Maxi investigation into the long-term X-ray variability from the very-high-energy γ-ray blazar Mrk 421

Abstract

The archetypical very-high-energy γ-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 f = 1 × 10-8-2 × 10-6 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 α = 1.60 ± 0.25. This index is smaller than that obtained in the higher-frequency range (f ≳ 1 × 10-5 Hz), namely, α = 2.14 ± 0.06 in the 1998 ASCA observation of the object. The MAXI data impose a lower limit on the PSD break at fb = 5 × 10-6 Hz, consistent with the break of fb = 9.5 × 10-6 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 (α = 1-2). The physical implications from these results are briefly discussed.