Sign Of Circular Polarization Research Articles

Sign reversals of the polarization of the microwave emission of sunspot radio sources

Thermal cyclotron emission features from a sunspot-associated source in a solar active region are considered in the framework of a three-level temperature model with a hot layer. The images of the source have a complex fine structure, with rings with different brightness temperatures and different signs of circular polarization. The proposed model suggests the possibility of a double or multiple reversal of the sign of polarization, as well as a significant increase of the fluxes in the 3–4 cm range, as is typical of active regions associated with proton flares. Energetic particles of the halo cannot provide the required temperatures of the layers. Alternative heating mechanisms are discussed.

Observation and Modeling of Anomalous CN Polarization Profiles Produced by the Molecular Paschen-Back Effect in Sunspots

We report novel spectropolarimetric observations of sunspots carried out with the Tenerife Infrared Polarimeter in a near-IR spectral region around 15410 A, which is known to contain two groups of prominent OH lines that show circular polarization signals of opposite polarity. Surrounding these well-known OH lines, we have discovered the presence of CN lines of the Δv = 1 band that show anomalous polarization profiles. Although the Stokes V signals of the OH lines are antisymmetric and of a sizable amplitude, the CN lines show almost negligible circular polarization. On the contrary, the linear polarization signals turn out to be much stronger in the CN lines than in the OH lines. Interestingly, these CN lines present striking antisymmetric linear polarization profiles, which we are able to explain and model via the Paschen-Back effect theory for diatomic molecules. The presence of such peculiar CN lines in the same spectral region of the OH lines may be useful to improve our empirical knowledge of solar magnetic fields via the simultaneous observation and modeling of the transverse and longitudinal Zeeman effects in two different molecular species.

Electron Spin Filtering in the GaAs∕AlGaAs Interface Space Charge Field

The circularly polarized recombination radiation produced by optically oriented electrons in GaAs and viewed in the direction of and opposite to the pump light propagation was found to have opposite signs of polarization. The excitation was effected by photons of energy E h ν ∪ E g + Δ through a transparent AlGaAs win- dow. The opposite signs of circular polarization and its complex dependence on the luminescence wavelength are accounted for by the influence of the space charge field created by the depleted layer near the interface. © 2005 Pleiades Publishing, Inc.

Magnetic fields in Herbig Ae stars

Herbig Ae stars are young A-type stars in the pre-main sequence evolutionary phase with masses of ~1.5–3 $M_\odot$. They show rather intense surface activity (Dunkin et al. [CITE], MNRAS, 290, 165) and infrared excess related to the presence of circumstellar disks. Because of their youth, primordial magnetic fields inherited from the parent molecular cloud may be expected, but no direct evidence for the presence of magnetic fields on their surface, except in one case (Donati et al. [CITE], MNRAS, 291, 658), has been found until now. Here we report observations of optical circular polarization with FORS 1 at the VLT in the three Herbig Ae stars HD 139614, HD 144432 and HD 144668. A definite longitudinal magnetic field at 4.8 σ level, $\left =-450\pm93$ G, has been detected in the Herbig Ae star HD 139614. This is the largest magnetic field ever diagnosed for a Herbig Ae star. A hint of a weak magnetic field is found in the other two Herbig Ae stars, HD 144432 and HD 144668, for which magnetic fields are measured at the ~1.6 σ and ~2.5 σ level respectively. Further, we report the presence of circular polarization signatures in the $\ion{Ca}{ii}$ K line in the V Stokes spectra of HD 139614 and HD 144432, which appear unresolved at the low spectral resolution achievable with FORS 1. We suggest that models involving accretion of matter from the disk to the star along a global stellar magnetic field of a specific geometry can account for the observed Zeeman signatures.

Observation of the Molecular Zeeman Effect in the G Band

Here we report on the first observational investigation of the Zeeman effect in the G band around 4305 A. Our spectropolarimetric observations of sunspots with the Zurich Imaging Polarimeter at the Istituto Ricerche Solari Locarno confirm our previous theoretical prediction that the molecular Zeeman effect produces measurable circular polarization signatures in several CH lines that are not overlapped with atomic transitions. We also find both circular and linear polarization signals produced by atomic lines whose wavelengths lie in the G-band spectral region. Together, such molecular and atomic lines are potentially important for empirical investigations of solar and stellar magnetism. For instance, a comparison between observed and calculated Stokes profiles suggests that the thermodynamical and/or magnetic properties of the photospheric regions of sunspot umbrae are horizontally structured with a component that might be associated with umbral dots.

Optical vortices in rotating weakly guiding ideal optical fibres

The structure of l = 1 modes of a weakly guiding ideal optical fibre rotating around its symmetry axis is studied. It is demonstrated that rotation brings forth a novel mode structure in which two circular optical vortices with coinciding signs of circular polarization and topological charge are the eigenmodes propagating with different phase velocities. Rotation-induced corrections to the propagation constants of these vortices are calculated in the first order in angular velocity. The absence of the rotational Doppler effect for optical vortices in such fibres and the implementation of the linear Doppler effect is demonstrated.

The self-inversion of the sign of circular polarization in “halo” microwave sources

The experimental and theoretic investigation of the halo (a detail of local source structure genetically associated with an active region magnetosphere) is presented by the example of the AR 5200 (October, 1988). Accordingly to the high space resolution observations of RATAN-600 and SSRT the emission of this region was almost completely (%) consist of halo emission. The peculiarities of the observed halo spectrum and polarization with the model explaining these peculiarities are described. It is shown that the inversion phenomenon for the halo is strongly affected by the propagation conditions influence on the polarized emission inside the large source of this emission. The term ‘self-inversion’ is introduced for this effect. Maximum in the halo spectrum at the wavelengths 5–10 cm is clarified by the scattering and by the strong suppression of the emissivity of non-thermal electrons at these and more longer wavelengths.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The Zeeman Effect in the G Band

We investigate the possibility of measuring magnetic field strength in G-band bright points through the analysis of Zeeman polarization in molecular CH lines. To this end we solve the equations of polarized radiative transfer in the G band through a standard plane-parallel model of the solar atmosphere with an imposed magnetic field and through a more realistic snapshot from a simulation of solar magnetoconvection. This region of the spectrum is crowded with many atomic and molecular lines. Nevertheless, we find several instances of isolated groups of CH lines that are predicted to produce a measurable Stokes V signal in the presence of magnetic fields. In part this is possible because the effective Lande factors of lines in the stronger main branch of the CH A2Δ-X2Π transition tend to zero rather quickly for increasing total angular momentum J, resulting in a Stokes V spectrum of the G band that is less crowded than the corresponding Stokes I spectrum. We indicate that, by contrast, the effective Lande factors of the R and P satellite subbranches of this transition tend to ±1 for increasing J. However, these lines are in general considerably weaker and do not contribute significantly to the polarization signal. In one wavelength location near 430.4 nm, the overlap of several magnetically sensitive and nonsensitive CH lines is predicted to result in a single-lobed Stokes V profile, raising the possibility of high spatial resolution narrowband polarimetric imaging. In the magnetoconvection snapshot we find circular polarization signals of the order of 1%, prompting us to conclude that measuring magnetic field strength in small-scale elements through the Zeeman effect in CH lines is a realistic prospect.

Detection of Polarization from the E 4 - A 4 System of FeH in Sunspot Spectra

Here we report the first detection of polarization signals induced by the Zeeman effect in spectral lines of the E 4Π-A 4Π system of FeH located around 1.6 μm. Motivated by the tentative detection of this band in the intensity spectrum of late-type dwarfs, we have investigated the full Stokes sunspot spectrum and have found circular and linear polarization signatures that we associate with the FeH lines of the E 4Π-A 4Π band system. We investigate the Zeeman effect in these molecular transitions and point out that in Hund's case (a) coupling, the effective Lande factors are never negative. For this reason, the fact that our spectropolarimetric observations indicate that the Lande factors of pairs of FeH lines have opposite signs prompts us to conclude that the E 4Π-A 4Π system must be in intermediate angular momentum coupling between Hund's cases (a) and (b). We emphasize that theoretical and/or laboratory investigations of this molecular system are urgently needed for exploiting its promising diagnostic capabilities.

High Levels of Circularly Polarized Emission from the Radio Jet in NGC 1275 (3C 84)

We present multifrequency, high-resolution, Very Long Baseline Array circular polarization images of the radio source 3C 84 in the center of NGC 1275. Our images reveal a complex distribution of circular polarization in the inner parsec of the radio jet, with local levels exceeding 3% polarization, the highest yet detected with VLBI techniques. The circular polarization changes sign along the jet, making 3C 84 also the first radio jet to show both signs of circular polarization simultaneously. The spectrum and changing sign of the circular polarization indicate that it is unlikely to be purely intrinsic to the emitted synchrotron radiation. The Faraday conversion process makes a significant and perhaps dominant contribution to the circular polarization, and the observed spectrum suggests that the conversion process is near saturation. The sign change in the circular polarization along the jet may result from this saturation or may be due to a change in magnetic field order after an apparent bend in the jet. From the small spatial scales probed here, 0.15 pc, and the comparably high levels of circular polarization inferred for the intraday variable source PKS 1519-273, we suggest a connection between small spatial scales and efficient production of circular polarization.

Rotation of the speckle pattern in an absorbing optical fiber when changing the circular-polarization sign of propagating light

Rotation of the speckle pattern in an absorbing optical fiber when changing the circular-polarization sign of propagating light

Linear and Circular Polarization Properties of Jets

I discuss the transfer of polarized synchrotron radiation in relativistic jets. I argue that the main mechanism responsible for the circular polarization properties of compact synchrotron sources is likely to be Faraday conversion and that, contrary to common expectation, a significant rate of Faraday rotation does not necessarily imply strong depolarization. The long-term persistence of the sign of circular polarization, observed in some sources, is most likely due to a small net magnetic flux generated in the central engine, carried along the jet axis and superimposed on a highly turbulent magnetic field. I show that the mean levels of circular and linear polarizations depend on the number of field reversals along the line of sight and that the gradient in Faraday rotation across turbulent regions can lead to`correlation depolarization'. The model is potentially applicable to a wide range of synchrotron sources. In particular, I demonstrate how the model can naturally explain the excess of circular over linear polarization in the Galactic Center (SgrA*) and the low-luminosity AGN M81*.

Measuring magnetic fields of early-type stars with $\vec{FORS{\it 1}}$ at the $\vec{VLT}$

This paper describes an experiment aimed at evaluating the capability of the FORS1 instrument at the VLT for measuring weak (0.1%) Zeeman circular polarization signatures in stellar H Balmer lines. We have obtained low-resolution polarized spectra at 3500–5800 Å of two bright A-type stars, HD 94660, and HD 96441. The former is a well known magnetic star, the latter an apparently non-magnetic, unpolarized star that was observed for comparison purposes. In order to test the possibility of performing multi-object spectropolarimetric measurements (e.g., for cluster studies), observations were taken both on-axis (i.e., at the CCD field center) and off-axis (i.e., at the edges of the CCD). In HD 94660 (the magnetic star), we detected a clear signal of circular polarization in all observed hydrogen Balmer lines (i.e., from Hβ to the Balmer limit), with a typical peak-to-peak amplitude of about 0.8%. From the analysis of Stokes V under the weak-field approximation, we have estimated a mean longitudinal field of G, a value fully consistent with previous studies of this star. Notably, we found that at wavelengths shorter than about 4100 Å, the polarization signal detected off-axis is about 20–30% smaller than that detected on-axis. No polarization due to the Zeeman effect was detected in the comparison star HD 96441, and we estimate that the contribution due to instrumental circular polarization is limited to the order of . This experiment demonstrates the effectiveness of FORS1 at the VLT as a tool for high-sensitivity diagnosis of the magnetic field in upper main sequence stars, providing the potential for measuring fields in fainter and more rapidly rotating stars than has previously been possible.

Circular Polarization from Stochastic Synchrotron Sources

The transfer of polarized radiation in stochastic synchrotron sources is explored by means of analytic treatment and Monte Carlo simulations. We argue that the main mechanism responsible for the circular polarization properties of compact synchrotron sources is likely to be Faraday conversion and that, contrary to common expectation, a significant rate of Faraday rotation does not necessarily imply strong depolarization. The long-term persistence of the sign of circular polarization, observed in many sources, is most likely due to a small net magnetic flux generated in the central engine, carried along the jet axis and superimposed on a highly turbulent magnetic field. We show that the mean levels of circular and linear polarizations depend on the number of field reversals along the line of sight and that the gradient in Faraday rotation across turbulent regions can lead to ``correlation depolarization''. Our model is potentially applicable to a wide range of synchrotron sources. In particular, we demonstrate how our model can naturally explain the excess of circular over linear polarization in the Galactic Center and the nearby spiral galaxy M81 and discuss its application to the quasar 3C 279, the intraday variable blazar PKS 1519-273 and the X-ray binary SS 433.

Influence of Rotation on Pulsar Radiation Characteristics

We present a relativistic model for pulsar radio emission by including the effect of rotation on coherent curvature radiation by bunches. We find that rotation broadens the width of the leading component compared to the width of the trailing component. We estimate the component widths in the average pulse profiles of about 24 pulsars and find that 19 of them have a broader leading component. We explain this difference in component widths by using the nested cone emission geometry. We estimate the effect of pulsar spin on the Stokes parameters and find that the inclination between the rotation and magnetic axes can introduce an asymmetry in the circular polarization of the conal components. We analyze the single-pulse polarization data of PSR B0329+54 at 606 MHz and find that in its conal components, one sign of circular polarization dominates in the leading component, while the other sign dominates in the trailing component. Our simulation shows that changing the sign of the impact parameter changes the sign of the circular polarization as well as the swing of the polarization angle.

LB 8827: A Variable, Magnetic DBA White Dwarf

We present accurate CCD spectropolarimetry and spectra that show that the hot DB star LB 8827 (PG 0853+164) is a weakly magnetic, variable DBA white dwarf. We confirm Putney's detection of circular dichroism in the He I lines, but on three occasions we measure a weaker field strength and opposite sign of circular polarization, Be 100 kG. The Balmer line strengths appear to vary with time, indicative of changes in the apparent H/He abundance ratio or irregularities over the stellar surface. Fits to the observed energy distribution and to the optical He I line spectrum with model atmospheres yield effective temperatures in the 21,200-27,700 K range, with lower values favored. This suggests that LB 8827 falls within or near the instability strip of the V777 Herculis stars. High time resolution photometric observations presented here rule out white-light brightness variations with amplitudes larger than 1.2 mmag in the period window 20 s to 2 hr. The possibility that the magnetic field might tend to suppress pulsations is discussed.

Observation of Linear Polarization in the Infrared CaiiTriplet Lines during Umbral Flashes

We report on Telescope Heliographique pour l'Etude du Magnetisme et des Instabilites Solaires (THEMIS) observations of linear polarization events associated with umbral flashes observed in the Ca II infrared (IR) triplet lines. The observed signals are usually delayed in time and shifted in space when compared to the intensity and circular polarization signals from the flash. The observations are compatible with a scenario whereby flashes are produced by a perturbation propagating along the magnetic field lines as they bend out toward the penumbra. Only a fraction of the resolution element appears to be emitting flashlike profiles, as if the waves were propagating only within localized magnetic field lines. This localization, however, does not impede the apparent propagation of the perturbation horizontally within the umbra.

Parsec‐Scale Circular Polarization Observations of 40 Blazars

We present circular polarization results from a 5 GHz survey of the parsec-scale polarization properties of 40 AGN made with the VLBA. We find 11 circular polarization detections at the 3 sigma level or higher. This nearly quadruples the number of sources detected in circular polarization at VLBI resolution. We find no correlation between fractional linear and circular polarization across our sample. A likely explanation is external Faraday depolarization in the cores of AGN which reduces linear polarization but leaves circular polarization unchanged. In comparing ours and other recent results to observations made 20 years ago, we find that, in five of six cases, sources have the same sign of circular polarization today as they did 20 years ago. This suggests the presence of a long term property of the jets, such as the polarity of a net magnetic flux, which is stable on time-scales much longer than those of individual outbursts.

Spectropolarimetric measurements of magnetic Ap and Bp stars in all four Stokes parameters

In this paper we begin an exploration of the potential of spectral line Zeeman linear and circular polarization signatures for reconstructing the surface magnetic field topologies of magnetic Ap and Bp stars. We present our first observational results, which include the very first high-quality measurements of stellar Zeeman spectral line linear polarization ever obtained. Using the new MuSiCoS spectropolarimeter at the Pic du Midi Observatory, over 360 spectra were obtained, in circular or linear polarization, of 14 magnetic Ap/Bp stars and six calibration objects. Zeeman circular polarization signatures are detected in most single lines in essentially all spectra of magnetic Ap stars, with typical relative amplitudes of a few per cent. Linear polarization Zeeman signatures are unambiguously detected in individual strong, magnetically sensitive lines in outstanding spectra of five objects. However, linear polarization is generally not detected in individual strong lines in our much more common moderate signal-to-noise ratio (S/N) spectra, and is essentially never detected in weak lines. In order to overcome the limitations imposed by the S/N ratio and the inherent weakness of linear polarization Zeeman signatures, we exploit the information contained in the many lines in our spectra by using the least-squares deconvolution (LSD) technique. Using LSD, mean linear polarization signatures are consistently detected within the spectral lines of 10 of our 14 programme stars. These mean linear polarization signatures are very weak, with typical amplitudes 10–20 times smaller than those of the associated mean circular polarization signatures. For 11 stars full or partial rotational phase coverage has been obtained in the Stokes I and V or the Stokes I, V, Q and U parameters. The rotational modulation of the LSD mean signatures is reported for these objects. Measurements of the longitudinal field and net linear polarization obtained from these LSD profiles show they are consistent with existing comparable data, and provide constraints on magnetic field models which are at least as powerful as any other data presently available. To illustrate the new information available from these data sets, we compare for four stars the observed Stokes profiles with those predicted by magnetic field models published previously in the literature. Important and sometimes striking differences between the observed and computed profiles indicate that the Zeeman signatures presented here contain important new information about the structure of the magnetic fields of Ap and Bp stars capable of showing the limitations of the best magnetic field models currently available.

Circular polarization in pulsar integrated profiles

We present a systematic study of the circular polarization in pulsar integrated profiles, based on published polarization data. For core components, we find no significant correlation between the sense change of circular polarization and the sense of linear position-angle variation. Circular polarization is not restricted to core components and, in some cases, reversals of circular polarization sense are observed across the conal emission. In conal double profiles, the sense of circular polarization is found to be correlated with the sense of position-angle variation. Pulsars with a high degree of linear polarization often have one hand of circular polarization across the whole profile. For most pulsars, the sign of circular polarization is the same at 50-cm and 20-cm wavelengths, and the degree of polarization is similar, albeit with a wide scatter. However, at least two cases of frequency-dependent sign reversals are known. This diverse behaviour may require more than one mechanism to generate circular polarization.