Helical magnetic fields through Faraday rotation and jet stratification studies José L. Gómez, Carolina Casadio, Mar Roca-Sogorb, Iván Agudo Instituto de Astrofísica de Andalucía (CSIC) Alan P. Marscher, Svetlana G. Jorstad Boston University
Jet formation and helical magnetic fields Recent numerical simulations show that helical magnetic fields should play an important role in the jet formation, providing the initial flow collimation and acceleration. How can we study the jet formation and observe these helical fields? A. Within the acceleration and collimation zone (ACZ), through multi-wavelength observations Animation by W. Steffen (UNAM & Cosmovision) • Previous talk by Marscher and next talk by Agudo • Posters by Jorstad and Molina B. Beyond the VLBI core, through Faraday r o t a t i o n s t u d i e s a n d e m i s s i o n asymmetries across the jet width
Faraday Rotation and Asymmetries Across the Jet Width Recent numerical simulations show that B pointing away helical magnetic fields should play an important role in the jet formation, providing the initial flow collimation and acceleration. How can we study the jet s d r a formation and observe these w o t B r e v r helical fields? e s b o e h t A helical magnetic field should lead to a jet B. Beyond the VLBI core, through Faraday stratification in RM across the jet width r o t a t i o n s t u d i e s a n d e m i s s i o n (e.g. Broderick & McKinney 2010) asymmetries across the jet width 3C273 Faraday rotation produces a rotation of the electric vector position angle ( χ ) ! = ! o + RM " 2 Where RM is the rotation measure, given by: ! (rad m " 2 ) RM = 812 n e B ! dl Aligned field Asada et al. (2002)
Faraday Rotation and Asymmetries Across the Jet Width Synchrotron emissivity: ε ! sin θ ’ B top where θ ’ is the angle between the magnetic Φ field and line of sight in the fluid frame. B bottom θʼ For a pitch angle of Φ , viewing angle θ , and flow velocity β : • Maximum asymmetry θʼ = Φ , θʼ = Π - Φ • Asymmetry reverses for β =cos θ Lorentz factor Internal energy Aloy, Gómez, Ibáñez, Martí & Polarized (log scale) Total flux (log scale) Müller 2000, ApJ, 528, L85 Different viewing angles see also Clausen-Brown et al. (2011)
Faraday Rotation and Asymmetries Across the Jet Width Synchrotron emissivity: ε ! sin θ ’ B top where θ ’ is the angle between the magnetic Φ field and line of sight in the fluid frame. B bottom θʼ For a pitch angle of Φ , viewing angle θ , and flow velocity β : • Maximum asymmetry θʼ = Φ , θʼ = Π - Φ • Asymmetry reverses for cos θ = β Numerical simulations of the linearly polarized emission at different pitch angles see also Clausen-Brown et al. (2011)
Faraday Rotation and Stratification in 3C273 Marscher’s Boston University Blazars Group Data 3C273 at 43 GHz Jet evolution from January 2008 to January 2011
Faraday Rotation and Stratification in 3C273 Marscher’s Boston University Blazars Group Data 3C273 at 43 GHz Jet evolution from January 2008 to January Progressive stacking of degree of 2011 polarization as components sample the jet
Faraday Rotation and Stratification in 3C273 0.6 mas 3 years mean total flux from core 1 2 3.1 mas 1 from core 2
Faraday Rotation and Stratification in 3C273 3 years mean total flux Total flux decay along the jet ridge line Stationary feature at ~0.8 mas. See Jorstad poster . S ! r " 1 Southern side is brighter
Faraday Rotation and Stratification in 3C273 3 years mean degree of polarization Stratification consistent with a helical magnetic field & shear
Faraday Rotation and Stratification in 3C273 Mojave’s data on 3C273 3C273 at 15 GHz Jet evolution from January 1996 to June 2010: 14 years of monitoring.
Faraday Rotation and Stratification in 3C273 Mojave’s data on 3C273 3C273 at 15 GHz Jet evolution from January 1996 to June Progressive stacking of degree of 2010: 14 years of monitoring. polarization as components sample the jet
Faraday Rotation and Stratification in 3C273 3.1 mas 14 years mean total flux from core 1 2 14 mas from core 1 43 GHz 2
Faraday Rotation and Stratification in 3C273 3.1 mas 14 years mean total flux from core 1 2 14 mas from core 1 Change in the brighter side of the jet may result from the measured acceleration of components (Jorstad et al. 2005; Lister et al. 2009). A reversal in the asymmetry is produced for β =cos θ . 2
Faraday Rotation and Stratification in 3C273 3.1 mas 14 years mean total flux from core 1 2 14 mas from core 1 Proper motions measure pattern s p e e d s , w h i l e c h a n g e s i n asymmetries depend on bulk flow velocity. We can relate both and estimate shock strengths. 2
Faraday Rotation and Stratification in 3C273 14 years mean degree pol. 15 GHz Consistent with the 43 GHz d e g r e e o f p o l a r i z a t i o n stratifciation. 43 GHz
Faraday Rotation and Stratification in 3C273 February 2009 RM map between 43 and 15 GHz 43 GHz 600 22 GHz RM slice at 3 mas 400 200 RM (rad/m2) 0 -200 Consistent with -400 Hovatta et al. -600 results 15 GHz -800 0.0 0.5 1.0 1.5 2.0 2.5 MOJAVE Distance (mas)
Faraday Rotation and Stratification in 3C273 February 2009 RM map between 43 and 15 GHz 43 GHz 600 22 GHz RM slice at 3 mas 400 Slice at 6 mas 200 from Zavala & RM (rad/m2) 0 Taylor in 2000 -200 Consistent with -400 Hovatta et al. -600 results 15 GHz -800 0.0 0.5 1.0 1.5 2.0 2.5 MOJAVE Distance (mas) • A change in velocity would also explain the change in the RM between the slices at 3 and 6 mas. • It also implies that some of the RM is produced within the emitting jet.
Faraday Rotation and Polarization gradients in 3C120 15, 22 and 43 GHz VLBA Observations during 2001 • Three frequency data set to study the Faraday rotation screen. • 12 monthly VLBA observations to obtain a detailed monitoring. Rich polarization structure at all three frequencies. Complex polarized structure evolution in the jet. Superluminal components sample the jet polarization as they travel downstream. Gómez et al. (2008)
Faraday Rotation and Polarization gradients in 3C120 Mean Rotation Measure During 2001 Mean Degree Polarization 2001 RM m (%) Gómez et al. (2008)
Faraday Rotation and Polarization gradients in 3C120 • Stratification in RM and degree of Mean Rotation Measure During 2001 polarization across the jet as expected for the case of a helical magnetic field . • Localized region of enhanced RM. A local process, such as interaction of the jet with the external medium or a cloud , is required to explain this region of enhanced RM. RM m (%) Gómez et al. (2008)
On the Stationarity of the RM Sheath in 3C120 New observations taken in November • Serendipitous discovery of a component at 80 2007 at all observing VLBA frequencies mas from the core, not seen previously, and with a T B about 600 times higher than expected. (1.7 to 43 GHz), to study: • Component cannot be produced entirely by jet • How stationary is the Faraday screen bending, moving, or standing shock without a • Wether similar jet/external medium significant flow acceleration (Roca-Sogorb et al. 2010, ApJ, 712, L160) interactions take place further downstream !"#$%&'()*+'%&",$-).#/$#0"/()123(45$%26 High brightness temperature in 3C120 !$% VLBA 10 12 43 GHz T b along the jet 10 11 !"# 22 GHz 10 10 T b ( K ) 10 9 15 GHz 10 8 10 7 10 6 0 20 40 60 80 100 Distance from the core (mas) 7$'%/"8$)7"9:/);0<$#0"+#)12%06 Roca-Sogorb et al. (2010)
Uncorrelated changes in the RM and RM-corrected EVPAs Comparison with observations before 2001 22 and 43 GHz VLBA observations starting in 1997 1998 (Gómez et al. 2000, 2001) January 1999 1999 Mean RM 2001 2000
Uncorrelated changes in the RM and RM-corrected EVPAs Comparison of Observations between 1999 and 2001 We find consistent values of RM and RM- corrected EVPAs between this 2-year interval, however There are two distinct regions in which the January 1999 RM-corrected EVPAs show a rotation close to 90 degrees. Therefore we find uncorrelated changes in RM = 5500 the linear polarization of the underlying jet ! = 40 o RM = 1700 emission and the Faraday rotation screen . ! = 46 o The emitting jet and the source of Faraday rotation are not closely connected RM = 5400 physically and have different configurations RM = 2000 ! = -28 o for the magnetic field. ! = -32 o Mean RM 2001 If we also consider the existence of the localized region of high RM, we conclude Gómez et al. (2011) that a significant fraction of the RM in 3C120 originates in foreground clouds.
Summary • Stacking of images proves very useful in revealing the full jet structure in total and polarized flux, allowing for the search of stratifications. • 3C273 shows a stratification across the jet in total and polarized flux (RM), which flips sides along the jet. This can be interpreted as produced by a helical magnetic field and jet flow acceleration along the jet. • Proper motions measure pattern speeds, while changes in asymmetries depend on bulk flow velocity. We can relate both and estimate shock strengths. • Polarization structure in 3C120 is compatible with the existence of a helical magnetic field, but a significant fraction of the RM appears to be originated in foreground clouds.
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