Probing the Circular Polarization of Relativistic Jets on VLBI Scales

High resolution studies of circular polarization allow us see where it arises in a jet, study its local fractional level and spectrum, and compare these results to local measures of linear polarization and Faraday rotation. Here we not only review past results from Very Long Baseline Array (VLBA) circular polarization studies, but we also present preliminary new results on two quasars. In the core of PKS 0607-157, we find strong circular polarization at 8 GHz and much weaker levels at 15 GHz. Combined with the linear polarization data, we favor a simple model where the circular is produced by Faraday conversion driven by a small amount of Faraday rotation. In the core of 3C345, we find strong circular polarization at 15 GHz in a component with distinct linear polarization. This core component is optically thick at 8 GHz, where we detect no circular polarization. With opposite trends in frequency for PKS 0607-157 and 3C345, it seems clear that local conditions in a jet can have a strong effect on circular polarization and need to be taken into account when studying inhomogeneous objects with multi-frequency observations.Comment: To appear in the proceedings of "Circular Polarization of Relativistic Jet Sources", eds R. P. Fender and J.-P. Macquart, in Astrophysics and Space Science. 11 pgs, 2 fig

Theoretical Models for Producing Circularly Polarized Radiation in Extragalactic Radio Sources

We discuss the production of circular polarization in compact radio sources both by the intrinsic mechanism and by Faraday conversion. We pay particular attention to the magnetic field structure, considering partially ordered fiel ds and Laing sheets, and distinguishing between uniform and unidirectional fields. (The latter can be constrained b y flux conservation arguments.) In most cases, Faraday conversion is the more important mechanism. Conversion opera tes on Stokes U, which can be generated by internal Faraday rotation, or by magnetic field fluctuations, which can therefore produce circular polarization even in a pure pair plasma. We also show that the spectrum of circular pola rization in an inhomogeneous jet can be quite different from that in a uniform source, being flat or even inverted.Comment: To appear in the proceedings of "Circular Polarization of Relativistic Jet Sources", eds R. P. Fender and J.-P. Macquart, in Astrophysics and Space Science. 12 pgs, 0 fig

Physical Properties of Jets in AGN

I review constraints on the physical properties of AGN jets revealed through Very Long Baseline Interferometry (VLBI) studies of the structure and time-evolution of parsec-scale jets, including recent results from the MOJAVE program. In particular I focus on constraints available from very long time baseline studies which probe a wide range of jet behavior over many outbursts. Kinematic studies of propagating jet features find an apparent speed distribution that peaks around 10c for blazars, with speeds up to 50c observed. These observed speeds require Lorentz factors at least as large, implying that parsec-scale Lorentz factors up to 10-20 are common for blazars with a tail up to ~ 50. Jet flows are still becoming organized on these scales as evidenced by the high incidence of non-radial motions and/or accelerations of jet features (including increases and decreases in apparent speed and direction). Changes in Lorentz factors of propagating jet features appear to play a significant role in the observed accelerations, and while the connection between acceleration of jet features and the underlying flow is not clear, the pattern of observed accelerations suggest the flow may increase in speed near the base of the jet and decrease further out. In some jets, ejections of new features span a range of ejection angles over many epochs, tracing out wider opening angles on parsec-scales than are apparent in single epoch observations.Comment: 10 pages, 2 figures, submitted to IJMPCS for proceedings of HEPROIII: High Energy Phenomena in Relativistic Outflows III, Barcelona, June 27 - July 1, 201

Concurrent 43 and 86 GHz Very Long Baseline Polarimetry of 3C273

We present sub-milliarcsecond resolution total intensity and linear polarization VLBI images of 3C273, using concurrent 43 and 86 GHz data taken with the Very Long Baseline Array in May 2002. The structure seen in the innermost jet suggest that we have fortuitously caught the jet in the act of changing direction. The polarization images confirm that the core is unpolarized (fractional polarization m < 1 %) at 86 GHz, but also show well ordered magnetic fields (m ~ 15 %) in the inner jet, at a projected distance of 2.3 pc from the core. In this strongly polarized region, the rotation measure changes across the jet by 4.2 x 10^{4} rad m^{-2} over an angular width of about 0.3 milliarcseconds. If the lack of polarization in the core is also attributed to a Faraday screen, then a rotation measure dispersion > 5.2 x 10^{4} rad m^{-2} must be present in or in front of that region. These are among the highest rotation measures reported so far in the nucleus of any active galaxy or quasar, and must occur outside (but probably close to) the radio emitting region. The transverse rotation measure gradient is in the same sense as that observed by Asada et al and by Zavala and Taylor at greater core distances. The magnitude of the transverse gradient decreases rapidly with distance down the jet, and appears to be variable.Comment: 4 pages, LaTeX, 3 postscript figures, submitted to Astrophysical Journal Letter

Aerosols in the tropical and subtropical UT/LS: in-situ measurements of submicron particle abundance and volatility

Processes occurring in the tropical upper troposphere (UT), the Tropical Transition Layer (TTL), and the lower stratosphere (LS) are of importance for the global climate, for stratospheric dynamics and air chemistry, and for their influence on the global distribution of water vapour, trace gases and aerosols. In this contribution we present aerosol and trace gas (in-situ) measurements from the tropical UT/LS over Southern Brazil, Northern Australia, and West Africa. The instruments were operated on board of the Russian high altitude research aircraft M-55 "Geophysica" and the DLR Falcon-20 during the campaigns TROCCINOX (Araçatuba, Brazil, February 2005), SCOUT-O3 (Darwin, Australia, December 2005), and SCOUT-AMMA (Ouagadougou, Burkina Faso, August 2006). The data cover submicron particle number densities and volatility from the COndensation PArticle counting System (COPAS), as well as relevant trace gases like N2O, ozone, and CO. We use these trace gas measurements to place the aerosol data into a broader atmospheric context. Also a juxtaposition of the submicron particle data with previous measurements over Costa Rica and other tropical locations between 1999 and 2007 (NASA DC-8 and NASA WB-57F) is provided. The submicron particle number densities, as a function of altitude, were found to be remarkably constant in the tropical UT/LS altitude band for the two decades after 1987. Thus, a parameterisation suitable for models can be extracted from these measurements. Compared to the average levels in the period between 1987 and 2007 a slight increase of particle abundances was found for 2005/2006 at altitudes with potential temperatures, theta, above 430 K. The origins of this increase are unknown except for increases measured during SCOUT-AMMA. Here the eruption of the Soufrière Hills volcano in the Caribbean caused elevated particle mixing ratios. The vertical profiles from Northern hemispheric mid-latitudes between 1999 and 2006 also are compact enough to derive a parameterisation. The tropical profiles all show a broad maximum of particle mixing ratios (between theta ~ 340 K and 390 K) which extends from below the TTL to above the thermal tropopause. Thus these particles are a "reservoir" for vertical transport into the stratosphere. The ratio of non-volatile particle number density to total particle number density was also measured by COPAS. The vertical profiles of this ratio have a maximum of 50% above 370 K over Australia and West Africa and a pronounced minimum directly below. Without detailed chemical composition measurements a reason for the increase of non-volatile particle fractions cannot yet be given. However, half of the particles from the tropical "reservoir" contain compounds other than sulphuric acid and water. Correlations of the measured aerosol mixing ratios with N2O and ozone exhibit compact relationships for the tropical data from SCOUT-AMMA, TROCCINOX, and SCOUT-O3. Correlations with CO are more scattered probably because of the connection to different pollution source regions. We provide additional data from the long distance transfer flights to the campaign sites in Brazil, Australia, and West-Africa. These were executed during a time window of 17 months within a period of relative volcanic quiescence. Thus the data represent a "snapshot picture" documenting the status of a significant part of the global UT/LS fine aerosol at low concentration levels 15 years after the last major (i.e., the 1991 Mount Pinatubo) eruption. The corresponding latitudinal distributions of the measured particle number densities are presented in this paper to provide data of the UT/LS background aerosol for modelling purposes

Intrinsic brightness temperatures of blazar jets at 15 GHz

We have developed a new Bayesian Markov Chain Monte Carlo method to deconvolve light curves of blazars into individual flares, including proper estimation of the fit errors. We use the method to fit 15GHz light curves obtained within the OVRO 40-m blazar monitoring program where a large number of AGN have been monitored since 2008 in support of the Fermi Gamma-Ray Space Telescope mission. The time scales obtained from the fitted models are used to calculate the variability brightness temperature of the sources. Additionally, we have calculated brightness temperatures of a sample of these objects using Very Long Baseline Array data from the MOJAVE survey. Combining these two data sets enables us to study the intrinsic brightness temperature distribution in these blazars at 15 GHz. Our preliminary results indicate that the mean intrinsic brightness temperature in a sample of 14 sources is near the equipartition brightness temperature of ~ 10^(11)K

Constraints on particles and fields from full Stokes observations of AGN

Combined polarization imaging of radio jets from Active Galactic Nuclei (AGN) in circular and linear polarization, also known as full Stokes imaging, has the potential to constrain both the magnetic field structure and particle properties of jets. Although only a small fraction of the emission when detected, typically less than a few tenths of a percent but up to as much as a couple of percent in the strongest resolved sources, circular polarization directly probes the magnetic field and particles within the jet itself and is not expected to be modified by external screens. A key to using full Stokes observations to constrain jet properties is obtaining a better understanding of the emission of circular polarization, including its variability and spectrum. We discuss what we have learned so far from parsec scale monitoring observations in the MOJAVE program and from multi-frequency observations of selected AGN.                                                                                                                </p

In situ measurements of tropical cloud properties in the West African Monsoon: upper tropospheric ice clouds, Mesoscale Convective System outflow, and subvisual cirrus

In situ measurements of ice crystal size distributions in tropical upper troposphere/lower stratosphere (UT/LS) clouds were performed during the SCOUT-AMMA campaign over West Africa in August 2006. The cloud properties were measured with a Forward Scattering Spectrometer Probe (FSSP-100) and a Cloud Imaging Probe (CIP) operated aboard the Russian high altitude research aircraft M-55 Geophysica with the mission base in Ouagadougou, Burkina Faso. A total of 117 ice particle size distributions were obtained from the measurements in the vicinity of Mesoscale Convective Systems (MCS). Two to four modal lognormal size distributions were fitted to the average size distributions for different potential temperature bins. The measurements showed proportionately more large ice particles compared to former measurements above maritime regions. With the help of trace gas measurements of NO, NOy, CO2, CO, and O3 and satellite images, clouds in young and aged MCS outflow were identified. These events were observed at altitudes of 11.0 km to 14.2 km corresponding to potential temperature levels of 346 K to 356 K. In a young outflow from a developing MCS ice crystal number concentrations of up to (8.3 ± 1.6) cm−3 and rimed ice particles with maximum dimensions exceeding 1.5 mm were found. A maximum ice water content of 0.05 g m−3 was observed and an effective radius of about 90 μm. In contrast the aged outflow events were more diluted and showed a maximum number concentration of 0.03 cm−3, an ice water content of 2.3 × 10−4 g m−3, an effective radius of about 18 μm, while the largest particles had a maximum dimension of 61 μm. Close to the tropopause subvisual cirrus were encountered four times at altitudes of 15 km to 16.4 km. The mean ice particle number concentration of these encounters was 0.01 cm−3 with maximum particle sizes of 130 μm, and the mean ice water content was about 1.4 × 10−4 g m−3. All known in situ measurements of subvisual tropopause cirrus are compared and an exponential fit on the size distributions is established for modelling purposes. A comparison of aerosol to ice crystal number concentrations, in order to obtain an estimate on how many ice particles may result from activation of the present aerosol, yielded low ratios for the subvisual cirrus cases of roughly one cloud particle per 30 000 aerosol particles, while for the MCS outflow cases this resulted in a high ratio of one cloud particle per 300 aerosol particles