  Strong X-ray flares from the blazar Mrk 421 were detected in 2010 January and
February through the 7 month monitoring with the MAXI GSC. The maximum 2 -- 10
keV flux in the January and February flares was measured as 120 +- 10 mCrab and
164 +- 17 mCrab respectively; the latter is the highest among those reported
from the object. A comparison of the MAXI and Swift BAT data suggests a convex
X-ray spectrum with an approximated photon index of about 2. This spectrum is
consistent with a picture that MAXI is observing near the synchrotron peak
frequency. The source exhibited a spectral variation during these flares,
slightly different from those in the previous observations, in which the
positive correlation between the flux and hardness was widely reported. By
equating 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 was
evaluated as B = 0.045 G $(\delta/10)^{-1/3}$, where $\delta$ is the jet
beaming factor. Assuming that the light crossing time of the emission region is
shorter 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. For
the February flare, the rise time, $t_{\rm r} < 1.3 \times 10^{5}$ s, gives a
loose 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, indicates
B = 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 a
superposition of unresolved events with a shorter timescale.

High Energy Astrophysical Phenomena;Cosmology and Nongalactic Astrophysics