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I. N. Reid - One of the best experts on this subject based on the ideXlab platform.

  • Circular Polarization in scattered light as a possible biomarker
    Journal of Quantitative Spectroscopy & Radiative Transfer, 2009
    Co-Authors: William B. Sparks, Ludmilla Kolokolova, J. H. Hough, Thomas A. Germer, Feng Chen, Shiladitya Dassarma, Priya Dassarma, Frank T. Robb, Nadine Manset, I. N. Reid
    Abstract:

    Biological molecules exhibit homochirality and are optically active. Therefore, it is possible that the scattering of light by biological molecules might result in a macroscopic signature in the form of Circular Polarization. If this is the case, then Circular Polarization spectroscopy, which may be utilized in remote sensing, can offer a powerful indicator of the presence of a universal biosignature, namely homochirality. Here, we describe laboratory experiments designed to investigate this idea. We focus on photosynthetic microorganisms, and also show results from macroscopic vegetation and control minerals. In the microorganisms, we find unambiguous Circular Polarization associated with electronic absorption bands of the photosynthetic apparatus. Macroscopic vegetation yields a stronger and more complex signature while the control minerals produce low-levels of Circular Polarization unrelated to their spectra. We propose a heuristic explanation of our results, which is that the Polarization is produced by Circular dichroism in the material after the light has undergone its last scattering event. The results are encouraging for the use of Circular Polarization spectroscopy in remote sensing of a generic biomarker from space or the ground.

  • Circular Polarization in scattered light as a possible biomarker
    Journal of Quantitative Spectroscopy and Radiative Transfer, 2009
    Co-Authors: W.b. Sparks, J. H. Hough, Thomas A. Germer, Feng Chen, Shiladitya Dassarma, Priya Dassarma, Frank T. Robb, Nadine Manset, L. Kolokolova, I. N. Reid
    Abstract:

    Original article can be found at : http://www.sciencedirect.com/ Copyright ElsevierBiological molecules exhibit homochirality and are optically active. Therefore, it is possible that the scattering of light by biological molecules might result in a macroscopic signature in the form of Circular Polarization. If this is the case, then Circular Polarization spectroscopy, which may be utilized in remote sensing, can offer a powerful indicator of the presence of a universal biosignature, namely homochirality. Here, we describe laboratory experiments designed to investigate this idea. We focus on photosynthetic microorganisms, and also show results from macroscopic vegetation and control minerals. In the microorganisms, we find unambiguous Circular Polarization associated with electronic absorption bands of the photosynthetic apparatus. Macroscopic vegetation yields a stronger and more complex signature while the control minerals produce low-levels of Circular Polarization unrelated to their spectra. We propose a heuristic explanation of our results, which is that the Polarization is produced by Circular dichroism in the material after the light has undergone its last scattering event. The results are encouraging for the use of Circular Polarization spectroscopy in remote sensing of a generic biomarker from space or the ground

G. J. Qiao - One of the best experts on this subject based on the ideXlab platform.

  • Circular Polarization in pulsar integrated profiles
    Monthly Notices of the Royal Astronomical Society, 1998
    Co-Authors: Jin-lin Han, Richard N. Manchester, G. J. Qiao
    Abstract:

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

Jin-lin Han - One of the best experts on this subject based on the ideXlab platform.

  • Circular Polarization in Pulsar Integrated Profiles: Updates
    Chinese Journal of Astronomy and Astrophysics, 2006
    Co-Authors: Xiao-peng You, Jin-lin Han
    Abstract:

    We update the systematic studies of Circular Polarization in integrated pulse profiles by Han et al (1998). Data of Circular Polarization profiles are compiled. Sense reversals can occur in core or cone components, or near the intersection between components. The correlation between the sense of Circular Polarization and the sense of position angle variation for conal-double pulsars is confirmed with a much large database. Circular Polarization of some pulsars has clear changes with frequency. Circular Polarization of millisecond pulsars is marginally different from that of normal pulsars.

  • A Summary of Pulsar Circular Polarization
    Chinese Journal of Astronomy and Astrophysics, 2006
    Co-Authors: Xiao-peng You, Jin-lin Han
    Abstract:

    We update the systematic studies of Circular Polarization in integrated pulse profiles of Han et al. Sense reversal can occur in either core or cone components, or near the intersection between the components. The correlation between the sense of Circular Polarization and the sense of position angle variation for conal double pulsars is confirmed with a much larger database. Some pulsars show clear changes of Circular Polarization with frequency. Circular Polarization is marginally different between millisecond and normal pulsars.

  • Circular Polarization in pulsar integrated profiles
    Monthly Notices of the Royal Astronomical Society, 1998
    Co-Authors: Jin-lin Han, Richard N. Manchester, G. J. Qiao
    Abstract:

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

John F. C. Wardle - One of the best experts on this subject based on the ideXlab platform.

  • Probing the Circular Polarization of Relativistic Jets on VLBI Scales
    Astrophysics and Space Science, 2003
    Co-Authors: Daniel C. Homan, John F. C. Wardle
    Abstract:

    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.

  • Parsec-Scale Circular Polarization Observations of 40 Blazars
    The Astrophysical Journal, 2001
    Co-Authors: Daniel C. Homan, Joanne M. Attridge, John F. C. Wardle
    Abstract:

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

  • The Nature of Jets: Evidence from Circular Polarization Observations
    arXiv: Astrophysics, 2000
    Co-Authors: John F. C. Wardle, Daniel C. Homan
    Abstract:

    We review recent observations of Circularly polarized radiation from AGN made with the VLBA and with the ATCA. We also discuss briefly the detections of the Galactic sources Sag A* and SS433. The origin of the Circular Polarization is still an open question in most cases, and we discuss four possible mechanisms. Detectable Circular Polarization is a common property of quasars, but not of radio galaxies, and is always associated with the compact core. There is growing evidence that the sign of the Circular Polarization stays the same over at least two or three decades in time, suggesting it is a fundamental property of the jet.

  • Detection and Measurement of Parsec-Scale Circular Polarization in Four AGNs
    The Astronomical Journal, 1999
    Co-Authors: Daniel C. Homan, John F. C. Wardle
    Abstract:

    We present five epochs of 15 GHz VLBA observations of 13 AGNs. These observations were specially calibrated to detect parsec-scale Circular Polarization and our calibration techniques are discussed and analyzed in detail. We obtained reliable detections of parsec-scale Circular Polarization in the radio jets of four AGNs: 3C 84, PKS 0528+134, 3C 273, and 3C 279. For each of these objects our detections are at the level of approximately 0.3%–1% local fractional Circular Polarization. Each individual detection has a significance in the range of 3 to 10 σ. Our observations are consistent across multiple epochs (and different calibration techniques) in the sign and magnitude of the Circular Polarization observed. 3C 273 and 3C 279 both undergo core outbursts during our observations and changes in the Circular Polarization of both sources are correlated with these outbursts. In general, we observe the Circular Polarization to be nearly coincident with the strong VLBI cores of these objects; however, in 3C 84 the Circular Polarization is located a full milliarcsecond south of the source peak, and in the 1996.73 epoch of 3C 273 the Circular Polarization is predominately associated with the newly emerging jet component. Our observations support the theoretical conclusion that emission of Circular Polarization is a sensitive function of opacity, being strongest when the optical depth is near unity. Circular Polarization may be produced as an intrinsic component of synchrotron radiation or by the Faraday conversion of linear to Circular Polarization. Our single-frequency observations do not easily distinguish between these possible mechanisms, but independent of mechanism, the remarkable consistency across epoch of the sign of the observed Circular Polarization suggests the existence of a long-term, stable, unidirectional magnetic field. Single-dish observations of 3C 273 and 3C 279 at 8 GHz by Hodge and Aller suggest that this stability may persist for decades in our frame of observation.

  • Detection of Parsec-Scale Circular Polarization with the VLBA
    International Astronomical Union Colloquium, 1998
    Co-Authors: Daniel C. Homan, John F. C. Wardle, R. Ojha, David H. Roberts
    Abstract:

    Circular Polarization has been detected in 3C 279 and 3C 84 at multiple epochs, using the VLBA at 15 GHz. This is the first detection of Circular Polarization in a compact radio source at VLBI resolution. In the case of 3C 279 the Polarization is most likely produced by the conversion of linear to Circular Polarization by low energy relativistic electrons in the emitting region. If this is so, the absence of Faraday rotation by the same electrons suggests that the jet consists of an electron-positron plasma.

Jean-pierre Macquart - One of the best experts on this subject based on the ideXlab platform.

  • Circular Polarization in relativistic jets
    New Astronomy Reviews, 2003
    Co-Authors: Jean-pierre Macquart
    Abstract:

    Circular Polarization is observed in some relativistic jet sources at radio wavelengths. It is largely associated with activity in the cores of the radio sources, is highly variable, and is strongest during ejection episodes. VLBI imaging and interstellar scintillation arguments show that the degree of Circular Polarization is as high a few percent in some sources, and is associated with milliarcsecond and microarcsecond source structure. The Circular Polarization most likely originates as a result of conversion of linear Polarization in the relativistic plasma surrounding the source. However, cyclotron or synchrotron emission can account for the magnitude of the Circular Polarization observed in some Galactic jet sources. (C) 2003 Elsevier B.V. All rights reserved.

  • Similarities between Circular Polarization in Galactic Jet Sources and AGN
    Astrophysics and Space Science, 2003
    Co-Authors: Jean-pierre Macquart, Robert J. Sault, Diana Hannikainen, David L. Jauncey
    Abstract:

    We compare the observational properties of the Circular Polarization in Galactic jet sources with that observed in AGN, and outline the constraints they place on the mechanism responsible for the Circular Polarization. We also discuss the implications of the time scale of Polarization variations on the mechanism responsible for the Circular Polarization.

  • Scintillation-induced Circular Polarization in Pulsars and Quasars
    The Astrophysical Journal, 2000
    Co-Authors: Jean-pierre Macquart, D. B. Melrose
    Abstract:

    We present a physical interpretation for the generation of Circular Polarization resulting from the propagation of radiation through a magnetized plasma in terms of a rotation measure gradient, or "Faraday wedges." Criteria for the observability of scintillation-induced Circular Polarization are identified. Application of the theory to the Circular Polarization in pulsars and compact extragalactic sources is discussed.