the birefringent filter. But the D3 helium line is mainly used. In 10 s of time, the polarimeter measures in a 5 diameter spot three quantities: monochromatic intensity, polarization ratio p, angular deviation from the limb: e. The instrument gives a polarimetric map of the prominence. Owing to its performances, it is possible to study fast moving objects such as active prominences.