Study of the Rapid Variability of the BL Lac Object MRK 421 in the Optical Range

MRK 421 is a giant elliptical galaxy in the Ursa Major constellation whose core has all the properties of a BL Lac object. It manifests a high activity and variability from the radio to gamma-ray ranges on time scales from a few hours to more than 10 years. MRK 421 has a large companion— a spiral, probably, also active galaxy (MRK 421-5). This paper presents the results of an analysis of observations (in the V, R, and I bands) as part of a joint Latvian-Ukrainian project for study of the variability of bright AGN. In addition, the light curves of MRK 421 from the AAVSO data base (American Association of Variable Star Observers) are analyzed to confirm the results of periodogram analysis. Ultimately, over the time interval studied here, MRK 421 is suspected of having a cyclical variability with a characteristic time in the range of 9.3-13.4 days determined by two different methods (approximation of the data by sinusoids and the CLEANest method), which is confirmed by an analysis of longer observations of MRK 421 in the V band (with AAVSO data). The longer variability of MRK 421 was also analyzed. It was found that in the V, R, and I light curves from AAVSO the maxima of the LS-periodograms correspond to periods of 1.6-1.7 years. This is close to earlier results from an analysis of historical light curves of MRK 421 obtained by other authors. The AAVSO data also indicate the presence in MRK 421 of a longer cycle of brightness variation with a characteristic time of 20-30 years.