High precision polarimetry Sources with stable linear and circular - - PowerPoint PPT Presentation

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
• emission processes
• variability mechanisms

Challenging due to

• low levels of LP and especially CP
• high levels of variability
• instruments specialize on either LP or CP

Design and development of a new 
 full-Stokes polarimetric data analysis methodology

Ipeak = 749 mJy LPpeak = 25 mJy EVPA = -6º Ipeak = 860 mJy LPpeak = 13 mJy EVPA = -68º

credit: VLBA-BU Blazar Monitoring Program

dT = 18 days

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3

Complete pipeline

• from telescope observables to I, Q, U, V

Several correction steps

• pointing, opacity, elevation-dependent gain

Careful treatment of telescope response

• Airy disk instead of gaussian beam pattern

Designed for CP feeds but easily applicable also to LP feeds Minimization of instrumental effects: LP , CP , EVPA

Linear and Circular polarimetry with Effelsberg

new, high-precision data analysis methodology

LCP RCP COS SIN

Instrumental linear polarization correction

Data fitting to extract amplitudes Cross-channel calibration (K or Jy) Pointing correction Opacity correction Gain-curve correction

Instrumental circular polarization correction

I V Q U Rotation

+

• mc

I V

Instrumental EVPA rotation correction

Myserlis et al. 2018, A&A, 609A, 68M Myserlis et al., Galaxies, vol. 4, issue 4, p. 58

• we recover significant Q and U from

corrupted measurements Instrument model

• parametrization with smooth functions

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Stokes Q Polarized source Stokes Q Unpolarized source

Instrumental LP correction

Amplitude

• ffset

Amplitude

Weakly polarized source uncorrected

Amplitude

Weakly polarized source corrected

Myserlis et al. 2018, A&A, 609A, 68M

Instrumental EVPA rotation correction

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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°

Poppi et al. 2002, AIP Conf. Proceedings, Vol. 609, 187–192 Myserlis et al. 2018, A&A, 609A, 68M

LP EVPA

Instrumental CP correction

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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 mc = m0

cr + m0 c + r − 1

m0

cr − m0 c + r + 1 Myserlis et al. 2018, A&A, 609A, 68M

Instrumental CP correction

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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 |Δmc|: 0.2 %

Myserlis et al. 2018, A&A, 609A, 68M

Principle 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 (>5σ) for low LP data

Comparison with Müller matrix correction

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Scorr = M−1 · Sobs

Myserlis et al. 2018, A&A, 609A, 68M

• 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 1σ regions around the mean values of the uncorrected and corrected data sets, respectively. The mean, µ, and standard deviation, σ, values

• f the corresponding data sets are shown in the legend.

Linear and circular polarimetry with Effelsberg

new, high-precision data analysis methodology

I (Jy) LP (%) EVPA (°) CP (%) MJD

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Uncertainties:

• LP degree: 0.1 %
• CP degree: 0.1—0.2 %
• EVPA: 1°

High-cadence, full-Stokes light curves using the F-GAMMA data set

Today at 15:30 “F-GAMMA: multi-frequency radio monitoring of Fermi blazars” by E. Angelakis

Myserlis et al. 2018, A&A, 609A, 68M

Sources with stable polarization over 5.5 yrs

July 2010 – April 2016

<σ> = 0.1 % <σ> = 1.7º <σ> = 0.1 %

10 Myserlis et al. 2018, A&A, 609A, 68M

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

Conclusions

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