1. There and back again by Mikhail Lisakov MPIfR
The moving core
There and back again
by Mikhail Lisakov
MPIfR

2. What is an apparent core? Can it move?
A standing shock?
A τ =1 surface?
In both cases one could expect it to move along the jet [Also see a poster by Nick MacDonald for a standing shock scenario]

3. Why is it important ? Core-shift Astrometry Kinematics
Talk by Petrov: [Use of VLBI/Gaia position offsets for AGN physics]
Astrometry
Kinematics
An example of a kinematic plot. Scatter of the kinem data could be reduced.
Core shift. Example plot with an arrow up-down at one of the points. Core_shuttle can effect the estimated k_r parameted => derived physical parameters of the jet
Sketch of a jet with VLBI and GAIA positions. Ref to poster by Petrov: Use of VLBI/Gaia position offsets for AGN physics
Petrov, L.,

4. How to detect (a core shuttle) ?
Phase-referencing
Self-referencing to moving components
[ Niinuma ]

5. Reference to moving components
Average core position
Moving components (can not jump together)
time
Core
Distance from the core
Time

6. Reference to moving components
Average core position
Moving components (can not jump together)
time
Core
Distance from the core
Time

7. MOJAVE → core shuttle sample
≥ 2 robust components
≥ 12 epochs ( 3.2 yr )
Resulted in 85 sources
Fit kinematics
→ residuals = (data – fit)
→ projection
→ average
→ smooth

8. Measured residuals (magnitude of the core shuttle)
40% of common sources with MOJAVE sample could have up to half of the 15 – 8 GHz core shift due to the core shuttle at 15 GHz
Core shuttle magnitude is comparable to that of the 15 – 8 GHz core shift

9. Spectacular results
3C 279

10. OJ 287, contrary behavior

11. Gate Cross-correlation
Significance is assessed by performing a permutation test on each chunk of data (12 measurements)
Sensitive to strong correlation only

12. Correlation in the sample
residuals and the core flux density anti-correlate

13. Correlation in the sample
The core is
brighter
when it is more
downstream
The core is
fainter
when it is more
downstream
residuals and the core flux density anti-correlate

14. Connection to component ejection
Significant correlation coefficients close to components’ ejection
Correlation coefficient

15. Summary
A τ=1 apparent core can move along the jet following changing physical parameters of the flow, presumably N and B
There is a 10% chance to observe the core offset from its average position > 0.1 mas
40% of 15 – 8 GHz core shift measurements could be significantly affected by the core shuttle
2/3 of significant correlations between the core position and its flux density could be explained by an ejection of denser matter into the jet