Circular Polarization Fraction Research Articles

MeerKAT Observations of Procyon at 815.5 MHz

We have conducted observations of the nearby (11.46 ly) star system Procyon, using MeerKAT’s UHF (544–1087 MHz) receivers. We produce full-Stokes time and frequency integrated continuum images, as well as total intensity time series imaging at 8 s cadence, and full-Stokes vector-averaged dynamic spectra from the visibilities in order to search for transient activity such as flaring events. We detect no significant radio emission from the system, and estimate an upper limit on the circular polarization fraction of 65% (3σ confidence level). A comparison with previous Very Large Array observations places a 3σ lower limit on the spectral index between 815.5 and 8400 MHz of 0.26, however long-term significant variability over the last 33 yr cannot be ruled out without further, regular radio monitoring of the system.

A Volume-limited Radio Search for Magnetic Activity in 140 Exoplanets with the Very Large Array

We present results from a search for radio emission in 77 stellar systems hosting 140 exoplanets, predominantly within 17.5 pc using the Very Large Array (VLA) at 4–8 GHz. This is the largest and most sensitive search to date for radio emission in exoplanetary systems in the GHz frequency range. We obtained new observations of 58 systems and analyzed archival observations of an additional 19 systems. Our choice of frequency and volume limit is motivated by radio detections of ultracool dwarfs (UCDs), including T dwarfs with masses at the exoplanet threshold of ∼13 M J. Our surveyed exoplanets span a mass range of ≈10−3–10 M J and semimajor axes of ≈10−2–10 au. We detect a single target—GJ 3323 (M4) hosting two exoplanets with minimum masses of 2 and 2.3 M ⊕—with a circular polarization fraction of ≈40%; the radio luminosity agrees with its known X-ray luminosity and the Güdel–Benz relation for stellar activity suggesting a likely stellar origin, but the high circular polarization fraction may also be indicative of star–planet interaction. For the remaining sources our 3σ upper limits are generally L ν ≲ 1012.5 erg s−1 Hz−1, comparable to the lowest radio luminosities in UCDs. Our results are consistent with previous targeted searches of individual systems at GHz frequencies while greatly expanding the sample size. Our sensitivity is comparable to predicted fluxes for some systems considered candidates for detectable star–planet interaction. Observations with future instruments such as the Square Kilometre Array and Next-Generation VLA will be necessary to further constrain emission mechanisms from exoplanet systems at GHz frequencies.

Detection and localization of the highly active FRB 20240114A with MeerKAT

ABSTRACT We report observations of the highly active FRB 20240114A with MeerKAT using the ultra-high frequency (UHF; $544\!-\!1088$ MHz) and L band ($856\!-\!1712$ MHz) receivers. A total of 62 bursts were detected in coherent tied-array beams using the MeerTRAP real-time transient detection pipeline. We measure a structure-optimizing dispersion measure of $527.65\pm 0.01\, \text{pc}\, \text{cm}^{-3}$ using the brightest burst in the sample. We find the bursts of FRB 20240114A are generally detected in part of the broad-band of MeerKAT, $\sim 40~{{\ \rm per\ cent}}$ in the UHF and $\sim 30~{{\ \rm per\ cent}}$ in the L band, indicating the band limited nature. We analyse the fluence distribution of the 44 bursts detected at UHF, constraining the fluence completeness limit to $\sim 1$ Jy ms, above which the cumulative burst rate follows a power law $R (\gt F)\propto (F/1\, \text{Jy}\, \text{ms})^\gamma$ with $\gamma =-1.8\pm 0.2$. Using channelized telescope data captured in our transient buffer we localize FRB 20240114A in the image domain to RA = 21h27m39.86s, Dec. = +04d19m45.01s with an uncertainty of 1.4 arcsec. This localization allows us to confidently identify the host galaxy of FRB 20240114A. Also using the transient buffer data, we perform a polarimetric study and demonstrate that most of the bursts have $\sim 100~{{\ \rm per\ cent}}$ linear polarization fractions and up to $\sim 20~{{\ \rm per\ cent}}$ circular polarization fractions. Finally, we predict the flux density of a potential persistent radio source (PRS) associated with FRB 20240114A is $\backsimeq [0.6\!-\!60]\, \mu \text{Jy}$ based on the simple relation between the luminosity of the PRS and the rotation measure arising from the FRB local environment.

SiO maser polarization and magnetic field in evolved cool stars

Magnetic fields, photospheric and atmospheric dynamics can be involved in triggering the high mass loss observed in evolved cool stars. Previous works have revealed that the magnetic field of these objects extends beyond their surface. The origin of this magnetic field is still debated. The possible mechanisms include a turbulent dynamo, convection, stellar pulsation, and cool spots. Our goal is to estimate the magnetic field strength in the inner circumstellar envelope of six evolved cool stars (five Miras and one red supergiant). Combining this work with previous studies, we tentatively constrain the global magnetic field type and shed light on the mechanisms that cause it. Using the XPOL polarimeter installed at the IRAM-30 m telescope, we observed the $^ SiO $ v = 1, J = 2-1 maser line emission and obtained simultaneous spectroscopic measurements of the four Stokes parameters. Applying a careful calibration method for Stokes $Q$, $U$, and $V$, we derived estimates of the magnetic field strength from the circular and linear polarization fractions considering the saturated and unsaturated maser cases under the Zeeman hypothesis. Magnetic field strengths from several Gauss up to several dozen Gauss are derived. These new and more accurate measurements constrain the field strength in the region of 2-5 stellar radii better than previous studies and appear to exclude a global poloidal magnetic field type. The combination of a toroidal and poloidal field is not excluded, however. A variation in the magnetic field strength over a two-month timescale is observed in one Mira star, which suggests a possible link to the stellar phase, that is, a link with pulsation and photospheric activity.

Deep Synoptic Array Science: Polarimetry of 25 New Fast Radio Bursts Provides Insights into Their Origins

We report on a full-polarization analysis of the first 25 as yet nonrepeating fast radio bursts (FRBs) detected at 1.4 GHz by the 110-antenna Deep Synoptic Array (DSA-110) during commissioning observations. We present details of the data-reduction, calibration, and analysis procedures developed for this novel instrument. Faraday rotation measures (RMs) are searched between ±106 rad m−2 and detected for 20 FRBs, with magnitudes ranging from 4 to 4670 rad m−2. Fifteen out of 25 FRBs are consistent with 100% polarization, 10 of which have high (≥70%) linear-polarization fractions and two of which have high (≥30%) circular-polarization fractions. Our results disfavor multipath RM scattering as a dominant depolarization mechanism. Polarization-state and possible RM variations are observed in the four FRBs with multiple subcomponents. We combine the DSA-110 sample with polarimetry of previously published FRBs, and compare the polarization properties of FRB subpopulations and FRBs with Galactic pulsars. Although FRB polarization fractions are typically higher than those of Galactic pulsars, and cover a wider range than those of pulsar single pulses, they resemble those of the youngest (characteristic ages <105 yr) pulsars. Our results support a scenario wherein FRB emission is intrinsically highly linearly polarized, and propagation effects can result in conversion to circular polarization and depolarization. Young pulsar emission and magnetospheric propagation geometries may form a useful analogy for the origin of FRB polarization.

First M87 Event Horizon Telescope Results. IX. Detection of Near-horizon Circular Polarization

Event Horizon Telescope (EHT) observations have revealed a bright ring of emission around the supermassive black hole at the center of the M87 galaxy. EHT images in linear polarization have further identified a coherent spiral pattern around the black hole, produced from ordered magnetic fields threading the emitting plasma. Here we present the first analysis of circular polarization using EHT data, acquired in 2017, which can potentially provide additional insights into the magnetic fields and plasma composition near the black hole. Interferometric closure quantities provide convincing evidence for the presence of circularly polarized emission on event-horizon scales. We produce images of the circular polarization using both traditional and newly developed methods. All methods find a moderate level of resolved circular polarization across the image (〈∣v∣〉 < 3.7%), consistent with the low image-integrated circular polarization fraction measured by the Atacama Large Millimeter/submillimeter Array (∣v int∣ < 1%). Despite this broad agreement, the methods show substantial variation in the morphology of the circularly polarized emission, indicating that our conclusions are strongly dependent on the imaging assumptions because of the limited baseline coverage, uncertain telescope gain calibration, and weakly polarized signal. We include this upper limit in an updated comparison to general relativistic magnetohydrodynamic simulation models. This analysis reinforces the previously reported preference for magnetically arrested accretion flow models. We find that most simulations naturally produce a low level of circular polarization consistent with our upper limit and that Faraday conversion is likely the dominant production mechanism for circular polarization at 230 GHz in M87*.

Characterizing Pulsars Detected in the Rapid ASKAP Continuum Survey

We present the detection of 661 known pulsars observed with the Australian SKA Pathfinder (ASKAP) telescope at 888 MHz as part of the Rapid ASKAP Continuum Survey (RACS). Detections were made through astrometric coincidence and we estimate the false alarm rate of our sample to be ∼0.5%. Using archival data at 400 and 1400 MHz, we estimate the power-law spectral indices for the pulsars in our sample and find that the mean spectral index is −1.78 ± 0.6. However, we also find that a single power law is inadequate for modeling all the observed spectra. With the addition of flux densities between 150 MHz and 3 GHz from various imaging surveys, we find that up to 40% of our sample show deviations from a simple power-law model. Using Stokes V measurements from the RACS data, we measured the circular polarization fraction for 9% of our sample and find that the mean polarization fraction is ∼10% (consistent between detections and upper limits). Using the dispersion-measure-derived distance, we estimate the pseudo-luminosity of the pulsars and do not find any strong evidence for a correlation with the pulsars’ intrinsic properties.

Dense forests of microshots in bursts from FRB 20220912A

ABSTRACT We report on exceptionally bright bursts (&amp;gt;400 Jy ms) detected from the repeating fast radio burst source FRB 20220912A using the Nançay radio telescope (NRT), as part of the ECLAT (Extragalactic Coherent Light from Astrophysical Transients) monitoring campaign. These bursts exhibit extremely luminous, broad-band, short-duration structures (∼16 μs), which we term ‘microshots’ and which can be especially well studied in the NRT data given the excellent signal-to-noise and dynamic range (32-bit samples). The estimated peak flux density of the brightest microshot is 450 Jy. We show that the microshots are clustered into dense ‘forests’ by modelling them as Weibull distributions and obtaining Weibull shape parameters of approximately 0.5. Our polarimetric analysis reveals that the bursts are nearly 100 per cent linearly polarized; have ≲10 per cent circular polarization fractions; a near-zero average rotation measure of 0.10(6) rad m−2; and varying polarization position angles over the burst duration. For one of the bursts, we analyse raw voltage data from simultaneous observations with the Westerbork RT-1 single 25-m dish. These data allow us to measure the scintillation bandwidth, 0.30(3) MHz, and to probe the bursts on (sub-)microsecond time-scales. Some important nuances related to dedispersion are also discussed. We propose that the emission mechanism for the broad-band microshots is potentially different from the emission mechanism of the broader burst components, which still show a residual drift of a few hundred MHz ms−1 after correcting for dispersion using the microshots. We discuss how the observed emission is phenomenologically analogous to different types of radio bursts from the Sun.

Periodic Radio Emission from the T8 Dwarf WISE J062309.94–045624.6

We present the detection of rotationally modulated, circularly polarized radio emission from the T8 brown dwarf WISE J062309.94−045624.6 between 0.9 and 2.0 GHz. We detected this high-proper-motion ultracool dwarf with the Australian SKA Pathfinder in 1.36 GHz imaging data from the Rapid ASKAP Continuum Survey. We observed WISE J062309.94−045624.6 to have a time and frequency averaged Stokes I flux density of 4.17 ± 0.41 mJy beam−1, with an absolute circular polarization fraction of 66.3% ± 9.0%, and calculated a specific radio luminosity of L ν ∼ 1014.8 erg s−1 Hz−1. In follow-up observations with the Australian Telescope Compact Array and MeerKAT we identified a multipeaked pulse structure, used dynamic spectra to place a lower limit of B > 0.71 kG on the dwarf’s magnetic field, and measured a P = 1.912 ± 0.005 hr periodicity, which we concluded to be due to rotational modulation. The luminosity and period we measured are comparable to those of other ultracool dwarfs observed at radio wavelengths. This implies that future megahertz to gigahertz surveys, with increased cadence and improved sensitivity, are likely to detect similar or later-type dwarfs. Our detection of WISE J062309.94−045624.6 makes this dwarf the coolest and latest-type star observed to produce radio emission.

Detecting chiral asymmetry in the interstellar medium using propylene oxide

Context.Life is distinctly homochiral. The origins of this homochirality are under active debate. Recently, propylene oxide has been detected in the gas-phase interstellar medium (ISM). The enantiomeric composition of ISM propylene oxide may be probed through circular polarization measurements, but accurate estimates of the circular dichroism properties of the microwave transitions of propylene oxide are not available.Aims.Our aim is to develop a model of the circular dichroic activity in torsion–rotation transitions of closed-shell chiral molecules such as propylene oxide. With this model we can estimate the viability, and optimize the observation strategies, of enantiomeric excess detection in ISM propylene oxide.Methods.Circular dichroism in spectral lines manifests through the simultaneous interaction of an electromagnetic radiation field with the molecular electric dipole moment and magnetic dipole moment. We developed techniques to quantify electric dipole and magnetic dipole moments of torsion–rotation transitions by expanding on earlier modeling of the electric and magnetic dipole properties of single torsion–rotation levels. To model the circular dichroism properties of propylene oxide, we used these techniques in combination with ab initio quantum chemical calculations.Results.The expressions for the dichroic activity of the microwave transitions of torsionally active molecules are derived. We find that the torsional motion of molecules exhibiting internal rotation contributes significantly to the total magnetic moment. We present estimates for the dichroic activity of the torsion–rotation transitions of propylene oxide. We predict that the circular polarization fractions of emission lines of enantiopure propylene oxide relevant to astronomical detections are on the order of 10−6.Conclusions.Due to the low predicted circular polarization fractions, we conclude that enantiomeric characterization of propylene oxide in the gas phase of the ISM is impossible with the current astronomical observation techniques. We suggest that only chiral radical species may be viably employed for purposes of enantiomeric excess detection. We estimate that laboratory experiments may be successful in detecting the enantiomeric composition of a mixture of propylene oxide through microwave dichroism spectroscopy.

Radio masers on WX UMa: hints of a Neptune-sized planet, or magnetospheric reconnection?

ABSTRACT The nearby M dwarf WX UMa has recently been detected at radio wavelengths with LOFAR. The combination of its observed brightness temperature and circular polarization fraction suggests that the emission is generated via the electron–cyclotron maser instability. Two distinct mechanisms have been proposed to power such emission from low-mass stars: either a sub-Alfvénic interaction between the stellar magnetic field and an orbiting planet, or reconnection at the edge of the stellar magnetosphere. In this paper, we investigate the feasibility of both mechanisms, utilizing the information about the star’s surrounding plasma environment obtained from modelling its stellar wind. Using this information, we show that a Neptune-sized exoplanet with a magnetic field strength of 10–100 G orbiting at ∼0.034 au can accurately reproduce the observed radio emission from the star, with a corresponding orbital period of 7.4 d. Due to the stellar inclination, a planet in an equatorial orbit is unlikely to transit the star. While such a planet could induce radial velocity semi-amplitudes from 7 to 396 m s−1, it is unlikely that this signal could be detected with current techniques due to the activity of the host star. The application of our planet-induced radio emission model here illustrates its exciting potential as a new tool for identifying planet-hosting candidates from long-term radio monitoring. We also develop a model to investigate the reconnection-powered emission scenario. While this approach produces less favourable results than the planet-induced scenario, it nevertheless serves as a potential alternative emission mechanism which is worth exploring further.

Magnetospheric Curvature Radiation by Bunches as Emission Mechanism for Repeating Fast Radio Bursts

Abstract Coherent curvature radiation as the radiation mechanism for fast radio bursts (FRBs) has been discussed since FRBs were discovered. We study the spectral and polarization properties of repeating FRBs within the framework of coherent curvature radiation by charged bunches in the magnetosphere of a highly magnetized neutron star. The spectra can be generally characterized by multisegmented broken power laws, and evolve as bunches move and the line of sight sweeps. Emitted waves are highly linear polarized and polarization angles are flat across the burst envelopes, if the line of sight is confined to the beam within an angle of 1/γ, while a circular polarization fraction becomes strong for off-beam cases. The spectro-temporal pulse-to-pulse properties can be a natural consequence due to the magnetospheric geometry. We investigate the relationship between drift rate, central frequency, and temporal duration. The radius-to-frequency mapping is derived and simulated within the assumptions of both dipolar and quadrupolar magnetic configurations. The geometric results show that FRBs are emitted in field lines more curved than open field lines for a dipolar geometry. This suggests that there are most likely existing multipolar magnetic configurations in the emission region.

EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions

Context. Although the role of magnetic fields in launching molecular outflows in massive young stellar objects has been convincingly demonstrated by theoretical arguments, observationally, the alignment of the magnetic field lines with the molecular outflows is still under debate. Aims. We aim to complete the measurements of the direction of the magnetic fields at milliarcsecond resolution around a sample of massive star-forming regions to determine whether the magnetic field and outflows are aligned. Methods. In 2012, we started a large very long baseline interferometry campaign with the European VLBI Network to measure the magnetic field orientation and strength toward a sample of 31 massive star-forming regions (called the flux-limited sample) by analyzing the polarized emission of 6.7 GHz CH3OH masers. In the previous papers of the series, we have presented 80% of the sample. Here, we report the linearly and circularly polarized emission of 6.7 GHz CH3OH masers toward the last five massive star-forming regions of the flux-limited sample. The sources are G30.70-0.07, G30.76-0.05, G31.28+0.06, G32.03+0.06, and G69.52-0.97. Results. We detected a total of 209 CH3OH maser cloudlets, 15% of which show linearly polarized emission (0.07–16.7%), and 2% of which show circularly polarized emission (0.2–4.2%). As reported in previous papers, in the last five sources of the flux-limited sample, we also measured well-ordered linear polarization vectors. Zeeman splitting was measured toward G30.70-0.07, G32.03+0.06, and G69.52-0.97. Conclusions. The statistical analysis of the entire flux-limited sample shows that the observations are consistent with a bimodal distribution in the difference between the 3D magnetic field direction and the outflow axis, with half the magnetic field directions being perpendicular and the other half being parallel to the outflow. In addition, we determined that typical values of the linear and circular polarization fractions for 6.7 GHz CH3OH masers are Pl = 1.0–2.5% and PV = 0.5–0.75%, respectively. From the circularly polarized spectra of the CH3OH maser features, we found that a typical Zeeman splitting is in the range between 0.5 m s−1 and 2.0 m s−1. This would correspond to 9 mG &lt; |B||| &lt; 40 mG if F = 3 → 4 is the most favored of the eight hyperfine transitions that might contribute to the maser emission.

Positron Effects on Polarized Images and Spectra from Jet and Accretion Flow Models of M87* and Sgr A*

Abstract We study the effects of including a nonzero positron-to-electron fraction in emitting plasma on the polarized spectral energy distributions and submillimeter images of jet and accretion flow models for near-horizon emission from M87* and Sgr A*. For M87*, we consider a semi-analytic fit to the force-free plasma regions of a general relativistic magnetohydrodynamic jet simulation, which we populate with power-law leptons with a constant electron-to-magnetic pressure ratio. For Sgr A*, we consider a standard self-similar radiatively inefficient accretion flow where the emission is predominantly from thermal leptons with a small fraction in a power-law tail. In both models, we fix the positron-to-electron ratio throughout the emission region. We generate polarized images and spectra from our models using the general relativistic ray tracing and radiative transfer from GRTRANS. We find that a substantial positron fraction reduces the circular polarization fraction at IR and higher frequencies. However, in submillimeter images, higher positron fractions increase polarization fractions due to strong effects of Faraday conversion. We find an M87* jet model that best matches the available broadband total intensity, and 230 GHz polarization data is a sub-equipartition, with positron fraction of ≃10%. We show that jet models with significant positron fractions do not satisfy the polarimetric constraints at 230 GHz from the Event Horizon Telescope (EHT). Sgr A* models show similar trends in their polarization fractions with increasing pair fraction. Both models suggest that resolved, polarized EHT images are useful to constrain the presence of pairs at 230 GHz emitting regions of M87* and Sgr A*.

Simple self-calibrating polarimeter for measuring the Stokes parameters of light

A simple, self-calibrating, rotating-waveplate polarimeter is largely insensitive to light intensity fluctuations and is shown to be useful for determining the Stokes parameters of light. This study shows how to minimize the in situ self-calibration time, the measurement time, and the measurement uncertainty. The suggested methods are applied to measurements of spatial variations in the linear and circular polarizations of laser light passing through glass plates with a laser intensity dependent birefringence. These are crucial measurements for the ACME electron electric dipole measurements, requiring accuracies in circular and linear polarization fraction of about 0.1% and 0.4%, with laser intensities up to 100 mW/mm2 incident into the polarimeter.

Constraining Theories of Polarized SiO Maser Transport: Multi-epoch Analysis of a π/2 Electric Vector Rotation Feature

Abstract The detailed polarization mechanisms of SiO masers originating from the near-circumstellar environment of asymptotic giant branch stars are not yet definitively known. Prevailing theories are broadly classified as either Zeeman or non-Zeeman in origin, the latter including effects such as anisotropic pumping or anisotropic resonant scattering. The predicted behavior of the linear and circular polarization fractions and the electric vector position angle (EVPA) vary by theory. In particular, individual maser features that exhibit a rotation in linear polarization of ∼π/2 as a function of frequency over their extent can be utilized as a test of several maser polarization transport theories. In this paper, we analyze one SiO (ν = 1, J = 1 − 0) maser feature toward the Mira variable, TX Cam, that exhibits this internal polarization rotation and persists across five epochs (spanning ∼3 months). We compare our results to the predictions by several maser polarization theories and find that the linear polarization across the feature is consistent with a geometric effect for a saturated maser originating when the angle between the projected magnetic field and the line of sight (θ) crosses the Van Vleck angle θ F ∼ 55°. However, the EVPA exhibits a smooth rotation across the spatial extent of the feature rather than the expected abrupt π/2 flip. We discuss possible explanations for this discrepancy and alternative theories. Circular polarization across the feature is also analyzed, and it is most accurately described by Zeeman effects giving rise to a circular polarization fraction of the form .

The SUrvey for Pulsars and Extragalactic Radio Bursts – III. Polarization properties of FRBs 160102 and 151230

We report on the polarization properties of two fast radio bursts (FRBs): 151230 and 160102 discovered in the SUrvey for Pulsars and Extragalactic Radio Bursts (SUPERB) at the Parkes radio telescope. FRB 151230 is observed to be 6 $\pm$ 11% circularly polarized and 35 $\pm$ 13 % linearly polarized with a rotation measure (RM) consistent with zero. Conversely, FRB 160102 is observed to have a circular polarization fraction of 30 $\pm$ 11 %, linear polarization fraction of 84 $\pm$ 15 % for RM = -221(6) rad m$^{-2}$ and the highest measured DM (2596.1 $\pm$ 0.3 pc cm$^{-3}$) for an FRB to date. We examine possible progenitor models for FRB 160102 in extragalactic, non-cosmological and cosmological scenarios. After accounting for the Galactic foreground contribution, we estimate the intrinsic RM to be -256(9) rad m$^{-2}$ in the low-redshift case and $\sim 2.4 \times 10^{2}$ rad m$^{-2}$ in the high-redshift case. We assess the relative likeliness of these scenarios and how each can be tested. We also place constraints on the scattering measure and study the impact of scattering on the signal's polarization position angle.

SAGITTARIUS A* ACCRETION FLOW AND BLACK HOLE PARAMETERS FROM GENERAL RELATIVISTIC DYNAMICAL AND POLARIZED RADIATIVE MODELING

We obtain estimates of Sgr A* accretion flow and black hole parameters by fitting polarized sub-mm observations with spectra computed using three-dimensional (3D) general relativistic (GR) magnetohydrodynamical (MHD) (GRMHD) simulations. Observations are compiled from averages over many epochs from reports in 29 papers for estimating the mean fluxes Fnu, linear polarization (LP) fractions, circular polarization (CP) fractions, and electric vector position angles (EVPAs). GRMHD simulations are computed with dimensionless spins a_*=0,0.5,0.7,0.9,0.98 over a 20,000M time interval. We perform fully self-consistent GR polarized radiative transfer using our new code to explore the effects of spin a_*, inclination angle \theta, position angle (PA), accretion rate Mdot, and electron temperature Te (Te is reported for radius 6M). By fitting the mean sub-mm fluxes and LP/CP fractions, we obtain estimates for these model parameters and determine the physical effects that could produce polarization signatures. Our best bet model has a_*=0.5, \theta=75deg, PA=115deg, Mdot=4.6*10^{-8}M_Sun/year, and Te=3.1*10^10K at 6M. The sub-mm CP is mainly produced by Faraday conversion as modified by Faraday rotation, and the emission region size at 230GHz is consistent with the VLBI size of 37microas. Across all spins, model parameters are in the ranges \theta=42deg-75deg, Mdot=(1.4-7.0)*10^{-8}M_Sun/year, and Te=(3-4)*10^10K. Polarization is found both to help differentiate models and to introduce new observational constraints on the effects of the magnetic field that might not be fit by accretion models so-far considered.

THE MAGNETOSPHERE OF THE ULTRACOOL DWARF DENIS 1048–3956

Ultracool dwarfs, the least-massive contributors to the stellar mass function, exhibit striking magnetic properties that are inconsistent with trends for more massive stars. Here, we present the widest-band radio observations to date of an ultracool dwarf, DENIS-P J104814.9-395604, in four 2 GHz bandwidths between wavelengths of 1 cm and 10 cm. These data were obtained with the Australia Telescope Compact Array using the new Compact Array Broadband Backend instrument. We detected a stable negatively-sloped power-law spectrum in total intensity, with spectral index alpha=1.71+-0.09. Circular polarization fractions between 0.25 and 0.4 were found at the low-frequency end of our detection band. We interpret these results as indicative of gyrosynchrotron emission. We suggest that the radio emission originates from beyond the co-rotation radius, R_C, of the star. Adopting this model, we find R_C between 1.2-2.9 R_*, and a non-thermal electron density and magnetic field strength between 10^(5)-10^(7.2) cm^(-3) and 70-260 G respectively at R_C. The model accounts for the violation of the Guedel-Benz relation between X-ray and radio luminosities of low-mass stars by DENIS-P J104814.9-395604.

General relativistic polarized radiative transfer: building a dynamics-observations interface

The rising number of polarized observations of relativistic sources necessitates a correct theory for proper model fitting. The equations for general relativistic (GR) polarized radiative transfer are derived starting from the Boltzmann equation and basic ideas of general relativity. The derivation is aimed at providing a practical guide to reproducing the synchrotron part of radio and submillimetre emission from low-luminosity active galactic nuclei (LLAGNs), in particular Sgr A*, and jets. A recipe for the fast exact calculation of cyclo-synchrotron emissivities, absorptivities, Faraday rotation and conversion coefficients is given for isotropic particle distributions. The multitude of physical effects influencing simulated spectra is discussed. The application of the prescribed technique is necessary to determine the black hole spin in LLAGNs. The observations of total flux, linear and circular polarization fractions, and electric vector position angle as functions of the observed frequency could substantially constrain the absolute value and orientation of spin.