2019ApJ...884..125Z

We present X-ray flux and spectral analyses of the three pointed Suzaku observations of the TeV high synchrotron peak blazar Mrk 421 taken throughout its complete operational duration. The observation taken on 2008 May 5 is, at 364.6 ks (i.e., 101.3 hr), the longest and most evenly sampled continuous observation of this source, or any blazar, in the X-ray energy 0.8-60 keV until now. We found large amplitude intraday variability in all soft and hard bands in all the light curves. The discrete correction function analysis of the light curves in soft and hard bands peaks on zero lag, showing that the emission in hard and soft bands are cospatial and emitted from the same population of leptons. The hardness ratio plots imply that the source is more variable in the harder bands compared to the softer bands. The source is harder when brighter, following the general behavior of high synchrotron peak blazars. Power spectral densities of all three light curves are red noise dominated, with a range of power spectra slopes. If one assumes that the emission originates very close to the central super massive black hole, a crude estimate for its mass, of ∼4 x 10⁸ M_☉, can be made; but if the variability is due to perturbations arising there that are advected into the jet and are thus Doppler boosted, substantially higher masses are consistent with the quickest seen variations. We briefly discuss the possible physical mechanisms most likely responsible for the observed flux and spectral variability.