# Time variability of the core-shift effect in the blazar 3C 454.3

**Authors:** Wara Chamani, Tuomas Savolainen, Eduardo Ros, Yuri Y. Kovalev, Kaj Wiik, Anne Lähteenmäki, Merja Tornikoski, Joni Tammi  
**arXiv:** [2209.13301](https://arxiv.org/abs/2209.13301)  
**Date:** 2022-09-27  
**Summary page:** https://www.summarizepaper.com/en/paper/2209.13301v1/

## Key points

- Researchers investigated time variability of the core-shift effect in blazar 3C 454.3 using VLBI techniques
- Core-shift effect used to estimate magnetic field strength in jets
- VLBI core refers to bright feature at base of jet, position changes with observed frequency
- Multi-frequency VLBA data analyzed from 2005 to 2010, significant variations in core shift observed between different frequencies
- Time variability of core-shift index ($k_r$) examined, values typically below one with average value of $0.85 \pm 0.08$ and standard deviation of $0.30$
- Commonly assumed conical jet shapes and equipartition conditions do not always hold simultaneously
- Magnetic field strengths often derived assuming $k_r = 1$, unreliable results if $k_r$ significantly deviates from unity
- Selecting epochs where $k_r = 1$ for analysis in case of 3C 454.3 resulted in consistent magnetic field estimates despite variations in core shift over time
- Jet's magnetic flux estimated and evidence found for magnetically arrested disk state in source
- Correlation observed between core position and core flux density, indicating increase in particle density during flares

## Summary

In this study, the researchers investigated the time variability of the core-shift effect in the blazar 3C 454.3 using Very Long Baseline Interferometry (VLBI) techniques. The core-shift effect is a commonly used method for estimating the magnetic field strength in jets. The VLBI core refers to the bright feature at the base of the jet, and its position changes with the observed frequency. The researchers analyzed multi-frequency VLBA data covering 19 epochs from 2005 to 2010. They employed self-referencing analysis to determine the core shift variability at frequencies ranging from 5 to 43 GHz. The results revealed significant variations in core shift, ranging from 0.27 to 0.86 milliarcseconds (mas) between different frequencies, confirming previous observations of core-shift variability. The study also examined the time variability of the core-shift index ($k_r$), which describes how the position of the core changes with frequency ($r_\mathrm{core} \propto \nu^{-1/k_r}$). The researchers found that $k_r$ values were typically below one, with an average value of $0.85 \pm 0.08$ and a standard deviation of $0.30$. These findings suggest that commonly assumed conical jet shapes and equipartition conditions do not always hold simultaneously. It is important to note that magnetic field strengths are often derived from core shift measurements assuming $k_r = 1$, which may lead to unreliable results if $k_r$ significantly deviates from unity. Therefore, it is crucial to verify that $k_r = 1$ holds before using core shift values and making assumptions about equipartition conditions when deriving physical parameters in jets. When selecting epochs where $k_r = 1$ for analysis in the case of 3C 454.3, consistent magnetic field estimates were obtained despite variations in core shift over time. Additionally, the researchers estimated the jet's magnetic flux in 3C 454.3 and found evidence that the source is in a magnetically arrested disk state. Furthermore, a correlation was observed between the core position and core flux density, with $r_\mathrm{core} \propto S_\mathrm{core}^{0.7}$, indicating an increase in particle density during flares.