Intrinsic Linear Polarization Research Articles

Polarization measurements of the polluted white dwarf G29-38

ABSTRACT We have made high-precision polarimetric observations of the polluted white dwarf G29-38 with the HIgh Precision Polarimetric Instrument 2. The observations were made at two different observatories – using the 8.1-m Gemini North Telescope and the 3.9-m Anglo-Australian Telescope – and are consistent with each other. After allowing for a small amount of interstellar polarization, the intrinsic linear polarization of the system is found to be 275.3 ± 31.9 parts per million at a position angle of 90.8 ± 3.8° in the SDSS g′ band. We compare the observed polarization with the predictions of circumstellar disc models. The measured polarization is small in the context of the models we develop, which only allows us to place limits on disc inclination and Bond albedo for optically thin disc geometries. In this case, either the inclination is near-face-on or the albedo is small – likely in the range 0.05–0.15 – which is in line with other debris disc measurements. A preliminary search for the effects of G29-38’s pulsations in the polarization signal produced inconsistent results. This may be caused by beating effects, indicate a clumpy dust distribution, or be a consequence of measurement systematics.

Optical polarization properties of February 2010 outburst of the blazar Mrk 421

In this paper, we explore the behavior of optical polarization during the multi-wavelength outburst of the blazar Mrk 421 detected in February 2010. We use optical polarization measurements in the wavelength range 500-700 nm from SPOL observations available between January 1, 2010 and March 31, 2010 (MJD 55197-55286) including the period of multi-wavelength flaring activity detected from the source around February 16-17, 2010 (MJD 55243-55244). We also use near simultaneous optical and radio flux measurements from SPOL in V and R bands and OVRO at 15 GHz respectively. We find that the emissions in the optical and radio bands do not show any significant change in the source activity unlike at X-ray and $\gamma$--ray energies during the outburst. The optical and radio flux measurements are found to be consistent with the long term quiescent state emission of the source. Moreover, the linear polarization in the wavelength range 500-700 nm decreases to a minimum value of 1.6$\%$ during the X-ray and $\gamma$--ray outburst which is significantly lower than the long term average value of $\sim$ 4.2$\%$. The angle of polarization varies between 114$^\circ$-163$^\circ$ with a preferred average value of $\sim$ 137$^\circ$ during this period. We estimate the degree of polarization intrinsic to the jet taking into account the host galaxy contamination in R band and compare this with the theoretical synchrotron polarization estimated for a power law distribution of relativistic electrons gyrating in an emission region filled with ordered and chaotic magnetic fields. The intrinsic linear polarization estimated for different epochs during the above period is found to be consistent with the theoretical synchrotron polarization produced by the relativistic electrons with power law spectral index $\sim$ 2.2.

Investigating the origin of the spectral line profiles of the Hot Wolf–Rayet Star WR 2

The hot WN star WR2 (HD6327) has been claimed to have many singular characteristics. To explain its unusually rounded and relatively weak emission line profiles, it has been proposed that WR2 is rotating close to break-up with a magnetically confined wind. Alternatively, the line profiles could be explained by the dilution of WR2's spectrum by that of a companion. In this paper, we present a study of WR2 using near-infrared AO imaging and optical spectroscopy and polarimetry. Our spectra reveal the presence of weak photospheric absorption lines from a ~B2.5-4V companion, which however contributes only ~5-10% to the total light, suggesting that the companion is a background object. Therefore, its flux cannot be causing any significant dilution of the WR star's emission lines. The absence of intrinsic linear continuum polarization from WR2 does not support the proposed fast rotation. Our Stokes V spectrum was not of sufficient quality to test the presence of a moderately strong organized magnetic field but our new modelling indicates that to confine the wind the putative magnetic field must be significantly stronger than was previously suggested sufficiently strong as to make its presence implausible.

Polarization study of massive binaries with the ARIES-1.04 m telescope

We present preliminary results obtained from optical (B, V, R, and I) linear polarimetric observations of the O+O massive binary DH Cep. The observations were made on six nights in 2017 with the Aries IMaging POLarimeter (AIMPOL) mounted at the back of the 1.04-m Sampurnanand Telescope of ARIES. The average intrinsic linear polarization in each of the B, V, R, and I photometric bands is found to be less than 1%. The degree of polarization as well as the polarization angle appear to be orbital phase dependent. The significant polarization variability noticed towards shorter wavelengths (i.e. B and V bands) is indicative of the asymmetric circumstellar envelope.

Inferring telescope polarization properties through spectral lines without linear polarization

Context. Polarimetric observations taken with ground- or space-based telescopes usually need to be corrected for changes of the polarization state in the optical path. Aims. We present a technique to determine the polarization properties of a telescope through observations of spectral lines that have no or negligible intrinsic linear polarization signals. For such spectral lines, any observed linear polarization must be induced by the telescope optics. We apply the technique to observations taken with the Spectropolarimeter for Infrared and Optical Regions (SPINOR) at the Dunn Solar Telescope (DST) and demonstrate that we can retrieve the characteristic polarization properties of the DST at three wavelengths of 459, 526, and 615 nm. Methods. We determine the amount of crosstalk between the intensity Stokes I and the linear and circular polarization states Stokes Q, U, and V, and between Stokes V and Stokes Q and U in spectropolarimetric observations of active regions. We fit a set of parameters that describe the polarization properties of the DST to the observed crosstalk values. We compare our results to parameters that were derived using a conventional telescope calibration unit (TCU). Results. The values for the ratio of reflectivities X = rs∕rp and the retardance τ of the DST turret mirrors from the analysis of the crosstalk match those derived with the TCU within the error bars. We find a negligible contribution of retardance from the entrance and exit windows of the evacuated part of the DST. Residual crosstalk after applying a correction for the telescope polarization stays at a level of 3–10% regardless of which parameter set is used, but with an rms fluctuation in the input data of already a few percent. The accuracy in the determination of the telescope properties is thus more limited by the quality of the input data than the method itself. Conclusions. It is possible to derive the parameters that describe the polarization properties of a telescope from observations of spectral lines without intrinsic linear polarization signal. Such spectral lines have a dense coverage (about 50 nm separation) in the visible part of the spectrum (400–615 nm), but none were found at longer wavelengths. Using spectral lines without intrinsic linear polarization is a promising tool for the polarimetric calibration of current or future solar telescopes such as the Daniel K. Inouye Solar Telescope (DKIST).

Scattering linear polarization of late-type active stars

Context. Many active stars are covered in spots, much more so than the Sun, as indicated by spectroscopic and photometric observations. It has been predicted that star spots induce non-zero intrinsic linear polarization by breaking the visible stellar disk symmetry. Although small, this effect might be useful for star spot studies, and it is particularly significant for a future polarimetric atmosphere characterization of exoplanets orbiting active host stars. Aims. Using models for a center-to-limb variation of the intensity and polarization in presence of continuum scattering and adopting a simplified two-temperature photosphere model, we aim to estimate the intrinsic linear polarization for late-type stars of different gravity, effective temperature, and spottedness. Methods. We developed a code that simulates various spot configurations or uses arbitrary surface maps, performs numerical disk integration, and builds Stokes parameter phase curves for a star over a rotation period for a selected wavelength. It allows estimating minimum and maximum polarization values for a given set of stellar parameters and spot coverages. Results. Based on assumptions about photosphere-to-spot temperature contrasts and spot size distributions, we calculate the linear polarization for late-type stars with Teff = 3500 K–6000 K, log g = 1.0–5.0, using the plane-parallel and spherical atmosphere models. Employing random spot surface distribution, we analyze the relation between spot coverage and polarization and determine the influence of different input parameters on results. Furthermore, we consider spot configurations with polar spots and active latitudes and longitudes.

Searching for Jet Emission in LMXBs: A Polarimetric View

We present results taken from a study aiming at detecting the emission from relativistic particles jets in neutron star-low mass X-ray binaries using optical polarimetric observations. First, we focus on a polarimetric study performed on the persistent LMXB 4U 0614+091. Once corrected for interstellar effects, we measured an intrinsic linear polarization in the r-band of ~3% at a 3σ confidence level. This is in-line with the observation of an infrared excess in the spectral energy distribution (SED) of the source, reported in a previous work, which the authors linked to the optically thin synchrotron emission of a jet. We then present a study performed on the transitional millisecond pulsar PSR J1023+0038 during quiescence. We measured a linear polarization of 1.09 ± 0.27% and 0.90 ± 0.17% in the V and R bands, respectively. The phase-resolved polarimetric curve of the source in the R-band reveals a hint of a sinusoidal modulation at the source orbital period. The NIR -optical SED of the system did not suggest the presence of a jet. We conclude that the optical linear polarization observed for PSR J1023+0038 is possibly due to Thomson scattering with electrons in the disc, as also suggested by the hint of the modulation of the R-band linear polarization at the system orbital period.

A real-time fast radio burst: polarization detection and multiwavelength follow-up

Fast radio bursts (FRBs) are one of the most tantalizing mysteries of the radio sky; their progenitors and origins remain unknown and until now no rapid multiwavelength follow-up of an FRB has been possible. New instrumentation has decreased the time between observation and discovery from years to seconds, and enables polarimetry to be performed on FRBs for the first time. We have discovered an FRB (FRB 140514) in real-time on 14 May, 2014 at 17:14:11.06 UTC at the Parkes radio telescope and triggered follow-up at other wavelengths within hours of the event. FRB 140514 was found with a dispersion measure (DM) of 562.7(6) cm$^{-3}$ pc, giving an upper limit on source redshift of $z \lesssim 0.5$. FRB 140514 was found to be 21$\pm$7% (3-$\sigma$) circularly polarized on the leading edge with a 1-$\sigma$ upper limit on linear polarization $<10%$. We conclude that this polarization is intrinsic to the FRB. If there was any intrinsic linear polarization, as might be expected from coherent emission, then it may have been depolarized by Faraday rotation caused by passing through strong magnetic fields and/or high density environments. FRB 140514 was discovered during a campaign to re-observe known FRB fields, and lies close to a previous discovery, FRB 110220; based on the difference in DMs of these bursts and time-on-sky arguments, we attribute the proximity to sampling bias and conclude that they are distinct objects. Follow-up conducted by 12 telescopes observing from X-ray to radio wavelengths was unable to identify a variable multiwavelength counterpart, allowing us to rule out models in which FRBs originate from nearby ($z < 0.3$) supernovae and long duration gamma-ray bursts.

TMD evolution and the Higgs transverse momentum distribution

The effect of the linear polarization of gluons on the transverse momentum distribution in Higgs production is studied within the framework of TMD factorization. For this purpose we consider the TMD evolution for general colorless scalar boson production, from the lower mass C-even scalar quarkonium states χc0 and χb0 to the Higgs mass scale. In the absence of an intrinsic nonperturbative linearly polarized gluon distribution the results correspond to the CSS formalism, indicating a rather rapid decrease with increasing energy scale. At the Higgs mass scale the contribution from linearly polarized gluons is in this case found to be on the percent level, somewhat larger than an earlier finding in the literature. At the lower mass scale of quarkonium states χc0 and χb0 we find contributions at the 15–70% level, albeit with considerable uncertainty. In the presence of an intrinsic linear gluon polarization, percent level effects are also found at the Higgs mass scale, but with a considerably slower evolution. Although these results were obtained using a model for the TMDs that are approximately Gaussian at small transverse momenta and have the correct perturbative power law fall-off at large transverse momenta, it illustrates well the differences that can exist between results obtained from a TMD formalism as compared to a CSS formalism. The behavior of the TMDs at small pT can affect the results for all transverse momenta of the produced boson, even for a particle as heavy as the Higgs. The TMD evolution from χc0 to χb0 may be used to constrain the nonperturbative contributions and improve on the prediction of the effect at the Higgs mass scale.

Starspots and active regions on IN Com: UBVRI photometry and linear polarization

The activity of the variable star IN Com is considered using the latest multicolor UBVRI photometry and linear polarimetric observations carried out during a decade. The photometric variability of the star is fully described using the zonal spottedness model developed at the Crimean Astrophysical Observatory (CrAO). Spotted regions cover up to 22% of the total stellar surface, with the difference in temperatures between the quiet photosphere and the spot umbra being 600 K. The spots are located at middle and low latitudes (40°–55°). The intrinsic broad-band linear polarization of IN Com and its rotational modulation in the U band due to local magnetic fields at the most spotted (active) stellar longitudes were detected for the first time.

COMPUTING THE CONTINUUM POLARIZATION FROM THOMSON SCATTERING IN GASEOUS CIRCUMSTELLAR DISKS

We investigate the computation of the intrinsic continuum linear polarization from electron scattering in optically thin and thick circumstellar disks of gas. We present the use of a non-LTE radiative transfer code, along with two different computational methods for obtaining the Stokes parameters, to reproduce the polarization levels that arise from disks of classical Be stars. Since the pioneering work of Poeckert & Marlborough, numerous improvements and refinements have been incorporated into computational radiative transfer models of classical Be stars. We present an assessment of the effect of several improvements on Poeckert & Marlborough's technique for calculating the polarization levels of the classical Be star γ Cas. We find that improvements to the sampling of the disk density and the inclusion of a non-isothermal structure for the gas in the disk yield polarization levels that differ from the levels expected by Poeckert & Marlborough. Principally, the inclusion of the self-consistent calculation of the thermal structure of the disk has a significant impact on the resulting polarization. In addition, we assess the importance of the inclusion of multiple scattering calculations in predicting the continuum polarization in classical Be stars. We confirm that multiple scattering calculations are necessary for studying the linear polarization levels from optically thick gaseous disks around classical Be stars.

ORBITAL ELEMENTS OF THE SYMBIOTIC STAR Z ANDROMEDAE FROM OPTICAL LINEAR POLARIZATION DURING THE QUIESCENT PHASE

We present low-resolution spectropolarimetry for the symbiotic star Z Andromedae at four different epochs during the quiescent phase. The linear polarization of the continuum showed a temporal variation; the difference between the maximum and the minimum is 0.3%–0.6% in Stokes q and is larger with shorter wavelengths. Applying scattering models to this variation, we found the variation in the continuum may be correlated with the orbital motion of the binary and estimated the orbital inclination angle ic = 73 ◦ ± 14 ◦ and the orientation angle Ωc = 80 ◦ ± 5 ◦ . We also confirmed that the intrinsic linear polarization of the Raman line λ683 varies with the orbital phase; from this modulation, the orbital elements were derived as ir = 41 ◦ ± 8 ◦ and Ωr = 82 ◦ ± 2 ◦ . The inclination derived from the continuum has a large error, and the value is larger by twice the error than the inclination angle value derived from the Raman line. The derived orientation, in contrast, is comparable with that derived from the Raman line. The possible inconsistency in the inclination may be due to the simplicity of our adopted model, or it may be caused by a bias effect due to the low quality of the observed continuum polarization data. An accurate estimation of the inclination from the continuum polarization could settle the question, but that estimation requires more frequent observations that cover at least more than a few orbital cycles during the quiescent phase when the observations are not interrupted by the activity of the hot component.

10.1007/s11443-008-1004-y

We demonstrate a situation where the wavelength dependence of the intrinsic linear polarization of stellar radiation matches that of the interstellar linear polarization described by the Serkowski law. Such a situation can arise when the radiation from a star with a dipole magnetic field is scattered in a circumstellar plasma shell with a uniform electron density distribution. As a result, we have estimated the magnetic field strength at the photospheric phase of Supernova 1999gi. We show that the existence of intrinsic polarization in Galactic stars disguised as interstellar polarization is possible in principle.

The Growth, Polarization, and Motion of the Radio Afterglow from the Giant Flare from SGR 1806-20

The extraordinary giant flare (GF) of 2004 December 27 from the soft gamma repeater SGR 1806-20 was followed by a bright radio afterglow. We present an analysis of VLA observations of this radio afterglow from SGR 1806-20, consisting of previously reported 8.5 GHz data covering days 7-20 after the GF, plus new observations at 8.5 and 22 GHz from day 24 to 81. We detect motion in the flux centroid of the afterglow, at an average velocity of (0.26 ± 0.03)c (assuming a distance of 15 kpc) at a position angle of -45°. This motion, in combination with the growth and polarization measurements, suggests an asymmetric outflow, mainly from one side of the magnetar. We find a deceleration in the expansion, from ~9 to <5 mas day-1. The time of deceleration is roughly coincident with the rebrightening in the radio light curve, as expected to result when the ejecta from the GF sweeps up enough of the external medium and transitions from a coasting phase to the Sedov-Taylor regime. The radio afterglow is elongated and maintains a 2 : 1 axis ratio with an average position angle of -40° (north through east), oriented perpendicular to the average intrinsic linear polarization angle.

Asymmetric explosions of thermonuclear supernovae

A type Ia supernova explosion starts in a white dwarf as a laminar deflagration at the center of the star and soon several hydrodynamic instabilities (in particular, the Rayleigh-Taylor (R-T) instability) begin to act. In previous work (Ghezzi, de Gouveia Dal Pino, & Horvath 2001), we addressed the propagation of an initially laminar thermonuclear flame in presence of a magnetic field assumed to be dipolar. We were able to show that, within the framework of a fractal model for the flame velocity, the front is affected by the field through the quenching of the R-T instability growth in the direction perpendicular to the field lines. As a consequence, an asymmetry develops between the magnetic polar and the equatorial axis that gives a prolate shape to the burning front. We have here computed numerically the total integrated asymmetry as the flame front propagates outward through the expanding shells of decreasing density of the magnetized white dwarf progenitor, for several chemical compositions, and found that a total asymmetry of about 50 % is produced between the polar and equatorial directions for progenitors with a surface magnetic field B ~ 5 x 10^{7} G, and a composition C12 = 0.2 and O16 = 0.8 (in this case, the R-T instability saturates at scales \~ 20 times the width of the flame front). This asymmetry is in good agreement with the inferred asymmetries from spectropolarimetric observations of very young supernova remnants, which have recently revealed intrinsic linear polarization interpreted as evidence of an asymmetric explosion in several objects,such as SN1999by, SN1996X, and SN1997dt. Larger magnetic field strengths will produce even larger asymmetries. We have also found that for lighter progenitors the total asymmetry is larger.

Broad-band multicolor photometry and polarimetry of spotted stars

We have confirmed the BY Dra-type variability of the active spotted stars MS Ser, LQ Hya, VY Ari, and EK Dra using simultaneous UBVRI photometric and polarimetric observations. We have also reliably detected the intrinsic linear polarization of their radiation and its rotational modulation in U due to the inhomogeneous distribution of active magnetized regions over the surfaces of the stars. Modeling of the linear polarization based on the Zeeman effect indicates that all the stars display strong local magnetic fields (about 2 kG, similar to those in sunspots), with filling factors of up to 40% of the total stellar surface. The magnetized regions coincide with cool photospheric spots detected in photoelectric observations.

Scattered light models of the dust shell around HD 179821

The dust shell around the evolved star HD 179821 has been detected in scattered light in near-IR imaging polarimetry observations. Here, we subtract the contribution of the unpolarized stellar light to obtain an intrinsic linear polarization of between 30 and 40 per cent in the shell which seems to increase with radial offset from the star. The J and K band data are modelled using a scattering code to determine the shell parameters and dust properties. We find that the observations are well described by a spherically symmetric distribution of dust with an inverse square density law, indicating that when mass-loss was occurring, the mass-loss rate was constant. The models predict that the detached nature of a spherically symmetric, optically thin dust shell, with a distinct inner boundary, will only be apparent in polarized flux. This is in accordance with the observations of this and other optically thin circumstellar shells, such as IRAS 17436+5003. By fitting the shell brightness we derive an optical depth to the star which is consistent with V band observations and which, assuming a distance of 6 kpc, gives an inner shell radius of 1.44 X 10^{15} m, a dust number density of 2.70 X 10^{-1} m^{-3}. and a dust mass of 0.08 Msun. We have explored axisymmetric shell models but conclude that any deviations from spherical symmetry in the shell must be slight, with an equator-to-pole density contrast of less than 2:1. We have not been able to simultaneously fit the high linear polarizations and the small (E(J-K)=-0.3) colour excess of the shell and we attribute this to the unusual scattering properties of the dust. We suggest that the dust grains around HD 179821 are either highly elongated or consist of aggregates of smaller particles.

Some Low‐Polarization B Stars

ABSTRACTWe present polarimetry of three B stars with very low interstellar polarization that may be useful in testing whether there is intrinsic linear polarization in B stars of moderate rotation rate in the UV.

Spectropolarimetry of Nova Cygni 1992: Evidence for an asymmetric geometry

We present the first study of the spectropolarimetric evolution of a fast neon nova from outburst through the decline of the light curve, showing the initial presence and gradual disappearance of intrinsic linear polarization. The observations of Nova Cygni 1992 were obtained at the Pine Bluff Observatory of the University of Wisconsin from 1992 March through 1993 May. The emission lines and the continuum show quite different linear polarization behaviors, with the line emission essentially unpolarized and the continuum showing intrinsic polarization. The data provide evidence for an asymmetric geometry in the initial explosion and evidence for clumping in the subsequent nova ejecta.

Broadband Measurements of the Transverse Magnetic Field of Cool Ap Stars

AbstractObservations of variable broadband linear polarization in magnetic Ap stars (due to the transverse Zeeman effect), when combined with measurements of the mean longitudinal field Bɩ can in some cases allow one to determine the angles i and β (which describe the inclination of the stellar axis of rotation and the obliquity of the magnetic axis to the rotation axis) much more accurately than these angles can be determined from observations of Bɩ alone. Such variable intrinsic linear polarization has been observed for a number of stars; the effect is generally detectable only in cool Ap stars of unusually large field strength. We discuss the data and simple modelling for the stars HD 24712 = HR 1217, HD 137909 = β CrB, and HD 62140 = 49 Cam.