1. 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
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2. Preliminary
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3. 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
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4. Raw EVPA (set to 50o -130o)
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5. 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
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6. Adjusted EVPA (a)
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7. Adjusted EVPA (b)
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8. 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
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9. 1984 – Event A (a)
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10. Expanded View of Event A (a)
One curve 3 freq
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11. 1984 – Event A (b)
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12. 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)
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13. 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.
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14. 1984 – 1988 Event A (a) EVPA peak, rapid rotation
at same time as minima
in F, PF
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15. 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.
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16. 1984 – 1988 Event A (a) Rapid swing in EVPA Two flux outbursts bracket
the double rotation in EVPA
Three bursts in PF
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17. Two Gaussian outbursts separated in time plus a steady background
3 Cpts, similar
amplitude
CCW CW
Step in EVPA
Deep minimum in PF
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18. 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.
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19. counter−rotating outbursts generated?
But,
how are the successive
counter−rotating outbursts
generated?
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20. 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
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21. 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.
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22. 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
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23. 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.
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24. 1.5-Dimensional Simulation
Of Super-Fast MHD Jet
Enhancements in Bϕ between slow & fast shocks Bϕ
Jet
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25. 1.5-Dimensional Simulation
Of Super-Fast MHD Jet
Opposite rotations of plasma Vϕ between slow & fast shocks Vϕ
Jet
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26. 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
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27. The End
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28. ~12 Year Intervals
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29. 1988 – Event B
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30. Events C and D
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31. Model 2
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32. 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
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