Four years of extreme ultraviolet observations of Markarian 421. II. Temporal analysis

The Extreme Ultraviolet Explorer (EUV E) satellite accumulated similar to1, 000,000 s of public data between 1994 and 1997 for the BL Lacertae object M arkarian 421. This is the second of two papers in which we present the resu lts of spectral and temporal analysis of this EUV E data set. We analyze in the present paper the imaging data by means of power spectrum and structur e function techniques, while the spectral analysis is presented in a compan ion paper. We find for Mrk 421 a power spectrum with slope -2.14 +/- 0.28 w ith a break at similar to3 days. This is the first time that a break in the power spectrum of a BL Lacertae object has been found. We also find eviden ce of nonstationarity for Mrk 421 EUV emission.