1. Frequency-dependent core shift
Jet apex is there
Jet at ν1
Jet at ν2 < ν1
The “core” of a VLBI jet is located in a region with τ ~ 1 and changes its position with the observing frequency

2. Core shift vectors: polar plots
med ( GHz): 128 μas
med ( GHz): 125 μas
med ( GHz): 88 μas
>80% sources within |θcore shift - θmed jet PA| < 45°

3. Magnetic field at 1 pc from jet apex
Magnetic field is derived for the sources with known
apparent speed
redshift
core shift
It total 102 sources:
84 QSO and 18 BLO
median BQuasar ~ 0.9 G
median BBL Lac ~ 0.4 G
Why the magnetic field at 1 pc is stronger in quasars?
more distant sources
faster jets
more massive central BHs and/or higher accretion rates? This scenario is
supported by narrower intrinsic opening angles (Pushkarev et al. 2009)

4. Multi-frequency MOJAVE obs.
extended sample of 192 sources
12 epochs (BL137A-L) during 2006
polarimetric observations at 8.1, 8.4, 12.1 and 15.3 GHz
Papers planned
RM (Hovatta et al., submitted in AJ)
Core shift (Pushkarev et al., submitted to A&A)
Spectral index (Hovatta et al. in prep.)

5. Core shift measurement method
core phase center offset
image shift
core shift = image shift – core phase center offset
(from 2D)
(from modelfit)

6. Accuracy of the method: approach 2
Let’s assume:
the core shift is along the jet
the accuracy is direction irrespective
SD of transverse projections of the core shift vectors  accuracy
σ ~ 50 μas, σ ~ 51 μas, σ ~ 35 μas

7. Core shift vs redshift
Kendall’s tau:
chance p: