Polarization Position Angle Research Articles

“The photopolarimetric activity and circumstellar environment of the T tauri star CO ORI”

We present the results of simultaneous UBVRI photometry and polarimetry of the classical T Tauri star CO Ori carried out at the Crimean Astrophysical Observatory during the 18 years between 1986 and 2004. We show that the variations of linear polarization accompanying the star’s brightness variations follow the law characteristic of UX Ori stars. This suggests that the brightness variations of the star are mainly due to changes of the circumstellar extinction due to non-uniform structure of the circumstellar environment, and to an “optimal” orientation of the circumstellar gas and dust disk relative to the observer, whose line of sight crosses the gas and dust atmosphere of the disk. We determine the star’s intrinsic polarization due to scattering of light in the circumstellar disk. The polarization position angle indicates the orientation of the disk’s symmetry axis in the plane of the sky. Our analysis of an archival light curve for CO Ori confirms the existence of a many-year cycle of photometric activity, suspected by us earlier. The refined period of this cycle is 12.4 years. The existence of such activity cycles of UX Ori stars testifies to considerable deviations of their circumstellar disks from axial symmetry, a reflection of either stellar binarity or the commencement of the process of planetary formation.

Extreme photopolarimetric behaviour of the blazar AO 0235+164

Abstract We present optical photopolarimetric observations with high temporal resolution of the blazar AO0235+164. Our data, the first to test the photopolarimetric behaviour of this object at very short time-scales, show significant micro-variability in total flux, colour index, linear polarization degree and position angle. Strong inter-night variations are also detected for these parameters. Although no correlation between colour index and total flux was found, our data seem to support the general bluer-when-brighter trend already known for this object. The polarization degree, in turn, shows no correlation with total flux, but a clear trend in the sense that colour index is redder (the spectrum is softer) when the measured polarization is higher.

Multiple Scattered Sight Lines to the Red QSO 2MASX J10494334+5837501

The reddened, highly polarized QSO 2MASX J1049334+5837501 (2M 1049+5837) shows a strong IR dust signature with a spectral energy distribution peaked at ~20 μm and a luminosity of ~1011 L☉, but the active nucleus is outshone at optical wavelengths by the host galaxy. Remarkable variations with wavelength in the degree and position angle of polarization require two primary sources of scattered flux from the active nucleus. These are: (1) a reddened component with P ≥ 8% that we identify with material near the axis of a presumed obscuring dust torus in the context of orientation-dependent unification schemes for active galactic nuclei (AGNs); and (2) a faint, very blue source that is only important at λ 4500 A. The latter component is only revealed because of its extremely high polarization (P = 20%-30%) and the fact that flux from the nucleus and the scattering is nearly completely extinguished in this spectral region. High-resolution images show extensions approximately orthogonal to the two position angles of polarization. Consideration of these characteristics suggests refinements to the standard, first-order picture offered by the unified scheme for AGNs. Namely, that there can be multiple scattered sight lines within some AGNs, and that it is possible that the polar scattering lobes can provide sufficient scattering optical depth that the emergent nuclear spectrum is highly reddened. These deviations are useful in explaining the existence of highly polarized, red, type 1 AGNs that form a significant fraction of the QSOs found in near-IR surveys. Importantly, the utility of the type 1/type 2 spectral signature as an indicator of orientation becomes less effective as the dust covering factor increases and nuclear radiation is more effectively scattered into all sight lines.

A survey of polarization in the JVAS/CLASS flat-spectrum radio source surveys - II. A search for aligned radio polarizations

We have used the very large Jodrell Bank VLA Astrometric Survey/Cosmic Lens All-Sky Survey 8.4-GHz surveys of flat-spectrum radio sources to test the hypothesis that there is a systematic alignment of polarization position angle vectors on cosmological scales of the type claimed by Hutsemekers et al. The polarization position angles of 4290 sources with polarized flux density ≥ 1 mJy have been examined. They do not reveal large-scale alignments either as a whole or when split in half into high-redshift (z ≥1.24) and low-redshift subsamples. Nor do the radio sources which lie in the specific areas covered by Hutsemekers et al. show any significant effect. We have also looked at the position angles of parsec-scale jets derived from very long baseline interferometry observations and again find no evidence for systematic alignments. Finally, we have investigated the correlation between the polarization position angle and those of the parsec-scale jets. As expected, we find that there is a tendency for the polarization angles to be perpendicular to the jet angles. However, the difference in jet and polarization position angles does not show any systematic trend in different parts of the sky.

The Magnetic Field Structure of the LMC 2 Supershell: NGC 2100

We present UBVRI imaging polarimetry of NGC 2100 and its surrounding environment, which comprise a part of the LMC 2 supershell. The morphology of the observed position angle distribution provides a tracer of the projected magnetic field in this environment. Our polarization maps detail regions exhibiting similarly aligned polarization position angles, as well as more complex position angle patterns. We observe regions of coherent fields on spatial scales of 42 × 24 to 104 × 83 pc, and infer projected field strengths of ~14-30 μG. We propose that the superposition of global outflows from the LMC 2 environment, as well as outflows created within NGC 2100, produce the unique field geometry in the region.

Absolute broad-band polarization behaviour of PSR B0329+54: a glimpse of the core emission process

In this paper we report multifrequency single-pulse polarization observations of the PSR B0329+54 normal mode using the Giant Metre-wave Radio Telescope at 325 and 610 MHz and the Effelsberg Observatory at 2695 MHz. Our observations show that towards the central part of the polarization position angle traverse there is an unusual ‘arc’-like structure, which comprises a broad-band ‘kink’ and a frequency-dependent ‘spot’. The features are intimately connected with the intensity dependence of the core component: the stronger emission arrives earlier and its linear polarization is displaced farther along the ‘kink’. Moreover, at high intensities, the circular polarization is −/+ antisymmetric; the nearly complete positive circular is characteristic of the weaker, later core subpulses. We find that the ‘kink’ emission is associated with the extraordinary (X) propagation mode, and hence propagation effects do not appear to be capable of producing the core component's broad-band, intensity-dependent emission. Rather, the overall evidence points to a largely geometric interpretation in which the ‘kink’ provides a rare glimpse of the accelerating cascade or height-dependent amplifier responsible for the core radiation.

Mid-infrared polarisation and inferred magnetic field direction toward YSOs with outflow

The position angle of mid-infrared polarisation can be directly related to the magnetic field direction projected onto the plane of the sky. Such observations, from both ground and space-based platforms, have been used to investigate the relation between the magnetic field and other “symmetry axes” associated with embedded young stars, such as the interstellar magnetic field, bipolar outflow and disk/toroid axes. Interpretation of the results in terms of hydromagnetic driving mechanisms of bipolar outflow is discussed.

Multiwaveband Polarimetric Observations of 15 Active Galactic Nuclei at High Frequencies: Correlated Polarization Behavior

We report on multifrequency linear polarization monitoring of 15 active galactic nuclei containing highly relativistic jets with apparent speeds from ~4c to >40c. The measurements were obtained at optical, 1 mm, and 3 mm wavelengths, and at 7 mm with the Very Long Baseline Array. The data show a wide range in degree of linear polarization among the sources, from 30%, and interday polarization variability in individual sources. The polarization properties suggest separation of the sample into three groups with low, intermediate, and high variability of polarization in the core at 7 mm (LVP, IVP, and HVP, respectively). The groups are partially associated with the common classification of active galactic nuclei as radio galaxies and quasars with low optical polarization (LVP), BL Lacertae objects (IVP), and highly optically polarized quasars (HVP). Our study investigates correlations between total flux, fractional polarization, and polarization position angle at the different wavelengths. We interpret the polarization properties of the sources in the sample through models in which weak shocks compress turbulent plasma in the jet. The differences in the orientation of sources with respect to the observer, jet kinematics, and abundance of thermal matter external to the jet near the core can account for the diversity in the polarization properties. The results provide strong evidence that the optical polarized emission originates in shocks, most likely situated between the 3 and 7 mm VLBI cores. They also support the idea that the 1 mm core lies at the edge of the transition zone between electromagnetically dominated and turbulent hydrodynamic sections of the jet.

Polarimetry toward theIRASVela Shell. II. Extinction and Magnetic Fields

We explore correlations between visual extinction and polarization along the western side of the IRAS Vela Shell using a published polarimetric catalog of several hundreds of objects. Our extinction maps along this ionization front (I-front) find evidence of clumpy structure with typical masses between 1.5 and 6 solar masses and a mean length scale L ~ 0.47pc. The polarimetric data allowed us to investigate the distribution of the local magnetic field in small (~pc) scales across the I-front. Using the dispersion of polarization position angles, we find variations in the kinetic-to-magnetic energy density ratio of, at least, one order of magnitude along the I-front, with the magnetic pressure generally dominating over the turbulent motions. These findings suggest that the magnetic component has a significant contribution to the dynamical balance of this region. Along the I-front, the mean magnetic field projected on the sky is [0.018 +/- 0.013]mG. The polarization efficiency seems to change along the I-front. We attribute high polarization efficiencies in regions of relatively low extinction to an optimum degree of grain alignment. Analysis of the mass-to-magnetic flux ratio shows that this quantity is consistent with the subcritical regime (lambda < 1), showing that magnetic support is indeed important in the region. Our data extend the overall lambda-N(H2) relation toward lower density values and show that such trend continues smoothly toward low N(H2) values. This provides general support for the evolution of initially subcritical clouds to an eventual supercritical stage.

Gemini Mid-IR Polarimetry of NGC 1068: Polarized Structures around the Nucleus

We present diffraction-limited, 10 μm imaging polarimetry data for the central regions of the archetypal Seyfert active galactic nucleus NGC 1068. The position angle of polarization is consistent with three dominant polarizing mechanisms. We identify three distinct regions of polarization: (1) north of the nucleus, arising from aligned dust in the narrow emission line region, (2) south, east, and west of the nucleus, consistent with dust being channeled toward the central engine, and (3) a central minimum of polarization consistent with a compact (≤22 pc) torus. These observations provide continuity between the geometrically and optically thick torus and the host galaxy's nuclear environments. These images represent the first published mid-IR polarimetry from an 8 m-class telescope and illustrate the potential of such observations.

The optical polarisation of the Vela pulsar revisited

Aims. In this work we present a revised measurement of the phase-averaged optical polarisation of the Vela pulsar (PSR B0833-45), for which only one value has been published so far (Wagner & Seifert 2000, ASP Conf. Ser., 202, 315). Methods. Our measurement has been obtained through an accurate reanalysis of archival polarisation observations with the FORS1 instrument at the VLT. Results. We measured a phase-averaged linear polarisation degree P L = 9.4% ± 4% and a position angle θ = 146° ±11°, very close to the ones for the axis of symmetry of the X-ray arcs and jets detected by Chandra and of the pulsar proper motion. Conclusions. We compared the measured phase-averaged optical polarisation with the expectations of different pulsars' magnetosphere models. We find that all models consistently predict values of the phase-averaged linear polarisation that are too high with respect to the observed one. This is probably a consequence of present models' limitations that neglect the contributions of various depolarisation effects. Interestingly, for the outer gap model we find that, when assuming synchrotron radiation for the optical emission, the observed polarisation position angle also implies an alignment between the pulsar rotational axis and the axis of symmetry of the X-ray arcs and jets.

Polarized Radio Emission from the Magnetar XTE J1810-197

We have used the Parkes radio telescope to study the polarized emission from the anomalous X-ray pulsar XTE J1810-197 at frequencies of 1.4, 3.2, and 8.4 GHz. We find that the pulsed emission is nearly 100% linearly polarized. The position angle of linear polarization varies gently across the observed pulse profiles, varying little with observing frequency or time, even as the pulse profiles have changed dramatically over a period of 7 months. In the context of the standard pulsar rotating vector model, there are two possible interpretations of the observed position angle swing coupled with the wide profile. In the first, the magnetic and rotation axes are substantially misaligned and the emission originates high in the magnetosphere, as seen for other young radio pulsars, and the beaming fraction is large. In the second interpretation, the magnetic and rotation axes are nearly aligned, and the line of sight remains in the emission zone over almost the entire pulse phase. We deprecate this possibility because of the observed large modulation of thermal X-ray flux. We have also measured the Faraday rotation caused by the Galactic magnetic field, RM = +77 rad m-2, implying an average magnetic field component along the line of sight of 0.5 μG.

OH (1720 MHz) masers: signposts of SNR/molecular cloud interactions

AbstractOver the last decade it has been demonstrated that supernova remnant (SNR) OH (1720 MHz) masers are unique tracers of SNR/molecular cloud interactions. Here I briefly review the current state of our observational understanding of these masers including results from recent MERLIN and VLBA full polarization studies of the masers in W28, W44, and W51C. Some of our findings include that (1) in accordance with theory, the linear polarization position angles are either parallel or perpendicular to the plane-of-sky magnetic field determined through other observations; (2) the maser spot sizes are fairly large (~ 1014 cm) and exhibit a core/halo morphology; and (3) while the magnetic field strengths do increase slightly with higher resolution, this effect can be completely explained by spectral/spatial blending.

Polarization of Quasars: Resonant Line Scattering in the Broad Absorption Line Region

Recent works showed that the absorbing material in broad absorption line (BAL) quasars is optically thick to major resonant absorption lines. This material may contribute significantly to the polarization in the absorption lines. In this paper, we present a detailed study of the resonant line scattering process using a Monte Carlo method to constrain the optical depth, the geometry, and the kinematics of the BAL region (BALR). By comparing our results with observed polarized spectra of BAL quasars, we find the following: (1) Resonant scattering can produce polarization up to 9% at the absorption trough for doublet transitions and up to 20% for singlet transitions in radially accelerated flows. To explain the large polarization degree in the C IV and N V absorption-line troughs detected in a small fraction of BAL QSOs, a nonmonotonic velocity distribution along the line of sight or/and additional contribution from the electron-scattering region is required. (2) The rotation of the flow can lead to the rotation of the polarization position angle (PA) in the line trough. A large extending angle of BALR is required to produce the observed large PA rotation in a few BAL QSOs. (3) A large extending angle of BALR is required to explain a subtrough in the polarized flux that was observed in a number of BAL QSOs. (4) The resonant scattering can contribute a significant part of N V emission line in some QSOs, and it may give rise to anomalous strong N V lines in these quasars. (5) The polarized flux and PA rotation produced by the resonant scattering regions is uniquely asymmetric, which may be used to test the presence of BALR in non-BAL QSOs.

Spectropolarimetric Study on Circumstellar Structure of Microquasar LS I+61°303

Abstract We present an optical linear spectropolarimetry study of the microquasar LS I$+61^{\circ}$303. The continuum emission is mildly polarized (up to 1.3%) and shows almost no temporal change. We found a distinct change of polarization across the $\mathrm{H}\alpha$ emission line, indicating the existence of a polarization component intrinsic to the microquasar. We estimated the interstellar polarization component from polarization of the $\mathrm{H}\alpha$ line and derived the intrinsic polarization component. The wavelength dependence of the intrinsic component is well explained by Thomson scattering in an equatorial disk of a Be-type mass donor. The position angle (PA) of the intrinsic polarization is $\sim 25^{\circ}$, and represents the rotational axis of the Be disk. This PA is nearly perpendicular to the PA of a radio jet found during quiescent phases. Assuming an orthogonal disk–jet geometry around the compact star, the rotational axis of the accretion disk is almost perpendicular to that of the Be disk. Moreover, according to the orbital parameters of the microquasar, the compact star is likely to get across the Be disk around their periastron passage. We discuss the peculiar circumstellar structure of this microquasar inferred based on our observation, and suggest a possible connection with its high-energy activities.

The Submillimeter Polarization of Sgr A*

We report on the submillimeter properties of Sgr A* derived from observations with the Submillimeter Array and its polarimeter. We ftid that the spectrum of Sgr A* between 230 and 690 GHz is slightly decreasing when measured simultaneously, indicating a transition to optically thin emission around 300–400 GHz. We also present very sensitive and well calibrated measurements of the polarization of Sgr A* at 230 and 345 GHz. With these data we are able to show for the frst time that the polarization of Sgr A* varies on hour timescales, as has been observed for the total intensity. On one night we fhd variability that may arise from a polarized "blob" orbiting the black hole. Finally, we use the ensemble of observations to determine the rotation measure. This represents the frst statistically significant rotation measure determination and the only one made without resorting to comparing position angles measured at separate epochs. We frid a rotation measure of (−5.6 ± 0.7) × 105 rad m2, with no evidence for variability on inter-day timescales at the level of the measurement error. The stability constrains interday flictuations in the accretion rate to 8%. The mean intrinsic polarization position angle is 167°±7° and we detect variations of 31+18−9 degrees. This separation of intrinsic polarization changes and possible rotation measure flictuations is now possible because of the frequency coverage and sensitivity of our data. The observable rotation measure restricts the accretion rate to the range 2 × 10−7 Mȯ yr−1 to 2 × 10−9 Mȯ yr−1, if the magnetic ffeld is near equipartition and ordered.

Optical Polarimetry of the Jets of Nearby Radio Galaxies. I. The Data

We present an overview of new HST imaging polarimetry of six nearby radio galaxies with optical jets. These observations triple the number of extragalactic jets with subarcsecond-resolution optical polarimetry. We discuss the polarization characteristics and optical morphology of each jet. We find evidence of high optical polarization, averaging 20%, but reaching upwards of ~50% in some objects, confirming that the optical emission is synchrotron, and that the components of the magnetic fields perpendicular to the line of sight are well ordered. We find a wide range of polarization morphologies, with each jet having a somewhat different relationship between total intensity and polarized flux and the polarization position angle. We find two trends in all of these jets. First, jet ``edges'' are very often associated with high fractional optical polarizations, as also found in earlier radio observations of these and other radio jets. In these regions, the magnetic field vectors appear to track the jet direction, even at bends, where we see particularly high fractional polarizations. This indicates a strong link between the local magnetic field and jet dynamics. Second, optical flux maximum regions are usually well separated from maxima in fractional polarization and often are associated with polarization minima. This trend is not found in radio data and was found in our optical polarimetry of M87 with HST. However, unlike in M87, we do not find a general trend for near 90° rotations in the optical polarization vectors near flux maxima. We discuss possibilities for interpreting these trends, as well as implications for jet dynamics, magnetic field structure, and particle acceleration

Polarization of High‐Energy Emission from the Crab Pulsar

We investigate polarization of high-energy emissions from the Crab pulsar in the frame work of the outer gap accelerator, following previous works of Cheng and coworkers. The recent version of the outer gap, which extends from inside the null charge surface to the light cylinder, is used for examining the synchrotron radiations from the secondary and the tertiary pairs, which are produced outside the gap. We calculate the light curve, the spectrum and the polarization characteristics, simultaneously, by taking into account gyration motion of the particles. The polarization position angle curve and the polarization degree are calculated to compare with the Crab optical data. We demonstrate that the radiations from inside the null charge surface make outer-wing and off-pulse emissions in the light curve, and the tertiary pairs contribute to bridge emissions. The emissions from the secondary pairs explain the main features of the observed light curve and spectrum. On the other hand, both emissions from inside the null charge surface and from the tertiary pairs are required to explain the optical polarization behavior of the Crab pulsar. The energy dependence of the polarization features is expected by the present model. For the Crab pulsar, the polarization position angle curve indicates that the viewing angle of the observer measured from the rotational axis is greater than $90^{\circ}$.

Evidence for Alignment of the Rotation and Velocity Vectors in Pulsars

We review our case for strong observational evidence for a relationship between the direction of a pulsar's motion and its rotation axis. The information comes from calibrated polarization data for 20 pulsars which display linearly polarized emission from the pulse longitude at closest approach to the magnetic pole. Of these 20 pulsars, 10 show an offset in the angle between the velocity vector and the polarisation position angle which is either less than 10° or more than 80°, a fraction which is very unlikely by random chance. We believe that the bimodal nature of the distribution arises from the presence of orthogonal polarisation modes in the pulsar radio emission. In some cases this orthogonal ambiguity is resolved by observations at other wavelengths so that we conclude that the velocity vector and the rotation axis are aligned at birth. Strengthening the case is the fact that 4 of the 5 pulsars with ages less than 3 Myr show this relationship, including the Vela pulsar.

The H2O super maser emission of Orion KL accretion disk, bipolar outflow, shell

The H2O super maser outbursts were observed in Orion KL in active periods 1979-1987 (F ≤ 8 MJy) and 1998-1999 (F ≤ 4 MJy). The line velocity was VLSR = 7.65 km/s and line width ΔV ~ 0.5 km/s. The emission was linear polarized m ≤ 75%. We studied structure of H2O super maser region with VLBI angular resolution 0.1 mas or 0.05 AU. The emission was determined by high organized structure: a chain of bright (Tb ~ 1016K) compact components (~ 0.05 AU), which are distributed along thin S-form structure 27× 0.3 AU, Tb ~ 1011K, Fig. 1. The brightest components have velocities V ~ 7.65 km/s. The components correspond to tangential direction of the rings, velocities of which are V ~ ΩR and rotation period is ~180 yrs. The highly collimated bipolar outflow 9x0.7 AU and comet-like bullets were observed in the quiescent period 1995 (F= 1 kJy) and second activity period 1998-1999. The central compact (0.05 AU) bright (Tb ~ 1016K) source is ejector of bipolar outflow, which surrounded by a torus. Compact bright features are located in the outflow, which velocities are V ~ 10 km/s in the beginning of activity, and V ~ 3 km/s in the end. The helix structure of outflow is determined by precession with period T ~ 10 yrs. The bullets were ejected in the first period activity. Extraordinary changing of the polarization position angle dX/dV ~ 23o/km/s is determined by nozzle emission.