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Double EVPA Rotations in OJ 287
M.H. Cohen, D.L. Meier Caltech H.D. Aller, M.F. Aller U. Michigan T. Hovatta Tuorla Observatory T. Savolainen Metsähovi Observatory 6/15/17 Crete June 2017
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Preliminary 6/15/17 Crete June 2017
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OJ 287 EVPA Data UMRAO 1974-2012 4.8 GHz, 856 points
MOJAVE 1996 – 2016 15.3 GHz, 89 points Kikuchi et al 1988, A&A, 190, L8 10 GHz, 15 points 6/15/17 Crete June 2017
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Raw EVPA (set to 50o -130o) 6/15/17 Crete June 2017
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EVPA Time Series Adjust by nπ
Make a smooth curve Adjacent points step < 90o unless there is a long time gap 3a. All frequencies fit together OR 3b. All frequencies do not fit together 6/15/17 Crete June 2017
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Adjusted EVPA (a) 6/15/17 Crete June 2017
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Adjusted EVPA (b) 6/15/17 Crete June 2017
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Double EVPA Rotations CCW followed by CW Amplitude 250 –400 degrees
Duration 1 – 2 years Three large and one small double rotation in 40 years 6/15/17 Crete June 2017
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1984 – Event A (a) 6/15/17 Crete June 2017
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Expanded View of Event A (a)
One curve 3 freq 6/15/17 Crete June 2017
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1984 – Event A (b) 6/15/17 Crete June 2017
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Advantages of the ‘Separated’ Solution
Elimination of the large jumps in EVPA Reduced overall EVPA range and easier to fit the model for rapid rotation (multiple Stokes vectors summing to a small resultant) 6/15/17 Crete June 2017
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Model for a Rapid Rotation
Several emission components, Stokes vectors sum to near zero. Small change in one of the vectors can make a large change in resultant. Easy to get 90o swing in EVPA, 180o is hard. Supported by observations: PF is in a deep minimum at epoch of rapid change. 6/15/17 Crete June 2017
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1984 – 1988 Event A (a) EVPA peak, rapid rotation
at same time as minima in F, PF 6/15/17 Crete June 2017
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Formation of Double-Rotation
Event A: Double rotation is bracketed by two outbursts in flux density. Suppose first outburst has EVPA rotating CCW, while second has EVPA rotating CW. Add a background component, and the combination can make a double EVPA rotation. 6/15/17 Crete June 2017
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1984 – 1988 Event A (a) Rapid swing in EVPA Two flux outbursts bracket
the double rotation in EVPA Three bursts in PF 6/15/17 Crete June 2017
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Two Gaussian outbursts separated in time plus a steady background
3 Cpts, similar amplitude CCW CW Step in EVPA Deep minimum in PF 6/15/17 Crete June 2017
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Successive Oppositely-Rotating Flux Outbursts
Two bursts plus a steady background. Bursts overlap at flux near that of background. Three Stokes vectors add to near zero. Small change can have a big effect, giving a rapid swing in resultant EVPA and a minimum in PF. 6/15/17 Crete June 2017
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counter−rotating outbursts generated?
But, how are the successive counter−rotating outbursts generated? 6/15/17 Crete June 2017
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Model that generates successive outbursts with opposite EVPA rotations
Highly relativistic jet Helical magnetic field, right-hand twist Forward and reverse subrelativistic shocks, in jet frame. Shocks follow field. Both shocks move downstream and are relativistic, in galaxy frame First shock rotates CCW, second rotates CW as seen by an observer on axis 6/15/17 Crete June 2017
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Jet: Γ = 10 Example Shocks: β = 0.1 (in jet frame)
Γ = 11.05, 9.05 (in galaxy frame) Near the axis, the shocks will have similar Doppler factors. The first one seen will be CCW, the second, CW. 6/15/17 Crete June 2017
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Super−Magnetosonic Jet A Plausible Physical Model
Numerical studies of a super-magnetosonic jet Nakamura et al 2010, ApJ, 721, 152 Nakamura and Meier 2014, ApJ, 785, 152 This jet has a helical magnetic field and advances into a background plasma. Vjet > Vms = (VA2 + Vs2)1/2 6/15/17 Crete June 2017
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Two pairs of fast/slow MHD waves are set up: one pair forward (FF/FS) and the other pair in reverse (RF/RS). Simulations show that the toroidal magnetic field Bφ is compressed between FF and FS, also between RF and RS, and that the sign of Vφ is opposite in the two regions. The increased angular momentum in the field requires counter−rotations of the plasma around the jet axis. This produces two successive radio bursts, with opposite senses of EVPA rotation. 6/15/17 Crete June 2017
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1.5-Dimensional Simulation
Of Super-Fast MHD Jet Enhancements in Bϕ between slow & fast shocks Bϕ Jet 6/15/17 Crete June 2017
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1.5-Dimensional Simulation
Of Super-Fast MHD Jet Opposite rotations of plasma Vϕ between slow & fast shocks Vϕ Jet 6/15/17 Crete June 2017
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Conclusions OJ 287 shows a new phenomenon − double EVPA rotations
Three major and one minor event in 40 years Successive, oppositely−rotating, flux bursts Rapid EVPA changes due to near-cancellation of several Stokes vectors Super−magnetosonic jet with helical field suggested as explanation 6/15/17 Crete June 2017
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The End 6/15/17 Crete June 2017
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~12 Year Intervals 6/15/17 Crete June 2017
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1988 – Event B 6/15/17 Crete June 2017
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Events C and D 6/15/17 Crete June 2017
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Model 2 6/15/17 Crete June 2017
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1984 – 1988 Event A 100o spike at peak Periodic gap because
source is near Sun EVPA peak at same time as F and PF minima EVPA Double rotation bracketed by two flux outbursts 6/15/17 Crete June 2017
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