Searching for Faraday Complexity in the ATCA Beta Test Fields

Transcription

1 Searching for Faraday Complexity in the ATCA Beta Test Fields A Brief Overview & Preliminary Results Craig Anderson SIfA CASS Collaborators: Bryan Gaensler (USYD) FACULTY OF SCIENCE Ilana Feain (CASS) Thomas Franzen (CASS)

2 Motivation: Dodging The Spatial Resolution Constraint Motivating Question: What can we determine about the physical structure of this blob of plasma and magnetism, given it is spatially unresolved by our telescope? 2

3 One Application: AGN Science Magnetic processes dominate AGN phenomenology behaviour of accretion disks jet launching and collimation particle acceleration cluster B field seeding Typically studied with VLBI Highly effective but limited - poor survey capability & small bandwidth coverage (Gomez et al. 211) Imagine we had a super-resolution technique for studying B fields > a complimentary approach to studying critical AGN processes. >> RM Synthesis gives us that capability. 3

4 Refresher: Faraday Effect The Faraday Effect 1 Polarisation Angle vs. Wavelength Squared (1) (2) Relative Polarisation Angle (Degrees) Observing Wavelength Squared [m 2 ] - The Faraday Effect > Polarisation angle of EM radiation rotated as a linear function of wavelength-squared (Eqn. 1) - Characterised by the Rotation Measure, or Faraday Depth > Measure of the depth of magnetic field from which radiation has emanated. 4

5 Refresher: RM Synthesis RM Synthesis 1 Polarisation Angle vs. Wavelength Squared 8 Relative Polarisation Angle (Degrees) Observing Wavelength Squared [m 2 ] - Gives Faraday Dispersion Function the intensity of polarised emission emanating from each Faraday Depth. Polarised Flux [Jy / (rad / m 2 )] Reconstructed Faraday Dispersion Function 1 - Resolves emission spectrally rather than spatially! Reconstructed Faraday Depth [rad / m 2 ] 5

6 ATCA Beta Test Fields Two 3 degree 2 mosaiced fields located in Circinus & Fornax observed with ATCA. My work currently focuses on Fornax field only. - ~34 pointings; GHz; 15m & 75m ATCA configs - Sensitivity ~ 15 ujy for mosaiced 512 MHz MFS image - ~125 sources detected to 1sig in Stokes I - ~2% significantly polarised 512MHz MFS mosaic of ATCA beta Fornax field Image credit: Thomas Franzen (CASS) 6

7 ATCA Beta Test Fields 1) Imaging: linear mosaics of 8 MHz MFS images through entire band. 2) Source finding (T. Franzen) 3) Stokes I, Q, U, QU noise extraction across full band (5x5 boxed extraction) 4) RM synthesis & rmclean using Q/I and U/I (decouples SI effects) 5) Sources classified as simple or complex/structured based on second moment ( spread ) of clean-component distribution in Faraday Depth space (algorithm: S. Brown) Figures adapted from POSSUM memo by Shea Brown Off-axis instrumental pol a major concern > several checks performed to date with available data, but more characterisation work to do. 7

8 Preliminary Results: Prevalence of Faraday Complexity Initial breakdown of Faraday structure classifications (probably too simplistic): - 77% effectively unpolarised - ~18% polarised with simple Faraday structure (to first order) - ~5% polarised with complexity apparent in Faraday structure Position of Current Source in Fornax Mosaic Position of Current Source in Fornax Mosaic Declination (deg) Declination (deg) RA (deg) RA (deg)

9 Example: A Simple Source Example of a source classified as simple : Reconstructed Emission per Unit Faraday Depth [Jy / rad.m 2 / beam] FDF RM Synth and CLEAN Data for Source Designation:217, (File Number:217), (SP PC sep:.1197) Dirty FDF (Magnitude) CLEANing Residuals (Magnitude).2.14 CLEANed FDF (Magnitude) CLEANing Cutoff (.17933) Clean Components Clean Components.1 FWHM of RMSF Main Beam Faraday Depth [rad / m 2 ] Q, U, P Flux (Jy) Q, U and P From Data + QU Clean Model Wavelength squared 9

10 Example: A Complex Source Example of a source classified as complex: Reconstructed Emission per Unit Faraday Depth [Jy / rad.m 2 / beam] FDF RM Synth and CLEAN Data for Source Designation:65, (File Number:65), (SP PC sep:.13376).12.5 Dirty FDF (Magnitude).45 CLEANing Residuals (Magnitude).1 CLEANed FDF (Magnitude).4 CLEANing Cutoff (.18911) Clean Components.8.35 Clean Components FWHM of RMSF Main Beam Faraday Depth [rad / m 2 ] Q, U, P Flux (Jy) Q, U and P From Data + QU Clean Model Wavelength squared 1

11 Example: A Complex Source Data Reconstructed Emission per Unit Faraday Depth [Jy / rad.m 2 / beam] FDF RM Synth and CLEAN Data for Source Designation:65, (File Number:65), (SP PC sep:.13376).5.12 Dirty FDF (Magnitude).45 CLEANing Residuals (Magnitude).1 CLEANed FDF (Magnitude).4 CLEANing Cutoff (.18911) Clean Components.8.35 Clean Components FWHM of RMSF Main Beam x Proscribed Faraday Dispersion Function Faraday Depth [rad / m 2 ] Real part of FDF.9 Complex part of FDF Amplitude of FDF.8 Q, U, P Flux (Jy) Q, U and P From Data + QU Clean Model Wavelength squared 7 x 1 3 Observable Complex Polarisation as a Function of Lambda Squared 6 Real part of Complex Polarisation Complex part of Complex Polarisation Amplitude of Complex Polarisation Toy Model Polarised Flux [Jy / (rad / m 2 )] Complex Polarised Flux [Jy] Faraday Depth [rad / m 2 ] Observing Wavelength Squared [m 2 ] 11

12 Example: Another Complex Source Another example of a source classified as complex: Reconstructed Emission per Unit Faraday Depth [Jy / rad.m 2 / beam] FDF RM Synth and CLEAN Data for Source Designation:372, (File Number:372), (SP PC sep:.1731).45 Dirty FDF (Magnitude).1.4 CLEANing Residuals (Magnitude) CLEANed FDF (Magnitude).8 CLEANing Cutoff (.21621).35 Clean Components.6 Clean Components.3 FWHM of RMSF Main Beam Faraday Depth [rad / m 2 ] Q, U, P Flux (Jy) Q, U and P From Data + QU Clean Model Wavelength squared 12

13 Example: Another Complex Source Data Reconstructed Emission per Unit Faraday Depth [Jy / rad.m 2 / beam] FDF RM Synth and CLEAN Data for Source Designation:372, (File Number:372), (SP PC sep:.1731).45.1 Dirty FDF (Magnitude).4 CLEANing Residuals (Magnitude).8 CLEANed FDF (Magnitude).35 CLEANing Cutoff (.21621) Clean Components.6 Clean Components.3 FWHM of RMSF Main Beam x 1 5 Proscribed Faraday Depth Dispersion [rad / mfunction 2 ] Q, U, P Flux (Jy) Q, U and P From Data + QU Clean Model Wavelength squared Toy Model Polarised Flux [Jy / (rad / m 2 )] Real part of FDF Complex part of FDF Amplitude of FDF Complex Polarised Flux [Jy] 3.5 x 1 3 Observable Complex Polarisation as a Function of Lambda Squared Real part of Complex Polarisation Complex part of Complex Polarisation Amplitude of Complex Polarisation Faraday Depth [rad / m 2 ] Observing Wavelength Squared [m 2 ] 13

14 How do we Interpret Complex FDFs? Reconstructed Emission per Unit Faraday Depth [Jy / rad.m 2 / beam] FDF RM Synth and CLEAN Data for Source Designation:372, (File Number:372), (SP PC sep:.1731) x Dirty FDF (Magnitude).4 CLEANing Residuals (Magnitude) 11 CLEANed FDF (Magnitude) 1 CLEANing Cutoff (.21621).35 Clean Components 9 Clean Components.3 FWHM of RMSF Main Beam Faraday Depth [rad / m 2 ] Polarised Flux [Jy / (rad / m 2 )] Proscribed Faraday Dispersion Function Faraday Depth [rad / m 2 ] Real part of FDF Complex part of FDF Amplitude of FDF 14

15 Speculative Interpretation of Example Polarised Flux [Jy / (rad / m 2 )] x Proscribed Faraday Dispersion Function Real part of FDF Complex part of FDF Amplitude of FDF Emission from within helical magnetic sheath + lobe? Faraday Depth [rad / m 2 ] Image: Hovatta et al. (212) Image: Nakamura 26 15

16 Just One Possibility for Interpretation of an Example Polarised Flux [Jy / (rad / m 2 )] x Proscribed Faraday Dispersion Function Real part of FDF Complex part of FDF Amplitude of FDF Emission from within helical magnetic sheath + lobe? Faraday Depth [rad / m 2 ] Image: Hovatta et al. (212) 16

17 Further checks of off-axis instrumental polarisation effects The Path Ahead Further analysis of data to shore up Faraday simple/complex classifications Attempt to identify different types of FDF complexity Spectral index of sources where is the emission coming from? Attempts to model Q and U behaviour with constructed FDFs and/or analytic models of spectropolarimetric behaviour More observations! Targeted radio observations of ~3 individual complex objects, 1-9 GHz (not drawn from Beta obs at this stage). Multi-wavelength obs. 17

18 Thankyou Thankyou for your time. Questions? 18

19 Median Fractional Pol vs. Dist. From Pointing Centre 19

20 Fractional Pol vs. Dist. From Pointing Centre FracPol Against SP-PC 3GHz, up to.4 degs sep 5. <.2Degs >.2Degs 1e P 1e e SP-PC sep (degrees) 5. FracPol Against SP-PC 3GHz, up to.2 degs sep Less than.2 Degs 5. 1e e 5. 1e1 Stokes I P 1e e e SP-PC sep (degrees) 5. 1e e 5. 1e1 Stokes I 2