# Optical Polarization and Spectral Variability in the M87 Jet

**Authors:** Eric S. Perlman, Steven C. Adams, Mihai Cara, Matthew Bourque, D. E. Harris, Juan P. Madrid, Raymond C. Simons, Eric Clausen-Brown, C. C. Cheung, Lukasz Stawarz, Markos Georganopoulos, William B. Sparks, John A. Biretta  
**arXiv:** [1109.6252](https://arxiv.org/abs/1109.6252)  
**Date:** 2011-09-28  
**Summary page:** https://www.summarizepaper.com/en/paper/1109.6252v1/

## Key points

- The M87 jet has exhibited extraordinary variability over the last decade
- One knot (HST-1) increased in brightness by over a factor of 100
- Nuclear flux also showed variability on timescales of months
- Study discusses optical-UV polarization and spectral variability of components with vastly different behavior
- HST-1 shows highly significant correlation between flux and polarization, with P increasing from around 20% at minimum to over 40% at maximum, while orientation of electric vector remained constant
- Optical-UV spectrum of HST-1 is very hard ($\alpha_{UV-O}\sim0.5$, $F_\nu\propto\nu^{-\alpha}$), displaying "hard lags" during epochs 2004.9-2005.5, including peak of flare, with soft lags at later epochs.
- Behavior of HST-1 interpreted as enhanced particle acceleration in a shock with cooling from both particle aging and relaxation of compression.
- Upper limits set on size and advance speed of flaring region at 0.5 $\delta$ parsecs and 1.02$c$, respectively.
- Slight deviation in electric vector orientation from jet PA suggests that on smaller scales, flaring region may have either double or twisted structure.
- -In contrast to HST-1's behavior, nucleus displays much more rapid variability with highly variable electric vector orientation and 'looping' in the $(I,P)$ plane but does not show UV-optical spectral variability.
- -Behavior can be interpreted as either a helical distortion to a steady jet or a shock propagating through a helical jet.
- -Study provides important insights into M87's jet dynamics and highlights how different regions within it exhibit vastly different behaviors even though they are part of one continuous structure.
- -Further studies will be necessary to fully understand complex dynamics of this jet and gain further insight into its dynamic properties such as size and speed limits for flaring regions as well as possible double or twisted structures on smaller scales than previously thought possible for this type of system.

## Summary

The M87 jet has been a site of extraordinary variability over the last decade, with one knot (HST-1) increasing in brightness by over a factor of 100. The nuclear flux also showed variability on timescales of months. In this study, the authors discuss the optical-UV polarization and spectral variability of these components, which exhibit vastly different behavior. HST-1 shows a highly significant correlation between flux and polarization, with P increasing from around 20% at minimum to over 40% at maximum, while the orientation of its electric vector remained constant. The optical-UV spectrum of HST-1 is very hard ($\alpha_{UV-O}\sim0.5$, $F_\nu\propto\nu^{-\alpha}$), displaying "hard lags" during epochs 2004.9-2005.5, including the peak of the flare, with soft lags at later epochs. The authors interpret the behavior of HST-1 as enhanced particle acceleration in a shock with cooling from both particle aging and relaxation of compression. They set upper limits on the size and advance speed of the flaring region at 0.5 $\delta$ parsecs and 1.02$c$, respectively. The slight deviation in electric vector orientation from jet PA suggests that on smaller scales, the flaring region may have either a double or twisted structure. In contrast to HST-1's behavior, the nucleus displays much more rapid variability with highly variable electric vector orientation and 'looping' in the $(I,P)$ plane but does not show UV-optical spectral variability. Its behavior can be interpreted as either a helical distortion to a steady jet or a shock propagating through a helical jet. This study provides important insights into M87's jet dynamics and highlights how different regions within it exhibit vastly different behaviors even though they are part of one continuous structure. Further studies will be necessary to fully understand the complex dynamics of this jet and gain further insight into its dynamic properties such as size and speed limits for flaring regions as well as possible double or twisted structures on smaller scales than previously thought possible for this type of system.