High precision polarimetry Sources with stable linear and circular polarization in the GHz regime Ioannis Myserlis, E. Angelakis & J. A. Zensus Max Planck Institute für Radioastronomie I. Myserlis, E. Angelakis, A. Kraus, C. Liontas, N. Marchili, M. Aller, H. Aller, V. Karamanavis, L. Fuhrmann, T. Krichbaum and A. Zensus 2018, A&A, 609A, 68M
Linear and Circular polarimetry powerful tool but challenging endeavor Investigate - physical conditions I peak = 749 mJy - emission processes LP peak = 25 mJy - variability mechanisms EVPA = -6º Challenging due to - low levels of LP and especially CP - high levels of variability dT = 18 days - instruments specialize on either LP or CP Design and development of a new I peak = 860 mJy full-Stokes polarimetric data analysis methodology LP peak = 13 mJy EVPA = -68º credit: VLBA-BU Blazar Monitoring Program 2
Linear and Circular polarimetry with E ff elsberg new, high-precision data analysis methodology Complete pipeline LCP RCP COS SIN - from telescope observables to I , Q , U , V Instrumental linear polarization correction Several correction steps Data fitting to extract amplitudes - pointing, opacity, elevation-dependent gain Cross-channel calibration (K or Jy) Careful treatment of telescope response Pointing correction - Airy disk instead of gaussian beam pattern Opacity correction Designed for CP feeds but easily applicable also to LP feeds Gain-curve correction Minimization of instrumental e ff ects: LP , CP , EVPA + - Rotation I V m c Instrumental circular Instrumental EVPA polarization correction rotation correction Myserlis et al. 2018, A&A, 609A, 68M Myserlis et al., Galaxies, vol. 4, issue 4, p. 58 I V Q U 3
Instrumental LP correction Stokes Q Instrument model Polarized source - parametrization with smooth functions - we recover significant Q and U from corrupted measurements Stokes Q Weakly polarized source Weakly polarized source uncorrected Unpolarized source corrected Amplitude Amplitude Amplitude offset Myserlis et al. 2018, A&A, 609A, 68M 4
Instrumental EVPA rotation correction Moon's linear polarization - LP degree maximized towards limb - radially oriented EVPA Scanning directions: 0°, 30°, 45°, 60°, 90°, 120°, 135°, 150° Instrumental rotation - 4.85 GHz: 1.26 ± 0.11° - 8.35 GHz: -0.50 ± 0.12° EVPA LP Poppi et al. 2002, AIP Conf. Proceedings, Vol. 609, 187–192 5 Myserlis et al. 2018, A&A, 609A, 68M
Instrumental CP correction Indications of instrumental CP - CP degree distributions centered at non-zero value - non-zero CP measurements of unpolarized sources Sensitivity imbalance between left- and right- circularly polarized feeds, r m c = m 0 c r + m 0 c + r − 1 m 0 c r − m 0 c + r + 1 Myserlis et al. 2018, A&A, 609A, 68M 6
Instrumental CP correction r estimation methods: A. Unpolarized sources B. Sources with stable CP - singular value decomposition (SVD) - no need to know their CP a priori Comparison with UMRAO dataset - 169 concurrent measurements - 5 sources - median | Δ m c |: 0.2 % Myserlis et al. 2018, A&A, 609A, 68M 7
Comparison with Müller matrix correction Principle S corr = M − 1 · S obs Fundamental advantages of our method - correct the spatial dependence of instrumental LP - individual instrumental LP correction for each measurement - no need to know the CP of stable sources a priori Results - reduced intra-session variability of Stokes I , Q and U - significant improvement (>5 σ ) for low LP data Fig. 6. Degree of linear polarization (top panel) and polarization angle (bottom panel) of the source 3C 48 at 4.85 GHz before (red circles) and after (blue triangles) applying the instrumental linear polarization correction. The data correspond to 23 sub-scans of the source within a single observing session. The dashed red and dotted blue lines indicate the 1 σ regions around the mean values of the uncorrected and corrected data sets, respectively. The mean, µ , and standard deviation, σ , values of the corresponding data sets are shown in the legend. 8 Myserlis et al. 2018, A&A, 609A, 68M
Linear and circular polarimetry with E ff elsberg new, high-precision data analysis methodology Uncertainties: - LP degree: 0.1 % I - CP degree: 0.1—0.2 % (Jy) - EVPA: 1° High-cadence, full-Stokes light curves using LP the F-GAMMA data set (%) Today at 15:30 “F-GAMMA: multi-frequency radio monitoring of Fermi blazars” EVPA by E. Angelakis (°) CP (%) MJD Myserlis et al. 2018, A&A, 609A, 68M 9
Sources with stable polarization over 5.5 yrs July 2010 – April 2016 < σ > = 0.1 % < σ > = 1.7º < σ > = 0.1 % Myserlis et al. 2018, A&A, 609A, 68M 10
Conclusions New full-Stokes polarimetric data analysis methodology Instrumental LP correction - across the whole beam - individually for each measurement Instrumental EVPA correction using lunar observations Instrumental CP correction - two independent methods - consistent results with completely independent data sets (UMRAO) More stable results than Müller matrix method, especially for low LP sources Precision reached: 0.1–0.2 % for LP and CP degrees, 1º for EVPA Sources with stable LP and CP over 5.5 yrs - 7 with stable LP degree, 8 with stable EVPA and 11 with stable CP degree - 3C 286, 3C 48, 3C 295, NGC 7027 are stable in LP , EVPA and CP - LP and CP calibrator candidates 11
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