MAXI INVESTIGATION INTO THE LONG-TERM X-RAY VARIABILITY FROM THE VERY-HIGH-ENERGY γ-RAY BLAZAR Mrk 421

The archetypical very-high-energy gamma -ray blazar Mrk 421 was monitored for more than three years with the Gas Slit Camera on board Monitor of All Sky X-ray Image (MAXI), and its long-term X-ray variability was investigated. The MAXI light curve in the 3-10 keV range was transformed into the periodogram in the frequency range [functionof] = 1 x 10 super(-8)-2 x 10 super(-6) Hz. The artifacts on the periodogram, resulting from data gaps in the observed light curve, were extensively simulated for variations with a power-law-like power spectrum density (PSD). By comparing the observed and simulated periodograms, the PSD index was evaluated as alpha = 1.60 + or - 0.25. This index is smaller than that obtained in the higher-frequency range ([functionof] [> ~] 1 x 10 super(-5) Hz), namely, alpha = 2.14 + or - 0.06 in the 1998 ASCA observation of the object. The MAXI data impose a lower limit on the PSD break at [functionof] sub(b) = 5 x 10 super(-6) Hz, consistent with the break of [functionof] sub(b) = 9.5 x 10 super(-6) Hz suggested from the ASCA data. The low-frequency PSD index of Mrk 421 derived with MAXI falls well within the range of typical values among nearby Seyfert galaxies ( alpha = 1-2). The physical implications from these results are briefly discussed.