We present the results of X-ray observations of the well-studied TeV blazar Mrk 421 with the Suzaku satellite in 2006 April 28 .
During the observation , Mrk 421 was undergoing a large flare and the X-ray flux was variable , decreasing by \sim 50 % , from 7.8 \times 10 ^ { -10 } to 3.7 \times 10 ^ { -10 } erg s ^ { -1 } cm ^ { -2 } in about 6 hours , followed by an increase by \sim 35 % .
Thanks to the broad bandpass coupled with high-sensitivity of Suzaku , we measured the evolution of the spectrum over the 0.4–60 keV band in data segments as short as \sim 1 ksec .
The data show deviations from a simple power law model , but also a clear spectral variability .
The time-resolved spectra are fitted by a synchrotron model , where the observed spectrum is due to a exponentially cutoff power law distribution of electrons radiating in uniform magnetic field ; this model is preferred over a broken power law .
As another scenario , we separate the spectrum into ‘ ‘ steady ’ ’ and ‘ ‘ variable ’ ’ components by subtracting the spectrum in the lowest-flux period from those of other data segments .
In this context , the difference ( ‘ ‘ variable ’ ’ ) spectra are all well described by a broken power law model with photon index \Gamma \sim 1.6 , breaking at energy \epsilon _ { brk } \simeq 3 keV to another photon index \Gamma \sim 2.1 above the break energy , differing from each other only by normalization , while the spectrum of the ‘ ‘ steady ’ ’ component is best described by the synchrotron model .
We suggest the rapidly variable component is due to relatively localized shock ( Fermi I ) acceleration , while the slowly variable ( ‘ ‘ steady ’ ’ ) component is due to the superposition of shocks located at larger distance along the jet , or due to other acceleration process , such as the stochastic acceleration on magnetic turbulence ( Fermi II ) in the more extended region .