Is the Inner Radio Jet of BL Lac
Precessing?
R. L. Mutel
University of Iowa
Astrophysics Seminar
17 September 2003
BL Lac VLBP monitoring log
•
•
•
•
9 epochs from 1998.7 – 2001.4
3-5 month separation between observations
15, 22, 43 GHz, VLBA, full polarization
EVPA calibration using VLBA calibration
Database
UI Astrophysics Seminar Sept
2003
Superluminal Component S10
1998.7 -2001.6: Trajectory fits helical model
N.B. Not
ballistic
Model line
predicted by
Denn et al.
2000
S10
98.7- 99.2
00.4-01.6
Helical model (Hardee 1987)
using adiabatic expansion of
light jet, LOS angle 5º ± 1º
(forward shock)
UI Astrophysics Seminar Sept
2003
S10 apparent speed
1998-7 -2001.4
UI Astrophysics Seminar Sept
2003
Bl Lac May 1999 PPOL maps
1. Core, component EVPA structure at high frequencies
2. Low Rotation measure in jet, vs. high RM of core
Core (high RM)
Core N
Core S
S11 (low RM)
Note:
hint of
gradient
S10 (low RM)
15 GHz
22 GHz
UI Astrophysics Seminar Sept
2003
43 GHz
BL Lac Sept 99 at 15, 22, 43GHz
IPOL, PPOL, FPOL maps
Note: low Core polarization (~2%)
Core Pol’n ~5%
Core
contains
emerging
component
S9
S9 only visible
on PPOL map
UI Astrophysics Seminar Sept
2003
Core, S10 Rotation measure vs. Epoch
S10
S10
Core
1998.97
UI Astrophysics Seminar Sept
2003
Core depolarization modeled with random
scattering screen
Core fractional
polarization is
approximately
quadratic with
wavelength
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2003
Foreground turbulent Rotation Measure
screen
Assume quadratic structure function of RM
fluctuations (Tribble 1991)
 s
DRM  s     
 s0 
2
Fractional polarization vs. wavelength
p ( ) 
Best-fit values:
so
t
23/ 2  2
 : 4000 rad m2
s0 : 0.03 AU
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2003
Evidence for sheath boundary layer
(parallel magnetic field)
UI Astrophysics Seminar Sept
2003
Cf. Blazar 1055+018 (1997.0, 5 GHz) (Attridge et al. 1999)
B vectors
If these are sheaths (jet-ambient medium interaction regions) with
(cold?) entrained material, why is the synchrotron emissivity so
high?
Perhaps due to helical structure of B field itself?
UI Astrophysics Seminar Sept
2003
Earth-Moon-Sun system
dL
 L
dt
 prec
 I orb
 orb

I spin
1
 I 
~
 I 
300
earth
 orb 
1
~


 spin  earth moon 30
 prec ~ 1013  Pprec ~ 25,000 y
UI Astrophysics Seminar Sept
2003
SS433
• Precession period 164
days
• Companion Wolf-Rayet
star (?)
• Ballistic jets at v =
0.25c
• Associated with SN
remnant (W50)
•
UI Astrophysics Seminar Sept
2003
UI Astrophysics Seminar Sept
2003
200 pc
4C12.50 (Lister et al 2003 astro-ph)
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2003
Is the jet nozzle of BL Precessing?
• Precession: physics summary
• Examples
– Earth-Moon-Sun system (companion torque)
– Galactic jets: SS433 (companion torque)
– Extragalactic Jets: 4C12.50 (Accretion disk torque?)
• Evidence for BL Lac Precession (Stirling et al.
MNRAS 2003)
• Independent test (Mutel et al. 2003)
• Possible problems with precession
– Binary hypothesis: Gravitational radiation timescale
– Accretion disk precession: Lens-Thirring timescale
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2003
Stirling et al. 1mm JCMT Observations
UI Astrophysics Seminar Sept
2003
Stirling et al 43 GHz radio map:
Periodic change in ‘structural
position angle between C1, C2
UI Astrophysics Seminar Sept
2003
Our SPA Observations, and comparison to
Stirling et al.
Our data, constant model 2 = 0.83
Our dataCombined
with best-fitSPA
model
with
Stirling
(solid, 2 =data
0.58),
Stirling
model
model
(dashed)
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2003
Model fit results
UI Astrophysics Seminar Sept
2003
Possible problems with precession period:
1. Companion hypothesis (binary BH):
A. Binary black hole
Assume 2 BH equal mass (Mbh ~ 3 •108 Msun, , P = 2.3
yr)
a = 103 AU (200 Rs)
B. Gravitational radiation:
LGR ~ 1040 W (1% Lrad!)
Timescale for coalescence:
P 96 G 3  M 2
5
  &

10
yrs
5
4
P 5 c a
This seems implausibly short
UI Astrophysics Seminar Sept
2003
Precession issues continued
• Warped accretion disk: Lense-Thirring
precession from warped accretion disk? (Bardeen
& Petterson 1975; Scheuer 1992)
3
LT
 R c  J 
 2 


 Rs  Rs  J max 
P ~ few years is only possible with maximally
rotating BH with accretion disk of radius r ~ 10 AU
(~20 Rs) (much longer for larger R, scales as R3)
UI Astrophysics Seminar Sept
2003
Summary
• Superluminal component S10 position, speed consistent
with prediction of helical model described in Denn et al.
2000 (ApJS).
• All jet components have B field orientation within 20º of 
to jet direction (consistent with perpendicular shocks,  <
1), low RM (-300 ± 300 rad-m-2)
• Core rotation measure high (RM~2000 rad-m-2),
probably variable (but very difficult to isolate from
emerging components)
• Core depolarization vs. wavelength can be modeled with
random RM fluctuating screen, s~ 0.3 AU, σ~ 4000 radm-2
• Strong evidence for weak sheath component with
parallel magnetic field [at least 3 epochs]
• 43 GHz maps do not agree with 2-yr core precession
claim of Stirling et al. 2003
UI Astrophysics Seminar Sept
2003