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Broadband radio circular polarization spectrum of the relativistic jet

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1 Broadband radio circular polarization spectrum of the relativistic jet
in PKS B2126–158 O’Sullivan, McClure-Griffiths, Feain, Gaensler, Sault (2013) MNRAS, in review Shane O’Sullivan, Relativistic Jets & Their Magnetic Fields, Spain, 13 Jun 2013

2 Broadband radio circular polarization spectrum of the relativistic jet
in PKS B2126–158 Deconstruct the title a little ATCA from 1.5 – 10 GHz at ~128 MHz intervals (with some gaps) O’Sullivan, McClure-Griffiths, Feain, Gaensler, Sault (2013) MNRAS, in review Shane O’Sullivan, Relativistic Jets & Their Magnetic Fields, Spain, 13 Jun 2013

3 Broadband radio circular polarization spectrum of the relativistic jet
in PKS B2126–158 Non-thermal synchrotron radiation, intrinsically highly linearly polarized but can also have a small amount of CP O’Sullivan, McClure-Griffiths, Feain, Gaensler, Sault (2013) MNRAS, in review Shane O’Sullivan, Relativistic Jets & Their Magnetic Fields, Spain, 13 Jun 2013

4 Broadband radio circular polarization spectrum of the relativistic jet
in PKS B2126–158 Very important word in the CP business: The first comprehensive CP spectrum of a quasar jet O’Sullivan, McClure-Griffiths, Feain, Gaensler, Sault (2013) MNRAS, in review Shane O’Sullivan, Relativistic Jets & Their Magnetic Fields, Spain, 13 Jun 2013

5 Australia Telescope Compact Array
6 x 22 m dishes with 6 km maximum baseline Linear feed system (sV/I ~ 0.01%) CABB (2 GHz b/w per IF), Wilson et al. (2011) 1 to 3 GHz, 4.5 to 6.5 GHz, 8 to 10 GHz (1 to 10 arcsec)

6 PKS B2126-158 VLBA 5 GHz Ghisellini+11 GPS quasar (Stangellini+98)
z = (Osmer+94) MBH ~ 1010 Msol (Ghisellini+11) VLBA 5 GHz Fomalont+95

7 Synchrotron from a single electron is elliptically polarized
CP can be positive/negative (left/right) Large CP not expected for an ensemble of electrons with an isotropic distribution of pitch angles Legg & Westfold (1968) Emission beamed into a cone defined by its Lorentz factor \chi is the pitch angle of the direction of emission with respect to the magnetic field For completely uniform magnetic field with an ensemble of electrons and random distribution of pitch angles gives LP ~ 75% Polarization => strength and degree of order of magnetic field

8 Conversion of LP to CP Gabuzda+08
Propagation effect due to finite width of the jet (Bconv induces a delay between E|| and Eperp) A change in the orientation of the magnetic field through the jet eg. helical magnetic field Conversion due to Faraday rotation: Linear polarization emitted from back of the jet rotated as it propagates through the jet

9 CP Spectrum Determine the CP generation mechanism
Using correct model allows us to infer: B-field geometry (& strength, flux) (Gabuzda+08, Homan+09) Jet particle composition (e-,p / e-,e+) (Wardle+98) Direction of rotation of accretion disk/black hole spin (Ensslin+03) Polarity of black hole magnetic field (Beckert & Falcke 2002)

10 Circular Polarization in AGN
Predicted spectrum: Intrinsic: Melrose (1972): sign change ~1/2 self-abs turnover freq Conversion: Jones (1988): inhomogeneous relativistic jets, conversion greatest is region where optical depth is unity, sign flip will occur regardless of mechanism Kennett & Melrose (1998): for co-spatial relativistic & cold plasma: purely relativistic: Wardle & Homan (2003): BK jet:

11 PKS B2126-128 mc ~ 1% at 6 GHz No sign change in Stokes V
Turnover in CP spectrum when emission becomes optically thin Not dominated by intrinsic CP Consistent with Faraday conversion due to uniform B-field in a purely relativistic plasma (no thermal electrons)

12 PKS B CP increases with frequency in optically thick regime (inverted CP spectrum) Consistent with B&K79 conically self-similar jet Unity optical depth surface moves further upstream in the jet at higher frequencies, where the uniform B- field component is stronger

13 Circular Polarization
PKS B Linear and circular polarization anti-correlated Spearman rank test: -0.8 Clear signature of Faraday conversion of LP to CP Circular Polarization Linear Polarization Thick Thin

14 PKS B2126-128 Circular Polarization Linear Polarization
Faraday rotation and depolarization Faraday thick or thin? Unconstrained for small range in l2 Circular Polarization Linear Polarization large increase in p towards longer wavelengths Change in sign of RM?

15 Macquart+01, Kennett & Melrose 98
RRM fit (8 – 10 GHz) Macquart+01, Kennett & Melrose 98 Fitting to 8-10 GHz: RRM = 647 +/- 12 rad/m3 RRMint = RRM ((1+z)/d)3 = 50 rad/m3 for d=10 Assuming equipartition: B = 15 mG (for L = 1 pc) Intrinsic CP for completely uniform B-field: ~ 0.3% at 8 GHz

16 SSA analysis Marscher+79,83,87
Angular size of emission region ~ 0.5 mas (using B = 15 mG) Ure >> UB (particles dominate jet energy density) No contraints on gmin or jet composition VLBI observations + radiative transfer simulations required

17 CP generation mechanisms (Homan+09)
Stochastic production of CP from tangled B-field: requires intrinsic or conversion or combination. CP should vary in sign and amount across different frequencies Intrinsic CP from a strong (tens of mG) vector-ordered B-field (assuming all emitting particles are e-s, “e-p jet”) Conversion of LP to CP by internal Faraday rotation (thermal or low energy e-). Small Faraday depth so that B-field at front of jet converts LP from back of jet into some CP. 2 and 3 will act together to some degree. High rotation depth version of 3, where no large scale B-field order is required and net CP can be produced in large amounts with very little net LP Faraday conversion from a helical B-field. No internal Faraday rotation required as field orientation at back of jet is already different to that at front of jet

18 CP generation mechanisms (Homan+09)
Stochastic production of CP from tangled B-field: requires intrinsic or conversion or combination. CP should vary in sign and amount across different frequencies Intrinsic CP from a strong (tens of mG) vector-ordered B-field (assuming all emitting particles are e-s, “e-p jet”) Conversion of LP to CP by internal Faraday rotation (thermal or low energy e-). Small Faraday depth so that B-field at front of jet converts LP from back of jet into some CP. 2 and 3 will act together to some degree. High rotation depth version of 3, where no large scale B-field order is required and net CP can be produced in large amounts with very little net LP Faraday conversion from a helical B-field. No internal Faraday rotation required as field orientation at back of jet is already different to that at front of jet

19 CP generation mechanisms (Homan+09)
Stochastic production of CP from tangled B-field: requires intrinsic or conversion or combination. CP should vary in sign and amount across different frequencies Intrinsic CP from a strong (tens of mG) vector-ordered B-field (assuming all emitting particles are e-s, “e-p jet”) Conversion of LP to CP by internal Faraday rotation (thermal or low energy e-). Small Faraday depth so that B-field at front of jet converts LP from back of jet into some CP. 2 and 3 will act together to some degree. High rotation depth version of 3, where no large scale B-field order is required and net CP can be produced in large amounts with very little net LP Faraday conversion from a helical B-field. No internal Faraday rotation required as field orientation at back of jet is already different to that at front of jet

20 GPS quasar z = 3.268 MBH ~ 1010 Msol Pjet ~ 1048 erg/s mc ~ 1% at 6 GHz Dominant CP generation mechanism: Faraday conversion of LP to CP Conclusions: little or no thermal plasma within the jet cannot yet distinguish between magnetic twist and internal Faraday rotation models relativistic particle content unconstrained Dominant CP generation mechanism: Faraday conversion of LP to CP Conclusions: little or no thermal plasma within the jet cannot yet distinguish between magnetic twist and internal Faraday rotation models relativistic particle content unconstrained Dominant CP generation mechanism: Faraday conversion of LP to CP Conclusions: little or no thermal plasma within the jet cannot yet distinguish between magnetic twist and internal Faraday rotation models relativistic particle content unconstrained Dominant CP generation mechanism: Faraday conversion of LP to CP Conclusions: little or no thermal plasma within the jet cannot yet distinguish between magnetic twist and internal Faraday rotation models relativistic particle content unconstrained Dominant CP generation mechanism: Faraday conversion of LP to CP Conclusions: little or no thermal plasma within the jet cannot yet distinguish between magnetic twist and internal Faraday rotation models relativistic particle content unconstrained

21 Thank You

22 PKS B Source to check calibration against CSS radio galaxy

23 Steeper slope for internal FR
Ensslin+03 Red: internal FR Green: no internal FR Steeper slope for internal FR

24 PKS B VLBA 5 GHz GPS quasar z = 3.268 MBH ~ 1010 Msol

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