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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Preliminary
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OJ 287 EVPA Data
• UMRAO 1974-2012
4.8 GHz, 856 points
8.0 GHz, 917 points
14.5 GHz, 1,207 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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Raw EVPA (set to 50o -130o)
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EVPA Time Series
Adjust by nπ
1. Make a smooth curve
2. 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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Adjusted EVPA (a)
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Adjusted EVPA (b)
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Double EVPA Rotations
•
•
•
•
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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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1984 – 1988 Event A (a)
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Expanded View of Event A (a)
One curve 3 freq
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1984 – 1988 Event A (b)
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Advantages of the ‘Separated’ Solution
•
•
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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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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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1984 – 1988 Event A (a)
EVPA peak, rapid rotation
at same time as minima
in F, PF
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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.
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1984 – 1988 Event A (a)
Two flux outbursts bracket
the double rotation in EVPA
Rapid swing in EVPA
Three bursts in PF
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Two Gaussian outbursts separated in time
plus a steady background
3 Cpts, similar
amplitude
Step in EVPA
Deep minimum in PF
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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.
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But,
how are the successive
counter−rotating outbursts
generated?
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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
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Example
•
Jet: Γ = 10
•
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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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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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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1.5-Dimensional Simulation
Of Super-Fast MHD Jet
Enhancements in Bϕ
between slow & fast
shocks
Bϕ
Jet
Nakamura et al. (2010), Fig. 3
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1.5-Dimensional Simulation
Of Super-Fast MHD Jet
Opposite rotations of
plasma Vϕ between
slow & fast shocks
Vϕ
Jet
Nakamura et al. (2010), Fig. 3
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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
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The End
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~12 Year Intervals
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1988 – 1995 Event B
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1994 - 2004 Events C and D
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Model 2
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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
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