The results of 8-year R-band photopolarimetric data of blazar Mrk 421 collected from February 2008 to May 2016 are presented , along with extensive multiwavelength observations covering from radio to TeV \gamma -rays around the flares observed in May 2008 , March 2010 , and April 2013 .
The most important results are found in 2013 when the source displayed in the R-band a very high brightness state of 11.29 \pm 0.03 mag ( 93.60 \pm 1.53 mJy ) on April 10th and a polarization degree of ( 11.00 \pm 0.44 ) % on May 13th .
The analysis of the optical data shows that the polarization variability is due to the superposition of two polarized components that might be produced in two distinct emitting regions .
An intranight photopolarimetric variability study carried out over 7 nights after the 2013 April maximum found flux and polarization variations on the nights of April 14 , 15 , 16 and 19 .
In addition , the flux shows a minimum variability timescale of \Delta t = 2.34 \pm 0.12 hours , and that the polarization degree presented variations of \sim ( 1 - 2 ) % in a timescale of \Delta t \sim minutes .
Also , a detailed analysis of the intranight data shows a coherence length of the large-scale magnetic field of l _ { B } \simeq 0.3 pc which is the same order of magnitude to the distance traveled by the relativistic shocks .
This result suggests that there is a connection between the intranight polarimetric variations and spatial changes of the magnetic field .
Analysis of the complete R-band data along with the historical optical light curve found for this object shows that Mrk 421 varies with a period of 16.26 \pm 1.78 years .