2009ApJ...696.2170R

We have performed a structure function analysis of the Rossi X-Ray Timing Explorer All-Sky Monitor data to search for variability in 24 blazars using data trains that each exceed 12 yr. Although 20 of them show nominal periods through this technique, the great majority of these "periods" are clearly related to yearly variations arising from the instrument. Nonetheless, an apparently real periodic component of about 17 days was detected for the blazar AO 0235+164 and it was confirmed by discrete correlation function and periodogram analyses. For 1ES 2321+419, a component of variability with a near periodicity of about 420 days was detected by all of these methods. We discuss several possible explanations for these nearly periodic components and conclude that they most likely arise from the intersections of a shock propagating down a relativistic jet that possesses a helical structure.