On the possible core shift break in relativistic jets

ABSTRACT Measurement of a jet geometry transition region is an important instrument of assessing the jet ambient medium properties, plasma bulk motion acceleration, parameters of a black hole, and location of a jet-launching radius. In this work, we explore the possibility of a presence of a core shift break, associated with the geometry and jet physical properties transition. We obtain the relations on the core shift offset jump due to a change in a core shift exponent. The condition of a proper frame magnetic field continuity and the core shift break can be used as an instrument to refine the magnetic field estimates upstream the break. This method is applied to the jet in NGC 315. We also argue that a local change in the plasma properties in the jet, for example due to the occurrence of a standing shock or a magnetic field reconnection, will also affect the dependence of the core shift on frequency. The properties of such a jump will be different than in the previous case. We propose to use the multifrequency core shift measurements to increase the number of sources with a detected jet shape break and to boost the accuracy of assessing the properties of a jet geometry transition region.