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Strong X-ray flares from the blazar Mrk 421 were detected in 2010 January andFebruary through the 7 month monitoring with the MAXI GSC. The maximum 2 -- 10keV flux in the January and February flares was measured as 120 +- 10 mCrab and164 +- 17 mCrab respectively; the latter is the highest among those reportedfrom the object. A comparison of the MAXI and Swift BAT data suggests a convexX-ray spectrum with an approximated photon index of about 2. This spectrum isconsistent with a picture that MAXI is observing near the synchrotron peakfrequency. The source exhibited a spectral variation during these flares,slightly different from those in the previous observations, in which thepositive correlation between the flux and hardness was widely reported. Byequating the halving decay timescale in the January flare, $t_{\rm d} \sim 2.5\times 10^{4}$ s, to the synchrotron cooling time, the magnetic field wasevaluated as B = 0.045 G $(\delta/10)^{-1/3}$, where $\delta$ is the jetbeaming factor. Assuming that the light crossing time of the emission region isshorter than the doubling rise time, $t_{\rm r} \lesssim 2 \times 10^{4}$ s,the region size was roughly estimated as $ R < 6 \times 10^{15}$ cm$(\delta/10)$. These are consistent with the values previously reported. Forthe February flare, the rise time, $t_{\rm r} < 1.3 \times 10^{5}$ s, gives aloose upper limit on the size as $ R < 4 \times 10^{16}$ cm $(\delta/10)$,although the longer decay time $t_{\rm d} \sim 1.4 \times 10^{5}$ s, indicatesB = 0.015 G $(\delta/10)^{-1/3}$, which is weaker than the previous results.This could be reconciled by invoking a scenario that this flare is asuperposition of unresolved events with a shorter timescale.

Bright X-ray flares from the BL Lac object Mrk 421, detected with MAXI in 2010 January and February